Physics
7. Australian
45 - northward moving plate that is subducting beneath the Eurasian and Pacific Plates, forming the volcanoes of Indonesia.
6. South American
45 - westward moving plate that is diverging from the African Plate at the Mid-Atlantic Ridge
13. Cocos
5 - small northeastward traveling plate that is subducting beneath the Caribbean and North American plates, forming the volcanoes of Central America and Mexico.
sierra
a high range of hills or mountains, especially one having jagged or irregular peaks that when projected against the sky resemble the teeth of a saw (e.g. the Sierra Nevada of California).
cliff
a high, steep rock face.
moulin
a hole in the ice of a glacier formed by swirling meltwater pouring down from the surface of the glacier.
well
a hole, generally cylindrical and usually walled or lined with pipe, that is dug or drilled into the ground to penetrate an aquifer within the zone of saturation.
wave-cut platform
a horizontal bench of rock formed beneath the surf zone as a coast retreats because of wave erosion.
*barrage
a low dam built across a tidal inlet for the purpose of capturing the energy of the changing tides (tidal energy).
levee
a low ridge of flood-deposited sediment formed on either side of a stream channel. Artificial levees are sometimes built by humans to contain a stream in its banks during heavy rain.
lateral moraine
a low, ridgelike pile of till along the side of a valley glacier.
KREEP
a lunar basalt enriched in potassium (K), the rare Earth elements (REE), and phosphorus (P).
gabbro
a mafic, coarse-grained igneous rock composed predominantly of ferromagnesian minerals and with lesser amounts of calcium-rich plagioclase feldspar. Gabbros are the result of plutonic crystallization of basaltic magma.
Mediterranean belt
a major volcanic belt of composite volcanoes, which includes Mount Vesuvius. It is the second largest belt of volcanoes, after the Ring of Fire.
topographic map
a map on which elevations are shown by means of contour lines.
altitude
a measure of height, usually taken to be the height above mean sea level.
butte
a narrow pinnacle of resistant rock with a flat top and very steep sides.
sediment trap
a natural or artificial device that serves to trap part of the sediments moving across or above the sea floor.
fringing reef
a reef attached directly to shore. It forms a shelf whose rough surface appears above the water at low tide.
skarn
a calcium carbonate-silicate mineral assemblage produced during the contact metamorphism of highly calcareous rocks. It is derived from nearly pure limestones and dolomites into which large amounts of Si, Al, Fe, and Mg have been introduced.
surf
a collective term for the breakers that breaks along a sure.
key bed
a bed (stratum) of material with sufficiently distinctive characteristics to make it easily identifiable in correlation. It often consists of a layer of volcanic ash that has been discovered throughout a wide region.
bajada
a broad, gently sloping, depositional surface formed at the base of a mountain range in a dry region by the coalescing of individual alluvial fans.
index fossil
a fossil from a very short-lived, but widespread, species known to have existed during a specific period of geologic time.
trace fossil
a fossilized remnants of the effects of an organism in the past, rather than the remains of the organism itself. Trace fossils may be the fossilized feces of an organism or structures produced by it that have been preserved in the sediment, such as tracks and burrows.
fault
a fracture in bedrock along the plane of which movement has taken place. The displacement of rock on one side relative to the other may occur in a horizontal, vertical, or oblique sense. See diagram below for types of faults.
joint
a fracture or crack in bedrock along which essentially no displacement has occurred.
inclusion
a fragment of rock that is distinct from the body of igneous rock in which it is enclosed. Examples of inclusions include xenoliths (a preexisting rock fragment caught up in a later rock), crystals of one mineral contained in another, fluid inclusions (volatiles trapped during crystal growth), and nodules (inclusions of gabbro and peridotite within alkali basalt).
marginal sea
a semi-enclosed sea adjacent to a continent.
coulee
a series of deep, branching channels that are formed as meltwater is suddenly released from a lake dammed by ice at the foot of a glacier. Well-known coulees occur on the Columbia plateau in Washington state.
point bar
a stream bar deposited on the inside curve of a steam, where the water velocity is low.
alpine
a type of climate found in mountainous areas above the timberline but below any permafrost level.
recessional moraine
an end moraine built during the retreat of a glacier, while the terminus remains temporarily stationary. A single receding glacier can build several recessional moraines.
plutonic rock
an igneous rock that crystallizes at depth and cools slowly, resulting in coarse grain size. Granite is an example.
trench suction
mechanism for plate motion that occurs when subducting plates fall into the mantle at angles steeper than their dip, causing trenches and the overlying plates to be pulled horizontally seaward toward the subducting plates.
discordant
not parallel to any layering or parallel planes. This term describes a rock unit that cuts across the bedding or foliation of adjacent rocks. Intrusive igneous rocks, such as dikes, show discordant relationships.
aphotic zone
ocean depth below which photosynthesis cannot occur because of lack of light. It normally stretches from about a depth of 200 m down to the bottom.
fluvial
of or relating to a river.
contour interval
on a topographic map, the vertical distance in elevation between one contour line and the next
horizon
one of a number of layers of soil arranged in a vertical sequence in the profile. Each layer is reasonably uniform.
disseminated ore deposits
ore deposits formed when ore-bearing solutions percolate upward between the grains of a rock and deposit very fine grains of or mineral throughout the rock. Most of the world copper comes from disseminated deposits.
parent rock
original rock before being metamorphosed.
carbon dating
the estimation of ages of archeological specimens of biological origin by comparing the ratio of radiocarbon (14C) atoms to stable carbon atoms (12C)
worm cast
the excretion of and earthworms, or a sinuous fossil trail of a worm, preserved as a sand cast on the bedding plane of an arenaceous rock.
*types of plate boundaries
three basic types of plate boundaries exist: convergent (plates move toward one another), divergent (plates move away from one another), and transform (plates slide past one another).
isostatic adjustment
vertical movement of sections of Earth's crust to achieve balance or equilibrium.
archipelago
a group of islands in fairly close proximity.
8. Nazca
15 - eastward moving plate that is subducting beneath the South American Plate, forming the Andes Mountains.
9. Indian
10 - northward moving plate that is colliding with the Eurasian Plate to the north, forming the Himalaya Mountains.
4. North American
60 - a westward moving plate.
Vesuvian (Type of Volcano)
Similar to strombolian an vulacanian, but with gas-charged lava being shot violently up into the air, emptying the lava column to a considerable depth.
submarine canyon
V-shaped valleys that run across the continental shelf and down the continental slope. Submarine canyons are caused by turbidity currents.
boulder
a sediment particle with a diameter greater than 256 mm.
seep
a spot where water or petroleum oozes from the Earth.
polar wandering curve
a theoretical line produced for a particular point on the Earth's surface by joining successive paleomagnetic pole positions through time.
lamination
a thin layer (less than 1 cm thick) in sedimentary rock.
reg
a type of arid desert plain consisting of gravel, especially in the Sahara.
soufrière
a volcano that emits sulfurous gases; a large solfatara.
inlier
an area of exposed rock that is surrounded by stratigraphically younger strata. Compare to outlier.
columnar jointing
basalt in the form of six-sided, parallel, vertical columns. These six-sided columns form as the basalt contracts as it cools after solidifying. Evenly space centers of contraction form, with tension cracks developing halfway between neighboring centers. A hexagonal fracture pattern is the most efficient way in which a set of contraction centers can share fractures. A classic example of columnar jointing is found at Devil's Postpile, California.
diapir
bodies of rock (e.g. rock salt) or magma that ascend within Earth's interior because they are less dense than the surrounding rock. If the rock is rock salt, a salt dome is formed by this method.
plastic
capable of being molded and bent under stress.
maria
lava plains on Moon's surface.
2. Regional metamorphism
metamorphism developed over large areas (entire regions) that have suffered intense deformation. The temperatures and pressures associated with this type of metamorphism are those incurred at depth within the Earth's crust.
meteorite
meteor that strikes Earth's surface.
Proterozoic
one of the eons of Precambrian time.
seismogram
paper record of the vibration of the Earth, usually generated by a seismograph.
concordant
parallel to layering or earlier developed planar structures.
pedogenesis
soil formation.
chronozone
the body of rock that has formed during one chron.
aeolian erosion
the direct erosive action of wind.
coprolite
the fossilized feces of an animal.
pedology
the scientific study of soils.
provenance
the source area from which the particles composing sediments are derived.
backshore
upper part of a beach, landward of the high-water line.
Darcy's law
the rate of flow of a fluid through a porous material (such as rock) is proportional to the product of the permeability of the material and the pressure causing the flow.
Poisson's ratio
the ratio between the fractional longitudinal strain and the fractional lateral strain in a deformed material. It is equal to the ratio of change in diameter divided by change in length.
bulk modulus
the ratio of compressive stress in rocks to the resulting change in volume.
axial modulus
the ratio of stress to strain in a material.
tortuosity
the ratio of the actual length of a river channel, measured along the middle of the main channel, to the axial length of the river.
EROEI (Energy Return on Energy Invested)
the ratio of the amount of energy extracted versus the amount of energy put into the extraction process. If it takes more energy to get petroleum out of the ground then is derived from the sale and consumption of that petroleum, that is no longer worth operating the oilfield.
S-wave shadow zone
the region on Earth's surface (at any distance more than 103 degrees from an earthquake epicenter) in which S waves from the earthquake are absent.
P-wave shadow zone
the region on Earth's surface, 103 to 142 degrees away from an earthquake epicenter, in which P waves from an earthquake are absent.
catena
the regular repetition of a characteristic sequence of soil profiles associated with a particular topography in the tropics.
slip
the relative displacement of formerly adjacent points on opposite sides of a fault, measured at the fault surface.
fossil
the remains of any organism that lived in the past. They are usually found in sedimentary rock.
plucking
the removal of a block of rock out of the bedrock by a glacier. It entails the freezing of the glacier ice onto bedrock and the subsequent plucking out of blocks on movement of the glacier. Already loosened blocks must be present, because the tensile strength of consolidated rock is far greater than that of ice. Freeze-thaw activity must be taking place to loosen the blocks, so this process is restricted to temperate glaciers.
sheet erosion
the removal of a thin layer of surface material, usually topsoil, by a flowing sheet of water.
deflation
the removal of clay, silt, and sand particles from the land surface by wind.
bioturbation
the reworking and further degradation of sediment by the action of organisms moving through it and feeding on it.
lithosphere
the rigid outer shell of Earth, composed of the crust and upper mantle (above the asthenosphere), that varies from 70 to 125 km or more thick.
*tide
the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of Earth. Tides are waves (tidal waves) with extremely long wavelengths of about 20,000 km (12,400 miles), which is half the circumference of the Earth. They are thus considered long waves and shallow-water waves, as they travel in depths of less than 1/20 their wavelength. Wind waves in the open ocean are considered deep-water waves, as they travel in depths greater than ½ their wavelength.
crustal rebound
the rise of Earth's crust after the removal of glacial ice.
magnetic stripes
the roughly parallel bands of ocean floor with alternating directions of magnetic induction (the bands alternate in terms of magnetic polarity). These bands result from the process of seafloor spreading.
geochemical (rock) cycle
the route followed by an element or group of elements through the Earth's crustal and subcrustal rocks. Weathering of igneous rocks produces particles that settle as sediments, which may become sedimentary rock or undergo metamorphism into metamorphic rocks. Igneous rocks can also be converted by heat and pressure into metamorphic rocks. These sedimentary, metamorphic, and igneous rocks may melt and form new magma, which rises to the surface and crystallizes as new igneous rock. This cycle is quite often called the rock cycle.
geodesy
the science concerned with determining the exact shape and size of the Earth and the exact position of points on the surface of the Earth.
eluviation
the washing out of fine material from a soil, especially from the upper part. The eluvial horizon is the A horizon, which has less clay than the rest of the profile as a result.
congelifraction
the weathering of rocks through the freezing of water within pore spaces. The expansion of the water upon freezing creates stresses which result in fracture of the rock.
denudation
the weathering of rocks, the entrainment of debris, and its subsequent transport and deposition. The highest rates of denudation recorded, reaching 3000 mm/1000 years, occur in glaciated areas; the lowest rates, about 1.2 mm/1000 years, occur in hot, dry lowlands.
Quarternary Period
the youngest geologic period, which includes the present time. It is formed of two epochs, the Pleistocene and Holocene, and extends from 1.6 mya until the present.
zone of aeration
the zone below the ground surface and above the water table in which the pore spaces and openings within the soil, sediments, and raw contain mainly air. Water in the zone is held by capillarity.
zone of saturation
the zone below the ground surface were all cracks and the pore spaces in sediments or rock are filled with water. The upper boundary of the zone of saturation forms the water table.
overturned fold
they fold in which the axial plane is inclined to such a degree that the fold limbs dip in the same direction.
thorium-232 dating
thorium-232 decays to lead-208 with a half-life of 14.1 billion years, and thorium-232 dating is effective for dating things that are between 10 million and 4.6 billion years old.
intermediate rocks
those rocks with a chemical content between that of felsic and mafic (their silica content is between 50 and 65%.
*spring tide
tides of large amplitude that occur when the sun, moon, and Earth are aligned in a straight line (a position called syzygy), which occurs during the new and full moons. Spring tides have a large tidal range, meaning the difference between high and low tide on a particular day is large.
16. Gorda
tiny, eastward moving plate that is sometimes just considered to be a southern extension of the Juan de Fuca Plate. The Gorda Plate is subducting beneath the North American Plate, and is responsible for volcanoes in Northern California, such as Mt. Shasta and Mt. Lassen.
rafting
transportation of sediment, rocks or soil by means of attachment to ice, plants, or other floating material.
truncated spur
triangular facet where the lower end of a ridge has been eroded or beveled by a moving glacier.
lava tube
tunnel-like cave within a lava flow that forms during the late stages of solidification of a mafic lava flow. It is created when lava below the surface continues to flow after the surface has become solid.
decompression melting
type of melting that occurs when a body of hot mantle rock moves upward and the pressure is reduced to the extent that the melting point drops to the temperature of the body. The melting point of a mineral decreases with decreasing pressure. Temperatures over 1000°C are required to create basaltic magma, whereas a granitic magma can be created at a temperature as low as 650°C. Mixed fragments of two minerals often melt at a lower temperature than either mineral alone.
flow
type of movement in mass wasting in which the descending mass is moving downslope as a viscous fluid.
British Thermal Unit (BTU)
unit used to measure the heat released by burning coal. One BTU is equivalent to 252 cal of heat (the amount of heat energy it takes to raise 1 pound of water from 62 to 63°F). A pound of ordinary bituminous coal typically contains 45 to 86% carbon and releases 10,500 to 15,000 BTUs. Anthracite produces about 15,000 BTUs per pound, whereas high octane gasoline produces 18,500 BTUs. A pound of diesel fuel, the most energy-dense of all fossil fuels, produces about 19,000 BTUs. It takes an especially strong, heavy engine to combusted diesel fuel; hence, its use in trucks, trains, and other bulky machinery.
stoping
upward movement of a body of magma by fracturing of overlying country rock. Magma engulfs the blocks of fractured country rock as it moves upward.
*eustatic change
a global change in sea level. An example would be during the height of the last ice age (the last glacial maximum - about 19,000 years ago), sea level dropped about 130 meters lower than it is today.
rift
a graben associated with divergent plate boundaries, either along mid-ocean ridges or on continents.
abyssal hill
a large, dome-shaped, submarine hill on the abyssal plain that rises to a height of up to 1000 meters. Abyssal hills are the most common geologic feature found on the abyssal plain.
inselberg
a large, domelike residual hill of hard rock, characterized by very steep slopes rising abruptly from the surrounding surface. They are more typical of humid, tropical climates. The main reason for their occurrence is differential deep weathering, followed by subsequent exhumation, which exposes the more massive, less weathered parts of the weathering profile as inselbergs.
glacier
a large, long-lasting mass of ice, formed on land by the accumulation, compaction, and recrystallization of snow, which moves because of its own weight.
isotopes
atoms of the same element that have different numbers of neutrons but the same number of protons.
Tethys Sea (Tethys Ocean)
before the northward movement of Africa a relative to Europe, a large ocean existed, of which the Mediterranean Sea is the surviving remnant. Within this ocean, which existed for a long period of time, the sentiments of the Alps and Himalayan mountains were deposited. As the ocean closed as a result of seafloor spreading this wedge of sediments was compressed into the present day mountain chains and associated orogenic features.
spheroidally weathered boulder
boulder that has been rounded by weathering from an initial blocky shape.
sound
a relatively long arm of the sea or ocean, forming a channel between an island and a mainland or connecting two larger bodies, as a sea and the ocean, or two parts of the same body; it is usually wider than a strait.
strait
a relatively narrow waterway between two larger bodies of water.
shoal
a relatively shallow place in a body of water, or a submerged ridge, bank, or bar of sand rising from the bed of a body of water to near the surface so as to constitute a danger to navigation. It may be exposed at low water.
right-lateral fault
a strike-slip fault in which a stream crossing the fault appears displaced to the right. In a left-lateral fault, a person walking along a stream displaced by the fault would see that it is displaced to the left.
land bridge
a strip of land that joins two larger landmasses and may provide a route for the migration or spread of plants and animals.
rock avalanche
a very rapidly moving, turbulent mass of broken-up bedrock. The only difference between a rock avalanche and he debris avalanche is that a rock avalanche begins its journey as bedrock.
ophiolite
distinctive rock sequence found in many mountain ranges on continents. Ophiolites are thought to represent slices of the basaltic oceanic crust that have been tectonically emplaced onto continental margins.
Mauna Kea
dormant shield volcano on the Big Island of Hawaii that is the world's tallest mountain (as measured from its base to its summit) - it is about 10,000 m (33,000 ft) tall, but only 4,205 m (13,796 ft) are above sea level. Mt. Everest attains the highest altitude above sea level (8,848 m, or 29,028 ft).
terrigenous sediment
land-derived sediment that has found its way to the sea floor.
Popocatepetl
large composite volcano located just east of Mexico City.
bomb
large spindle- or lens-shaped pyroclast. A bomb is formed when lava is ejected into the air, and the molten blob becomes streamlined during flight, solidifies, and falls to the ground.
barrier island
ridge of sand paralleling the shoreline and extending above sea level. The lagoon in between the island and the mainland is of mangrove type in tropical areas and salt marsh type in temperate areas.
ultramafic rock
rock composed entirely or almost entirely of ferromagnesian minerals.
organic sedimentary rock
rock composed mostly of the remains of plants and animals. An example is coal.
talus
rock fragments, usually coarse and angular, lying at the base of the cliff or steep slope from which they have been derived.
coarse-grained rock
rock in which most of the grains are larger than 1 mm (igneous) or 2 mm (sedimentary).
evaporite
rock that forms from crystals precipitating during evaporation of water. Evaporation from virtually closed bodies of saline water is the most common process involved. Evaporites are a form of sedimentary rock. Rock gypsum and rock salts are common evaporates. Modern evaporate deposits are forming in the Persian Gulf and Red Sea, and were formed in the past where shallow, continental seas existed in hot, arid climates (such as the Bonneville salt flats in Utah).
sedimentary rock
rock that has formed from 1) lithification of sediment, 2) precipitation from solution, or 3) consolidation of the remains of plants or animals. Sedimentary rocks include clastic rocks formed of land sediments, including clay, silt, sand, and gravel; consolidated types are sandstone and shale. Organic sedimentary rocks, usually lacking land sediment, include limestone (from shell material of animals) and coal (derived from plants). Chemical sedimentary rocks are those formed by chemical processes, and include the evaporites.
jetty
rock wall protruding above sea level, designed to protect the entrance of a harbor from sediment deposition and storm waves; jetties are normally built in pairs.
lapilli
round fragments of solid lava thrown out by an erupting volcano which are between 2 and 65 mm across in size. They are one type of ejecta, and are considered to be small volcanic bombs.
chondrule
round silicate grain within some stony meteorites. It is usually made of olivine or enstatite.
gravel
rounded particles coarser than 2 mm in diameter (pebble, cobble, and boulder)
pillow basalt (pillow lava)
rounded, closely shaped masses formed when lava enters the ocean. It has the appearance of pillows piled one upon another. The outer skin of the lava is chilled on extrusion and a bubble of lava grows, flattening under its own weight to produce the characteristic shape.
rubidium-87
rubidium-87 decays to strontium-87 with a half-life of 49 billion years, and rubidium-87 dating is effective for dating things that are between 10 million and 4.6 billion years old.
poorly sorted
said of a clastic sediment or rock that consists of particles of many sizes mixed together in an unsystematic manner so that no one size class predominates (it is the opposite of well-sorted).
sour
said of crude oil or natural gas containing significant fractions of sulfur compounds.
sweet
said of crude oil or natural gas to contains few or no sulfur compounds.
unda
said of the environment of sedimentation that lies in the zone of wave action.
*cosmogenous sediment
sediment composed of particles from space, such as meteorites.
geoid
the Earth pictured as a smooth, oblate spheroid, coinciding with sea level and taken to continue across continents at the same level. At all points on Earth, the geoid surface is at right-angles to the downward direction of the pull of gravity.
sima
the Earth's oceanic crust, which is composed of basaltic rock types that are rich in both silica (Si) and magnesium (Mg).
hinge line
the axis of a fold. Anticlines and synclines are plunging folds, which means that the hinge lines are not horizontal.
foreshore
the beach zone that is regularly covered and uncovered by the rise and fall of the tides.
refraction
the bending of a wave as it comes into shallow water, causing its direction to become more nearly parallel to the shoreline.
bathyal zone
the benthic zone in the ocean that encompasses the continental slope. It normally embraces depths of about 200 m down to about 1000 m.
streambed
the bottom of the channel through which a stream is flowing.
grade
the degree of metamorphic change undergone by a rock. High-grade metamorphic rocks are produced under conditions of high temperatures or pressures, whereas low-grade rocks form at low temperatures and pressures.
aquitard
a body of rock that prevents the movement of groundwater. Aquitards retard the flow of groundwater. Shale and crystalline rocks such as granite are aquitards.
aquifer
a body of saturated rock or sediment through which water can move readily, yielding economic supplies of groundwater, either as springs or in wells. Aquifers are both highly permeable and saturated with water. A well must be drilled into an aquifer to reach an adequate supply of water. Good aquifers include sandstone, conglomerate, well-jointed limestone, bodies of sand and gravel, and some fragmental or fractured volcanic rocks such as columnar basalt. Shale is relatively impermeable, and thus is a poor aquifer. Crystalline rocks such as granite, gabbro, gneiss, and schist are not good aquifers.
delta
a body of sediment deposited at the mouth of a river when the river velocity decreases as it flows into a standing body of water (sea or lake). Deltas are a result of the decrease in load-carrying capacity following the deceleration of river water on entering the comparatively static sea or lake. For the sediment to accumulate, the amount of material deposited by the river must exceed that removed by coastal erosion. The river tends to bifurcate (divide) into distributaries after a certain amount of deposition has occurred, which then deposit material over a wider area. A wave-dominated delta is one that contains barrier islands along its oceanward side; the barrier islands form by waves actively reworking the deltaic sediments. The Nile delta is an example of a wave-dominated delta. In tide-dominated delta is, strong ties reshape the sediment into tidal bars that are aligned parallel to a tidal current. The Ganges-Brahmaputra Delta in Bangladesh is a good example. In a stream-dominated (or river-dominated) delta, very large amounts of sediment are carried into relatively quiet water. The stream sedimentation forms fingerlike distributaries, which look like long fingers projecting out into the sea. Because of their appearance, stream-dominated deltas are sometimes called bird-foot deltas. The Mississippi River Delta is an example of a stream-dominated delta. Many deltas, particularly small ones in freshwater lakes, are built up from three types of deposits: foreset beds, topset beds, and bottomset beds. Foreset beds form the main body of the delta, and are deposited at an angle to the horizontal. On top of the foreset beds are the topset beds, which are nearly horizontal beds of varying grain size form by distributaries shifting across the delta surface. Out in front of the foreset beds are the bottomset beds, which are deposits of the finest silt and clay carried out into the lake by the river water flow or by sediments sliding downhill on the lake floor.
moraine
a body of till either being carried on a glacier or left behind after a glacier has receded.
lake
a body of water completely surrounded by land, lying in a depression in the Earth's surface. Most contain fresh water, although some large ones, such as the Caspian Sea and Dead Sea, have saline water.
ventifact
a boulder, cobble, or pebble with flat surfaces caused by the abrasion of wind-blown sand. If a ventifact becomes worn in such a way that a roughly triangular cross-section involves, it is known as a dreikanter.
convergent plate boundary
a boundary separating two plates that are moving toward each other. If an oceanic plate and a continental plate converge, the oceanic plate subducts (dips below) the continental plate (because the oceanic plate is denser). If two oceanic plates meet, the older, and therefore denser, of the two subducts beneath the other. If two continental plates collide, neither subducts. Instead, the two plates buckle and deform, with significant uplift (mountain building) occurring (an example is the Indian Plate colliding with the Eurasian Plate, forming the Himalaya Mountains).
selenology
a branch of astronomy that deals with the study of the Moon, including lunar geology.
paleobiology
a branch of paleontology dealing with the study of fossils as organisms rather than as features of historical geology.
hiatus
a break in a stratigraphic sequence, either as a result of nondeposition or erosion. It represents the period of time missing between beds above and below an unconformity.
kimberlite
a brecciated, carbonate-rich peridotite found in pipes and diatremes piercing very old metamorphic rocks. Many kimberlites contain diamonds.
saturation
the degree to which the pores in a rock contain oil, gas, or water, generally expressed in percent of total pore space.
*illuviation
the deposition of substances such as iron, clay, aluminum, and organic compounds in the B horizon via leaching from the E horizon.
delamination
the detachment of part of the mantle portion of the lithosphere beneath a mountain belt. During an orogeny, the crust as well as the underlying lithosphere mantle thickens. The lithosphere mantle is cooler and denser than the asthenosphere mantle. The thickened portion of the lithosphere mantle is gravitationally unstable, so after it is softened from convicting asthenosphere, it breaks often sinks through the asthenosphere to a lower level in the mantle. Hot asthenosphere mantle flows into replace the foundered, colder mantle. Heating of the crust follows, allowing the lower crust to flow. The once-sick crust becomes thinner than that of adjoining regions of the mountain belt. Extension results in block-faulting in the upper part of the crust.
correlation
the determination of time equivalency of rock units. It involves matching rocks of the same age at different locations. Rock units may be correlated within a region, a continent, and even between continents. Strata may be correlated on the basis of similarities in lithology, or biostratigraphically on the basis of similar contained fossils. Other methods of correlation include radiometric age determinations or paleomagnetic information.
hypsometric tinting
the differential coloring of elevation bands on a map to enable the user to determine quickly the higher and lower areas of the land portrayed. Greens usually portray lowland areas; as the land rises, the colors change from yellows to browns to purples. This technique is commonly used on in atlases and wall maps.
depth of focus
the distance between the focus and epicenter of an earthquake. Earthquakes are classified into three groups according to their depth of focus: shallow focus (0-70 km deep), intermediate focus (70-350 km deep), and deep focus (350-670 km deep). Shallow focus earthquakes are most common (about 85% of earthquakes). Deep focus quakes account for only 3% of earthquakes, and are rarer because most people rocks flow in a ductile manner when stressed or deformed (they are unable to store and suddenly release energy as brittle surface rocks do).
rock slide
the rapid sliding of a massive bed rock along an inclined surface of weakness, such as a bedding plane, a major fracture in the rock, or a foliation plane. Once sliding begins, a rock slab but usually breaks up into rubble.
superposition, law of
the order in which sedimentary rocks occur in strata one above the other, the highest bed being the youngest.
petrogenesis
the origin or mode of formation of rocks.
crust
the outer layer of rock, forming a thin skin over Earth's surface. It varies from 5 km thick under the oceans to 60 km under mountain ranges. Oceanic crust averages 7 km in thickness, and varies from 5 to 8 km. Oceanic crust is composed of four layers. The upper layer is made of sediments and sedimentary rock, the second layer is composed of basalt, the third layer is made of gabbro, and the deepest layer is made primarily of peridotite. Oceanic crust is denser than continental crust. It has a density of about 3.0 g/cm3, whereas continental crust has a density of about 2.7 g/cm3. Continental crust is much thicker than oceanic crust, averaging 30 to 50 km in thickness, though it varies from 20 to 70 km thick. Continental crust is thickest under geologically young mountain ranges, such as the Andes and Himalayas. Continental crust is sometimes called granitic, because it contains granite, as well as other plutonic rocks, schist, and gneiss. The lower boundary of the crust is marked by the Mohorovicic discontinuity. The crust is composed of two units, the continental crust (also known by the acronym sial, standing for silica/aluminum) and the oceanic crust (also known as sima, standing for silica/magnesia).
ebb tide
the outgoing of the tidal stream (the retreating tide). That part of a tidal cycle following the high-water stage and preceding the low-water stage. Compare to flood tide.
hanging (head) wall
the overlying surface of an inclined fault plane. It is the surface of rock above a fault plane or ore body. Compare to footwall.
bedding
the parallel layering of sedimentary rocks.
estuary
the part of a river mouth within which tides have an effect, and therefore where fresh and saline water are mixed. Most present-day estuaries are drowned river valleys, owing their existence to the post-glacial rise in sea level.
dissolved load
the part of a river's load carried in chemical solution. Rainwater, being mildly acidic, can dissolve rocks, especially limestone, and then feed that dissolved content into a river via groundflow. One estimate is that rivers in the United States carry about 250 million tons of solid load and 300 million tons of dissolved load each year.
intertidal
the part of the seashore between the high- and low-water marks.
porosity
the percentage of a rock's volume that is taken up by openings. Porosity is a measurement of the ability of a rock to hold water. Some sedimentary rocks, such as sandstone, conglomerate, and many limestones, tend to have a high porosity and therefore can hold a considerable amount of water.
Ordovician
the period in the Paleozoic Era that followed the Cambrian and preceded the Silurian.
Jurassic Period
the period of the Mesozoic Era that followed the Triassic and preceded the Cretaceous. It lasted from 208 to 144 mya, and was named for the Jura Mountains in Europe, where rocks of this age were first studied. Continental rifting of Pangaea began toward the end of the Triassic, and by the Middle Jurassic n. America was moving away from Eurasia and Africa and the Atlantic Ocean was opening. Fossils of both Archaeopteryx, the earliest known bird, and the first mammals come from Jurassic rocks. Dinosaurs continued to flourish during the Jurassic, and aerial pterosaurs and marine reptiles evolved. Important marine invertebrates included corals, brachiopods, bivalves, and ammonites.
Carboniferous Period
the period of the Paleozoic Era that followed the Devonian and preceded the Permian. It began 360 mya, and lasted until about 286 mya. North American geologists divide the Carboniferous into two periods, the Mississippian and the Pennsylvanian, corresponding to the Lower and Upper Carboniferous, respectively. During this period, Gondwanaland (present-day Africa, South America, India, the Middle East, Australia, and Antarctica), the southern supercontinent, and Laurasia (present-day North America, Greenland, and N. Europe) had formed. By the end of the period, these continental plates had collided, closing the Tethys Sea ad forming one large supercontinent, Pangaea. Large, tropical swamps extended across what is now N. America, Europe, and Siberia, from which the coal deposits of the Upper Carboniferous formed (the reason behind the name).
Silurian
the period of the Paleozoic coming after the Ordovician Period and before the Devonian Period.
Devonian Period
the period of the upper Paleozoic Era, which began 408 mya and ended about 363 mya. The Devonian followed the Silurian and was followed by the Carboniferous Period. It is named for Devon, England, where these rocks were first recognized as a major group. Cephalopod mollusks were common during this period. Jawed fishes, including placoderms, were also common. By the end of the period, primitive amphibians had evolved from certain crossopterygian fishes. Ferns and horsetails were present on land, as well as insects and spiders.
sea arch
a bridge of rock left above openings eroded in headlands or stacks by waves. The openings are eroded in spots where the rock is weaker than normal.
saddle
a broad shallow depression in a mountain ridge forming a pass.
rise
a broad, elevated area of the sea floor, similar to a mid-ocean ridge but less steep and lacking a median rift valley. Like mid-ocean ridges, rises are spreading centers, but the rate of spreading is faster than at a ridge, resulting in a less steep profile.
mesa
a broad, flat-topped hill bounded by cliffs and capped with a resistant rock layer.
pediment
a broad, gently sloping erosional surface or plain of low relief, typically developed by running water, in an arid or semiarid region at the base of an abrupt and receding mountain front. It is underlain by bedrock and is usually mantled with a thin veneer of gravel or alluvium. It can be difficult to distinguish a pediment from the surface of the associated bajada downhill, because both have the same slope and gravel cover. The pediment, however, is an erosional surface, usually underlain by solid rock, while the bajada surface is depositional and may be underlain by hundreds of meters of sediment.
peat
a brown, lightweight, unconsolidated or semi-consolidated deposit of plant remains. The formation of peat is the first step in coalification (formation of coal).
fold
a buckling of bedded sedimentary rocks due to deformation processes or the effect of gravity. Folds are bends or wavelike features in layered rock. Folds may be anticlines (upward arching) or synclines (downward arching).
tectogene
a buckling of the earth's crust that results in the down-buckling of granitic rocks deep into the earth's crust to form the roots of mountains. It also results in the upward-buckling of the shallower levels in the crust to form orogenic mountain belts.
paleosol
a buried soil, that may become buried by volcanic ash, wind-blown dust, glacial deposits or other sediment, or lava.
sea cave
a cavity eroded by wave action at the base of a cliff.
vug (vugh)
a cavity within a volcanic rock lined with crystals of a different mineral composition from the enclosing rock.
cordillera
a chain of mountains, such as the parallel chains of the Andes in South America or the Rockies, Sierra Nevada, and Coast Ranges of North America.
reverse fault
a fault in which the hanging wall block moved up relative to the footwall block. Horizontal compressive stresses cause reverse faults. Reverse faults tend to shorten the crust.
normal fault
a fault in which the hanging-wall block moved down relative to the footwall block. The angle of dip is normally 45-90°. Normal faults are caused by rocks being pulled apart, whereas reverse faults are due to compression all forces. Normal faults result in extension or lengthening of the crust.
transform fault
a fault in which two plates are sliding past each other, or the portion of a fracture zone between two offset segments of a mid-ocean ridge crest. Transform faults have strike-slip displacement and are oriented at right angles to the mid-ocean ridges, which they offset for several tens or even hundreds of kilometers.
hinge (pivot) fault
a fault on which the movement of one side pivots (hinges) about an axis perpendicular to the fault plane. Displacement increases with distance from the hinge (pivot point).
oblique-slip fault
a fault with both strike-slip and dip-slip components.
honeycomb weathering
weathering within jointed rocks in which joint infillings are more resistant to weathering than the main mass of the rock, so that after erosion they project and surround recesses in the rock.
eustasy
worldwide movements of sea level, attributed to the withdrawal and release of water due to growth and decay of ice masses. Eustasy can also be caused by tectonic movements of the sea floors or land masses.
amber
yellow, translucent, fossilized resin that was once exuded by trees and often entrapped insects prior to hardening.
striations
1) on minerals, extremely straight, parallel lines; 2) in glaciology, straight scratches in rock caused by abrasion by a moving glacier. Glacial striations are produced by sharp-edged, larger particles that are carried along the base of the glacier. Striations indicate the direction of ice movement.
15. Juan de Fuca
2 - small, eastward moving plate that is subducting beneath the North American plate, forming the Cascade Mountains of Oregon and Washington (this is called the Cascadia Subduction Zone).
12. Caribbean
5 - small plate covering the region of the Caribbean Sea. The plate is responsible
14. Scotia
5 - small plate wedged in between the South American Plate to the north and the Antarctic plate to the south.
11. Philippine
6 - small plate that is subducting beneath the Eurasian Plate to the west, forming part of the Japanese archipelago
5. Antarctic
60 - large plate covering Antarctica and extending out into the Pacific, Atlantic, and Indian oceans.
3. Eurasian
70 - this eastward moving plate it is diverging from the North American Plate at the Mid-Atlantic Ridge.
10. Arabian
8 - northward moving plate that contains the Arabian Peninsula. It is diverging from the African plate at the Red Sea Rift, and it is colliding with the Eurasian Plate to the north.
2. African
80 - the second largest plate. Its collision with the Eurasian Plate to the north formed the European Alps.
Plinian (Type of Volcano)
A culmination of vesuvian in which a violent blast of gas rises to a height of several kilometers. The gas and vapor on reaching this height spreads into a large cloud. The fallout of ash is low, being confined to material removed from the conduit.
geosyncline
A linear part of the crust of the Earth that sagged deeply through time, that is, a great trough hundreds of kilometers long and tens of kilometers wide that subsided as it received thousands of meters of sedimentary and volcanic rocks through millions of years. It normally consists of sediment deposited by rivers within a long, narrow, subsiding belt of the sea which is usually parallel to a plate margin (along the shore).
Bombs and blocks.
Bombs are large fragments with a rounded to subangular shape and erupted in a plastic condition. During flight, aerodynamically modified shapes are produced, the most common being a spindle form. Blocks are large angular fragments ejected in a solid condition.
Vulcanian (Type of Volcano)
Characterized by viscous lava whose surface rapidly solidifies. Beneath this crust gas accumulates and builds up pressure until the crust shatters. This results in large quantities of pyroclastic deposits ranging from large bombs to fine ash.
Hawaiian (Type of Volcano)
Fairly quiet eruptions in which fluid lava (basaltic) is erupted from fissures or pits, with gas being liberated freely. When the eruption is accompanied by spurting gases, incandescent spray is thrown into the air.
Airy's hypothesis of isostasy
George Airy proposed that in order for isostatic equilibrium to exist, mountain ranges must have roots proportional to their height (the highest mountains have the deepest roots). These roots cause the gravity anomalies present near mountain chains).
Tambora
Indonesian volcano whose 1815 eruption was the largest eruption in recorded history, and the largest since the Lake Taupo eruption in about 180 A.D. The eruption scored a 7 on the Volcanic Explosivity Index.
Krakatoa (Krakatau)
Indonesian island volcano that erupted in 1883 with the force of several hydrogen bombs. Two-thirds of the island was destroyed, and only 1/3 remained above sea level. The destroyed section became a 300-meter deep, underwater depression. The explosion was heard 5,000 km away. About 34,000 people died as a result of the tsunamis generated by the explosion.
Strombolian (Type of Volcano)
More violent than the Hawaiian type, the eruption taking place more spasmodically as trapped gas escapes from a more viscous lava confined in a crater. Eruptions may occur every few minutes. During violent activity bombs are ejected, which may be accompanied by lava flows.
Methane (CH4)
a component of natural gas that is used in fertilizer manufacture and a source of hydrogen for fuel cells.
Lapilli.
Round to angular fragments of diameter between 64 and 2 mm. Most of the irregular vesicular fragments known as cinders and scoriae are of lapilli size. A special form of lapilli is known as Pele's tears.
Ash.
Tephra fragments less than 2mm in diameter are known as ash. Consolidated ash is known as tuff.
lava
magma on Earth's surface.
intrusion
a body of intrusive rock classified on the basis of size, shape, and relationship to surrounding rocks. Also, intrusion is the process by which intrusive rocks are formed.
Pelean (Type of Volcano)
Very violent eruptions accompanied by nuées ardente (swiftly flowing, turbulent gaseous clouds that contain ash and other pyroclasts in their lower reaches; synonymous with pyroclastic flow) and hot avalanches of incoherent constantly expanding self-explosive lava, lubricated by hot gases and vapors. Such eruptions are characteristic of highly viscous lava, whose gas content cannot readily escape.
peninsula
a body of land nearly surrounded by water, and connected with a larger body of land by an isthmus.
diapir (mantle diapir)
a body of mantle rock, hotter than its surroundings, that ascends because it is less dense than the surrounding rock.
foliation
a banded or laminated structure within a metamorphic rock, resulting from the metamorphic segregation of minerals into compositionally different layers, parallel to the schistosity. Variations of foliation exist, such that three very different textures can be found in foliated metamorphic rocks. The three textures, order from lowest to highest degree of metamorphism, are: 1) slaty, in which the rock splits easily along nearly flat and parallel planes, indicating that pre-existing, microscopic, platy (flat) minerals were realigned during metamorphism - this type of rock is said to possess slaty cleavage, 2) schistose, is visible platy or needle-shaped minerals have grown essentially parallel to a plane due to differential stress, and 3) gneissic, if the rock became very ductile and the new minerals separated into distinct (light and dark) layers or lenses.
tombolo
a bar of marine sediment connecting a former island or sea stack to the mainland. It is a type of coastal spit.
scoria
a basalt that is highly vesicular. It is actually a vesicular crust on the surface of lava flows, the cellular nature of which is due to the escape of volcanic gases before solidification. It is heavier, darker, and more crystalline than pumice. Cinder is sometimes used synonymously.
crater
a basin-like depression over a vent at the summit of a volcanic cone. The bowl-shaped depression of a crater results from volcanic activity, outgassing, or the impact of a meteorite.
Burgess shale
a bed of black, middle Cambrian shale in British Columbia, Canada. It is the side of significant discoveries of the fossils of invertebrate animals, which occur as films of carbon between the bedding planes of rock.
marker bed
a bed of rock having some distinctive characteristic of lithology or contained fossils that permits its easy recognition wherever it occurs. Such a bed can often be inferred to represent a very short period of time. Marker beds have great use in stratigraphy and in the correlation of strata. It is equivalent to a key bed.
monocline
a bending of sedimentary rock strata due to a normal fault. Two areas of horizontally bedded sediments are left at different elevations but still connected by a steeply inclined series of the same beds.
oil shale
a black or brown shale with a high content of organic matter from which oil may be extracted by distillation. The best known oil shale in the United States is the Green River Formation which covers more than 40,000 km² in Colorado, Wyoming, and Utah. The oil shale, which includes numerous fossils of fish skeletons, formed from mud deposited on the bottom of large, shallow Eocene lakes. The organic matter came from algae and other organisms that lived in the lakes. The Green River formation yields about 25 gallons of oil per Of Rock.
bituminous coal
a black or dark brown coal with a carbon content intermediate between anthracite and lignite. It is the type of coal most commonly used as fuel. Bituminous coal and sub-bituminous coal are sometimes called soft coal. These types of coal are often banded with layers of different plant material. They are dusty to handle, ignite easily, and burn with a smoky flame.
ground moraine
a blanket of till deposited from the base of a glacier on the melting of the ice. It forms a cover, usually tens of meters thick, that tends to mask former bedrock features.
iceberg
a block of glacier-derived ice floating in water which exceeds 5 m in height. Many result from calving, the breaking off of fragments of glacier ice, which fall into the sea. Icebergs float partly above, but largely beneath the surface of the sea. Usually 80-90 percent of an iceberg is below sea level.
magnetic reversal
a change in Earth's magnetic field between normal polarity and reversed polarity. In normal polarity, the north magnetic pole, where magnetic lines of force enter the Earth, lies near the geographic North Pole. In reversed polarity, the south magnetic pole, where lines of force leave Earth, lies near the geographic North Pole (magnetic poles have exchanged positions). The magnetic field on earth reverses on average about once every 500,000 years. The present normal orientation has lasted for the past 700,000 years.
*shear vorticity
a change in wind speed over a change in distance.
ice fall
a chaotic jumble of crevasses that split a glacier ice into pinnacles and blocks. Ice falls occur when a section of a glacier moves down a very steep slope.
landform
a characteristically shaped feature of Earth's surface, such as a hill or a valley.
geologic timescale
a chronological series of events in the geologic history of the Earth, beginning in the Precambrian and extending to the present (the Holocene epoch of the Quaternary period). it is divided into eons, eras, periods, epochs, and ages, each ascribed dates in terms of millions of years ago (mya). You should know in detail the geologic timescale shown in Figure 1 at the back of this packet (anything on this chart is fair game for the test, so make lots of flashcards on this!).
salt dome
a circular dome-like structure formed by the upward movement of a column of salt (halite), generally below strata of sedimentary rock. The core of the dome is called a salt plug, which is usually 1-2 km in diameter. The rocks below the dome are deformed, and often hold deposits of oil and natural gas. Salt domes are commonly found on the coastal plain adjacent to the Gulf of Mexico in the U.S.
atoll
a circular reef surrounding a deeper lagoon, that is formed by subsidence of the central island.
Shand's classification
a classification of igneous rocks based on crystallinity, degree of saturation with silica, degree of saturation with alumina, and color index.
arenaceous
a clastic sedimentary rock or deposit in which the constituent fragments are of sand grade size. Arenaceous rocks are also referred to as sandstones.
argillaceous
a clastic sedimentary rock or deposit in which the constituent fragments are of silt or clay size. Siltstones and mudstones are argillaceous rocks.
pavement
a closely packed, smooth, natural bare-rock surface that resembles a paved road (e.g. desert pavement).
gneiss
a coarse-grained metamorphic rock composed of light and dark layers of minerals or lenses. Gneiss is a very high grade metamorphic rock, forming at the highest temperatures and pressures, which have changed the rock so that minerals have separated into layers. The dark layers contain minerals such as mica or amphibole, and the light layers contain feldspar or quartz. Coarse granular light-colored bands of quartz and feldspar alternate with thin, dark, often undulating, schistose bands in which micas and amphiboles are concentrated. Gneiss is formed during high-grade, regional metamorphism.
marble
a coarse-grained rock composed of interlocking calcite or dolomite crystals. It is produced by regional or contact metamorphism of limestones (the limestone recrystallizes during the metamorphism). Marble, being composed of calcite, is highly susceptible to chemical weathering.
sedimentary breccia
a coarse-grained sedimentary rock (grains coarser than 2 mm) formed by the cementation of angular rubble.
breccia
a coarse-grained sedimentary rock in which the constituent clasts or fragments are angular. Because grains are rounded so rapidly during transport, it is unlikely that the angular fragments with breccias have moved very far from their source.
submergent coast
a coast in which formerly dry land has been recently drowned, either by land subsidence or a rise in sea level.
fjord
a coastal inlet that is a glacially carved valley, the base of which is submerged by the ocean. They are formed when sea level rises. Because of glacial action, they are characterized by the existence of gouged basins separated from the open sea by sills of solid rock, often capped by moraine, marking the point where the glacier that cut the valley lost much of its erosive power, either through melting or slowing down. The cross-section of a fjord is often U-shaped.
facies
a collection of metamorphic rocks that have formed over the same range of physical conditions.
tephra
a collective term for all clastic materials thrown out by a volcano, including bombs, blocks, scoria, ash, cinders, pumice, and lapilli.
firn
a compacted mass of granular snow, a half-way stage in the transformation of fresh snow into glacier ice. Fresh snow is very loosely packed and has a density between 0.1 and 0.2. Compaction and recrystallization reduce the pore space between grains, increasing the density. Firn exists when the density reaches 0.5. The word means "of last year", a reference to snow that has survived one summer. Further compaction continues until glacier ice of density 0.89 to 0.90 is formed, although this may take many years. Firn is analogous to a sedimentary rock such as sandstone.
Precambrian shield
a complex of old, Precambrian metamorphic and plutonic rocks that are not covered by sedimentary rocks and are exposed over a large area. The Precambrian Shield in eastern and northern Canada is known as the Canadian Shield.
Ethane (C2H6)
a component of natural gas that is also used in fertilizer manufacture and as a source of hydrogen for fuel cells.
P wave (Primary wave)
a compressional wave (seismic wave) in which the rock vibrates parallel to the direction of wave propagation. Since the vibration is parallel to direction of propagation, they are classified as longitudinal waves. They are called primary waves because they are the first to arrive at a given location after an earthquake (faster than S waves). P waves are very fast, traveling through surface rocks at speeds of 4 to 7 km/s (9000 to 15,000 mi./h). P waves can travel through solids, liquids, and gases, whereas S waves can only travel through solids.
continental drift
a concept suggesting that continents move over Earth's surface. Continental drift is the result of sea-floor spreading. Continental drift was hypothesized by Alfred Wegener in 1910. Wegener hypothesized the existence of a vast supercontinent called Pangaea, which broke up to form the present continents. His evidence included distributions of rock types, flora, fauna, geologic structures, and the similarity of the shape of the coastlines on either side of the Atlantic (S. America and Africa). He also used from the distribution of late Paleozoic glaciations, as well as locations of glacial till and striations. Plate tectonics is the mechanism that explains continental drift and sea-floor spreading.
laccolith
a concordant intrusive structure similar to a sill, with the central portion thicker and domed upward (lens-shaped). Laccoliths are less than 5 km in diameter. They have a lens shape because of the fact that as the magma is injected from its dikelike feeder, it has sufficient pressure to arch up the overlying strata (it has a flat base with a convex upper surface).
isostasy
a condition of theoretical balance for all large portions of the Earth's crust, which assumes that they are floating on an underlying more dense medium (mantle). As a result of erosion or deposition, this balance is put out of equilibrium and has to be compensated for by movements of the Earth's crust. Areas of deposition sink, whereas areas of erosion rise. Just as a block of wood in water floats due to the weight of the displace water buoying them up, the lithosphere tends to rise or sink until it is balanced by the weight of displaced asthenosphere.
seamount
a conical undersea mountain rising 1,000 meters or more above the sea floor.
passive continental margin
a continental margin that is characterized by a lack of tectonic activity. It usually consists of a continental shelf, a continental slope, and a continental rise, which generally extends down to an abyssal plain at a depth of about 5000 meters.
index contour
a contour line that is accentuated by its width, in order to facilitate the reading of elevations on a map. For example, where the contour interval is 10 m, the 50 m and 100 m contours may be accentuated.
conglomerate
a course-grained sedimentary rock (grains coarser than 2 mm) formed by the cementation of rounded clasts (gravel). There must have been some transport involved before the formation of the rock because the particles are rounded.
impact crater
a crater formed with a meteorite hits the ground.
barchan
a crescent-shaped dune with the horns of the crescent pointing downwind. It occurs in desert areas with a unidirectional wind regime. The horns, or wings, develop because the rate of advance of the dune is inversely proportional to its height, so the lower sides extend downwind faster than the center. Barchans have a steep slip face on the inward or concave side (downwind side). Barchans usually develop where the sand supply is limited, and they tend to move across a barren surface.
chatter mark
a crescent-shaped fracture seen in hard rock that was subjected at some point to the passage of a glacier. These marks are believed to be created by the impact and rolling of boulders held loosely within the base of a glacier.
oxbow lake
a crescent-shaped lake occupying the abandoned channel of a stream meander that is isolated from the present channel by a meander cutoff and sedimentation. With time, and oxbow lake may fill with sediment and vegetation.
shale oil
a crude oil obtained from oil shale by submitting it to destructive distillation.
xenocryst
a crystal that resembles a phenocryst in igneous rock but is foreign to the body of rock in which it occurs.
hypsographic curve
a curve showing the proportion of a landscape lying at, above, or below particular elevations. The percentage of the landscape between successive contours is calculated from a relief map, and the results expressed either as simple proportions between different heights, or as a cumulative measure showing the percentage of area lying above each level It is used to display the proportions of the Earth's surface at particular elevations above and depressions below sea level.
island arc
a curved line of islands. Many island arcs are bordered on their ocean flank by trenches. The eastern seaboard of Asia, for example, has a whole series of island arcs - the Aleutians, the Kurils, the Japanese, and the Marianas - flanked on their ocean side by a series of trenches. Island arcs usually include lines of active volcanoes, and are formed at subduction zones, places where one plate is being subducted (moving under) another. The friction created during subduction accounts for the large number of earthquakes, and this friction increases the thermal energy, leading to partial melting of the plates. If this melt (magma) reaches the surface, a volcano will form.
Aitoff's equal-area projection
a cylindrical map projection of a hemisphere in which the major axis, the Equator, is twice the length of the minor, central meridian, axis. The projection is bounded by an ellipse. Landmasses near the center of the are covered are of quite good shape, although distortion increase towards the east and west limits of the projection.
cosmogenic isotope dating
a dating method that uses the effects of constant bombardment of material at Earth's surface by neutron radiation coming from deep space. The high-energy particles hit atoms in minerals and alter their nuclei. For instance, when the atoms in quartz are hit, oxygen is converted to beryllium-10. The concentrations of these isotopes increase at a constant rate once the rock surfaces exposed to the atmosphere because the influx of cosmogenic radiation is uniform over time. The length of time a rock surfaces been exposed can be calculated by knowing the rate of increase of a cosmogenic isotope and determining the amount of that isotope in a mineral at the surface of a rock. This technique has been used to determine how long ago boulders were deposited by advancing glaciers. This technique is sometimes called surface exposure dating.
finger lake
a deep, long, narrow lake, usually occupying a valley carved by glacial erosion. The Finger Lakes in New York State are well-known examples.
canyon
a deep, steepsided section of a river valley, the depth of which considerably exceeds its width, normally found in arid or semiarid regions. The continuous external source of water enables efficient downward erosion to proceed, while the local lack of precipitation hinders weathering of the sides and hence there degradation.
pluton
a deep-seated major intrusive body of coarse-grained igneous rock, generally of granitic composition.
parabolic dune
a deeply curved dune in a region of abundant sand. The horns point upwind (the opposite direction of a barchans) and are often anchored by vegetation. This type of dune has the approximate form of a parabola, with the concave side facing the wind. Because they require abundant sand and strong winds, parabolic dunes are typically found inland from an ocean beach. The three types of dunes that develop in areas having steady wind direction are barchans, transverse dunes, and parabolic dunes. All three have steep slip faces on the downwind side.
ejecta blanket
a deposit of ejecta that surrounds an impact crater after a meteorite hits the ground.
sinter
a deposit of silica that forms around some hot springs and geysers.
kettle (kettlehole or kettle lake)
a depression caused by the melting of a stagnant block of ice left behind by a glacier that was surrounded by sediment. Kettles usually form small lakes.
cone of depression
a depression of the water table formed around a well when water is pumped out; it is shaped like an inverted cone. As a result, the water level in a well gets lower and lower. The local lowering of the water table, called drawdown, tends to change the direction of groundwater flow by changing the slope of the water table.
gravity anomaly
a deviation from the average gravity value at a point on the Earth's surface. A gravity reading higher than the normal regional gravity is called a positive gravity anomaly. It can indicate that tectonic forces are holding a region up out of isostatic equilibrium. When the forces stop acting, the land surface sinks until it reestablishes isostatic balance. The gravity anomaly then disappears. Positive gravity anomalies are also caused by local concentrations of dense rock such as metal ore. A gravity reading lower than the normal regional gravity is called a negative gravity anomaly. Negative gravity anomalies indicate that a region is either being held down by tectonic forces or that local mass deficiencies exist for other reasons.
magnetic anomaly
a deviation from the average strength of Earth's magnetic field at a point on the Earth's surface.
bornhardt
a dome-shaped mound of rock; a large inselberg.
syncline
a downward arching fold in rock (the opposite of an anticline). It is generally concave upward, and the core of the fold contains the stratigrapically younger rocks.
trellis drainage pattern
a drainage pattern consisting of parallel main streams with short tributaries meeting them at right angles. Trellis patterns form where tilted layers of resistant rock such as sandstone alternate with nonresistance rock such as shale. Erosion of such a region results in a surface topography of parallel ridges and valleys.
radial drainage pattern
a drainage pattern in which streams diverge outward like spokes of a wheel. This pattern is formed on high conical mountains, such as composite volcanoes and domes.
rectangular drainage pattern
a drainage pattern in which tributaries of a river change direction and join one another at right angles. This pattern develops on the rock that is fractured in a regular pattern.
accordant
a drainage pattern that is controlled by the structures over which the stream or river flows.
estuary
a drowned river mouth where fresh water from a river mixes with the seawater, forming brackish water.
ria
a drowned river valley, eroded by the river at a time when the sea level was lower than it is now. Like present river valleys, rias have a V-shaped cross section, which deepens seaward and narrows inland.
Himalaya Mountains
a fairly young mountain range that began forming around 45 million years ago as India began colliding with Asia. The Himalaya mountains are still rising.
submarine fan
a fan-shaped mass of sediment normally found at the lower end of a submarine canyon. Much of this material may have been deposited under the action of turbidity currents.
dip-slip fault
a fault in which movement is parallel to the dip of the fault surface.
strike-slip fault
a fault in which movement is parallel to the strike of the fault surface. The San Andreas Fault in California is an example of a strike-slip fault (it is a right-lateral fault) that forms a portion of the boundary between the Pacific plate and the North American plate. The average rate of movement along the San Andreas Fault is about 2 cm per year.
granite
a felsic (light-colored), coarse-grained, intrusive igneous rock containing quartz, and composed mostly of potassium feldspars and sodium-rich (plagioclase) feldspars, and a bit of mica (biotite usually). Potassium feldspar normally dominates over plagioclase feldspar. Granite is the coarse-grained, intrusive equivalent of the fine-grained, extrusive igneous rock rhyolite. Granite (and its extrusive equivalent rhyolite) has a greater variation in its composition than other igneous rocks. Granite is much more common that its extrusive equivalent rhyolite because silicic magma is very viscous, so it travels upward through the crust more slowly and with more difficulty than mafic magma - unless it is exceptionally hot, it will solidify before it reaches the surface of the Earth. Granite is the most abundant igneous rock in mountain ranges, and it is also the most commonly found igneous rock in the interior lowlands of continents.
patina
a film or skin on the surface of a boulder, produced by chemical weathering. Patinas are distinctive in color and physical properties, their thicknesses often reflecting age.
shale
a fine grained sedimentary rock (grains finer than 1/16 mm) formed by the cementation of silt and clay (mud). Shale has thin layers called laminations, and the ability to split into small chips (fissility). The fissility is related to the disposition of clay minerals within the rock.
slate
a fine-grained metamorphic rock that splits easily along flat, parallel lines (slaty cleavage). Most slate is formed by the low-grade, regional metamorphism of shale (or any other fine-grained argillaceous sediment).
rhyolite
a fine-grained, felsic, igneous rock made up mostly of feldspar and quartz. Rhyolite is the extrusive equivalent of granite.
basalt
a fine-grained, mafic, igneous rock composed predominantly of ferromagnesian minerals and with lesser amounts of calcium-rich plagioclase feldspar (contrast with andesite and rhyolite). Basalt is relatively low in silica (compared to andesite and rhyolite), which gives basaltic lava low viscosity and the ability to flow long distances (ex - Hawaiian Island volcanoes erupt basalt). Basalt is the extrusive, fine-grained equivalent of the intrusive igneous rock gabbro. Alkali basalts contain abundant olivine, both as phenocrysts and in the groundmass. Alkali basalts are found on oceanic islands and continents and contain large amounts of Na2O and K2O. Tholeiite basalts have sufficient silica to convert much of the olivine into other minerals (it is still present in phenocrysts, just not in the groundmass). Tholeiite basalts occur on continents, especially along continental margins - most of the flood basalts of the world are tholeiitic, as are the ocean floor basalts. Tholeiite basalts contain small amounts of Na2O and K2O. A third type of basalt, high-alumina basalt, is found in island arcs and orogenic (mountain-building) belts. Its composition is intermediate to the alkali and tholeiite basalts, and therefore contains intermediate amounts of Na2O and K2O. It also has large amounts of Al2O3.
mudstone
a fine-grained, sedimentary rock that lacks the laminations and fissility of shale.
loess
a fine-grained, yellowish sedimentary deposit of wind-blown dust. It is very soft and easily eroded to form gullies. It is porous but can form extremely fertile soil.
spit
a fingerlike ridge of sediment attached to land but extending out into open water. Spits are formed due to the deposition of sediment from a longshore current off of a point of land.
volcano
a fissure or vent on the Earth's surface connected by a conduit to the Earth's interior, from which lava, gas, and pyroclastic material are erupted. The term also refers to the hill or mountain that is produced by the ejected material.
guyot
a flat-topped seamount, formed primarily by wave erosion. The depth of the summit is usually between 1000 and 2000 m. Few guyots with summit depths of less than 1000 m have been located.
flash flood
a flood of sudden occurrence that results from rapid runoff after heavy rainfall. Flash floods are commonest in desert areas, where there is no vegetation to prevent fast runoff.
debris flow
a flow involving soil in which coarse material (gravel, boulders) is predominant.
turbidity current
a flowing mass of sediment-laden water that is heavier than clear water and therefore flows downslope along the bottom of the sea or lake.
mudflow
a flowing mixture of soil and water, usually moving down a channel.
isoclinals fold
a fold in which the limbs are nearly parallel to one another. The presence of parallel limbs implies larger shortening strain or shear strain.
recumbent fold
a fold overturned to such an extent that the limbs are essentially horizontal.
open fold
a fold that has limbs that dip gently. The more open the fold, the less it has been strained by shortening. Open folds are the opposite of isoclinal folds.
stress
a force acting on a body, or rock unit, that tends to change the size or shape of that body. Stress is also described as being the force per unit area within a body. Stress causes rocks to undergo strain. Stress can take the form of tension (forces pull rocks away from each other), compression (rocks are pushed together from opposite directions), or shear (when stresses act parallel to a plane - a shear stress results in shear strain parallel to the direction of the stresses - shear stresses occur along actively moving faults).
shear
a force tending to deform a rock mass through the movement of one part of it relative to another, as for example at a fault. Shear causes contiguous parts of a body to slide relative to each other in a direction parallel to their plane of contact.
spheroidal weathering
a form of chemical weathering in which concentric shells of decayed rock are successively loosened and separated from a block of rock by water penetrating the bounding joints or other fractures and attacking the block from all sides.
coke
a form of coal that has been lightly burned, releasing the most volatile ingredients in the coal (mostly sulfur). When burned, coke releases more intense heat in a furnace than does ordinary coal. It is hardly smoky at all. It serves as the main fuel for producing steel in foundries.
*sleet
a form of precipitation composed of frozen water droplets that forms when a warm horizontal layer of air is sandwiched between two cold layers of air. The precipitation begins as snow in the upper cold layer, which falls into the warm layer and melts into rain droplets. These rain droplets then enter the lower cold layer and freeze into small ice pellets (sleet) which strike the ground. Sleet is different from freezing rain in that freezing rain comes down as raindrops, which freeze when they hit the ground. Freezing rain also begins as snow, but then enters the warm layer and is converted into rain droplets. Unlike the situation in sleet, the warm layer of air extends almost all the way to the ground. If the ground is cold enough, the rain will freeze on contact with it (freezing rain). Freezing rain forms a layer of ice on the Earth's surface, which is dangerous for driving due to the slick surfaces it creates.
pumice
a frothy, volcanic glass that is highly vesicular (contains many rounded spaces) and commonly has the composition of rhyolite. It is often sufficiently buoyant to float on water and is economically useful as a light-weight aggregate and as an abrasive. Pumice is formed in viscous lava, where the gas cannot escape as easily.
natural gas
a gaseous mixture of naturally occurring hydrocarbons; methane is usually the most abundant component of natural gas.
lutite
a general name for rocks composed of material that was once mud (e.g. shale, mudstone, etc.). Lutaceous is the adjective used to refer to sedimentary rock formed from mud.
hardpan
a general term for a hard layer of Earth material that is difficult to dig or drill. Geologists usually restrict the term to a hard, often clayey, layer of cemented soil particles. Hardpan layers in wet climates are usually formed of clay minerals, silica, and iron compounds, whereas in arid climates the hardpan forms from the cementing of soil by calcium carbonate and other salts that precipitate in the soil as water evaporates.
graben
a generally elongated block of rock that has been downthrown between two parallel normal faults relative to the surrounding area. The Rhine Valley in Germany in the Red Sea are examples of grabens.
piedmont
a gentle slope leading down from the foot of a mountain range down to comparatively flat land. In arid and semiarid areas where these features most frequently occur, they consist of an eroded upper segment (known as a pediment), which makes an abrupt angle with the mountain front, followed by an accumulation form, or bajada, consisting of transported debris from the mountains. This merges into a flat inland basin (playa).
coastal plain
a gently sloping plain leading from the foot of inland upland areas down to the coast, and largely continuous with the continental shelf under the sea.
trap
a geologic structure in which hydrocarbons accumulate. Hydrocarbons migrate away from their source rocks under pressure and accumulate where an effective barrier to their further migration exists.
Lake Vostok
a giant lake that lies 4,000 m below the East Antarctic Ice Sheet. The lake is about 200 km long and 50 km wide, about the size of Lake Ontario. It is 510 m deep, placing it among the 10 deepest lakes in the world. The lake was discovered in the 1970s using ice-penetrating radar. The lake is named after the Russian research station, Vostok, which lies above the lake.
mid-ocean ridge
a giant mountain range, approximately 70,000 km long, that lies under the ocean and extends around the world. It is the source of new crustal material (according to the seafloor spreading model). It is here that basaltic lava from the Earth's mantle reaches the surface to be added to the oceanic crust in the form of pillow lavas and dikes (the rocks near the ridge, therefore, are quite young).
roche moutonnée
a glacially sculptured knob of bedrock, with its long axis oriented in the direction of ice movement, an upstream (stoss) side that is gently inclined, rounded, and striated, and a downstream (lee) side that is steep, rough, and hackly. This elongated mound of bedrock is more resistant to glacial erosion than the surrounding rock. These mounds are elongated parallel to the direction of glacier flow.
valley glacier
a glacier confined to a valley. The ice flows from a higher to a lower elevation.
temperate glacier
a glacier containing considerable amounts of water above, within, and beneath the glacier ice. Water at the ice-rock boundary promotes easy sliding and consequently such glaciers flow faster than other types. This movement produces erosion of the abrasive type, while the presence of water allows freezing and thawing at the ice margins, an important factor in glacial plucking.
ice sheet
a glacier covering a large area (more than 50,000 km2) of land. They are hundreds of meters thick, and many developed during the Pleistocene period. Ice sheets are associated with continental glaciation. At present two exist, the Greenland and Antarctic ice sheets. Antarctica actually has to ice sheets The West Antarctic Ice Sheet and the much larger East Antarctic Ice Sheet (the largest ice sheet in the world). The East Antarctic Ice Sheet is about 4700 m thick at its thickest point. About 75% of Earth's freshwater is contained in the ice sheets of Antarctica and Greenland.
surging glacier
a glacier that alternates between brief periods (usually one to four years), of very rapid flow, called surges, and longer periods (usually 10 to 100 years) of near stagnation. During a search, a large volume of ice from an ice reservoir area reaches a critical amount and is suddenly displaced downstream at speeds up to several meters per hour into an ice receiving area, and the affected portion of the glacier is chaotically crevassed.
ice cap
a glacier that covers a relatively small area of land and is not restricted to a valley. It is a small-scale ice sheet. Ice caps are found in a few mountain highlands in Iceland and on islands in the Arctic Ocean, off Canada, Russia, and Scandinavia. An ice cap or ice sheet flows downward and outward from a central high point.
receding glacier
a glacier that grows smaller added edges and melts back. Receding glaciers have negative but and lose more snow than they gain over time.
advancing glacier
a glacier that pushes outward and downward at its edges. Advancing glaciers have positive budgets, meaning they gain more snow over a period of time than they lose.
Wentworth scale
a grade scale for sediment particles in which the size limits for the common grade terms are modified but the geometric interval or constant ratio of one half is retained. The scale ranges from clay (diameter less than 1/256 mm), to silt (diameter between 1/256 and 1/16 mm) to sand (diameter between 1/16 and 2 mm) to pebble (diameter 2 to 64 mm) to cobble (between 64 and 256 mm) to boulder (diameter greater than 256 mm). It is the grade scale generally used by North American sedimentologists.
Bouguer anomaly
a gravity anomaly that takes into account the effect of topography but not isostatic compensation, first observed by P. Bouguer in 1735.
abyssal (submarine) fan
a great, fan-shaped deposit of sediment on the deep-sea floor at the base of many submarine canyons (usually deposited by turbidity currents).
mountain range
a group of closely spaced mountains or parallel ridges.
concretion
a hard, rounded mass of cementing material, such as silica or calcite, that is carried into place by ground water, typically precipitating around an organic nucleus such as a leaf, tooth, or other fossil.
altiplano
a high plateau that is surrounded by mountains. An example is the South American Altiplano between two extensions of the Andes, mainly in Bolivian and southwestern Peru.
sturzstrom
a huge mass of rapidly moving rock debris and dust, derived from the collapse of a cliff or mountainside, flowing down steep slopes and across low ground, often for several kilometers at speeds of more than 100 km/hr. It is the most catastrophic of all forms of mass movement, and several have been identified on the moon.
sapropel
a jelly-like ooze or sludge composed of plant remains, most often algae, putrefying in an anaerobic environment on the shallow bottoms of lakes and seas. This sediment, when compacted, may change into shale containing coal, oil, or natural gas. Initially, buried hydrocarbons will partly disintegrate thanks to the activity of hungry bacteria. Chemical reactions then set up with clay minerals in the sediment. Eight do we, hydrocarbon-rich sediment, termed a sapropel, forms. The total hydrocarbon content of a sapropel layer is typically no more than a few percent of the total sedimentary deposit, but this is enough to supply future oil reservoir. As burial deepens the sapropel, it heats up, at a rate of 8°C per every thousand feet beneath the surface. At a depth of 2300 m it is hard enough, around 82°C, for the complex hydrocarbon molecules to crack (break up) into petroleum. Ever-simpler molecules form with greater natural cooking, until at around 4600 m deep, the hydrocarbons have broken down all together into natural gas. The petroleum window is the space between 2300 and 4,600 m (7000 to 15,000 feet) down through which a sapropel must pass if it is to transform into oil. The Trillium geologists look for places where the pressure due to deep burial has forced the oil upward into a place where it can concentrate underground. Geologists seek to identify three specific features before drilling: 1) a source rock - the original sapropel, such as an oil shale, containing organic matter that is converted to petroleum; 2) a reservoir rock, usually sandstone or limestone, that is sufficiently permeable and porous to transmit and store the petroleum as it migrates toward the surface; and 3) an oil trap (or structural trap), a place where in permeable rock (called trap rock) prevents any further upward percolation of petroleum. There are many types of oil traps. Examples include anticlines, domes, faults (common in California), and salt domes (common in the Gulf of Mexico)
black smoker
a jet of very hot water that rises from a vent in the ocean floor, darkly colored due to dissolved sulfides of copper, iron, manganese, and zinc.
pluvial lake
a lake formed during an earlier time of abundant rainfall.
*eutrophic lake
a lake with a large supply of plant nutrients, such as nitrate and phosphate. Eutrophic lakes are shallow, have murky brown or green water (due to large populations of phytoplankton and other algae), a high rate of sediment accumulation, and high net primary productivity. Eutrophic lakes can be contrasted with oligotrophic lakes, which have a small supply of plant nutrients. Oligotrophic lakes are generally deep, high elevation lakes with crystal clear water and low net primary productivity.
stromatolite
a laminated concentric structure formed of calcium carbonate and produced by cyanobacteria. Fossilized stromatolites dating back to Precambrian times have been found.
Canadian Shield
a large area of Precambrian rock that occupies 5 million square km of Canada. Most of the shield consists of granite and banded gneiss. There are extensive deposits of metal-bearing minerals, including those of copper, gold, iron, nickel, and silver.
impact basin
a large impact crater with a diameter in excess of 200 km that forms when a meteorite strikes the ground. The best examples can be seen on the Moon and on Mercury.
mogote
a large limestone hill that rises on a tropical or subtropical karst landscape.
*Permian Basin
a large sedimentary basin in Texas and the namesake of the Permian period. It is so named because it has one of the world's thickest deposits of rocks from the Permian geologic period.
Mount Pinatubo
a large volcano in the Philippines that erupted in 1991 and spewed out enough ash and dust to lower the worldwide average temperature a half a degree Celsius for a couple of years.
lopolith
a large, concordant, typically layered igneous intrusion which is saucer-shaped in cross section. It is sunken in its central part owing to sagging of the underlying country rock.
batholith
a large, discordant pluton with an outcropping area greater than 100 square kilometers. Batholiths compose the cores of many mountain ranges. A batholith is formed when many plutons converge together to form a huge expanse of granitic rock. Some batholiths are mammoth, paralleling past and present subduction zones and other heat sources for hundreds of kilometers in continental crust. One such batholith is the Sierra Nevada Batholith, which is a continuous granitic formation that forms much of the Sierra Nevada mountain range in California. An even larger batholith extends the length of the mountain ranges of western Canada and southeastern Alaska. Smaller batholiths are found in eastern N. America in the Piedmont east of the Appalachian Mountains and in New England and the coastal provinces of Canada.
pingo
a large, dome-shaped frost mound of soil-covered ice, 30-50 m high and up to 400 m in diameter. The raised hill/bulge forms in Arctic areas (esp. Canada) in part by subsurface hydrostatic pressure build-up within isolated groundwaters before the complete extension of permafrost over the area.
plate (tectonic)
a large, mobile slab of lithosphere making up part of the Earth's surface. These plates may be made of continental or oceanic material, and their base is marked by the asthenosphere. They move across the surface of the Earth as a result of seafloor spreading. The 7 major plates, listed in order of size (numbers are in million km2), are:
seif (longitudinal dune)
a large, symmetrical ridge of sand parallel to the wind direction. Seifs form in areas where the wind regime is 2-directional, with a gentle prevailing wind that supplies the sand and short-term cross winds, often of greater strength, that help build the dune, producing the curved faces. Its crest in profile consists of a series of peaks, and it bears on one side a succession of curved slip faced produced by strong but infrequent cross winds. A seif dune may up to 200 m high and from 400 m to more than 100 km long (300 km in Egypt).
mascon
a large-scale high-density lunar mass concentration (hence the term mascon) below a ringed mare.
unaka
a larger residual mass rising above a peneplain and sometimes displaying on its surface the remnants of a peneplain older than the one above which rises. It is erosional remnant of greater size and height than a monadnock. The name is derived from the Unaka Mountains of eastern Tennessee and western North Carolina.
pahoehoe
a lava flow characterized by a ropy or billowy surface.
aa
a lava flow that solidifies with a spiny, rubbly surface.
cap rock
a layer of shale or other impervious rock that overlies porous rocks containing deposits of oil or natural gas; also used to refer to a layer of anhydrite, gypsum, calcite, and sulfur that forms a hard covering on top of a salt dome. In the Gulf Coast region of the U.S., cap rock is a major source of sulfur.
pycnocline
a layer of water in the ocean characterized by a rapid change of density with depth.
soil
a layer of weathered, unconsolidated material on top of bedrock; often also defined as containing organic matter and being capable of supporting plant growth.
E horizon
a light-colored soil layer that is sometimes present between the A and B horizons. The E horizon is sometimes referred to as the zone of leaching, and is characterized by the downward movement of water and removal or eluviation (what the 'E' stands for) of fine-grained soil components such as clay. The E horizon is not present in younger soils, so the zone of leaching in these zones is located in the A horizon.
coquina
a limestone consisting of coarse shells of animals.
oolitic limestone
a limestone formed from oolites.
isotherm
a line along which the temperature of rock (or other material) is the same.
cotidal line
a line drawn on certain hydrographic charts joining points at which average high water occur simultaneously.
agonic line
a line joining all points on the surface of the Earth where the angle of declination is zero. Along such a line, magnetic north and true north coincide.
meridian
a line of longitude passing through any given point and the North and South Poles.
contour line
a line on a topographic map connecting points of equal elevation.
slurry
a liquid mixture of mud and water, which readily slips downhill. It most commonly occurs in clays and shales. The term also refers to any highly fluid mixture of water and finely divided material.
phi scale
a logarithmic transformation of the Wentworth scale of particle sizes in which the negative logarithm to the base 2 of the particle diameter (in mm) is substituted for the diameter value. It has integers for the particle class limits, increasing from -5 (for 32 mm) to +10 (for 1/1024 mm).
Rocky Mountains
a long mountain range in North America that stretches in a north-south direction from northern British Columbia and Alberta in Canada through Montana, Wyoming, Colorado, and New Mexico in the United States. The Rocky Mountains were formed due to flattened subduction of an oceanic plate underneath a continental plate. It is hypothesized that the shallow angle of the subducting plate greatly increased the friction and other interactions with the thick continental mass above it. Tremendous thrusts piled sheets of crust on top of each other, building the Rocky Mountain range.
Andes Mountains
a long mountain range that extends in a North-South direction for almost the entire length of the western portion of South America. The Andes Mountains were formed by the subduction of the Nazca plate underneath the South American plate.
fosse
a long, narrow depression between the side of a glacier and the valley that contains it. The depression forms as some ice melts because of heat absorbed by or reflected from the walls of the valley.
esker
a long, sinuous ridge of sediment deposited by glacial meltwater streams, which may be underneath, within, or above the ice mass. Eskers consist primarily of sands and gravels, and can range in length from tens of meters to hundreds of kilometers long. Eskers are composed of cross-bedded and well-sorted sediment.
rock flour
a powder of fine fragments of rock produced by glacial abrasion. It is composed largely of very fine silt and clay-sized particles of unaltered minerals that are pulverized from chemically on weathered bedrock. When meltwater washes rock flour, glacier, the streams draining the glacier appear milky, and lakes into which glacial meltwater flows often appear a milky green color.
drumlin
a long, streamlined hill made of till deposited by a glacier. Drumlins tend to be found in groups, all with their long axes parallel with the direction of ice movement. They may be composed of till, preexisting drift, or of rock. Drumlins composed of rock or preexisting drift material must have formed through erosion, whereas those made of newly deposited till were formed by deposition. In a drumlin, one end is steeper than the other. The gentler end points in the downglacier direction.
rubble
a loose mass of angular rock fragments (fragments larger than 2 mm in diameter).
compaction .
a loss in overall volume and pore space of a sediment as the particles are packed closer together by the weight of overlying material
mudflat
a low area of fine silt that lies along the shore of an estuary or on the lee side of an island. It supports no vegetation and is generally covered and uncovered by the tide.
hydrographic chart
a map of the seabed, showing depths of water, heights of underwater features, and sometimes geologic information.
Mercator projection
a map projection in which the equator is represented by a straight line true to scale, the meridians by parallel straight lines perpendicular to the equator and equally spaced according to their distance apart at the equator, and the parallels by straight lines perpendicular to the meridians and the same length as the equator. There is great distortion of distances, areas, and shapes in the polar regions. Because any line of constant direction on the sphere is truly represented on the projection by a straight line, the Mercator projection is of great value in navigation and for plotting trajectories of Earth satellites in oblique orbits.
Mollweide projection
a map projection showing the entire Earth's surface on an ellipsoidal base. The major axis (the Equator) is twice the length of the minor axis (central meridian). The parallels are straight lines, as is the central meridian. The other meridians are curved, the curvature increasing toward the outside limits of the projection. The distortion of shapes is therefore considerable, especially at the margins of the map. This projection is sometimes interrupted: each continental area has its own central meridian, with different ones north and south of the Equator (the interruptions are in the oceans).
active (continental) margin
a margin consisting of a continental shelf, a continental slope, and an oceanic trench. Subduction of oceanic crust under continental crust is normally occurring here.
relict sediment
a marine deposit, usually present on shelf areas, that is incompatible with contemporary marine environments. For example, much of the relict sediment on the continental shelf surrounding Britain was initially deposited there under glacial conditions.
bench mark
a mark indicating a point of known position and height, which has been surveyed extremely accurately by a national surveying body.
salt marsh
a marsh periodically flooded by seawater.
bayou
a marshy area of an estuary or where a lake outflows. The water normally flows sluggishly or may be stagnant. This term is used in the Southeastern states of the U.S. (e.g. Louisiana).
massif
a massive block of rock, usually in an orogenic belt, formed of rocks that are more rigid than those of the surroundings.
petrified wood
a material that forms as the organic matter of buried wood is either filled in or replaced by inorganic silica carried in by ground water.
incised meander
a meander of a stream that is cut deeply into bedrock.
hill shading
a means of showing relief on maps, shading the east and south-facing slopes (the steeper the slope, the darker the shading) to give the effect of an oblique light shining from the northwest over a relief mode.
sounding
a measure of the depth of water. Before the advent of modern echo sounding techniques, a sounding line with a lead weight on the end was used.
magnitude
a measure of the energy released during an earthquake.
intensity
a measure of the size of an earthquake by its effect on people and buildings.
saltation
a mechanism of sediment transport whereby individual grains move by bouncing off the surface of the land or bed of a stream. It is the most common form of transport by wind, but is less significant in rivers because of the viscosity of water as compared with air.
sandstone
a medium-grained sedimentary rock (grains between 1/16 mm and 2 mm) formed by the cementation of sand grains with a material such as calcite or various iron minerals, or alternatively by welding together by pressure. Types of sandstone include quartz sandstone, arkose, and greywacke.
pterosaur
a member of the order Pterosauria, reptiles highly adapted to flight. They were characterized by extreme elongation of the fourth digit of the hand for support of a membranous wing, and by reduction of the hind limbs. The term pterodactyl is a colloquial term for one of the Pterosauria, taken from the name of one of the genera of this order.
metasomatism
a metamorphic process by which mineralogical and chemical changes occur in rocks as a result of interaction with ions introduced from an external source. The ions may be brought in by migrating hot water (during regional metamorphism) or from an adjacent cooling magma (during contact metamorphism).
amphibolite
a metamorphic rock consisting predominantly of amphibole.
phyllite
a metamorphic rock in which clay minerals have recrystallized into microscopic micas, giving the rock a silky sheen. Phyllite is a higher grade metamorphic rock then slate
floodplain
a relatively level area bordering a river, subject to periodic flooding, and made up of sediments deposited by the river. A floodplain consists mainly of fine-grained silt and clay deposited during episodes of flooding.
schist
a metamorphic rock that is strongly foliated and in which mica minerals are abundant. It is a higher grade metamorphic rock than phyllite. At least 50% of the mica minerals are parallel, producing a marked schistosity, in which the minerals are oriented in a parallel fashion. There are many types of schists, and they are named after their dominant minerals. Two schists that form from shale are mica schist and garnet-mica schist. Schists that form from basalt include amphibole schist and chlorite schist (or greenschist). Greenschist is of lower grade than amphibole schist.
iron meteorite
a meteorite composed principally of iron-nickel alloy.
radiometric dating
a method of dating rocks by determining the relative proportions present of parent and daughter isotopes of a radioactive element.
hectare
a metric unit of area equal to 10,000 m2 (100 ares), and equivalent to 2.47 acres.
strip mine
a mine in which the valuable material is exposed at the surface by removing a strip of overburden. Strip mining involves the complete removal of overlying rock and vegetation. Strip mining is an environmentally harmful activity that destroys topsoil and leaves behind open pit that must be filled back in and replanted to curb further erosion and water pollution. But strip mining is the only way much of the world's coal supply can be safely mind. Shaft and Connell mining provide access to deeper coal deposits, but this form of mining is especially dangerous because of the weakness of coal beds and high concentrations of flammable gas and coal dust.
ore mineral
a mineral of commercial value.
fluid-induced melting
a mineral's melting temperature is significantly lowered by water under high pressure (water sealed in under high pressure helps break the silicon-oxygen bonds causing the crystal to liquefy). Experiments have shown that, under moderately high pressure, water mixed with granite lowers the melting point of granite from over 900°C (when dry) to as low as 650°C when saturated with water under a pressure of 10 kilobars (corresponding to a depth in the earth of approximately 35 km).
rill
a minute ephemeral channel at the head of drainage systems, forming at the point where unconfined sheet wash becomes concentrated into definite channels. Rills carry water only during storms.
migmatite
a mixed igneous and metamorphic rock, which forms when metamorphic rock partially melts.
adobe
a mixture of silt and clay, commonly used to make bricks because it dries to a hard, weatherproof mass. Commonly used in Mexico and the Southwestern U.S.
Young's modulus
a modulus of elasticity in tension or compression, equal to the ratio of the stress on a cross-sectional area of a rod of material to the longitudinal strain.
sand dune
a mound of loose sand grains piled up by the wind.
dune
a mound or hill of sand. The two major types of desert dunes are barchans (crescent-shaped) and seifs (longitudinal dunes). Coastal and riverside dunes are also common.
kame
a mound or knob or stratified sand and gravels deposited by a subglacial stream as a fan or delta at the margin of a melting stream. A kame can also form when material is deposited in meltwater pools on the surface of a stagnant glacier - the accumulated sediment will be lowered to ground level upon melting of the glacier, forming a mound called a kame.
orogeny
a mountain building period that occurs during an episode of intense deformation of the rocks in a region, generally accompanied by metamorphism and plutonic activity.
fault-block mountain range
a mountain range bounded by normal faults on each side of the range, or, more commonly, are tilted fault blocks in which the uplift has been great along one side of the range, while the other side of the range has exhibited as if hinged. The Sierra Nevada and Teton Range are tilted fault-block mountains. The first step in forming a fault-block mountain range is that the crust must break into fault-bounded blocks. The blocks are then uplifted. The normal faulting found in these mountain ranges implies a horizontal extension strain, the regional pool in a part of the crust. Isostatic vertical adjustment of a fault block probably occurs at the same time. Block-faulting is taking place in much of the Western United States, such as the Basin and Range province (also called the Great Basin) of Nevada and parts of Utah, Arizona, New Mexico, Idaho, and California. Hundreds of small, block-faulted mountain range are in evidence in this region.
stream
a moving body of water, confined in a channel, and running downhill under the influence of gravity.
longshore current
a moving mass of water that develops parallel to a shoreline, caused by waves arriving at an angle to the shore.
lahar
a mudflow composed mainly of volcanic debris. If the unstable debris accumulating on the sides of a volcano is lubricated by heavy rain, it will flow under gravity. The flow that covered Herculaneum in AD 79 may have been a lahar.
mantle plume
a narrow column of hot mantle rock that rises and spreads radially outward. They may account for the formation of volcanoes and constructive plate boundaries. Mantle rock begins to melt, thereby becoming less dense. As a result this material rises toward the surface as a plume. Mantle plumes are thought to give rise to hotspots in the crust.
isthmus
a narrow strip of land that connects two large land masses. For example, North and South America are connected by the isthmus of Panama.
trench
a narrow, deep trough parallel to the edge of a continent or an island arc. The continental slope on an active margin forms the landward wall of the trench, its steepness often increasing with depth. The slope is typically 4 to 5° on the upper part, steepening to 10 to 15° or even more near the bottom of the trench. The deepest spots on earth, more than 11 km below sea level, are in oceanic trenches in the western Pacific Ocean.
gorge
a narrow, steep-sided valley, generally formed in hard rocks by the erosive action of a flowing stream or river. A large gorge is called a canyon.
Pele's hair
a natural spun glass formed by blowing-out during quiet fountaining of fluid lava, cascading lava falls, or turbulent flows. A single strand, with a diameter of less than half a mm, may be as long as 2 meters.
cave (cavern)
a naturally formed underground chamber, usually formed in rock by erosion, particularly by the action of slightly acidic groundwater on limestone.
petroleum
a naturally occurring complex mixture of flammable liquid hydrocarbons. After distillation and removal of impurities petroleum yields a range of combustible fuels, petrochemicals, and lubricants. Some geologists use petroleum as a synonym for oil or crude oil. Unlike coal, which originates in a swampy environment, petroleum originates in well-lit, coastal seawater, or a sparkling, tropical lagoon. Plankton-rich coastal seas are the "rain forests" of the ocean. Dead organic matter shed from this floating rain forest can decay underwater just as readily as it does on sure, but some of it will also settle to the seabed where sediment accumulates and becomes sealed in a shallow, oxygen-deprived burial. This is where new oil is born. Petroleum normally forms in regions with high nutrients, which are trapped near shore by current. Sediment from a river mouth may sea the nutrient debris in place, forming a nutrient trap. The only good, large nutrient trap existing today is the Eastern Mediterranean basin. Presently, the world consumes about 100 million barrels of oil per day, giving us 30 to 40 years before the current reserve disappears.
ore
a naturally occurring, usually rocky material from which a useful product, such as a metal or one of its minerals, can be economically extracted.
sill
a near-horizontal tabular intrusive body of igneous rock, usually dolerite, of roughly uniform thickness but thin relative to its area. It is concordant with the planar structures of the rock types into which it intrudes.
peneplain
a nearly flat erosional surface of considerable area presumably produced as mass wasting, sheet erosion, and stream erosion reduce a region almost to base level. Mountains worn down by erosion become peneplains.
bedding plane
a nearly flat surface separating two beds or layers (strata) of sedimentary rock.
meander cutoff
a new, shorter channel across the narrow neck of a meander. The old meander may be abandoned as sediment separates it from the new, shorter channel. The cutoff meander becomes a crescent-shaped oxbow lake. With time, and oxbow lake may fill with sediment and vegetation.
Richter scale
a numerical scale of earthquake magnitudes devised by Richter in 1935. Very small earthquakes can have negative magnitudes. In theory there is no upper limit to the magnitude of an earthquake, but the strength of crustal rocks produces an actual upper limit of 9 on the scale. Because the Richter scale is logarithmic, the difference between two consecutive whole numbers on the scale means an increase of 10 times in the amplitude of the Earth's vibrations. This means that if the measured amplitude of vibration for certain rocks as 1 cm during a magnitude 4 quake, these rocks will move 10 cm during a magnitude 5 earthquake. It has been estimated that a tenfold increase in the size of Earth vibrations is caused by an increase of roughly 32 times in terms of energy released. A quake of magnitude 5, for example, releases 32 times more energy than one of magnitude for. A magnitude 6 quake is about 1000 times (32 x 32) more powerful in terms of energy released than a magnitude 4.
unconfined aquifer
a partially filled aquifer exposed to the land surface and marked by a rising and falling water table. Unconfined aquifers have water tables because they are only partly filled with water. And unconfined aquifer is recharged by petition, has a rising and falling water table during wet and dry seasons, and has relatively rapid movement of groundwater through it.
aerosol
a particle of matter that is larger than a molecule but small enough to remain suspended in the atmosphere. Aerosols may be solid or liquid. Over the ocean, sea spray provides salt nuclei, whereas over land the major source of nuclei is probably the weathering dusts of clay particles.
geode
a partly hollow, globe-shaped rock that is partly hollowed out inside. Inside, a geode contains an outer layer of amorphous silica, with well-formed crystals of quartz, calcite, or other minerals projecting towards the central cavity.
*deranged drainage pattern
a pattern characteristic of recently glaciated areas, such as the Canadian Shield, where the drainage pattern has not yet adjusted to the structures in the relatively recent, glacially deposited surface. As a result, drainage is not coordinated and has no discernible pattern, and is characterized by many small, local drainage basins and lakes.
reclamation
restoration of the land to usable condition after mining has ceased.
Widmanstätten patterns
a pattern of iron-nickel alloys crystallized in a distinctive pattern inside of iron meteorites that is revealed upon treating a cut cross-section of the meteorite with nitric acid. The pattern forms only deep inside planetary bodies that take millions of years to cool. Iron never crystallizes this way on Earth's surface, so any piece of metal showing this pattern on Earth is definitely from a meteorite. The crystal patterns in iron meteorites range from very coarse to extremely fine. The thickness of the crystals in the pattern depends mainly on the amount of nickel they contain, and how slowly the metal cooled. With this information, one can estimate the size of the meteorite's parent body, because larger asteroids cool more slowly than small ones.
pisolith
a pea-sized accretion that occurs in some sedimentary rocks and is often formed of calcium carbonate, although some are thought to be produced by a biochemical algal-encrustation process. A pisolith is larger and less regular in form than an oolith, although it has the same concentric and radial internal structure.
dreikanter
a pebble with 3 facets, formed by the erosive action of windblown sand in desert regions. The wind moves the pebbles back and forth because they are too heavy to be lifted entirely.
ooze
a pelagic sediment consisting of at least 30% skeletal remains of calcareous or siliceous pelagic organisms, the rest being clay minerals. Oozes are defined by their characteristic organisms (such as diatomaceous ooze).
Little Ice Age
a period between 1550 and 1850 in which temperatures in much of the N. hemisphere fell to their lowest since the last ice age.
Ice Age
a period in the Earth's history when ice spread toward the Equator, accompanied by a general lowering of surface temperatures, especially in temperate latitudes. The Pleistocene period, which ended about 10,000 years ago, experienced at least four major ice advances, with the margin reaching about 52 degrees north latitude over NW Europe and about 45 degrees north latitude in NE North America. Ice ages also existed in Permo-Carboniferous times (about 250 mya), one in late Precambrian times (about 500 mya), and possibly several earlier.
Tertiary Period
a period of the Cenozoic era that followed the Cretaceous Period nearly 65 million years ago, and lasted some 63 million years until the beginning of the Quaternary Period.
Pennsylvanian
a period of the Paleozoic era that occurred after the Mississippian and before the Permian. It is the approximate equivalent of the Upper Carboniferous of European usage.
pluvial period
a period of time experiencing greater rainfall than preceding or succeeding periods, usually on a geologic timescale.
Mississippian
a period within the American classification of geologic time that extends from the end of the Devonian period to the beginning of the Pennsylvanian period. The Mississippian and Pennsylvanian periods together are called the Carboniferous period elsewhere in the world.
plastic deformation
a permanent change in shape of a solid that does not involve failure by rupture/fracture.
sial
a petrologic name for the upper layer of the Earth's continental crust, which is composed of granitic rocks that are rich in silica and alumina.
guano
a phosphate or nitrate deposit formed by the leaching of bird excrement accumulated in arid regions. It is most commonly found on Pacific and West Indies islands, and along the arid part of the west coast of South America. It is processed for use as a fertilizer, due to its high phosphate and nitrate content.
palisade
a picturesque, extended rock cliff, rising precipitously from the margin of the sea, a stream, or a lake. Palisades usually have a columnar structure.
oasis
a place in a dry landscape (such as a desert) that has water and so can support vegetation.
placer deposit
a place in a stream where the running water has mechanically concentrated heavy sediment. The heavy sediment is concentrated in the stream where the velocity of the water is high enough to carry away lighter material but not the heavy sediment. Such places include river bars on the inside of meanders, plunge pools below waterfalls, and depressions on the streambed. Grains concentrated in this manner include gold dust and nuggets, native platinum, and diamonds and other gemstones.
spring
a place where water flows naturally out of rock onto the land surface because the water table intersects the surface. Spring can also occur where water flows out from caverns or long fractures, faults, or rock contact that come to the surface.
calcifuge
a plant that grows best on acid (i.e. lime-free) soils. Also called acidophiles.
calciole
a plant that grows best on chalky (i.e. alkaline) soils. Also called calciphiles.
granodiorite
a plutonic igneous rock containing plagioclase feldspar, subordinate potassium feldspar, and biotite (or hornblende). It is intermediate in composition between granite and diorite, and it is the intrusive equivalent of dacite.
needle
a pointed, elevated, and detached mass of rock formed by erosion (formed in a similar manner as a sea stack, by erosion of less resistant material).
puddingstone
a popular name applied chiefly in Great Britain to a type of conglomerate consisting of well-rounded pebbles whose colors are in marked contrast with the fine-grained matrix or cement (it supposedly resembles a fruit pudding).
acid rock
a rock that contains greater than 66% silica (SiO2). In current usage, an acid rock contains greater than 10% free quartz (e.g. granite, granodiorite, rhyolite).
Lake Bonneville
a prehistoric pluvial lake that covered much of North America's Great Basin region. Most of the territory it covered was in present-day Utah. The lake was nearly as large as Lake Michigan and significantly deeper. With the change in climate, the lake began drying up, leaving the present-day Great Salt Lake as a remnant.
principle of cross-cutting relationships
a principle or law stating that a disrupted pattern is older than the cause of the disruption
principle of lateral continuity
a principle that states that an original sedimentary layer extends laterally until it tapers or thins at its edges. This is expected at the edges of a depositional environment.
principle of original horizontality
a principle that states that most sediment deposited in water is deposited in horizontal or near-horizontal layers that are essentially parallel to Earth's surface. In many cases, this is also true for sediments deposited by ice or wind. Using this principle, one can infer that a tilting event must have occurred after the formation was deposited on a seafloor.
principle of superposition
a principle that states that within a sequence of undisturbed sedimentary rocks, the oldest layers are on the bottom, the youngest on the top.
frost heaving
a process in which rock and soil is lifted vertically. Solid rock conducts heat faster than the soil, so on a cold winter day, the bottom of a partially buried rock will be much colder than soil at the same depth. As the ground freezes in winter, ice forms first under large rock fragments in the soil. The expanding ice layers push boulders out of the ground. Frost heaving bulges the ground surface upward in winter, breaking up roads and leaving lawns bungees and misshapen after the spring thaw.
regelation
a process of thawing and refreezing within a glacier, which contributes toward the down-valley movement of ice. It is believed that the pressure within the ice of a glacier causes the melting of some ice crystals. The resulting meltwater will move to locations at which pressure is less, i.e. normally downslope, and then refreeze. This process has been classed as the primary cause of ice motion, but many authorities on the subject consider it to be only a secondary factor arising from flow.
meander
a pronounced sinuous curve along a stream's course. These curves develop because a stream's velocity is highest on the outside of curves, promoting erosion there. The low velocity on the inside of the curve promotes sediment deposition. The sediment deposited on the inside of curves forms point bars. The simultaneous erosion on the outside of the curve and deposition on the inside can deepen a gentle curve into a hairpin-like meander. Incised meanders are meanders that retain their sinuous pattern as they cut vertically downward below the level at which they originally formed. The result is a meandering valley with essentially no floodplain, into the land as a steep-sided canyon. Uplift of the region through which a river is flowing is a key factor in the formation of many incised meanders.
Pratt's hypothesis
a proposed mechanism of hydrostatic support for the Earth's crust. It involves the concept of isostasy, which postulates an equilibrium of crustal blocks of varying density; thus the topographically higher mountains would be less dense than topographically lower units, and the depth of crustal material would be everywhere the same.
radon
a radioactive gas produced by the radioactive decay of uranium.
avalanche
a rapid movement of snow en masse down steep slopes, which must usually have an angle greater than 22 degrees.
barrier reef
a reef separated from the shoreline by the deeper water of a lagoon. It is thought to form as a result of the submergence of a flat surface as the postglacial sea level rose, with the growth of the reef keeping pace with the rise in sea level.
shadow zone
a region 103°-143° from the epicenter of an earthquake in which, owing to refraction from the low-velocity zone inside the core boundary, there is no direct penetration of seismic waves. Within this area seismic waves are only received after they have been reflected at the Earth's surface.
terrane
a region in which the geology is markedly different from that in adjoining regions. The geology in one terrane is markedly different from a neighboring terrane. Terrane boundaries are usually faults. Typically, a terrane covers thousands of square kilometers, but some terranes are considerably smaller. Alaska and western Canada have been subdivided by some geologists into over 50 terranes. Terranes are named after major geographic features; for instance, Wrangellia (in Canada and Alaska) was named after the Wrangell Mountains of Alaska. Many terranes appear to have formed essentially in place as a result of accumulation and orogeny along the continent's margin. Other terranes have rock types and ages that do not seem related to the rest of the geology of the mountain belt and have been called suspect terranes, that is, terranes that may not have formed at their present site. If evidence indicates that a terrane did not format its present site on a continent, is regarded as an accreted terrane. Accreted terranes that can be shown to have traveled great distances are known as exotic terranes.
alp
a region of grassland on a high mountainside. It is named after the Alps in west-central Europe.
rain shadow
a region on the downwind side of mountains that has little or no rain because of the loss of moisture on the upwind side of the mountains.
fracture zone
a region that lies astride a line along which faulting has occurred. It is often the site of earthquakes and volcanic activity.
oil field
a region underlain by one or more oil reservoirs. Over 60% of the world's oil comes from exploitation of oil fields in the Middle East.
desert
a region with low precipitation (usually defined as less than 25 cm per year).
continental slope
a relatively steep slope extending from a depth of 100 to 200 meters at the edge of the continental shelf down to oceanic depths. The slope begins at the end of the continental shelf (at the shelf break) and ends at the continental rise. Globally, it makes up about 8.5% of the total ocean floor.
transverse dune
a relatively straight, elongate dune oriented perpendicular to the wind. That forms when more sand is available than for barchan formation.
spall
a relatively thin, curved piece of rock produced by exfoliation. As a verb, spall means to break off in layers parallel to a surface.
reef
a resistant ridge of calcium carbonate formed on the sea floor by corals and coralline algae.
thrust fault
a reverse fault in which the dip of the fault plane is at a low angle to the horizontal. In general, the dip is 45° or less, and the hanging wall appears to have moved upward relative to the footwall. Horizontal compression rather than vertical displacement is its characteristic feature.
sand bar
a ridge of sand built up to or near to the surface of the water by currents in a river or by wave action in coastal waters.
baymouth bar
a ridge of sediment that cuts a bay off from the ocean.
bar
a ridge of sediment, usually sand and gravel, deposited in the middle or along the banks of a stream.
bar
a ridge of sediment, usually sand or gravel, that has been deposited in the middle or along the banks of a stream by a decrease in stream velocity, or in the ocean deposited parallel with the coastline.
end moraine
a ridge of till piled up along the front edge of a glacier. An actively flowing glacier brings debris to its terminus. If the terminus remains stationary for a few years or advances, a distinct end moraine piles up along the front edge of the ice.
spur
a ridge that projects sharply from the crest or side of a mountain. It can also mean an underwater ledge or projection from an ice wall, iceberg, or coral reef face.
cuesta
a ridge with a steep scarp face or cliff on one side, a well-defined crest, and a gentle backslope (called a dip slope) on the other side. Cuestas form by gentle tilting of sedimentary rock strata.
misfit river
a river that is apparently too small for its valley. It is characterized by meander bends in its channel of smaller amplitude and greater intricacy than the meanders in the valley itself. There is an established relation between meander length and discharge in rivers, and on that basis it has been calculated that the rivers that eroded the valley meanders must have had a bankfull discharge 80-100 times the current streams.
ductile (plastic)
a rock capable of being banded and molded under stress.
petroglyph
a rock carving done by ancient humans.
chemical sedimentary rock
a rock composed of material precipitated directly from solution. An example is the formation of rock salt as seawater evaporates. Limestone is also a chemical sedimentary rock, that forms from cementation of broken pieces of shells, or crystallized as solid rock by corals and coralline algae in reefs.
quartzite
a rock composed of sand-sized grains of quartz that have been welded together during metamorphism caused by high temperatures. It is the product of the metamorphism of pure sandstone, during which process the quartz grains recrystallize and become interlocking. The welding together of the grains of quartz makes quartzite very difficult to crush or break, and it is the most durable of common rocks used for construction due to its hardness and because quartz is not susceptible to chemical weathering.
biogenic rock
a rock created by living organisms, their remains, or activities. Sediment created by living organisms is called biogenous sediment (or biogenic sediment).
pyroclastic rock
a rock formed by the accumulation of fragmental materials thrown out by volcanic explosions. Such material is known usually as tephra, and may be expelled as solid fragments or in the molten state, chilling in the air and producing vitreous material. All volcanic eruptions are the result of the release of gas that has been confined under pressure. The main kinds of pyroclastic materials and rocks are as follows:
tuff
a rock formed from fine-grained pyroclastic particles, such as consolidated ash and dust.
igneous rock
a rock formed from solidification of magma. Igneous rocks include intrusive igneous rocks (formed by cooling of magma inside the earth) and extrusive igneous rocks (formed by cooling of lava at the Earth's surface). The compositional range of igneous rocks is described by the terms acid, intermediate, basic, and ultrabasic.
fine-grained rock
a rock in which most of the mineral grains are less than 1 mm across (igneous) or less than 1/16 mm (sedimentary).
nunatak
a rock peak sticking out above the surface of an ice sheet. In many instances, these peaks were once covered with ice, and only subsequent reduction in the extent of ice cover has brought about their emergence. Nunataks are common along the coast of Greenland.
metamorphic rock
a rock produced by metamorphism, or changes to rocks that take place in Earth's interior. Well-known metamorphic rocks include gneiss, amphibolite, marble, schist, shale, slate, hornfels, and quartzite.
ductile rock
a rock that bands while under stress and does not return to its original shape after the stress is removed.
pedestal rock
a rock that has been shaped like a mushroom by the erosive action of windblown sand. In desert regions, the corrasive (yes, this is spelled correctly) action of sand is strongest about a meter above the ground, and therefore most pedestal rocks have "stems" of this height.
volcanic breccia
a rock that includes larger pieces of volcanic rock, such as cinder, blocks, and bombs.
reservoir rock
a rock that is sufficiently porous and permeable to store and transmit petroleum. Because of the surface tension of fluids cleaned the walls of the pores within this rock, only a certain fraction of the petroleum may ever be extracted and, in most cases, this is less than half of the oil present. The gaseous pocket at the top of a reservoir is an especially valuable feature that drillers can put to good use. When a drill hole first penetrates an oil reservoir, the pent up gases within may drive the petroleum all the way to the surface so that no pumping has to be done whatsoever. This fluid-pressure of fact saves oil companies a tremendous amount of money. In time, fluid pressure diminishes and oilfield becomes less economical to operate remaining oil may be flushed out of the ground by flooding the reservoir with injected groundwater. The groundwater drives the petroleum I had of it from the area of injection wells toward oil well for removal. Developers have also use steam to drive out the oil. As much as one third of the original reserve and oilfield may be extracted using these secondary recovery methods.
crag
a rocky outcrop on a hillside, characterized by very steep edges. Most crags occur in hard-rock regions, and result from erosion and weathering of the surrounding rock.
headland
a rocky promontory that projects outward into the ocean. It is composed of material that is more resistant to erosion than adjacent areas of the coast. Headlands are usually separated from each other by small coves containing pocket beaches.
eddy
a rotational feature of a fluid, which retains its identity for a limited time while moving within the main body of the fluid, but eventually combines and amalgamates with it.
boss
a roughly circular igneous intrusion that has a diameter of less than 25 km.
nodule
a rounded concretion in sedimentary rock, or a sediment mass of hydrogenous formation which precipitates from chemical reactions in seawater on the sea floor (as in manganese nodules).
cobble
a rounded sediment particle that is larger than a pebble but smaller than a boulder, with a diameter of 64 to 256 mm.
sabkha
a salt flat that has a surface encrusted with halite and runs along the coast just inland. It is formed under arid or semiarid conditions just above normal high tide level, and found in places such as along the coasts of the Persian Gulf and the Gulf of California.
arkose
a sandstone in which more than 25% of the grains are feldspar. It is usually composed of coarse, angular grains.
quartz sandstone
a sandstone in which more than 90% of the grains are quartz. The quartz grains in a quartz sandstone are usually well-sorted and well-rounded because they have been transported great distances. Most quartz sandstone was deposited as beach sand or dune sand.
sinkhole
a saucer-like hollow, typical of limestone areas, varying from 1 to 1000 m in diameter and from a few cm to 300 m in depth, produced by solution or by rock collapse. The ratio of diameter to depth is usually about 3:1. Limestone regions in Florida, Missouri, Indiana, and Kentucky are heavily dotted with sinkholes.
modified Mercalli scale
a scale expressing earthquake intensity based on the amount of damage done. The scale ranges in Roman numerals from I to XII. Higher numbers indicate greater damage. An earthquake within intensity of XII exhibits total damage, with waves seen on the ground surface, lines of sight and level are distorted, and objects are thrown upward into the air.
volcanic explosivity index (VEI)
a scale that classifies the explosivity of volcanoes based on amount of material thrown out, how high the eruption goes, and how long it lasts. The scale goes from 0 (non-explosive) to 8 (mega-colossal). The eruption of Mauna Loa was non-explosive, and scored a 0 on the VEI. The Mt. St. Helens eruption in Washington in 1980 scored a 5, the eruption of Krakatoa in 1883 and Mt. Pinatubo in 1991 scored a 6, the eruption of Tambora in 1815 scored a 7, and the eruption of Taupo (26,500 years ago) scored an 8. This eruptions was the most recent 8, but a couple of eruptions in Yellowstone in the U.S. have also scored 8 (these occurred hundreds of thousands of years ago). An eruption with a VEI of 8 only occurs once every 10,000 years.
turbidite
a sediment deposited in water by turbidity currents. Because the speed of a turbidity current is proportional to the square root of effective density, its lower part will be more heavily charged with sediment and will move faster than the less heavily charged upper part. For this reason the particle size of the sediment transported decreases from the base of the flow upward, a consequence being that a form of sediment grading, or graded bedding, occurs in the deposit ultimately laid down. A typical sequence in such a deposit is for course material at the base to give way to finer material above.
pebble
a sediment particle with a diameter of 2 to 64 mm.
arête
a sharp ridge that separates adjacent glacially carved valleys (often a cirque is present on either side of the ridge, producing the sharp edge).
hogback
a sharp-topped ridge formed by the erosion of steeply dipping beds. It generally results from the folding or faulting of strata, followed by differential erosion that removes softer rocks from alongside it.
pressure release
a significant type of mechanical weathering that causes rocks to crack when overburden is removed. When the weight of the rock above is removed (this is called unloading), cracks called sheet joints develop parallel to the outer surface of the rock as the outer part of the rock expands more than the inner part.
graded bed
a single bed with coarse grains at the bottom of the bed and progressively finer grains toward the top of the bed. It is often found in turbidity deposits.
limestone
a sedimentary rock composed mostly of calcite but including other minerals such as dolomite. Two types of limestone exist: biochemical and inorganic. Biochemical limestones are precipitated through the actions of organisms. This type of limestone is usually formed on continental shelves in warm, shallow water. This limestone may be deposited directly in the core of a reef by corals, encrusting algae, or other shell-forming organisms. But the great majority of limestones are biochemical limestones formed of wave-broken fragments of algae, corals, and shells. The fragments may be any size and are often sorted and rounded during transport by waves and currents. The fragments are then cemented, giving these bioclastic (or skeletal) limestones a great variety of appearances , such as the coarse-grained coquina in which individual shell fragments are easily recognizable, or the fine-grained chalk. Inorganic limestones are precipitated directly as the result of inorganic processes. One example is oolitic limestone, formed by the cementation of sand-sized ooids, which are small spheres of calcite inorganically precipitated in warm, shallow seawater. Tufa and travertine are inorganic limestones that form from fresh water.
clastic sedimentary rock
a sedimentary rock composed of fragments (clasts) of preexisting rock. Clastic sediments range from large boulders to fine-grained sediments, such as silt. Clastic rocks include breccia, conglomerate, mudstone, sandstone, and shale.
siltstone
a sedimentary rock consisting mostly of silt grains. It resembles shale but contains less clay.
mélange
a sedimentary rock consisting of a jumbled mass of various rock fragments including schists, limestones, cherts, quartzites, graywackes, and a wide range of other rock types. They are thought to have been deposited as a result of the slumping of large masses of unstable rock debris, associated with destructive plate boundaries.
coal
a sedimentary rock formed from the consolidation of plant material that has not completely decayed. It is rich in carbon, usually black, and burns readily. Coalification, the formation of coal, proceeds from peat, through lignite, bituminous coal, and finally to anthracite (the highest grade of coal). During the process of coalification, the percentage of carbon increases and volatiles and moisture are gradually eliminated. Coal forms in swampy, tropical, coastal environments. Dead plant matter settles into stagnant, oxygen-depleted water and becomes buried by sediment. Under pressure and heat, the fossil plant remains transform it coal.
bioclastic rock
a sedimentary rock that contains the fragmentary remains of once-living organisms, such as shell fragments.
seismic gap
a segment of a fault that has not experienced earthquakes for a long time; such gaps may be the site of large, future quakes.
S wave (Secondary wave)
a seismic wave propagated by a shearing motion, which causes rock to vibrate perpendicular to the direction of wave propagation. S waves are classified as transverse waves, since the direction of vibration is perpendicular to the direction of propagation. S-waves do not travel through liquids or gases, and hence do not travel through the outer core of the earth.
surface wave
a seismic wave that travels on the surface of the Earth. Surface waves include Love and Rayleigh waves. Surface waves are the slowest waves set off by earthquakes. Surface waves cause more property damage than body waves because surface waves produce more ground movement and travel more slowly, so that they take longer to pass.
Conrad discontinuity
a seismically detectable boundary within the Earth's continental crust, separating it into a lower (basic) layer and an upper (granitic) layer. Beneath the ocean floors, the upper, granitic layer is missing.
seismograph
a seismometer with a recording device that produces a permanent record of Earth motion.
reaction series
a series of minerals in which any early-formed mineral phase tends to react with the melt, later in the differentiation, to yield a new mineral further down the series. For example, early-formed crystals of olivine react with later liquids to form pyroxene crystals, and these in turn may react with still later liquids to form amphiboles. The common rock-forming minerals have been arranged by Bowen in the order in which the reactions take place and constitute two series.
paternoster lakes
a series of tarns (rock-basin lakes) carved by glacial erosion. They occupy depressions in a glacial valley, and are connected by streams, rapids, or waterfalls.
nematath
a series of volcanoes that form as one of the Earth's lithospheric plates moves slowly across a hot spot. The farther a volcano is from the hot spot, the older it is. The youngest volcanoes are usually still active, whereas the older ones may be dormant. The Emperor Seamount Chain in the Pacific is a good example.
oil trap
a set of conditions that hold petroleum in a reservoir rock and prevent its escape by migration. It allows oil and natural gas to accumulate and normally occurs where an upward convex stratum of porous and permeable rock is sealed from above by an impermeable rock cap.
*Marcellus formation (Marcellus shale)
a shale formation located in Pennsylvania, New York, and Pennsylvania, is a huge shale deposit that is though to contain 400 trillion cubic feet of natural gas. The natural gas here is being extracted by a technique called hydraulic fracturing (fracking)
maar
a shallow, low-relief, volcanic crater not associated with a single vent or cone, but rather formed by multiple shallow explosive eruptions. Many maars fill with water and form lakes.
playa lake
a shallow, temporary lake (following a rainstorm) on a flat valley floor in a dry region.
trimline
a sharp boundary line delimiting the maximum upper level of the margins of a glacier that has receded from an area.
horn
a sharp peak formed where cirques cut back into a mountain on several sides.
medial moraine
a single, long ridge of till on a glacier, formed by adjacent lateral moraines joining and being carried downglacier. Upon melting of the glacier, the medial moraine may be left as a ridge running down the middle of a valley. In a large trunk glacier that has formed from many tributaries, the numerous medial moraines give the glacier the appearance from the air of a multilane highway.
joint set
a situation in which a bunch of joints are oriented approximately parallel to one another. It's
adit
a slightly upward-sloping, closed-ended passage into a mine, usually constructed to intersect a seam of coal or vein of mineral, and sloped so that water drains out of the adit.
slough
a sluggish body of water in a tidal flat, marsh, bottomland, or shallow undrained depression.
oolith
a small accretionary body in sedimentary rock formed of calcium carbonate in concentric layers around a nucleus such as a sand grain.
stock
a small discordant pluton with an outcrop area of less than 100 km2 (if the outcrop area is greater than 100 km2 the pluton is called a batholiths).
aftershock
a small earthquake that follows the main earthquake or shock.
cirque glacier
a small glacier found in a valley head or depression on a mountain slope. These glaciers are usually the first glaciers to form and the last to disappear during a period of glaciation.
tor
a small hill consisting of a pile of rocks, much-jointed and usually granitic, formed by weathering. Tors are prominent masses on the moors of Devon and Cornwall, England.
ravine
a small narrow deep-sided valley usually carved by the erosive action of a river.
promontory
a small peninsula or headland that projects into the sea. Wave action can cause erosion on both sides, which are likely to have cliffs and possibly caves.
clast
a small piece of rock that has been removed from a larger mass by some fragmentation process.
sea stack
a small rock island that is an erosional remnant of a headland left behind as a wave-eroded coast retreats inland.
vesicle
a small rounded cavity within lava that is formed by bubbles of gas coming out of solution during the solidification of the lava.
oolite (ooid)
a small sphere of calcite (limestone) precipitated from seawater that is formed predominantly from ooliths.
tributary
a small stream flowing into a large stream, or any stream that contributes water to another stream.
tarn
a small, deep, commonly circular glacial lake occupying a cirque. Tarns occupy depressions carved out of the weaker rock. Tarns are fed by run-off from the surrounding slopes and damned by a lip of bedrock or by a small moraine. Tarns are sometimes called rock-basin lakes.
gully
a small, narrow channel on a hillside lacking vegetation, generally formed by the rapid runoff of surface water following heavy rainfall. Gullies are dry most of the time.
knoll
a small, rounded hill.
cay
a small, sea island that is generally low-lying and formed in sand or coral. For example, the term is applied to the tiny islands or islets off the South Florida coast.
hanging valley
a smaller valley that terminates abruptly, high above a main valley. A large glacier will cause greater deepening of a preexisting valley than will a smaller glacier, and therefore where glaciers invade a major valley and it s tributaries, the major valley will be deepened to a greater extent than the tributary valleys, which become hanging valleys upon disappearance of the ice. Subsequent rivers will reach the main valley from these hanging valleys as waterfalls.
kaolin (china clay)
a soft, white clay used in medicine, as a filler for paper, and in the manufacture of china. It consists of deposits of minerals of the kaolinite group of clays, which are produced by the weathering and hydrothermal alteration of feldspars in granitic rocks.
podzol
a soil characterized by an ashen-colored acid eluviated A horizon and a B horizon illuviated with iron or humus, possibly in the form of a compact pan. The A horizon is depleted of bases by leaching processes. Podzols are zonally developed soils in the taiga zones or Russia and North America, developing under coniferous or mixed forests in cool to temperate climates.
C horizon
a soil layer composed of incompletely weathered parent material that lies beneath the B horizon. It is the material from which soil is formed, lying between the upper subsoil (B horizon) and the bedrock. It contains no humus and will not support plant life.
zonal soil
a soil occurring over a wide area because of the dominance of the bioclimatic factor in soil formation, which determines soil-forming processes.
andisol
a soil that has developed recently on base-rich volcanic ash. The upper A horizon of these soils is dark owing to the presence of organic matter that is well-mixed by earthworm activity. In the U.S., andisols are found in northwestern Washington.
acid soil
a soil with a pH of less than 7.0. These soils form where there is a lack of exchangeable bases in the soil, such as calcium and sodium. These bases are largely replaced by the two cations hydrogen and aluminum, which control soil acidity. Possible factors leading to this type of soil formation are leaching, organic matter containing few bases, and an acid parent material. Acid soils are common in the humid tropics and the humid temperate lands. Examples of acid soils are podzols, brown earths, and latosols. Most cultivated crops thrive on mildly acid soils.
meteor
a solid body from an extraterrestrial source that passes through the Earth's atmosphere, heated to incandescence by friction (sometimes called shooting or falling stars).
hot spring
a spring in which the water is warmer than human body temperature. The usual way that water gains heat is by circulating the air and underground magma chamber or a body of cooling igneous rock. Groundwater can also heat if it circulates unusually deep in the earth, perhaps along joints or faults. The hot springs in the Western U.S. are heated in the first manner, whereas the famous springs in Georgia or Hot Springs, Arkansas, are warmed by deep circulation.
clapotis
a standing wave formed when incident sea waves meet waves being reflected from a vertical barrier and where the depth of water in front of the barrier exceeds the wavelength of the incident waves.
scarp (escarpment)
a steep, cliff-like slope often of considerable size, which rises above the surrounding land surface. Such structures result from faulting or the differential erosion of gently inclined strata. Scarp is an abbreviation of escarpment, and the two have essentially the same meaning.
bysmalith
a steep-sided vertical igneous body, roughly cylindrical in form, and following a steeply inclined fault. It arches up the overlying country rock or becomes exposed at the surface.
cirque
a steep-sided, amphitheater-like hollow carved into a mountain at the head of a glacial valley. Cirques are enclosed by high and steep headwalls and sidewalls which are frequently frost-shattered. Cirques are created by freeze-thaw action on the headwall, which enlarges the cirque, abrasion by rotationally slipping ice, which deepens the rock basin, and joint-block removal.
volcanic dome
a steep-sided, dome- or spine-shaped mass of volcanic rock formed from viscous lava that solidifies in or immediately above a volcanic vent. A volcanic dome grew within the crater of Mount St. Helens after the climactic eruption of 1980.
bluff
a steeply sloped river bank on the outside of a meander, caused by erosion by the faster-flowing water on the outside of the bend.
gastrolith
a stone or pebble ingested by reptiles and birds to assist in the breakdown of food in the gizzard.
aerolite
a stony meteorite made up of silicate minerals.
carbonaceous chondrite
a stony meteorite that contains chondrules in a claylike matrix of silicates. Their carbon content may be up to 3%, and is mainly in the form of hydrocarbons.
chondrite
a stony meteorite that contains chondrules.
achondrite
a stony meteorite that does not contain chondrules.
reef flat
a stony platform of dead reef-rock, commonly strewn with coral fragments and coral sand, generally dry at low tide and formed as the summit of the reef above low tide.
perennial stream
a stream or river that flows permanently throughout the year.
ephemeral stream
a stream that contains water only immediately after rainfall, found mainly in arid and semiarid areas. For most of the year, its channel is dry.
graded stream
a stream that exhibits a delicate balance between its transporting capacity and the sediment load available to it. This balance is maintained by cutting and filling any irregularities in the smooth longitudinal profile of the stream. As a stream smooths out its longitudinal profile to a characteristic concave-upward shape, it becomes graded.
braided stream
a stream that flows in a network of many interconnected rivulets around numerous bars before rejoining farther downstream. A stream tends to become braided when it is heavily loaded with sediment (particularly bed load) and has banks that are easily eroded. The load that can't be carried by the river is dumped mid-channel, forming the characteristic bars in a braided stream. The braided pattern develops in deserts as a sediment-laden stream loses water through evaporation and percolation into the ground. It also occurs in front of a melting glacier. In meltwater streams flowing off glaciers, braided patterns tend to develop when the discharge from the melting glaciers is low relative to the great amount and ranges of size of sediment the stream has to carry.
intermittent stream
a stream that flows only certain times of the year, as after heavy rain or when it receives water from springs. They usually dry up in the dry season because the water table becomes lower.
effluent
a stream that flows out of a lake or other stream. In general usage, the term describes waste material discharged as a liquid from a sewer, nuclear power plant, or waste treatment plant. It is a potential source of pollution.
antecedent stream
a stream that maintains its original course despite later deformation of the land.
gaining stream
a stream that receives water from the saturated zone. The surface of gaining streams coincides with the water table. Water from the saturated zone flows into the stream through the stream bed and banks that lie below the water table. Because of the added groundwater, the discharge of these streams increases downstream. Gaining streams are normally found in rainy regions.
tensional stress
a stress due to a force pulling away on a body.
Hooke's law
a stressed body deforms to an extent that is proportional to the force applied. Materials that obey Hooke's law are said to be elastic.
rapids
a stretch of fast-flowing turbulent broken water at a break in the long profile of a river that is not a vertical drop. The river is broken by obstructions, as where the stream descends over a series of small steps.
lagoon
a stretch of water that is more or less enclosed and often shallow, protected from the open sea by a spit, tombolo, bay-mouth bar, or other kind of barrier. The barrier is not complete, for lagoons invariably possess a free connection with the open sea.
fold and thrust belts
a structure characteristic of the many mountainous regions that is characterized by large thrust faults stacked one upon another. Begin to routine rock usually was folded while it was being transported during faulting. Fold and thrust faults in a mountain belt suggest tremendous squeezing or crustal shortening and crustal thickening.
structural dome
a structure in which the beds dip away from a central point. In cross-section, a dome resembles an anticline.
structural basin
a structure in which the beds dip toward a central point. In cross-section, it is comparable to a syncline.
continental shelf
a submarine platform at the edge of a continent, inclined very gently seaward generally at an angle of less than 1 degree. The continental shelf is the part of a continent that is shallowly submerged by the sea. It extends from the shoreline down to the continental slope, which begins at the shelf break, or down to a depth of 200 m if the continental slope is absent. Its average width is 70 km, but it ranges from less than 1 to more than 1000 km wide. Globally, shelves occupy about 7.6% of the ocean floor.
volatile
a substance, normally gaseous, that is dissolved under pressure in a magma. The main the volatile constituents of magmas are water (highest) and carbon dioxide (lesser amounts). The solubility of volatile substances increases with pressure so that during the ascent of magma and the attendant lowering of pressure, the magma is unable to retain these constituents in solution. These volatiles concentrate at the top of the magma chamber, and may build in pressure until they cause a volcanic eruption. The explosive release of these gases produces pyroclastic rocks.
MTBE (methyl tributyl ether)
a substitute for octane that is put into reformulated fuels. It has been used in gasoline as an additive at low levels since 1979 to replace tetra-ethyl lead and to increase its octane rating, which helps prevent engine knocking. It is an oxygenating agent, which helps gasoline burn more completely, reducing tailpipe emissions.
saturated zone
a subsurface zone in which all rock openings are filled with water. Most rivers and lakes intersect the saturated zone.
vadose (unsaturated) zone
a subsurface zone in which rock openings are generally unsaturated and filled partly with air and partly with water. The vadose zone lies just above the saturated zone. Underground streams in the zone flow with free air surfaces.
Pangaea
a supercontinent that broke apart 180-200 million years ago to form the present continents. It is thought to have fragmented initially into two segments, Laurasia to the north and Gondwanaland to the south.
placer
a surface deposit of sand or gravel formed by mechanical concentration of mineral particles from weathered debris. The mineral concentrated is usually a heavy mineral, such as gold, platinum, diamonds, tin, or chromite. Small amounts of the minerals can be removed by panning. Large-scale extraction usually involves dredging and concentration of the mineral by various processes.
outcrop
a surface exposure of bare rock, not covered by soil or vegetation.
unconformity
a surface that represents a break in the geologic record, with the rock unit immediately above it being considerably younger than the rock beneath. It is a surface that represents a period of non-deposition and/or erosion separating rocks of different ages. Unconformities can be classified into three types: disconformities, angular unconformities, and nonconformities. In a disconformity, the contact representing missing rock strata that separates beds that are parallel to one another. Probably what happened is that older rocks were eroded away parallel to the bedding plane; renewed deposition later buried the erosional surface. Because it often appears to be just another sedimentary contact (or bedding plane) in a sequence of sedimentary rock, a disconformity is the hardest type of unconformity to detect in the field. An angular unconformity is a contact in which younger strata overlie any erosional surface on tilted or folded layered rock. An angular unconformity implies the following sequence of events, from oldest to youngest: 1) deposition and electrification of sedimentary rock, 2) uplift accompanied by folding or tilting of the layers, 3) erosion, and 4) renewed deposition on top of the erosional surface. A nonconformity is a contact in which an erosional surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock. A nonconformity generally indicates deep or long-continued erosion before subsequent burial, because metamorphic or plutonic rocks format considerable depths. The geologic history implied by a nonconformity implies the following sequence of events: 1) crystallization of igneous or metamorphic rock at depth; 2) erosion of at least several kilometers of overlying rock, and 3) deposition of new sediment, which eventually become sedimentary rock, on the ancient erosion surface.
disconformity
a surface that represents missing rock strata, but beds above and below that surface are parallel to one another.
alidade
a surveying instrument used for sighting onto objects of detail and for defining the rays to be drawn to them in plane tabling. The alidade is basically a ruler of metal or wood with a vertical slit at the observer's end and vertical stretched wire sight at the other.
L wave
a synonym for surface wave. Types include Love waves and Rayleigh waves. These are seismic waves that travel in the thin uppermost layer of the Earth's crust. L waves cause multiple reflections between the surface and the top of the layer of the rock below.
dike
a tabular, roughly vertical, discordant, intrusive igneous body of rocks.
brea
a tar and oil seep. Under deep burial conditions, pressure usually squeezes the fluid petroleum up into overlying permeable rocks, and it may continue to migrate all the way to the surface to issue from the earth as a brea.
*fracking (hydraulic fracturing)
a technique which involves pumping high pressure water, sand, and thickening agents down a well in order to break a shale formation apart, allowing natural gas and oil trapped within it to flow out/escape from the fissures created. Fracking is controversial, as it has been shown to contaminate ground water with natural gas and fracking chemicals, and it has been shown to weaken the rock, precipitating earthquakes.
polar wandering
the apparent movement of the North and South Poles. Alfred Wegener used polar wandering as evidence for continental drift.
rift valley
a tensional valley bounded by normal faults. Rift valleys are found at diverging plate boundaries on continents and along the crest of the mid-ocean ridges. they have steep sides and a flat bottom, the rocks of the valley floor having subsided between two parallel faults. The most famous is the East African Rift Valley.
mafic rock
a term describing igneous rock composed chiefly of dark-colored, ferromagnesian minerals. It is silica-deficient igneous rock with a relatively high content of magnesium, iron, and calcium. The word mafic is a contraction of magnesium and ferric. Mafic rock is the complement of felsic (light-colored) rock.
llano
a term for an extensive plain, with or without vegetation, applied especially to the generally treeless plains of northern South America and the southwestern U.S. (such as the Llano Estacado in west Texas).
mud
a term loosely used for silt and clay, usually wet.
watershed
a term synonymous with drainage basin.
wadi
a term used in Arabia and N. Africa for a narrow, normally dry valley in a desert or semidesert environment. Normal fluvial activity does not exist, but when a downpour occurs, large amounts of weathering debris accumulate in situ. Wadis may be relicts from times of wetter climate.
suspect terrane
a terrane that may not have formed at its present site.
schistose
a texture of rock in which visible platy or needle-shaped minerals have grown essentially parallel to each other under the influence of directed pressure.
Euler's theorem
a theorem that provides a mathematical explanation for the distribution of conservative plate boundaries. Plate tectonics requires that all conservative plate boundaries (transform faults) lie on small circles, the axes of which form the axis of rotation for the relative motion of the plates on each side. It also indicates that the velocity of relative motion across a destructive or constructive plate boundary is proportional to the angular distance of the particular point from the axis of rotation and to the angular velocity about the axis of rotation for the motion of the plates. Thus, velocities will vary along plate boundaries.
rock cycle
a theoretical concept relating tectonism, erosion, and metamorphism to the formation of the three main types of rocks (igneous, sedimentary, metamorphic). In the rock cycle, igneous rocks are formed by solidification of magma. Weathering and erosion of igneous, metamorphic, and sedimentary rock results in the formation of sediment, which can be transformed into sedimentary rock. Metamorphism of igneous or sedimentary rock due to high temperature and pressure results in the formation of metamorphic rock (which sometimes melts within the earth to form magma, which can solidify into igneous rock once again).
plate tectonics
a theory developed in the early 1960s in which the lithosphere is divided into a number of lithospheric plates, whose pattern of horizontal movement is that of torsionally rigid bodies that interact with one another at their boundaries, causing seismic and tectonic activity along these boundaries. The motion of these plates is caused by convection currents in the mantle. Only the oceanic parts of plates grow or are destroyed; the continents ride along on these plates and grow only slowly by the addition of volcanic material and sediment deposition along their margins. These sediments are compressed and folded when 2 continental regions collide by orogenic processes, which results in the formation of fold mountain chains. These have a central nucleus of older rocks, frequently cratons.
mantle
a thick shell of rock that separates Earth's crust above from the core below. It is found roughly between 30 and 2900 km beneath the Earth's surface. It is thought to consist of ferromagnesian silicate minerals such as olivine and pyroxene. Essentially it behaves as a solid, although within the asthenosphere (which lies below the lithosphere) it may be partly molten.
varve
a thin bed of sediment in glacial lakes that represents a seasonal increment. Spring and summer glacial melting produces a sudden influx of coarse sediment upon which finer material settled during the remainder of the year. A glacial varve normally includes a lower summer layer consisting of light colored sand or so, which grades upward into a thinner winter layer, consisting of clayey, often organic, dark sediment. Therefore, two layers of sediment (one light and one dark) represent one year's deposition. Counting and correlation of varves have been used to measure the ages of Pleistocene glacial deposits.
desert varnish
a thin coating, varying in color from pale yellow to very dark red, found on the surfaces of pebbles and blocks in stony deserts. It is thought to be caused by the deposition of iron and manganese oxides from solutions drawn to the surface by capillarity and then evaporated. Highly polished surfaces of this type can occur through the extremely abrasive effect of fine sand carried by strong winds.
desert pavement
a thin layer of closely packed gravel that protects the underlying sediment from deflation (also called pebble armor). The pavement is created by the abrasive effect of wind-blown sand. The individual stones may be cemented together by precipitated salts drawn to the surface in solution by capillarity.
sheetwash
a thin layer of unchanneled water flowing downhill. Sheetwash is particularly common in deserts, where the lack of vegetation allows rainwater to spread quickly over the land surface. Sheetwash can produce sheet erosion, in which a thin layer of surface material, usually topsoil, is removed by the flowing sheet of water.
polder
a tract of flat, low-lying land (as in the Netherlands and Belgium) reclaimed and protected from the sea or other body of water by embankments, dikes, dams, or levees. The term is usually reserved for coastal areas that are at or below sea level and that are constantly protected by an organized system of maintenance and defense.
radiocarbon dating
a type of radiometric dating that involves looking at the relative proportions of carbon-14 and nitrogen-14, into which carbon-14 decays. The half-life of carbond-14 is 5730 years, and radiocarbon dating is effective for objects that are between 100 and 40,000 years old.
earthquake
a trembling or shaking of the ground caused by the sudden release of energy stored in the rocks beneath the surface. They result from the accumulation of forces within the rocks until they are strained to a point beyond which they fracture. The magnitude of an earthquake is the amount of energy involved. The intensity of an earthquake refers to the degree of violence of an earthquake at a particular point on the Earth's surface, expressed on a descriptive scale called the modified Mercalli scale, which ranges from I (only detected by seismograms) to XII (total destruction).
yazoo stream
a tributary stream that flows for a considerable distance parallel to the mainstream, from which it is separated by a natural levee, before joining the main stream. Yazoo streams are named for the Yazoo River, which flows alongside the Mississippi River for 320 km before joining it near Vicksburg, Mississippi.
pyroclastic flow
a turbulent mixture of hot gases and pyroclasts that moves under gravity along the surface from the vent of an erupting volcano (down the flanks of a volcano). Such flows can move quickly (up to 200 km/hr) and be extremely dangerous, destroying anything in their path. Most of the deaths caused by volcanic eruptions are due to pyroclastic flows.
hammada
a type of arid desert plain, consisting of an extensive almost bare rock surface, especially in the Sahara.
muskeg
a type of bog that occurs in tundra areas. It forms when the surface permafrost melts in summer, and generally supports lichens and mosses, such as sphagnum moss. The best-known muskegs are found in N. Canada.
suevite
a type of breccia consisting of angular fragments of rock in a glass matrix.
asphalt
a type of brown or black bituminous material consisting mainly of carbon disulfide (CS2) and hydrogen. It varies from a thick viscous liquid to a tarry solid, and probably represents an early stage in the formation of petroleum. It is often used to surface roads.
fault scarp
a type of cliff formed when a block of rocks is forced upward by a fault.
basic lava
a type of dark-colored lava containing basic ferromagnesian minerals and less than 50% silica. Because of its low silica content, it flows freely, and becomes basalt upon solidification.
corrasion
a type of erosion involving abrasion of a rock surface by small fragments of rock carried along by a river or glacier.
cavitation
a type of erosion seen in fast-flowing streams whereby the speed of flow lowers the pressure of the water, which leads to the formation of bubbles in the stream. When the channel widens and the velocity of the water slows, the bubbles collapse and give off shock waves that exert stress on the channel walls.
accordion folding
a type of folding in rocks in which the beds of the hinge area are markedly thickened and sharply folded, while on the limbs the beds are straight and of uniform thickness.
cast
a type of fossil consisting of a pseudomorph in which the skeletal parts of the organism have been dissolved and replaced by a secondary material, producing a replica of the original form.
frost wedging
a type of frost action in which the expansion of freezing water pries a rock apart.
solfatara
a type of fumarole in which the gases are sulfurous. It is named after the Solfatara volcano in Italy.
liquefaction
a type of ground failure in which water-saturated sediment turns from a solid to a liquid as a result of shaking, often caused by an earthquake.
scablands
a type of highly eroded terrain that was formed by sudden extensive flooding after the melting of glaciers and ice sheets. There is little or no soil on the bare rock surface, and so very few plants grow. A well-known example is the Columbia Plateau in the Pacific Northwest of the U.S.
mud pot
a type of hot spring containing boiling mud, usually sulfurous and often multicolored, as in a paint pot. Mud pots are commonly associated with geysers and other hot springs in volcanic areas.
geyser
a type of hot spring that periodically erupts hot water and steam, which is formed when groundwater comes into contact with hot rock underground.
castle kopje
a type of inselberg developed by deep weathering and exhumation in a rock divided by evenly spaced vertical and horizontal joints. Stacks of these fairly regular blocks give a castle-like, stepped appearance.
rockfall
a type of landslide involving purely dry materials. It involves the detachment of a segment of bedrock from a cliff, steep slope, cave, or arch.
rotational slide
a type of mass wasting along a curved surface in which the upper part moves vertically downward while the lower part moves outward (also called slumping).
frost action
a type of mechanical weathering that results in the breakup of rock through repeated freezing and thawing of water in its crevices.
slide
a type of movement in mass wasting in which the descending mass remains relatively intact, moving along one or more well-defined surfaces.
fall
a type of movement in mass wasting in which the material free-falls or bounces down a cliff.
rille
an elongate, trenched or crack-like valley on the lunar (Moon) surface.
intermediate rock
a type of rock containing 55-66% silica by weight. Very little of the silica is quartz - most is in the form of plagioclase feldspar. Diorite is a coarse-grained example, whereas andesite is a fine-grained example of intermediate rock.
hydrolith
a type of rock that was formed by chemical precipitation from water, such as gypsum or halite.
black sand
a type of sand found on beaches or in alluvial deposits that contains dark, heavy minerals such as ilmenite and magnetite (containing iron) and rutile (containing titanium).
Rayleigh waves
a type of surface seismic wave that behaves like a rolling ocean wave and causes the ground to move in an elliptical path. In Rayleigh waves, the motion of surface particles is in the vertical plane containing the direction of wave propagation, whereas in Love waves the motion of surface particles has a horizontal motion that is shear or transverse to the direction of propagation. The elliptical path in which the ground moves is opposite to the direction the wave is moving. Rayleigh waves tend to be incredibly destructive to buildings because they produce more ground movement and take longer to pass.
Love wave
a type of surface seismic wave that causes the ground to move side to side in a horizontal plane perpendicular to the direction the wave is traveling. Love waves, like S waves, do not travel through liquids and gases (only solids), so they are not on a body of water. Because of the horizontal movement, Love waves tend to knock buildings off their foundations and destroy highway bridge supports.
amphidromic system
a type of tidal system in which the high water rotates around a central point called the amphidromic point. Tidal range is very small or zero at the amphidromic point, but increases outward from this point. The high water rotates counterclockwise in the N. Hemisphere and clockwise in the S. Hemisphere.
Strombolian eruption
a type of volcanic eruption characterized by jetting of clots or "fountains" of fluid basaltic lava from a central crater. It is named after the Stromboli volcano, of the Lipari Islands of Italy.
wacke
a type of young sandstone with poorly sorted grains in a matrix of clay and fine silt. Specifically it contains more than 10% silt and clay (argillaceous) matrix.
acre
a unit of area equal to 4840 square yards (equivalent to 4047 square meters of 0.4047 hectares).
acre foot
a unit of volume used for lakes and reservoirs equal to an area of 1 acre to a depth of 1 foot.
darcy
a unit used to measure the porosity (permeability coefficient) of a rock. It represents the resistance to flow through the rock of a fluid (gas or liquid).
downcutting
a valley-deepening process caused by erosion of a stream bed. The limit of downcutting is known as the base level; it is a theoretical limit for erosion of the Earth's surface.
fumarole
a vent in the ground in a volcanic region which emits steam and other hot gases. They occur mainly on lava flows and in the calderas and craters of active volcanoes.
soil profile
a vertical cross section showing the arrangement of soil horizons found between the ground surface and the parent material. The normal depth of a profile in temperate latitudes is about a meter. The surface horizon is known by the letter A and is usually rich in organic matter and plant nutrients. Because material is often leached from this horizon down the profile to the subsoil B horizon, it is known as the eluvial horizon. The B horizon (illuvial horizon) contains less organic matter and more closely resembles the parent material owing to the lesser effect of the soil-forming processes. This horizon usually merges into the C horizon, which is the weathered parent material. Well-developed soils consist of more than these 3 horizons and subdivision occurs.
waterfall
a vertical fall of water at a steep break in the long profile of a stream.
blowhole
a vertical fissure in the roof of a sea cave or cliff through which sea water is forced as a jet at high tide.
chalk
a very fine-grained bioclastic limestone which is pure white and formed predominantly from coccoliths (made of calcite) but may include other invertebrate skeletal fragments. The White Cliffs of Dover, in southeast England, are a famous example of a chalk deposit.
hornfels
a very fine-grained, unfoliated metamorphic rock whose parent rock is either shale or basalt. Hornfels are the product of contact metamorphism.
abyssal plain
a very flat region of the seafloor found seaward of the continental rise. Abyssal plains are the flattest features on earth. They generally have slopes less than 1:1,000 (less than 1 m of vertical drop for every 1000 meters of horizontal distance).
playa
a very flat surface underlain by hard, mud-cracked clay. Playas form when temporary playa lakes dry up. There may be deposits of evaporates just below the surface, and similar deposits on the surface when the ephemeral lake dries up.
shield
a very large rigid area of the Earth's crust made up of Precambrian rocks, which have been unaffected by later orogenic episodes. Shields represent areas of the Earth's earliest formed continental crust. A well-known example is the Canadian Shield.
Ural Mountains
a very old mountain range that resulted from the collision of Asia and Europe (continent-continent convergence). The Ural Mountains still form the border between Europe and Asia.
convection (convection current)
a very slow circulation of a substance driven by differences in temperature and density within that substance; convection currents in the mantle are thought to drive the motion of the lithospheric plates, and these convection currents drive seafloor spreading. Uneven heating of the Earth's surface sets up convection currents in the atmosphere (hot air rises at the equator, then descends at 30 degrees north and south).
phreatic eruption
a violent volcanic eruption caused by escaping steam generated when a lava flow comes into contact with groundwater.
*caldera
a volcanic depression much larger than the original crater of the volcano. Calderas normally form when the magma chamber of a volcano is drained/emptied, causing the roof of the magma chamber to collapse (no magma = no support for the roof of the chamber). The diameter of a caldera exceeds one km, and may reach up to 20 km. Calderas usually result from the collapse or explosive removal of the top of a volcano. Crater Lake is a famous example of a caldera.
spatter cone
a volcanic structure consisting of a small, steep-sided mound of lava that has issued from a central vent or along a fissure. The lava takes the form of lumps and cinders, adhering together, that have come from sprays of frothing molten magma.
diatreme
a volcanic vent, often filled by brecciated material, that has been cut from the sides of the conduit by high-pressure gas charged with particles. The best-known examples are the diamond-bearing kimberlite pipes of South Africa.
stratovolcano
a volcano constructed of alternating layers of pyroclastic material and rock solidified from lava flows (also called a composite volcano).
composite volcano (stratovolcano)
a volcano constructed of alternating layers of pyroclastic material and rock solidified from lava flows. Composite volcanoes are generally developed by volcanoes with fairly viscous lavas, such as andesites. They have a slope of less than 33°, and they are considerably larger than cinder cones. They are typically 1000 to 4000 m in height.
cinder cone
a volcano constructed of loose rock fragments (pyroclasts) ejected from a central vent. They are the smallest of the three types of volcanoes (shield and composite volcanoes are the other types), and have steep slopes of 30-33°. Few cinder cones exceed a height of 500 m.
dormant volcano
a volcano that has erupted within recorded history but is apparently not active at present. Because nobody can predict if it will one day become active again, it is best described as an inactive volcano.
seawall
a wall constructed along the base of retreating cliffs to prevent wave erosion.
nugget
a water-worn lump of placer gold or other metal.
bog
a waterlogged area of land, resulting from poor drainage, in which vegetation becomes partly decomposed. The ground is spongy and wet, and eventually the vegetation forms an acid peat.
shock wave
a wave of compression created when the speed of something exceeds the speed of sound in the medium in which it is traveling. Within a rock, a shock wave (as from a meteorite impact) may deform, melt, or even vaporize the material, perhaps altering its composition at the same time.
seismic wave
a wave of energy produced by an earthquake or explosion. There are 4 main types of seismic waves: primary (P) waves, secondary (S) waves, Rayleigh waves, and Love waves. P waves arrive first at a seismograph station, then the S waves, and finally the surface waves (Rayleigh and Love waves).
breaker
a wave that has become so steep that the crest of the wave topples forward, moving faster than the main body of the wave.
marine terrace
a wave-cut platform that has been exposed by uplift along a seacoast, or by lowering of sea level.
carbonic acid
a weak acid with the formula H2CO3. It forms when carbon dioxide reacts with water. Even though carbonic acid is a weak acid, it is so abundant at Earth's surface that it is the single most effective agent of chemical weathering.
continental rise
a wedge of sediment that extends from the lower part of the continental slope to the deep sea floor (abyssal plain); it is formed by sediments cascading down the continental slope, and finally piling up at the bottom of the slope.
artesian well
a well in which water rises to a level above the top of the aquifer. Artesian wells are drilled into confined aquifers, where the water is under pressure, causing it to rise above the top of the aquifer. In some artesian wells, the water rises above the land surface, producing a flowing well that spouts continuously into the air unless it is.
Devil's Tower
a well-known volcanic neck locate in Wyoming. The formation possesses many vertical lines resulting from intersections of fractures called columnar joints.
standard geologic time scale
a worldwide relative scale of geologic time divisions.
root
according to the Airy hypothesis, the downward extension of lower-density crustal material as isostatic compensation for its greater mass and topographic elevation.
divide
area of higher ground dividing one drainage basin from another
Mauna Loa
active shield volcano on the Big Island of Hawaii that is the world's most massive mountain (it is not quite as tall as Mauna Kea, but it is more massive). It last erupted in 1984. Kilauea and Mauna Loa are the only currently active volcanoes in Hawaii. Loihi is active, but still beneath the surface of the ocean, offshore from the Big Island of Hawaii.
Kilauea
active volcano on the Big Island of Hawaii that has been erupting since 1983.
barysphere
all of the Earth's interior beneath the lithosphere.
hydrosphere
all the waters of the Earth, as opposed to gases (atmosphere), rocks (lithosphere), and living organisms (biosphere). In all, water covers about 74% of the Earth's surface.
burden
all types of rock or earthy materials that overly the bedrock of a given area.
meander scar
an abandoned meander filled with sediment and vegetation.
Mohole project
an abandoned project originally proposed in order to obtain rocks of the upper mantle. It was hoped to drill down through the Earth's crust to the Mohorovicic discontinuity.
scree
an accumulation of loose rock fragments on a hillside or at the base of a weathered cliff. Scree that slips downhill (when it is wet) becomes highly sorted, with the larger pieces at the bottom. It is a term commonly used in Great Britain as a loose equivalent of talus.
smoker
an active volcanic vent on the ocean floor which emits hydrothermal fluids at high pressure. A black smoker produces sulfides of copper, iron, and manganese; a white smoker releases silica and barites. Sometimes a vertical chimney of deposited minerals forms around the site of the vent.
ped
an aggregate of soil particles, forming a crumb of soil.
soda lake
an alkali lake whose waters contain a high content of dissolved sodium salts, chiefly sodium carbonate accompanied by the chloride and the sulfate. Examples occur in Mexico and Nevada.
glass
an amorphous (non-crystalline) material that is formed by the rapid cooling of lava or magma, such as obsidian.
astrobleme
an ancient crater on the Earth's surface resulting from the impact of an extraterrestrial body.
dome
an anticlinal fold in which the beds dip in all directions away from the central point of folding.
confined (artesian) aquifer
an aquifer completely filled with pressurized water and separated from the land surface by a relatively impermeable confining bed or aquitard, such as shale. Confined aquifers are recharged slowly through confining shale beds. With very slow movement of groundwater, a confined aquifer may have no response at all to wet and dry seasons.
Mercalli scale
an arbitrary scale for measuring the intensity of earthquakes in terms of the damage they cause. It ranges from I (no damage or visible effects) to XII (total destruction). Its adaptation to North American conditions is known as the modified Mercalli scale. It has been superseded by the Richter scale.
Dust Bowl
an area in the western USA that has suffered extensive wind erosion, which has removed the fertile topsoil. It lies mainly within Kansas, Oklahoma, and Texas, and extends into Colorado and New Mexico.
quicksand
an area of mud and sand containing a large amount of water and thus almost liquid in composition. It consists of smooth rounded grains with little tendency to mutual adherence and is usually saturated with water flowing upward through the voids, forming a soft, shifiting, semiliquid that yields easily to pressure and tends to suck down and swallow heavy objects resting on its surface.
ice floe
an area of sea ice that measures 20 m or more across and is essentially flat. Giant floes over 10 km across have been found.
swamp
an area of soft, wet land that has poor drainage and is generally waterlogged. The dominant vegetation consists of trees, such as eucalyptus, mangroves, maples, palms, and willows, depending on the climate.
ice shelf
an area of very thick ice floating on the sea, which is attached to the land and built up by accumulated snow and outward moving glaciers. Ice shelves, such as the Ross Ice Shelf, surround much of Antarctica.
*hotspot
an area of volcanic eruptions and high heat flow above a rising mantle plume. A hotspot was responsible for generating the Hawaiian Islands, as well as many other volcanic islands, such as the Galapagos Islands and the Azores. Hotspots are isolate regions of volcanic island that can be in the ocean, or on a continent, such as the Yellowstone Hotspot at Yellowstone National Park. Iceland is extremely volcanically active as it sits on both a hotspot and a mid-ocean ridge.
subduction zone
an area on the earth's surface where ocean floor is destroyed by one lithospheric plate overriding another. Normally an oceanic plate is descending beneath a continental plate, but an oceanic plate may be subducting beneath another oceanic plate, where it gets destroyed within a trench.
magnetic pole
an area where the strength of the magnetic field is greatest and where the magnetic lines of force appear to leave or enter the Earth.
karst topography
an area with many sinkholes and a cave system beneath the land surface, and usually lacking a surface stream. These caves and sinkholes are usually limestone solution landforms. The name stems from the regional name for the massive limestone area on the Dalmation coast of the Adriatic Sea. Prerequisites for a fully developed karst topography include a considerable thickness of strong, soluble, well-bedded and well-jointed limestone, moderately heavy rainfall, and sufficient altitude to allow an extensive flow of underground water. If a surface stream does exist, it can disappear down a sinkhole to flow through caves beneath the surface, forming a true underground stream.
graywacke
an arenaceous (sandstone) sedimentary rock in which fairly angular particles of sand grade are suspended in a matrix of much finer material. Any sandstone with more than 15% fine-grained matrix (silt and clay) between the sand grains is classified as a greywacke. Most graywackes probably formed from sediments transported by turbidity currents, and are usually dark gray or green in color.
marl
an argillaceous sedimentary rock in the form of mudstone that has a high proportion of calcareous material in its composition. It is formed under marine and freshwater conditions, and is usually a mixture of clay and calcium carbonate, usually including shell fragments.
cross-bedding
an arrangement of relatively thin layers of rock inclined at an angle to the more nearly horizontal bedding planes of the larger rock unit. Cross-bedding results from changes in the direction of water or wind currents during deposition and is most commonly developed in sandstones.
clastic texture
an arrangement of rock fragments bound into a rigid network by cement.
oil sand
an asphalt-cemented sand or sandstone deposit. The asphalt is solid, so oil Sands are often mined rather than drilled into.
thanatocoenosis
an assemblage of fossils composed of the remains of animals that have been accumulated by the various agencies after their death and thus may not have lived together in life. Most fossil occurrences are thanatocoenoses.
moment magnitude
an earthquake magnitude calculated from the strength of the rock, surface area of the fault rupture, and the amount of rock displacement along the fault. The moment magnitude is the most objective way of measuring the energy released by a large earthquake.
deep-focus earthquake
an earthquake whose focus is at a depth of more than 300 km. Most are found along the Benioff zones.
horst
an elongated block of rocks bounded by normal faults that is uplifted relative to the surrounding rocks. If the block of rock is uplifted sufficiently, it becomes a fault-block mountain range. The Teton Mountains in Wyoming in the Sierra Nevada mountains in California are spectacular examples of fault-block mountain ranges. The Basin and Range province of Nevada is also characterized by numerous mountain ranges (horsts) separated from adjoining valleys by normal faults. Compare to graben.
terminal moraine
an end moraine marking the farthest advance of a glacier. It is an accumulation of till material that develops against the front of a glacier. It is finally deposited on the retreat of the glacier, thereby marking its farthest limit from the source.
arroyo
an ephemeral (temporary) stream of the semi-arid U.S. and Latin America. Arroyos are sometimes called gullies.
Pleistocene Epoch
an epoch of the Quaternary Period characterized by several glacial ages.
Miocene
an epoch of the Tertiary Period extending from the end of the Oligocene epoch to the beginning of the Pliocene epoch.
badlands
an eroded, furrowed landscape in a dry region, such as parts of Nebraska and South Dakota in the U.S. There is little or no vegetation, so rainwater runs off quickly along short steep slopes, further eroding any exposed or soft rocks.
flank eruption
an eruption in which lava pours from a vent on the side of a volcano.
rock salt
an evaporite composed of halite. The term is used for the mineral halite when it occurs as granular or massive aggregates.
supervolcano
an exceptionally large volcano that begins as a boiling reservoir of magma risen from the mantle to within the earth's crust, building in pressure until it finally erupts in a massive and devastating explosion. Unlike the majority of volcanoes, which are cone shaped, supervolcanoes may be immense calderas and can be hard to detect. For example, the magma-filled caldera (about 70 km by 30 km) of Yellowstone National Park was only detected in the 1960s through infrared satellite imagery. Yellowstone has been on a regular eruption cycle of approximately 600,000 years, the last eruption being some 640,000 years ago. Calculations have been made to indicate that during the 20th century parts of the caldera rose by over 70 cm, raising concerns that the overdue eruption may be imminent. The last supervolcano to erupt was Toba, in Sumatra, 74,000 years ago. The volcanic winter that ensued due to sulfur dioxide forming sulfuric acid (which forms a reflective barrier to incoming solar radiation) created a global catastrophe and is believed by some geneticists to a pushed human life to the brink of extinction.
wildcat well
an exploratory well drilled for oil or gas on a geologic feature not yet proven to be productive.
plain
an extensive region of low-lying land, which is generally flat or gently undulating. Most plains are formed by deposition of eroded sediments; others are created by the wearing away of higher land (denudation), forming a peneplain.
turtleback
an extensive smooth curved topographic surface, apparently unique to the Death Valley region, California, that resembles the carapace of a turtle and is produced by faulting. It is a large, elongate dome with an amplitude of up to a few thousand meters.
prairie
an extensive tract of level to rolling grassland, generally treeless, in the temperate latitudes of the interior of North America (Canada and the USA). The prairies have summer rain and fairly dry winters. Prairies are characterized by fertile soils (mollisols), but have been so extensively plowed in some regions that a Dust Bowl has formed.
pediplain
an extensive, thinly alleviated erosion surface formed in a desert region by the coalescence of two or more adjacent pediments.
steppe
an extensive, treeless grassland area in the semi-arid mid-latitudes of southeastern Europe and Asia. It is generally considered drier than the prairie, which develops in the subhumid mid-latitudes of the U.S.
pegmatite
an extremely coarse-grained igneous rock with interlocking crystals. Most grains are 1 cm in diameter or more. Most pegmatites are of granitic composition. Pegmatites are usually found as irregular dikes, lenses, or veins, usually at the margins of batholiths. The largest crystals occur in pegmatites, occasionally measuring meters in length. The growth of large crystals may be ascribed to slow cooling, rapid diffusion, and low viscosity due to the high concentration of volatiles.
chert
an extremely hard, compact, fine-grained sedimentary rock formed almost entirely of silica. It is a type of chalcedony that occurs as masses or layers in limestone, and can be white, gray, or black. The nodular form of chert is known as flint. The nodules, often found in limestone, probably form from inorganic precipitation as underground water replaced part of the original rock with silica. The layered deposits typically from the accumulation of delicate, glasslike shells of microscopic marine organisms on the sea floor.
sérac
an extremely irregular ice surface, usually at the foot of an icefall on a glacier. It is formed when the slowly flowing ice decreases speed, and crevassed ice piles up into impenetrable pillars and pinnacles.
dacite
an extrusive rock that is intermediate in composition between rhyolite and andesite. It is the extrusive equivalent of granodiorite. Dacite is the rock associated with the Mount St. Helens eruption of 1980.
stagnant ice
an ice mass that no longer receives an adequate supply of ice in the accumulation zone to maintain movement. The ice melts downward from the surface, meltwater frequently forming lakes marginal to the ice, which may subsequently be evidenced by deltaically-bedded terrace deposits. Debris- covered ice will melt more slowly than that exposed, and in this way large masses can be isolated, resulting in kettle hole formation at a later stage. The large amounts of meltwater present are responsible for the characteristic ice stagnation features, namely kames and eskers.
erratic
an ice-transported boulder that does not derive from bedrock near its present site (they were often carried quite a distance, and deposited by the glacier in a new area).
Gaia hypothesis
an idea championed in the 1970s by the British scientist James Lovelock which suggests that all the living and nonliving systems on Earth form a unity that is regulated, and kept suitable for life, by the organisms themselves. The whole planet can therefore by regarded as a single, huge organism. This hypothesis stresses the interdependence of living things and the environment.
porphyritic rock
an igneous rock in which large crystals called phenocrysts are enclosed in a matrix or ground mass of much finer-grained minerals or obsidian.
mold
an impression of the original fossil, which is left behind after the fossil was dissolved away by pore waters within sedimentary rock.
pyroclast
an individual particle ejected during a volcanic eruption. It is usually classified according to size. The size-based names of pyroclasts are: dust (<1/8 mm), ash (1/8 to 2 mm), cinder or lapilli (2 to 64 mm), and blocks and bombs (>64 mm)
bay
an inlet or indentation in the shore of a lake or the sea.
travertine
an inorganic limestone that forms in caves when carbonate-rich water loses CO2 to the atmosphere. It has a crystalline texture, and tends to be more dense than tufa, which is generally more porous and open.
sugarloaf
an inselberg, especially in the coastal area of Brazil. It is a conical hill or mountain comparatively bare of timber.
seismometer
an instrument designed to detect seismic waves or Earth motion.
seismic profiler
an instrument that measures and records the sub-bottom structure of the sea floor.
clinometer
an instrument that measures angles of slope or inclination, such as dip.
magnetometer
an instrument that measures the strength of Earth's magnetic field.
Brunton pocket transit
an instrument used by geologists for measuring the strike and dip of a bedding plane.
gravimeter
an instrument used to measure slight variations in the Earth's gravitational field. It is commonly employed in prospecting for oil and other minerals, whose deposits cause local anomalies in the gravitational field.
epoch
an interval of geologic time in the Chronomeric Standard scale of chronostratigraphic classification. The equivalent Stratomeric Standard term is the series. Several epochs together form a period. An epoch contains a number of ages.
era
an interval of geologic time in the Chronomeric Standard scheme of chronostratigraphic nomenclature. An era contains several periods, and a number of eras may be compounded to form an eon.
volcanic neck
an intrusive structure that apparently represents magma that solidified within the throat of an inactive volcano. Devil's Tower in Wyoming and Ship Rock in New Mexico are famous examples of this type of pluton.
manganese nodule
an irregular potato-shaped mass of manganese-rich material that occurs on the ocean floor. They are the only deep-sea mineral deposit that could be economically recovered from the deep ocean floor utilizing present-day technology. They are largely confined to deep-sea clays, but have also been found associated with oozes. They are surprisingly heavy, averaging 24% manganese, 14% iron, 1% nickel, 0.5% cobalt, and 0.5% copper. They are composed of concentric layerings of minerals that build up by slow precipitation from seawater (authigenic).
stack
an isolated, pillar-like rocky island, detached from a headland by wave erosion. The waves may first form a cave that is gradually enlarged, leading perhaps to the formation of a natural arch before complete detachment finally occurs. Stacks are arose and all remnants of headlines that are left behind as the coast retreats inland.
breakwater
an offshore structure built to absorb the force of large breaking waves and provide quiet water near the shore.
Appalachian Mountains
an old mountain range running north to south in the eastern portion of the United States that was formed due to continent-continent convergence. The appellations, the Caledonian Mountains of Great Britain and Norway, and the Atlas Mountains of north Africa were once part of a single mountain belt within the supercontinent Pangaea.
pedalfer
an old term for leached soil typical of humid climates.
crevasse
an open fissure or crack in a glacier. Crevasses occur in valley glaciers due to the fact that the edges of the glacier move more slowly than the center, owing to the frictional effect of the valley sides. The stress set up by this differential movement causes crevasses to form which are parallel to the direction of flow of the glacier. They are also formed when a glacier moves over a steeper section of ground (they are then transverse to the glacier, cutting across it).
gap
an opening in a ridge, usually formed by the action of a glacier or river. Gaps provide routes through mountain ranges (e.g. the Cumberland Gap).
neck (plug)
an ore-bearing pipe. It is an erosional remnant filling the conduit that fed a volcano. The conduit was filled with lava and pyroclastic material that was more resistant to erosion than that of the enclosing volcanic cone, which has since been eroded away.
seiche
an oscillation of a body of water in an enclosed or semi-enclosed basin that varies in period, depending on the physical dimensions of the basin, from a few minutes to several hours, and in height from several cm to a few meters. It is caused chiefly by local changes in atmospheric pressure, aided by winds, tidal currents, and occasionally earthquakes.
peridotite
an ultramafic rock consisting wholly or largely of olivine, together with other ferromagnesian minerals, and devoid of feldspar.
nonconformity
an unconformity in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.
ultralow-velocity zone (ULVZ)
an undulating zone at the core-mantle boundary where P-wave velocities decrease dramatically. The transition zone between the lower mantle and the ULVZ is about 200 km thick, and is called the D'' layer (D double prime layer). The ULVZ may be due to the lowermost mantle being partially melted by the core, or the core may be chemically reacting with the mantle.
gas hydrates (methane hydrates)
an unusual mixture of ice and gas in which methane is trapped in ice crystals. Gas hydrates are found in extreme environments, notably permafrost in polar regions and in the deep ocean floor. If lit, a piece of gas hydrate ice will burn with a red flame. The amount of gas hydrate in the ocean floors is staggering. Most of the deposits of gas hydrates are in lenses frozen in sediment at deep ocean floors, but needs a Kalama their or more of water. There, gas hydrate is stable because of the cold and high pressure. If pressure is reduced or the substance heated, it becomes unstable and the gas escapes. This makes it difficult to get to the surface without losing the gas. Gas hydrates could significantly exasperate global warming. Methane reacts with oxygen in the atmosphere and produces carbon dioxide, which remains in the atmosphere indefinitely. Significant volumes of methane could be released if gas hydrates sediments are disrupted by submarine landslides or other means.
anticline
an upward arching fold in rock (contrast with syncline).
Western Cordillera
and essentially continuous sequence of mountain ranges that stretches from north to south, from Alaska to Mexico. The Western Cordillera include the Rocky Mountains, a Sierra Nevada mountains, the Cascades, and the Coast Ranges in the U.S. and Canada, as well as the Sierra Madre Occidental and the Sierra Madre Oriental in Mexico.
ultrabasic rock
and igneous rock containing less than 45% silica by weight. Most ultramafic rocks (consisting largely of ferromagnesian minerals) are ultrabasic.
steptoe
and isolated hill or mountain of older rock surrounded by a lava flow.
pack ice
any area of sea ice formed by the jamming or crushing together of pieces of floating ice. The mass covers the sea surface, with little or no open water.
fissure
any extensive break, cleft, or fracture in the Earth's surface, generally caused by earth movements (earthquakes and faulting) or volcanic action.
monadnock
any hill or mountain rising conspicuously above the general level of a flat or gently undulating plain in a temperate climate. These hills may be composed of rocks that are more resistant to weathering than those around them; alternatively they may be located in former drainage divide positions.
extrusive rock
any igneous rock that forms at Earth's surface, whether it solidifies directly from a lava flow or is pyroclastic. The rapid cooling involved generally results in a fine-grained type of rock.
fossil fuel
any naturally occurring hydrocarbon fuel such as coal, natural gas, oil (petroleum), and peat. These fuels form slowly underground by the action of pressure on the remains of dead plants and marine animals. Fossil fuels are a nonrenewable resource.
phenocryst
any of the large crystals in porphyritic igneous rock. Phenocrysts are the first crystals to form as the melt cools and are thus minerals having the highest crystallization temperatures.
lithospheric plate (also called tectonic plate)
any of the large plates of lithosphere that cover the surface of the Earth.
soil horizon
any of the layers of soil that are distinguishable by characteristic physical or chemical properties, as well as appearance. Boundaries between soil horizons are usually transitional rather than sharp.
resin
any of various hard, brittle, transparent or translucent substances formed especially in plant secretions and obtained as exudates of recent or fossil origin by the condensation of fluids on the loss of volatile oils. Resins are yellowish to brown, with a characteristic luster. Amber is an example.
fertilizer
any organic substance or inorganic salt applied to the soil to improve crop production. The nutrient elements most important for plant growth and are therefore included in fertilizers are nitrogen, phosphorus, and potassium.
siliceous ooze
any pelagic deep-sea sediment containing at least 30% siliceous skeletal remains, e.g. radiolarian ooze or diatom ooze.
consolidation
any process that forms firm, coherent rock from sediment or from liquid.
plug
any roughly cylindrical vertical body of intrusive igneous rock, usually relatively small.
transform plate boundary
boundary between two plates that are sliding past each other. The San Andreas fault in California is a classic example (the Pacific Plate is sliding in a northwesterly direction past the North American Plate). Earthquakes result from this motion.
divergent plate boundary
boundary separating two plates that are moving away from each other. Divergent boundaries are usually associated with mid-ocean ridges but can also be found on continents (such as the Great Rift Valley in Africa).
plateau
broad, flat-topped area elevated above the surrounding land and bounded, at least in part, by cliffs.
shield volcano
broad, gently sloping volcanic cone constructed of solidified lava flows. They are usually several tens or hundreds of square miles in extent. They generally have slopes between 2 and 10°, and are composed of basalt. Typical examples are the volcanoes Mauna Loa and Kilauea on the island of Hawaii.
strain
change in size (volume) or shape of a body (or rock unit) in response to stress.
U-shaped valley
characteristic cross-profile of a valley carved by glacial erosion.
V-shaped valley
characteristic profile of a stream-cut valley. This type of valley is characteristic of young and mature rivers, but by the time the river reaches senility the broadening floodplain produces more of a U-shape.
anthracite
coal that has a very high fixed carbon content and a low amount of volatiles; the highest grade of coal. Anthracite is sometimes called hard coal. It has the most concentrated stored solar energy, it's hard to ignite, but is dust-free and smokeless. If anthracite is squeaking and needed any further, its hydrocarbon molecules break down altogether under essentially metamorphic conditions, and all that remains is pure carbon, which is called graphite (the stuff that we put in pencil lead).
diorite
coarse-grained igneous rock of intermediate composition. Up to half of the rock is plagioclase feldspar and the rest is ferromagnesian minerals such as hornblende and biotite. Diorite is the intrusive igneous equivalent of the extrusive igneous rock andesite.
Mount Erebus
composite volcano in Antarctica that is the southernmost active volcano in the world.
Mount Etna
composite volcano on the island of Sicily that is Europe's largest volcano and one of the world's most active volcanoes.
sluicing
concentrating heavy minerals, e.g. gold and cassiterite, by washing unconsolidated material through boxes (sluices) equipped with riffles that trap the heavier minerals on the floor of the box.
stalagmite
cone-shaped mass of dripstone formed on cave floors, generally directly below a stalactite. It is usually composed of calcite. If a stalagmite and its associated stalactite join together, they form a column.
steinkern
consolidated mud or sediment that filled the hollow interior of a fossil shell, such as a bivalve shell, or other organic structure. It also means the fossil thus formed after dissolution of the mold.
geophysics
the application of physical laws and principles to the study of the Earth.
heavy crude
dance, viscous, petroleum that may flow into it well, but it's rate of flow is too slow to be economical. As a result, heavy crude is left out of reserve and resource estimates of less viscous "light oil," or regular oil. Heavy crude can be made to flow faster by injecting steam or solvents down well, and if it can be recovered, it can be refined into gasoline and many other products just as light oil is. Most California oil is heavy crude.
humus
decomposed plant material rich in organic content found in the A horizon of the soil profile. It is an amorphous, colloidal material in the soil, dark in color and composed of resistant plant tissues, such as lignin, and new compounds, such as polysaccharides, synthesized by microorganisms. It is important to the soil both physically and chemically (its cation exchange capacity far exceeds that of the clays). Humus is so well decomposed that its original sources cannot be identified.
rheidity
deformation of a substance by plastic flow as a result of an applied stress over a long period.
aphanitic
denoting an igneous rock that is so fine-grained that individual crystals cannot be resolved with the naked eye.
dripstone
deposits of calcite (and rarely other minerals) built up by dripping water in caves.
tufa
deposits of calcium carbonate formed by precipitation from water and including stalagmites, stalactites, and flowstone. Tufa normally precipitates from solution in the water of a continental spring or lake, or from percolating ground water.
pothole
depression eroded into the hard rock of a stream bed by the abrasive action of the stream's sediment load.
rudaceous
describing a clastic sedimentary deposit or rock in which the constituent fragments are of relatively large size. It is formed of gravel, i.e. the clasts are greater than 2 mm in diameter.
elastic
describing a deformed material that recovers its original shape after the stress is reduced or removed.
oligotrophic
describing a lake or other body of water that is deficient in plant nutrients and so has very clear water (because of the scarcity of plankton). The water in the bottom layers is normally oxygen-rich, but the deposits on the bottom contain little organic material.
turbid
describing a liquid that is cloudy in appearance due to stirred up or disturbed sediment.
montane
describing a mountain or mountainous area, or something associated with it, such as a montane tropical forest.
vesicular
describing a rock that contains small cavities formed by the expansion of bubbles of gas or steam during the solidification of the rock.
brittle
describing a rock that fractures at a stress higher than it elastic limit, or once the stresses are greater than the strength of the rock.
pervious
describing a rock through which water can pass along cracks and fissures.
impermeable
describing a type of rock that is nonporous and therefore does not absorb water. Igneous rocks such as granite are good examples. Clay is impermeable to water after it has become saturated with water.
impervious
describing a type of rock that, because it contains no cracks or fissures, does not allow water to pass through it. Metamorphic rocks such as shale and slate are good examples.
authigenic
describing rock constituents that were formed in situ (in place), coming into existence during or after the formation of the rock in which they lie. Also used to describe sediments that form in place.
slaty
describing rock that splits easily along nearly flat and parallel planes.
calcareous
describing rocks or soils that contain calcium carbonate. For example, limestone and chalk are calcareous.
artesian
describing water that has moved underground from its original source, usually by percolation upward along a sloping aquifer, resulting in the water ending up above the level of the water table.
freeze-thaw
describing weathering processes involving the freezing and thawing of water within preexisting rock fissures. Upon freezing, water expands and tends to enlarge cracks. Upon thawing, the enlarged cracks can contain more water than before, which will cause increase enlargement on renewed freezing.
isotopic dating
determining the age of a rock or mineral through its radioactive elements and decay products (also called radiometric dating)
Benioff zone
distinct earthquake zone that begins at an oceanic trench and slopes landward and downward into Earth at an angle of about 30 to 60 degrees (deep focus earthquakes originate here). Benioff zones are found along subduction zones, where one plate moves below a more dense plate.
catchment area
the area from which a river and its tributaries obtain their water.
dendritic drainage pattern
drainage pattern of a river and its tributaries which resembles the branches of a tree or veins in a leaf. This pattern develops where structural controls of slope, variable lithology, or fault and joint patterns are absent - as a result, the drainage net is entirely random, with equal probability of stream flow in all directions. This pattern is found in areas with flat rocks and uniform lithology, notably plains and plateaus.
speleothem
dripstone deposit of calcite that precipitates from dripping water in caves. Most consist of calcite, and the most typical are stalactites and stalagmites.
period
each era of the standard geologic time scale is subdivided into periods (e.g. the Cretaceous Period). A period is formed of a number of epochs grouped together.
turbulent flow
eddying, swirling flow in which water drops travel along erratically curved paths that cross the paths of neighboring drops.
fold limb
either of the two flanks on either side of the axis of a fold.
bank
either side of a river channel.
*geothermal energy
energy produced by harnessing naturally occurring steam and hot water. In a geothermal area, wells can tap steam, or superheated water that can be turned into steam, that is then piped to a powerhouse, where it turns a turbine that spins a generator, creating electricity. To effectively produce electricity, the water must be hot and under high pressure, and thus must come from miles beneath the surface. The largest geothermal field in the world is at The Geysers in Northern California. It produces about 1000 megawatts of electricity, enough for 1 million people. Iceland is well-known for utilizing its abundant geothermal sources.
Phanerozoic Eon
eon that includes all time following the Precambrian period (it is divided into the Paleozoic, Mesozoic, and Cenozoic eras).
lateral erosion
erosion and undercutting of stream banks caused by a stream swinging from side to side across its valley floor.
ammonite
extinct cephalopod mollusks, which had shells similar to the present-day nautilus (coiled shell with several chambers). Ammonites are valuable as Mesozoic zone fossils, and became extinct at the end of the Cretaceous Period.
Ship Rock
famous volcanic neck located in New Mexico. Several well-formed dikes also radiate outward from this formation.
ash
fine pyroclasts (less than 4 mm).
andesite
fine-grained igneous rock of intermediate composition. Andesite is the fine-grained volcanic equivalent of diorite. Up to half of the rock is plagioclase feldspar, with the rest being ferromagnesian minerals. Andesites are porphyritic, and are the intermediate members of the calc-alkaline volcanic suite. They are associated with basalts and rhyolites in island arcs and orogenic regions.
matrix
fine-grained material found in the pore space between larger sediment grains.
dust (volcanic)
finest-sized pyroclasts.
hydrothermal process
following the pegmatitic stage during the crystallization of an igneous melt is the hydrothermal stage, when the residual fluid is a relatively low-temperature aqueous solution. Many mineral deposits are formed by precipitation from hydrothermal solutions, and the fluid may effect considerable alteration of the crystallized portion of the magma.
tectonic forces
forces generated from within the Earth that result in uplift, movement, or deformation of part of Earth's crust.
subterranean
formed or occurring beneath the earth's surface, or situated within the earth.
xenolith
fragment of rock distinct from the igneous rock in which it is enclosed. The inclusion may be a block of country rock that has been caught up in the intrusion, but has not yet been completely assimilated. It may alternatively be a block of the igneous body itself that solidified an earlier. And therefore has a slightly different composition.
drift
glacial and fluvioglacial deposits. Great thicknesses of this drift accumulated during the Pleistocene Epoch, although much has subsequently been removed by erosion. An area of glacial deposition may be referred to as one of drift topography.
alpine glaciation
glaciation that occurs in a mountainous area (contrast with continental glaciation).
positive magnetic anomaly
greater than average strength of the Earth's magnetic field. For example, a region in with a lot of gabbro, surrounded by a region with granite, will have a positive magnetic anomaly because gabbro contains more ferromagnesian minerals than granite, and thus gabbro is more magnetic. Magnetic anomalies can also be caused by a variation in the direction of magnetism.
positive gravity anomaly
greater than normal gravitational attraction.
permafrost
ground that remains permanently frozen for many years.
maculose
having a spotted or knotted appearance, usually describing a metamorphic rock. The irregularities (spots) are thought to result from the growth of new minerals during heating (contact metamorphism usually).
storm surge
high sea level caused by the low pressure and high winds of hurricanes. Storm surge is responsible for the majority of damage caused by a hurricane.
laterite
highly leached soil that forms in regions of tropical climate with high temperatures and very abundant rainfall. Laterites are normally oxisols. Laterites are usually red and are composed almost entirely of iron and aluminum oxides, which are generally the least soluble products of rock weathering in tropical climates. Pure layers of bauxite (aluminum ore) are often found in laterites, as well as limonite, a hydrated iron ore that is seldom rich enough to mine.
Lake Agassiz
huge lake that covered much of modern-day Manitoba and Ontario, Canada. It was formed as the Laurentide ice sheet retreated at the end of the last ice age. Lake Winnipeg in Manitoba is a remnant of this large lake.
tsunami
huge ocean wave produced by displacement of the sea floor. They are also called seismic sea waves. They result when a large section of seafloor suddenly rises or falls during a quake, causing all the water over the moving area to be lifted or dropped for an instant. As the water returns to sea level, it sets up long, low waves that spread very rapidly over the ocean. Tsunamis may have a wavelength of 160 km and may be moving at more than 800 km/h. Only a few localities have a combination of gently sloping offshore shelf and funnel-shaped bay that force tsunamis to awesome heights (the record high was 85 m in 1971 in the Ryukyu islands south of Japan).
*snow
ice crystals that form and fall to earth when tiny supercooled cloud droplets freeze. Snowflakes nucleate around particles in the atmosphere by attracting supercooled water droplets, which freeze in hexagonal-shaped crystals.
white ice
ice that contains trapped air, located at or near the surface of the ground, often on a glacier. Is it the sentence under further layers of ice, the pressure forces out the air and the ice appears blue.
stalactite
icicle-like pendant of dripstone formed on cave ceilings. It is usually composed of calcite.
intrusive rock
igneous rock that has crystallized beneath the Earth's surface from magma that was injected into the Earth's crust.
firth
in Scotland, a lengthy estuary or arm of the sea. Firths bear some similarity to fjords, and some have the closed and deep depressions and rock sills typical of fjords. They developed from river or glacial valleys that later experienced the postglacial flooding by the sea.
topset bed
in a delta, a nearly horizontal sediment bed of varying grain size formed by distributaries shifting across the delta surface.
dip
in a stratified rock, the angle made between the horizontal plane and that of the bedding plane. With respect to magnetic fields, dip is the angle between the Earth's magnetic field at any point on the Earth's surface and the horizontal (it is 90 degrees at the magnetic poles and 0 degrees at the equator).
discharge
in a stream, the volume of water that flows past a given point in a unit of time. Discharge is found by multiplying the cross-sectional area of the stream by its velocity. Discharge is reported in cubic feet per second (cfs) in the U.S., or in cubic meters per second elsewhere. In humid climates, discharge increases downstream for two reasons: 1) water flows out of the ground into the river through the stream bed, and 2) small tributary streams flow into the larger stream along its length, adding water to the stream as it travels. To handle the increased discharge, the streams increase in width and depth downstream. In a dry climate, the stream discharge can decrease in a downstream direction as river water of apparatus into the air and soaks into the dry ground (or is used for irrigation). As the discharge decreases, the load of sediment is gradually deposited.
chill zone
in an intrusion, the finer-grained rock adjacent to a contact with country rock. Rock in the chill zone cools quickly, so is fine-grained.
bore (tidal bore)
in areas with a large tidal range, the initial flooding of the tide may cause a tidal bore, which is a surging inflow that travels rapidly some kilometers up a river or estuary. Well-known examples occur in the Bay of Fundy and the Amazon River.
erg
in geology, a type of arid landscape consisting of a very extensive sand cover, especially in the Sahara desert.
slump
in mass wasting, movement along a curved surface in which the upper part moves vertically downward while the lower part moves outward (also called a rotational slide).
translational slide
in mass wasting, movement of a descending mass along a plane approximately parallel to the slope of the surface.
shear force
in mass wasting, the component of gravitational force that is parallel to an inclined surface.
shear strength
in mass wasting, the resistance to movement or deformation of material.
caliche
is a hardened deposit of calcium carbonate. This calcium carbonate cements together other materials, including gravel, sand, clay, and silt. It is found in aridisol and mollisol soil orders. Caliche occurs worldwide, generally in arid or semi-arid regions, including in central and western Australia, in the Kalahari Desert, and in the High Plains of the western USA. Caliche is also known as hardpan, calcrete, kankar (in India), or duricrust.
*alluvial fan
large, fan-shaped pile of sediment that usually forms where a stream's velocity decreases as it emerges from a narrow mountain canyon onto a flat plain at the foot of the mountain range. The decreased river velocity causes the river to drop its load of sediment, forming the alluvial fan (alluvial plain). The apex of the fan is at the point of emergence from the mountains.
exfoliation domes
large, rounded landforms developed in massive rock, such as granite, by exfoliation. Famous examples include Half Dome in Yosemite and Stone Mountain in Georgia.
1. Pacific - 105
largest plate. It is traveling in a northwestward direction past the North American plate (the boundary between the two is the San Andreas Fault, a transform fault boundary).
metastasy
lateral (sideways) adjustments of the Earths crust, as opposed to vertical movements (isostasy).
silicic lava
lavas such as rhyolite that are relatively high in silica (65-70% silica). Silicic lavas are much more viscous than mafic lavas (silicic is 100 million times the viscosity of water, whereas mafic is only 10,000 times the viscosity of water, because its silica content is only about 50%). Lavas rich in silica are more viscous because even before they have cooled enough to allow crystallization, silicon-oxygen tetrahedrons have linked to form small, framework structures in the lava. Because silicic magmas are most viscous, they are associated with the most violent eruptions and are the most dangerous. Increased viscosity limits gases from escaping. These gases, upon heating or rising near the surface, expand and create pressure within the gas pocket. If this pressure is not relieved, a huge explosion can result. Surface water introduced into a volcanic system can greatly increase the explosivity of an eruption, as the water gets heated to gaseous vapor, which then expands and creates pressure. This is what occurred during the eruption of Krakatoa. Intermediate magmas, which have silica contents of around 60%, can produce either violent eruptions or lava flows. Andesite is an example of an intermediate magma.
mafic lava
lavas, such as basalt, that are relatively low in silica (around 50% silica content), and thus have low viscosity and tend to flow easily. Mafic magmas, being less viscous than silicic magmas, commonly erupt as lava flows, such as in Hawaii. Ultramafic magmas have a silica content of only about 40%, but these are very rare at the surface of the Earth (komatiite is an example).
plateau basalts
layers of basalt flows that have built up to great thicknesses.
negative magnetic anomaly
less than average strength of Earth's magnetic field.
negative gravity anomaly
less than normal gravitational attraction.
slurry
less-refined liquefied coal.
tillite
lithified till.
scour
localized erosion, for example tidal scour in estuaries, removing sediment periodically and depositing it again at another stage; in rivers, periodic bed scour occurs during periods of high flow, compensated at low flow by infill of depressions, a process sometimes called scour-and-fill. In connection with glacial erosion, scour refers to the etching and polishing of solid rocks by rock material incorporated in the ice.
swell
long, rounded waves on shores that may be thousands of kilometers from the storms that generated the waves.
sediment
loose, solid particles that can originate by 1) weathering and erosion of preexisting rocks, 2) chemical precipitation from solution, usually in water, and 3) secretion by organisms. The general definition is any particulate material that has been deposited in a fluid medium. The fluid concerned is usually water, but aeolian sediments (wind-blown) are not uncommon.
regolith
loose, unconsolidated rock material resting on bedrock. The upper, biochemically weathered portion of the regolith is the soil. The regolith may up to 60 m thick in the tropics.
low-velocity zone
mantle zone at a depth of about 100 km where seismic waves travel more slowly than in shallower layers of rock. Velocities are about 6% lower than in the outermost mantle. It is probably caused by the near-melting point temperature of the material. The low-velocity zone is synonymous with the asthenosphere.
Laurentide ice sheet
massive ice sheet that covered most of Canada and a large portion of the northern United States, between 95,000 and 20,000 years before present, during the Pleistocene epoch (ice age). It extended for a time as far as the Missouri and Ohio River valleys. Cape Cod in Massachusetts and Long Island in New York are both terminal moraines left by the Laurentide ice sheet. This ice sheet also left hundreds of kettle lakes in Minnesota and the upper Midwest, and produced the Finger Lakes in New York (which still valleys modified and dammed by recessional moraines at their southern ends). The water for the Great Lakes was produced as a meltwater left behind by the retreat of the Laurentide Ice sheet.
outwash
material deposited by debris-laden meltwater from a glacier. It is layered and sorted, unlike till.
outwash (plain)
material deposited by debris-laden meltwater streams emerging from the margins of a glacier.
ejecta
material that is thrown out of an erupting volcano, or material that is thrown out when a meteorite impacts with the ground.
partial melting
melting of the components of a rock with the lowest melting temperatures. For example, partial melting of the lower continental crust likely produces silicic magma. The magma rises and eventually solidifies at a higher level in the crust into granite, or rhyolite if it reaches Earth's surface. Partial melting of the upper mantle (asthenosphere), forms mafic magma. This magma rises and eventually solidifies at a higher level in the crust into gabbro, or basalt if it reaches Earth's surface. Partial melting of the mantle (asthenosphere), followed by differentiation, assimilation, or magma mixing, results in the production of intermediate magma. Intermediate magma rises and eventually solidifies a higher level in the crust into diorite, or andesite if it reaches Earth's surface.
regional metamorphism
metamorphism that takes place over an area of wide extent, usually at considerable depth underground. Regional metamorphic rocks are almost always foliated, indicating differential stress during recrystallization. It is associated with large-scale tectonic processes such as mountain-building (continental collision) and seafloor spreading.
contact (thermal) metamorphism
metamorphism under conditions in which high temperature is the dominant factor. It is caused by heat transfer from an intruded magma body into the country rocks. The thermal effect of the magma causes a recrystallization of minerals in the country rock, and the growth of new minerals. Contact metamorphism usually takes place at depths of less than 10 km, and thus the confining pressure is usually relatively low. Contact metamorphism can be thought of as the taking of country rocks adjacent to an intrusive contact. The zone of contact metamorphism (also called the aureole) is usually quite narrow, and (generally from 1 to 100 m wide). The most common rocks found in an aureole are nonfoliated rocks, because differential stress is rarely significant in contact metamorphism. For example, marble forms when igneous rock intrudes limestone, quartzite forms when quartz sandstone is metamorphosed, and hornfels forms when shale is scorched.
coal bed methane
methane that is trapped in the fine pores, pockets, and fractures that speckle and laced the interior of coal as it forms. The methane is trapped in the cold during its formation along with water. Pumping the water out lowers pressure and releases the gas in huge quantities. Coal can store 6 to 7 times more gas than an equivalent amount of rock in an ordinary natural gas field.
open-pit mine
mine in which ore is exposed at the surface in a large excavation.
gangue
minerals found in an ore deposit that have no commercial value and must be removed during the refining process.
coccoliths
minute, round, calcareous plates that cover coccolithophores, unicellular algae of the phylum Haptophyta. Especially during the Cretaceous Period, coccoliths contributed largely to the formation of chalk.
magma
molten rock, usually composed mostly of silica. Magma may contain dissolved gases as well as some solid minerals. It is capable of intrusion and extrusion, from which igneous rocks have been derived through solidification and related processes. Mafic rocks have a silica content of 50% or less, whereas felsic rocks have a silica content of 65% or more.
traction
movement by rolling, sliding, or dragging of sediment fragments along a stream bottom. Particles that move this way are part of the bed load
longshore drift
movement of sediment parallel to shore when waves strike a shoreline at an angle, causing the development of a longshore current.
plastic flow
movement within a glacier in which the ice is not fractured. Ice is a crystalline solid and as such changes shape (deforms) at temperatures near its melting point. Movements between adjacent crystals are limited but individual crystals can deform internally, although little change in shape appears to occur over a period of time: this indicates that recrystallization accompanies the deformation. Ice is molded as it moves across its bed, and resultant features are maintained within the ice beyond the location molding. This suggests that the ice acts plastically under pressure, but that it remains rigid on removal of the pressure.
mass wasting
movement, caused by gravity, in which bedrock, rock debris, or soil moves downslope in bulk.
rip current
narrow currents that flow straight out to sea in the surf zone, returning water seaward that has been pushed ashore by breaking waves.
hard water
natural water that contains dissolved ions of calcium and magnesium. These ions from a scum with soap.
rock
naturally formed, consolidated material composed of grains of one or more minerals. The surface of the Earth is made up of rocks. They are classified into 3 major groups: igneous, sedimentary, and metamorphic rocks.
needle ice
needles of ice, often several cm long, growing beneath the surface layer of debris and able to lift frost-shattered rock fragments and soil particles perpendicularly from the ground surface. On melting, the fragments are deposited a little way downslope.
foreshock
one of a series of seismic waves recorded before the principal shock of an earthquake, resulting from small slips or fractures in brittle rocks as they reach their yield point. Foreshocks can sometimes be used to predict upcoming large quakes.
aridisol
one of the 10 soil orders of the Seventh Approximation classification. It includes saline and alkaline mineral soils of dry, desert areas. Aridisols are characterized by low organic matter contents and a horizon of calcium or sodium accumulation within 1 meter of the surface, and carbonate horizons. They are infertile due to their lack of moisture, coarse particle size, and susceptibility to erosion due to lack of vegetation. In the U.S., aridisols are dominant in southern Arizona and New Mexico, as well as northern Nevada.
mollisol
one of the 10 soil orders of the Seventh Approximation classification. This soil type characteristically forms under grass in climates that have a moderate to severe seasonal moisture deficit (subhumid to subarid grasslands). It is a dark-colored soil with a relatively high cation-exchange capacity dominated by calcium, and is rich in organic matter and plant nutrient ions. Mollisols are very productive agricultural soils. In the U.S., mollisols cover most of the Great Plains, such as the agricultural areas found in Kansas, Iowa, etc.
Long Valley Caldera
one of the largest calderas on Earth (20 miles long by 11 miles wide). It is adjacent to Mammoth Mountain, on the eastern side of the Sierra Nevada Mountains. It formed 760,000 years ago after a huge volcanic eruption. The eruption was so colossal that the magma chamber under the now destroyed volcano was significantly emptied to the point of collapse, forming the caldera.
Hudson canyon
one of the numerous submarine canyons that cut into or across the wide continental shelf lying off the east coast of the U.S. It is one of the best-known submarine canyons and has been carefully surveyed several times. It cuts into the shelf opposite the Hudson River, and has a V-shaped cross section, like most submarine canyons.
ultisol
one of the ten soil orders from the Seventh Approximation classification, denoting highly weathered soil that is low in plant nutrient ions and is formed in the subtropical to tropical climates, which has a surface horizon containing residual iron oxides and an illuvial horizon beneath it that is rich in clay. Soil fertility is low, and it is so named because it contains soils at the ultimate stage of weathering. It includes red-yellow and red-brown soil types. In the U.S., ultisols are the predominant soil type covering the entire southeast (Alabama, Georgia, the Carolinas, and Virginia).
spodosol
one of the ten soil orders in the Seventh Approximation classification, consisting of ashy soils that are acid soils that are low in plant nutrient ions with subsurface accumulation of organic matter and compounds of aluminum and iron. They are found in cool, humid forests. In the U.S., spodosols are found in the northeast states of Maine, Vermont, and New Hampshire, as well as in northern Michigan and central Florida.
entisol
one of the ten soil orders of the Seventh Approximation classification, approximately equivalent to the azonal category of the old classification. Entisols are recent soils without natural horizons, and include lithosols (shallow, stony soils), regosols (thin soils on unconsolidated drift), and alluvial soils (soils that are being added to because of their site on active floodplains). The lack of soil horizons means these are recent soils. Entisols are found in desert areas of southern California and western Arizona, as well as parts of New Mexico, Montana, and Wyoming.
histosol
one of the ten soil orders of the Seventh Approximation classification, which includes wet, organic soils such as peat in swamps and marshes. Histosols are characterized by accumulations of organic matter, which remains more or less undecomposed because of the water-logged conditions. Histosols occupy the smallest area of the 10 soil orders. They tend to develop in cooler, poorly drained, more humid areas. In the U.S., histosols are found in southern Florida and southern Louisiana.
oxisol
one of the ten soil orders of the Seventh Approximation classification. Oxisols are characterized by mixtures of quartz, clay (kaolin primarily), free iron and aluminum oxides, and organic matter. They are highly weathered and leached soils, generally lacking in fertility, and are found in tropical and subtropical regions, such as the Amazon basin in Brazil.
inceptisol
one of the ten soil orders of the Seventh Approximation, covering very young soils that are better developed than entisols, but not so advanced as the alfisols. They are young soils in which the horizons are weak. In the U.S., inceptisols are found along the coasts of Oregon and Washington, as well as in the northeast in states such as Pennsylvania, New York, Connecticut, and Massachusetts.
alfisol
one of the ten soil orders of the Seventh Approximation. It is found in humid regions of the world under deciduous woodland or grassland vegetation. The dominant soil-forming process is leaching, which is more intense in these soils than in the inceptisols but less than in the spodosols. Alfisols are productive soils and favor the more common agricultural crops. Alfisols are examples of pedalfers, and possess a gray to brown surface horizon, with a subsurface horizon of clay accumulation. In the U.S., alfisols are found in the northeastern Midwestern states, such as Indiana, Ohio, Wisconsin, and southern Michigan.
vertisol
one of the tens soil orders of the Seventh Approximation classification, denoting inverted soils that are clayey soils that swell when wet and shrink when dry, forming wide, deep cracks. They contain the volcanic clay montmorillonite, which forms in soils derived from basic rocks such as basalt. In the U.S., vertisols are found in Texas.
*eutrophication (cultural eutrophication)
over-nourishment of an aquatic ecosystem by nutrients such as nitrates and phosphates due to human activities, such as agriculture (fertilizer use) and sewage discharge. Fertilizer runoff from agriculture in the Midwest U.S. has created a dead zone of hypoxic water in the northern Gulf of Mexico, near the mouth of the Mississippi River. Fertilizer runoff provides the nutrients for an algal bloom, which is when algae reproduce quickly. These algae have a short lifespan and quickly die and begin to decay. Decay organisms multiply and the decay process uses up the dissolved oxygen in the water. Hypoxic conditions result, causing the death of other organisms, such as fish and invertebrates
shingle
pebbles that make up a beach on a seashore. They have been rounded by rolling back and forth up the sloping beach with the tides and waves. It is coarser than ordinary gravel.
riparian
pertaining to or situated on the bank of a body of water, usually a river.
littoral
pertaining to the benthic environment or depth zone between high water and low water, or to the organisms of that environment. It is a synonym of intertidal.
pelagic
pertaining to the water of the ocean as an environment.
crude oil
petroleum as it occurs in the ground, a liquid mixture of naturally occurring hydrocarbons
recurrence interval
the average time between floods of a given size. A 100 year flood is one that can occur, on the average, every 100 years.
cracked hydrocarbons
petroleum hydrocarbons that have been heated to a high enough temperature so that they break down, or crack, into simpler molecules very useful for human purposes. Major cracked petroleum-related hydrocarbons and their uses are listed below, in order of increasing complexity:
berm
platform of wave-deposited sediment at the landward portion of a beach that is either flat or slopes slightly landward. The berm is found landward of the usual high water line and is usually dry, being covered by waves only during severe storms.
mud crack
polygonal crack formed in very fine-grained sediment as it dries.
craton
portion of a continent that has been structurally stable for a prolonged period of time, usually the last 1 billion years. Cratons frequently represent the roots of deeply eroded, ancient mountain chains. The central part of the United States and Canada is all part of a single craton. Other continents similarly have a craton at their core. Most of the craton in the central United States has a very thin blanket of only about 1000 2000 m of sedimentary rock overlying its basement rock. This sentiment was mostly deposited in shallow inland seas during Paleozoic time.
limb
portion of a fold shared by an anticline and a syncline.
potassium-40 dating
potassium-40 decays to argon-40 with a half-life of 1.3 billion years, and potassium-40 dating is effective for dating things that are between 100,000 and 4.6 billion years old.
geothermal power
power derived from heat inside the earth. The world's largest geothermal power plant is at the Geysers in the Coast Range of Northern California. This 1000-megawatt facility provides the energy needs for 1 million people, though its level of production has been declining steadily since 1980. The most important reason for this is that groundwater supplies are withdrawn faster than nature can replenish and reheat them. Another problem with developing geothermal power is that hot groundwater often carries with it dissolved minerals, brines, and acid that corrode or clog pipes and turbines.
hydrostatic pressure
pressure that is exerted by a liquid, often water.
uniformitarianism
principle that geologic processes operating at present are the same processes that operated in the past. It states that all geologic changes have occurred by the gradual effect of processes that have been operating over a long period of time and are still going on today. The principle is sometimes stated as "The present is the key to the past." Uniformitarianism was first postulated by James Hutton, the Scottish "father of geology". It was popularized in a book by Charles Lyell called "The Principles of Geology." Some prefer the term actualism in place of uniformitarianism, because the word uniforrmitarianism suggests that changes take place at a uniform rate, which is not always true.
riparian rights
principle that says that all landowners whose property is adjacent to a stream have the right to make reasonable use of it. If there is not enough water to satisfy all users, allotments are generally fixed in proportion to frontage on the water source. These rights cannot be sold or transferred other than with the adjoining land.
principle of inclusion
principle that states that fragments included in a host rock are older than the host rock.
polishing
process by which the bedrock underlying a glacier is smooth and polished by fine particles within the glacier.
cinder
pyroclast approximately the size of a sand grain. Sometimes defined as between 4 and 32 millimeters in diameter, cinders generally consist of basalt or andesite. Cinders are semifluid when ejected from a volcano, but are solid by the time they fall to the ground.
viscosity
resistance to flow of a liquid, such as lava. Basalt, for example, has low viscosity, so it can flow more quickly and over longer distances than rhyolite (which has high viscosity).
renewable resource
resources that are replenished by natural processes fast enough that people can use them continuously. Water is a good example. Nonrenewable resources are not replenished fast enough in order to be used continuously. They form very slowly, often over millions of years under unusual conditions in restricted geographic settings. Humans extract nonrenewable resources much faster than nature replaces them. A good example is crude oil.
sand
sediment composed of particles with a diameter between 1/16 mm and 2 mm. Most sands are predominantly made of quartz, other material being too easily eroded to survive for long. The quartz is derived by weathering of quartz-bearing rocks and subsequently reduced in size by water or ice abrasion. Desert sand grains are more rounded than fluvial or marine types.
silt
sediment composed of particles with a diameter of 1/256 to 1/16 mm. Like clays, silts may include clay minerals, and also hydroxides and oxides of iron, silica dioxide, and numerous other fine material particles. Clay and silt together form the argillaceous division of sediments.
clay
sediment composed of particles with diameter less than 1/256 mm (the smallest sediment particles, below the size of silt). Clay particles are usually small, flaky clay minerals formed during the weathering of older rocks. Clay and silt particles together form the argillaceous division of clastic sediments.
*hydrogenous sediment
sediment formed by chemical reactions and precipitation in water.
*biogenous sediment
sediment formed from the hard parts of organisms, such as shells, coral fragments, or other hard skeletal parts.
*lithogenous sediment
sediment formed from the weathering and erosion of preexisting rocks.
unconsolidated
sediment grains that are loose, separate, or unattached to one another.
suspended load
sediment in a stream that is light enough in weight to remain lifted indefinitely above the bottom by water turbulence. The muddy appearance of a stream during a flood or after a heavy rain is due to a large suspended load. Silt and clay usually are suspended throughout the water, while the coarser bed load moves on the stream bottom.
pelagic sediment
sediment made up of fine-grained clay and the skeletons of microscopic organisms that settle slowly down through the ocean water.
detrital sedimentary rocks
sedimentary rocks that are formed from cemented sediment grains that are fragments of preexisting rocks.
red beds
sedimentary strata composed largely of sandstone, siltstone, and shale, that are predominantly red due to the presence of ferric oxide (hematite).
body wave
seismic wave that travels through Earth's interior. Types of body wave include P (primary) and S (secondary) waves.
hachures
short lines drawn on a map to show the relief of an area without the use of contours. They point downhill and are thicker and closer together where the gradient is steepest.
groin
short wall built perpendicular to shore to trap moving sand and widen a beach. They are effective at widening the beach just up current from the groin, but they may increase erosion down-drift from the groin. They widen the beach by trapping sediment carried by longshore transport.
felsic rock
silica-rich igneous rock with a relatively high content of potassium and sodium. Word combines the words feldspar and silica. Felsic rocks contain felsic minerals that are lighter in color and contains silicon, oxygen, aluminum, sodium, and potassium. Felsic rocks have a silica content of 65% or more. It is the complement of mafic.
subsidence
sinking or downwarping of a part of the Earth's surface.
laminar flow
slow, smooth flow, with each drop of water traveling a smooth path parallel to its neighboring drops.
acid lava
slow-moving, viscous lava containing a high proportion of silica. It is produced by acidic volcanoes, and generally solidifies very quickly.
*neap tide
small amplitude tides that occur when the sun, moon, and Earth are at right angles to each other, which occurs during the first and third quarter moons. Neap tides have a small tidal range, meaning the difference between high and low tide on a particular day is small.
frazil ice
small crystals of ice that form at the edges of fast-flowing streams or in moving seawater.
ripple marks
small ridges formed on sediment surfaces exposed to moving wind or water. The ridges form perpendicularly to the motion. They are the most common minor beach morphological form, consisting of fairly regular and generally small ridges formed in sediment.
distributary
small shifting river channel that carries water away from the main river channel and distributes it over a delta's surface. Distributaries flow away from the main course of the river and do not rejoin it.
meteoroid
small solid particles of stone and/or metal orbiting the Sun.
Pele's tears
small solidified drops of volcanic glass behind which trail strands of Pele's hair.
tektite
small, rounded bit of glass formed from rock melting and being thrown into the air due to a meteorite impact.
17. Anatolian
small, westward moving plate wedged in between the Eurasian Plate to the north, the Arabian plate to the east, and the African Plate to the south. It consists mostly of the country of Turkey. The East Anatolian Fault is a transform fault that forms the boundary between the Anatolian and Arabian plates, whereas the North Anatolian Fault forms the boundary between the Anatolian and Eurasian plates.
loam
soil containing approximately equal amounts of sand, silt, and clay, and organic matter. It has characteristics between those of sand and clay, and is usually rich in humus. It drains reasonably well, and yet still retains enough moisture to promote good plant growth. Loamy soils are often very fertile.
azonal soil
soil lacking a B horizon due to insufficient time for complete pedogenesis. In the Seventh Approximation classification system, azonal soils are classified as entisols.
gelisol
soil order consisting of frozen soils that have permafrost within 2 meters of the surface.
residual soil
soil that develops directly from weathering of the rock below.
transported soils
soils which did not form from local rock, but from regolith brought in from some other region. Transported soils usually form on sediment deposited by running water, wind, or glacial ice. For example, mud deposited by a river during times of flooding can form excellent agricultural soil after the waters recede (the Nile River in ancient Egypt is an example). Transported wind deposits are called loess and are the parent material for some of the most valuable food-producing soils in the Midwest and Pacific Northwest.
block
solid fragments of rock that are greater than 64 mm in size and are formed when solid rock has been blasted apart by a volcanic explosion. Blocks are angular, with no rounded edges or corners.
bedrock
solid rock that underlies soil.
losing stream
stream that loses water to the zone of saturation.
stream cross section
streams normally produce V-shaped valleys in cross section, whereas glaciers produce U-shaped valleys.
shear stress
stress due to forces that tend to cause movement or strain parallel to the direction of the forces.
beach
strip of sediment, usually sand but sometimes pebbles, boulders, or mud, that extends from the low-water line inland to a cliff or zone of permanent vegetation.
aseismic ridge
submarine ridge with which no earthquakes or volcanoes are associated.
placer mine
surface mines in which valuable mineral grains are extracted from stream bar or beach deposits.
hypocenter
synonym for the focus of an earthquake.
dolomite
term used to refer to both a sedimentary rock and the mineral that composes it, CaMg(CO3)2. Some geologists call the rock dolostone. Dolomite often forms from limestone as the calcium in calcite is partially replaced my magnesium, usually as water solutions move through the limestone. This process is called dolomitization, and causes recrystallization of the preexisting limestone, resulting in dolomite rock that is hard and very finely crystalline.
exotic terrane
terrane that did not form at its present site on a continent and traveled a great distance to get to its present site.
reserve
that portion of a resource that has been discovered and is economically and legally extractable.
trough
the lowest point of a wave.
Pele
the Hawaiian goddess that, according to myth, controls when and where a volcanic eruption will take place.
loch
the Scottish name for a lake.
Seventh Approximation
the U.S. soil classification system, published in 1960. Its name comes from the fact that it represented the seventh attempt by its authors to find an ideal classification. The emphasis in the classification system is placed on properties of soils and diagnostic horizons rather than on pedogenesis (like earlier classifications). There are 10 soil orders in this classification system: alfisols, aridisols, entisols, histosols, inceptisols, mollisols, oxisols, spodosols, ultisols, and vertisols.
slaty cleavage
the ability of a rock to break along closely spaced parallel planes. It is due to a parallel alignment of platy minerals in fine-grained rocks.
hydraulic action
the ability of flowing water to dislodge and transport rock particles and sediment.
litter
the accumulation of leaves and twigs on the surface of the soil. It is the raw material for the formation of the soil humus. Beneath the fresh litter there is a fermenting layer, and below this the humus.
triangulation
the accurate location of a number of points by dividing the area containing them into a series of triangles for which the values of internal angles and the lengths of sides are ascertained.
outgassing
the action of heat in removing occluded gases from rocks. The outgassing of water vapor and other gases from molten rocks in the primeval Earth is believed to be the source of the atmosphere.
rejuvenation
the action of stimulating a stream to renewed erosive activity, as by geologic uplift upstream or by a drop of sea level downstream.
leaching
the action of water moving downward through a soil profile, carrying soil materials with it in suspension and solution. This process can carry nutrients below the level that plants can reach with their roots, adversely affecting the plant. It is most prominent in areas with high rainfall (e.g. tropical rainforest).
recharge
the addition of new water to an aquifer or to the saturated zone.
absolute age
the age of a fossil, rock formation, or individual rock. It is usually determined by dendrochronology or radiometric dating (compare to relative dating).
deformation
the alteration of rock formations by bending, tilting, and breaking that generally results from tectonic plate movements.
magnetic dip (magnetic inclination)
the angle at which magnetic-field lines dip.
angle of declination
the angle between geographical north and the direction of the magnetic meridian.
angle of inclination (angle of dip)
the angle between the horizontal and the direction of the Earth's magnetic field at a given place. It is 90 degrees at the magnetic poles and zero along the magnetic equator.
hade
the angle measured between the vertical plane and that of the incline of the bedding plane of a fault. The term is usually applied to faults, but can be applied to any structural surface. Compare to dip.
Gutenberg discontinuity
the boundary between the Earth's core and mantle, at a depth of about 2900 km, at which seismic primary (P) waves slow down and secondary (S) waves disappear. The behavior of these waves is thought to be caused by the change from solid rock in the mantle to molten (liquid) rock in the core.
Mohorovicic discontinuity
the boundary separating the crust from the underlying mantle (also called the Moho).
contact
the boundary surface between two different rock types or ages of rocks. In sedimentary rock formations, the contacts are usually bedding planes.
petrography
the branch of geology dealing with the description and systematic classification of rocks by means of microscopic examination of thin sections.
geochemistry
the branch of geology that deals with the chemical elements and compounds in the atmosphere, water, soil, and rocks of the Earth, particularly their composition and how they are formed and distributed.
stratigraphy
the branch of geology that uses interrelationships between layered rock or sediment to interpret the history of an area or region. Stratigraphy uses for principals to determine the geologic history of a locality or a region. These principles are 1) original horizontality, 2) superposition, 3) lateral continuity, and 4) cross-cutting relationships.
actuopaleontology
the branch of paleontology in which investigations into modern organisms, including their effects and remains in modern environments, are directed toward the understanding of fossil analogs.
lithostratigraphy
the branch of stratigraphy concerned solely with lithological features and with naming and elucidating the spatial relations of rock units. It is the most purely descriptive of the systems of stratigraphic classification.
chronostratigraphy
the branch of stratigraphy linked to the concept of time, rather than being limited only to considerations of lithology and spatial distribution. The Chronomeric Standard terms are applied to intervals of geologic time. The Chronomeric Standard hierarchy is as follows (listed from longest to shortest time period): eon, era, period, epoch, age, and chron.
frost-shattering
the breaking apart of masses of rock by the continued enlargement of cracks within them through freeze-thaw action, in areas where the temperature fluctuates for considerable periods around the freezing point. The resultant outcrops and debris are all highly angular in appearance, which is the major characteristic of frost-shattered forms.
bittern
the brine that remains when useful, more soluble minerals have been removed from sea water by evaporation.
permeability
the capacity of a rock to transmit a fluid, such as water or petroleum through pores and fractures. A rock that allows water to flow easily through it is described as permeable, whereas an impermeable rock is one that does not allow water to flow through easily.
core
the central sphere within the Earth, separated into inner and outer units. The inner core has the properties of a solid, whereas the outer core prevents the passage of S waves, suggesting that it is a liquid. It is composed of a mixture of iron and nickel. The core is separated from the mantle above at a depth of 2900 km beneath the Earth's surface by the Gutenberg discontinuity.
angle of repose
the characteristic maximum angle of slope at which a pile of unconsolidated material is stable. This angle depends on the particle size of the material. Larger material has a higher angle than fine material. In most rocks and sands, the angle is 30 to 35 degrees. The angle of repose for loose, dry sand is about 34°. Above this angle, the slope will be unstable and subject to slides.
cementation
the chemical precipitation of material in the spaces between sediment grains, binding the grains together into a hard sedimentary rock. Silica is the most common natural cement, although other examples include calcite, carbonates, and iron oxides.
oxidation
the chemical weathering process involving the reaction between rocks and atmospheric oxygen, the oxygen usually being dissolved in water. The products are oxides and hydroxides, iron being the mineral most frequently affected and its oxidation products give many weathered rocks their reddish or yellowish color. The iron oxide normally formed is hematite (Fe2O3), which gives rocks a reddish color. If water is present, as it usually is at Earth's surface, the iron oxide combines with water to form limonite, which is the name for a group of mostly amorphous, hydrated iron oxides, which are yellowish-Brown when powdered. The brown, yellow, or red color of soil and many kinds of sedimentary rock is commonly the result of small amounts of hematite and limonite released by the weathering of iron-containing minerals (such as pyroxenes).
relative dating
the chronological ordering of features, fossils, or events with respect to the geologic time scale without reference to their absolute age.
Ring of Fire
the circum-Pacific system of earthquakes and volcanoes that surrounds the Pacific basin's oceanic plates where they are subducting below other plates. It extends around the Philippines, Japan, and the west coast of North and South America.
elastic rebound theory
the classic explanation of why earthquakes take place. It involves the sudden release of progressively stored strain in rocks, causing movement along a fault. Tectonic forces act on a mass of rock over many decades. Initially, the rock bends but does not break. More and more energy is stored in the rock as the bending becomes more severe. Eventually, the energy stored in the rock exceeds the breaking strength of the rock, and the rock breaks suddenly, causing an earthquake. Two masses of rock move past one another along a fault. The movement may be vertical, horizontal, or both. The strain on the rock is released; the energy is expended by moving the rock into new positions and by creating seismic waves.
stream order
the classification of a stream in an integrated drainage pattern that is broken down into stream segments and the segments then ranked in a hierarchy according to their allotted order. At the head of the basin, the first small tributaries are first orders; two or more first order tributaries unite to give a second order tributary; two or more second order tributaries unite to produce a third order tributary, etc.
direction of dip
the compass direction in which the angle of dip is measured. If you could roll a ball down a bedding surface, the compass direction in which the ball rolled would be the direction of dip.
strike
the compass direction of a line formed by the intersection of an inclined plane (such as a bedding plane) with a horizontal plane. It also means the direction taken by a structural surface, such as a bedding or fault plane, as it intersects the horizontal.
ridge push
the concept that oceanic plates diverge as a result of sliding down the sloping lithosphere-asthenosphere boundary.
slab pull
the concept that subducting plates are pulled along by their dense leading edges. Cold lithosphere sinking at a steep angle through hot mantle cools the surface part of the plate away from the ridge crest and then down into the mantle as it cools. A subducting plate sinks because it is denser than the surrounding mantle. This density contrast is partly due to the fact that the sinking lithosphere is cold (it is a long distance from the mid-ocean ridge, and has a thus had time to cool). Slab pull the stock to be important in moving an oceanic plate away from a ridge crest, and it results in rapid plate motion.
seafloor spreading
the concept that the ocean floor is moving away from the mid-ocean ridge and across the deep ocean basin, eventually disappearing beneath continents and island arcs. It is a satisfactory mechanism for continental drift. The theory was devised by Harry Hess. Magma rising from the Earth's mantle reaches the surface along the mid-ocean ridges, where it cools and becomes part of the Earth's crust. Repeated rifting of the area accompanied by the addition of further magma causes the older material to be displaced sideways. If the rate of creation of seafloor exceeds its destruction at trenches, the ocean basin will widen (such as the Atlantic Ocean and Red Sea). If destruction exceeds creation, the basin will close (such as the Mediterranean Sea). The spreading rate ranges from about 1 to 18 cm per year (about the growth rate of a fingernail). Vine and Matthews showed that magnetic anomalies that are symmetrical around mid-ocean ridges confirm the theory of seafloor spreading.
lithification
the consolidation of sediment into sedimentary rock. It involves the following 3 steps in order: compaction (of the sediments), cementation (of the sediment grains together), and crystallization (of the cemented grains).
surveying
the construction of maps and plans by accurately recording the relative positions and heights of features on the earth's surface, and plotting them to some suitable scale.
hypolimnion
the cool, lower layer of water in a lake or shallow sea. Insufficient light penetrates for photosynthesis, so there are no green plants and there is little dissolved oxygen in the water. Compare to epilimnion.
baked zone
the country rock adjacent to an igneous intrusion that appears metamorphosed, or baked, due to the high temperatures resulting from the nearby intrusion. The chill zone is the part of the intrusion next to the contact between the intrusion and the country rock, whereas the baked zone is the part of the country rock next to the contact between the country rock and the intrusion.
continental glaciation
the covering of a large region of a continent by a sheet of glacial ice; the only current continental glaciers exist on Antarctica and Greenland.
spreading center (spreading axis)
the crest of a mid-ocean ridge, where sea floor is moving away in opposite directions on either side.
bergschrund
the crevasse that develops where a glacier is pulling away from a cirque wall.
Wilson cycle
the cycle of splitting of a continent, opening of an ocean basin, followed by closing of the basin and collision of the continents.
chemical weathering
the decomposition of rock resulting through a number of chemical reactions, which may weaken the rock through removal of cement, or result in the formation of secondary minerals that are less resistant to erosion. Types of chemical action important in weathering include carbonation, hydration, hydrolysis, limestone solution, oxidation, and reduction. As a rock is decomposed by these agents, new chemical compounds formed.
*ocean acidification
the decrease in ocean pH over time, due to increasing levels of carbon dioxide in the atmosphere. Average surface ocean pH has dropped from 8.2 in pre-industrial times to 8.1 now (a decrease of 30%, due to the fact that the pH scale is a log scale). Carbon dioxide in the atmosphere enters the ocean and combines with water to form carbonic acid (H2CO3). Carbonic acid gives of a hydrogen ion in solution, lowering the pH.
basement rock
the deep-seated roots of the former Precambrian Mountain belts that are the staple, central part of the North American continent. Layers of Paleozoic and younger sedimentary rock cover most of the basement rock.
dredging
the deepening of a river, port-approach channel, or other such area by excavating loose sediment or rock from the river or seabed.
cataclasis
the deformation of rocks by the mechanical process of shearing and granulation. Cataclastic rocks range from coarsely broken breccias to intensely deformed mylonites. Cataclasis normally occurs due to fracturing and folding that occurs at fault zones.
fetch
the distance that a wind blows over water. The height of a wave is controlled by the wind speed, the length of time that the wind blows, and the fetch. The largest waves form where high winds blow over a long expanse of open water for an extended period of time.
stream velocity
the distance water travels in a stream per unit time. A moderately fast river flows of about 5 km/h (3 mi/h). A stream reaches its maximum velocity near the middle of its channel. When a stream goes around a curve, the region of maximum velocity is displaced by inertia toward the outside of the curve. High velocity means increased erosion, whereas low velocity causes sediment deposition.
stream gradient
the downhill slope of the bed of a stream. Stream gradient is usually measured in feet per mile in the U.S. a gradient of 5 feet per mile means that the river drops 5 feet vertically for every mile that it travels horizontally. Mountain streams have high or steep gradients (as steep as 50 to 200 feet per mile). The lower Mississippi River has a very low for gentle gradient of about 0.5 foot per mile or less. A stream's gradient usually decreases downstream, and is therefore high-risk in the headwater regions and lowest at the mouth of the stream. Local increases in the gradient of the stream are usually marked by rapids.
toe
the downslope edge of a landslide or slot. The term is also used to describe the lowest part of a slope or cliff, or the downslope end of an alluvial fan.
Archaeopteryx
the earliest known bird. Feather impressions can be seen in its fossils. Its reptilian features include wings with claws, teeth, a long tail, and solid bones.
Cambrian
the earliest period of the Phanerozoic eon and of the Paleozoic Era. Rocks laid down during this time were the first to show an abundance of fossils, which consist of primitive representatives of most of the invertebrate animal phyla known today. The Cambrian began about 570 million years ago, following the Precambrian, and ended about 505 million years ago. The name Cambrian is derived from the ancient name for Wales ("Cambria"), where rocks containing the earliest fossils were first studied. Cambrian rocks are predominantly sedimentary in origin. Trilobites were especially abundant during the Cambrian.
active continental margin
the edge of a continent that includes a continental shelf, a continental slope, and an oceanic trench. An active margin usually lacks a continental rise and in abyssal plain and is associated with con virgin plate boundaries. Active margins are typically characterized by the presence of earthquakes and a young mountain belt and volcanoes on land.
passive continental margin
the edge of a continent that includes a continental shelf, continental slope, and continental rise, and generally extends down to the abyssal plain had a depth of about 5 km. It is called a passive margin because it usually develops on geologically quiet coast that lack earthquakes, volcanoes, and young mountain belts. Passive margins are found on the edges of most landmasses bordering the Atlantic Ocean.
Oligocene
the epoch of the Tertiary Period that followed the Eocene and preceded the Miocene. Many mammalian groups common in the Eocene became extinct, but others continued to flourish.
Mesozoic Era
the era of geologic time that followed the Paleozoic Era and preceded the Cenozoic Era. The Mesozoic Era is often called the Age of Reptiles.
Paleozoic Era
the era that followed the Precambrian era and began with the appearance of complex life, as indicated by fossils. The first era into which Phanerozoic time is divided. It followed the Precambrian and consists of the following periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.
mofette
the exhalation of carbon dioxide in an area of late-stage volcanic activity. Also, the term is used for the small opening from which the carbon dioxide is emitted (it is a type of fumarole).
desertification
the expansion of barren deserts into once-populated regions. Desertification occurs due to human intervention, or changes in climate. The usual cause is overgrazing by farm animals or deforestation, resulting in erosion and infertility.
original horizontality (law of)
the fact that the deposition of most water-laid sediment occurs in horizontal or near-horizontal layers that are essentially parallel to the Earth's surface.
debris avalanche
the fastest variety of debris flow, which consists of a very rapidly moving, turbulent mass of debris, air, and water.
topsoil
the fertile, dark-colored surface soil, or A horizon. It is more fertile than the underlying
Pliocene
the final epoch of the Tertiary Period, preceded by the Miocene and followed by the Pleistocene.
Cretaceous Period
the final period of the Mesozoic Era, beginning about 135 mya and lasting until about 65 mya. It followed the Jurassic Period, and preceded the Tertiary Period (the first period of the Cenozoic Era). The name derives from creta, the Latin word for chalk, the characteristic rock of the period. During the Cretaceous, Africa and South America separated, and North America was moving away from Eurasia. By the end of the period, the Atlantic Ocean extended as far as the Arctic Ocean. On land, angiosperms (flowering plants) appeared, and dinosaurs reached their peak of development. At the end of the Cretaceous (the so-called K/T boundary), the ammonites and many other invertebrate groups, as well as most of the reptiles, became extinct.
Permian
the final period of the Paleozoic Era. The majority of the invertebrate species around at that time went extinct at the end of the Permian Period.
Cambrian explosion
the massive evolutionary radiation of invertebrates that produced many of the animal groups that we recognize today
komatiite
the fine-grained, extrusive equivalent of peridotite. Komatiite is very rare on Earth because it has the composition of the mantle rock peridotite, but it was clearly formed from a molten lava - this would have taken several hundred degrees more heat than Earth is capable of today (such as in the Archean Eon, when the radioactive heating of the Earth was greater.
Paleocene
the first epoch of geologic time in the Tertiary Period. It followed the cretaceous Period and was succeeded by the Eocene Epoch.
Triassic Period
the first period of the Mesozoic era, beginning about 246 million years ago, at the end of the Permian period, and lasting for some 40 million years until the beginning of the Jurassic period.
solifluction
the flow of water-saturated debris over impermeable material. It is the slow downslope movement of waterlogged soil that occurs especially at high elevations in regions underlain by frozen ground (permafrost) that acts as a downward barrier to water percolation. In spring, melting takes place in the top layer of the soil, but the water produced cannot drain away because beneath this active layer the ground remains frozen and hence impermeable. Consequently, the upper soil becomes highly saturated, and its cohesion may be reduced to such an extent that flowage can take place an material be moved downslope.
solifluction
the flow of water-saturated soil over impermeable material. The impermeable material is most commonly permafrost, but may be impenetrable bedrock. In areas with permafrost, the upper portion of the soil thaws in the summer, and the water produced, along with water from rain and runoff, cannot percolate downward through the permafrost, and so the slope becomes susceptible to solifluction.
schistosity
the foliation in schist or other coarse-grained, crystalline rock due to the parallel arrangement of mineral grains of the platy or prismatic types, usually mica.
*albedo
the fraction of incoming solar radiation reflected by a surface. Albedo varies between 0 and 1, where a value of 0 means the surface is a perfect absorber of radiation and 1 means the surface is a perfect reflector. The albedo of ice covered with snow is very high 0.85 (white objects reflect most of the incident radiation), whereas the ocean has a low albedo of 0.06 (its dark color means it absorbs 94% of the incoming radiation and reflects only 6% of the incoming radiation). Sea ice albedo varies between 0.5 and 0.7, so as it melts and is turned to ocean water, albedo goes down. Lower albedo means more absorbance of light, causing warmer temperatures and even more melting of ice (a positive feedback loop).
topography
the general configuration of a land surface, including its relief and the position of its natural and man-made features.
heat flow
the gradual loss of heat through Earth's surface. Higher than normal heat flow values are usually caused by the presence of a magma body or a still-cooling pluton near the surface.
glacial maximum
the greatest extent of Pleistocene ice, which existed about 200,000 years ago, during the penultimate (second to last) glacial period. During this time, the ice extended across the Arctic Ocean and joined with the ice sheets that covered much of N. America, Greenland, N. Russia, and NW Europe. The Baltic and Scandinavian countries were completely covered. The ice engulfed all but the very south of the British Isles.
abrasion
the grinding and wearing away of rocks by an agent of transportation (such as water, wind, or ice) that is charged with a load of already eroded material such as sediment, which acts as a tool for cutting, grinding, scratching, and polishing. Abrasion is the most effective form of religion on rocky streambed. The coarsest sediment is the most effective in stream erosion.
induration
the hardening of porous rocks or of soils through the deposition of minerals, which act as a cement, on or within the surface layers. The cement is usually calcite, hematite, or silica.
timberline (treeline)
the highest altitude on a mountainside or the highest latitude in which trees will grow. Trees do not grow beyond this line because it is too cold, there is insufficient soil, or there is not enough rainfall.
crest
the highest point of the wave.
offset
the horizontal component of displacement on a fault, measured perpendicular to the disrupted horizon.
heave
the horizontal displacement along a fault.
wavelength
the horizontal distance between two successive wave crests (or two successive wave troughs).
clay mineral - a hydrous aluminum silicate mineral that is formed by the chemical weathering of feldspar. Clay minerals form when feldspar is attacked by the hydrogen ion of carbonic acid
the hydrogen moves into the crystal structure of the feldspar, causing it to release its cation (either potassium, sodium, or calcium).
bed load
the material that is carried along the bed (bottom) of a stream or the sea by moving water, or along the ground by the wind. It consists of particles too large to be transported in suspension.
snowball Earth hypothesis
the idea that a late Precambrian ice age was so extensive that the surfaces of the world's oceans were frozen. Evidence for the hypothesis includes tillites that must, at the time, have been deposited near the equator. The hypothesis proposes that the extreme cold was due to the Sun being weaker at the time and the absence of carbon dioxide and other greenhouse gases in the atmosphere.
flood tide
the incoming of the tidal stream. That part of the tide cycle following the low-water stage and preceding the high-water stage.
headward (headwater) erosion
the increase in length of a gully or valley, or a stream within it, caused by erosion at the upper end (head) of the valley.
geothermal gradient
the increase in the Earth's temperature with depth from its surface. The geothermal gradient averages about 2.5°C for each 100 m of depth in the upper part of the crust (about 25 degrees Celsius per km). The geothermal gradient is not the same everywhere. It is higher in a volcanic region than it is for the continental interior. You would have to go deeper in the continental interior to reach the same temperature that is reached at a shallower depth at a volcanic region.
dendrochronology
the interpretation of former climates from changes in width of annual growth rings of certain tree species. Temperature and amount of rainfall are the most important factors affecting the width of a tree ring.
coast
the land near the sea, including the beach and a strip of land inland from the beach.
coastline
the landward limit of the beach, the boundary between the coast and the shore.
competence
the largest size of grain that a stream can move as bed load.
polje
the largest type of solution depression found in limestone areas, extending up to 250 square miles. The floor of the depression is alluvium-covered, and often uneven, reflecting the collapse of former cave systems.
eon
the largest unit of geological time in the Chronomeric Standard scheme. An eon consists of several eras grouped together.
Wisconsin glacial episode
the last major event of continental glaciers in the North American Laurentide ice sheet. This episode lasted from approximately 110,000 years ago to 10,000 years ago. The maximum ice extent occurred approximately 21,000 years ago during the last glacial maximum, also known as the Late Wisconsin in North America.
Gondwanaland
the late Paleozoic, southern part of the previous supercontinent Pangaea, that gave rise to South America, Africa, India, Australia, and Antarctica. It was named by Suess after the Gondwana system of India, thought to have existed over 200 mya.
thermocline .
the layer of a lake or ocean where temperature decreases rapidly with depth. It lies beneath the mixed layer
outer core
the layer of the Earth between 2900 km and 5000 km beneath the surface. It is bounded by a seismic discontinuity (the Gutenberg discontinuity) that separates it from the overlying mantle. It is thought to have a composition of iron alloy and nickel, and to behave essentially as a liquid, unlike the inner core, which is solid.
basement
the level below which sedimentary rocks do not occur.
lysocline
the level or ocean depth at which the rate of solution of calcium carbonate just exceeds its combined rate of deposition and precipitation.
seismicity
the likelihood of an area being subject to earthquakes, or the phenomenon of earth movements.
elastic limit
the limit to a rock's elastic behavior. Once the elastic limit is exceeded by the stress applied, the rock will deform in a permanent way.
fault line
the line along which a fault intersects the surface of the ground.
thalweg
the line connecting the lowest points along a stream bed or valley. It is a longitudinal profile. It can also be defined as the line of continuous a maximum to send from any point on land surface, or the deepest or best navigable channel, used in defining water boundaries between states.
ablation
the loss of glacial ice or snow by melting, evaporation, or breaking off into icebergs (also called wastage).
Caledonian orogeny
the lower Paleozoic orogeny in which the Caledonian Mountains, extending from Ireland via Scotland to Scandinavia, were formed. It resulted from the closure of the Proto-Atlantic Ocean, between the Baltic and Canadian Shields.
terminus
the lower edge of a glacier.
zone of ablation
the lower portion of a glacier from which ice is lost (ablated) by melting, evaporation, and calving.
drawdown
the lowering of the water table near a pumped well.
lignite
the lowest grade of coal (lowest carbon content). It is brownish-black in color and is intermediate in coalification between peat and bituminous coal. Lignite is sometimes called brown coal, and it is formed by the compaction of peat.
load
the material that is moved or carried by a natural transporting agent, such as a stream, a glacier, or the wind. It is formed by the erosive nature of the transporting agent, but it can also be supplied by groundwater flow or from the sides of a valley by landslides or wash. It can be divided into suspended load, dissolved load and bed load. The bed load is the large or heavy sediment particles that travel on the streambed. Sand and gravel, which form the usual bed load of streams, moved by either traction or saltation. In general, bed load is the least important (for example, only 8% of the sediments in Mississippi River moves as bed load).
field capacity
the maximum amount of water a soil can retain after gravitational water has drained away.
lessivage
the mechanical movement of clay down a soil profile under the influence of water and its redeposition lower down. It occurs in soils subject to leaching but where acidity is neutral or slight, so that the clay is not broken down into its constituents but moves as a whole.
halokinesis
the mechanism of formation of salt domes, which involves salt moving upward from deep sources. The term also refers to the study of salt dome formation.
capillarity
the mechanism whereby capillary water moves vertically up the soil profile from the groundwater table or moist subsoil; it is a process typical of arid and semiarid zones where evaporation of water exceeds precipitation. The capillary "current" brings with it dissolved salts, which are precipitated out at the point where the current finally dries out, forming a salt accumulation at or near the surface.
carnotite
the most common uranium-containing ore in the United States. It is a yellow, complex, hydrated oxide found as incrustations in the sedimentary rocks. The principal use of uranium at present is to provide power for electricity-generating nuclear reactors, although uranium was also used to make tens of thousands of nuclear warheads during the Cold War.
Cenozoic Era
the most recent of the eras; it follows the Mesozoic Era, and began about 65 million years ago. The Cenozoic includes the Tertiary and Quaternary Periods. It is sometimes called the Age of Mammals, due to the evolutionary radiation of the mammals following the extinction of most reptiles at the end of the Mesozoic.
helical flow
the most significant type of turbulence in streams. Superimposed on the primary downstream flow, a secondary flow moves across the surface of the stream toward the outside of the meander beds, compensated for by a reverse flow along the bed toward the inside of the meander bends. This gives the streamflow a net corkscrew movement, concentrating erosion on the outside of the meander beds, with deposition on the inside, causing downstream propagation of the meanders.
swash
the movement of a fairly thin layer of turbulent water up a beach, following the breaking of a wave.
transportation
the movement of eroded particles by agents such as rivers, waves, glaciers, or wind.
aeolian transport
the movement of sediment by wind. This transport can occur by suspension (buoyed up by the air), saltation (bouncing of grains along the surface), or surface creep (movement of coarse sand grains due to impact of saltating grains).
hydrologic cycle
the movement of water and water vapor from the sea to the atmosphere, to the land, and back to the sea and atmosphere again. Water evaporates from bodies of water, forming water vapor in the atmosphere, which may condense to form clouds, which may release precipitation, returning the water to the Earth's surface.
infiltration
the movement of water downward into the ground.
stream piracy
the natural diversion of the headwaters of one stream into the channel of another stream having greater erosional activity.
*sorting
the natural process of selection and separation of sediment grains according to their grain size by agents of transportation, especially by running water. Glaciers deposit all sediment sizes in the same place, so glacial sediments usually consist of a mixture of clay, silt, sand, and gravel. Glacial sediments are therefore considered to be poorly sorted. Sediment is considered to be well-sorted when the grains are nearly all the same size. Rivers are good sorting agents, separating the different sized particles from each other. Sorting takes place because of the greater weight of larger particles (a river must flow more rapidly to move boulders than to move pebbles). This process can be observed as a river flows out of steep mountains onto a gentle flood plain. As a river slows down, the heaviest particles are deposited first (such as boulders). As the river continues to slow, smaller particles begin to settle out. When a river enters a large body of water, such as a lake or stream, the water slows down and drops its sediment load. The largest particles settle out first (near shore), whereas the smaller particles settle out and are deposited farther from shore (the open ocean).
elutriation
the natural sorting of rock fragments into finer and coarser particles. It most commonly occurs when the fragments are transported by water, but may also happen during pyroclastic flow down the side of an erupting volcano.
geosphere
the nonliving part of the Earth, as opposed to the living biosphere. It includes the Earth's crust (lithosphere), all bodies of water (hydrosphere), and the air (atmosphere).
Laurasia
the northern hemisphere supercontinent formed upon the breakup of Pangaea about 200 mya (the southern supercontinent is called Gondwanaland). It subsequently fragmented to form present-day North America, Greenland, Europe, and Asia (excluding India).
Panthalassa
the ocean that surrounded Pangaea before its fragmentation.
vent
the opening in Earth's surface through which a volcanic eruption takes place. Long, ashes, and the vapor, are discharged at the opening.
gleying
the permanent or seasonal presence of either perched water or groundwater within a soil profile. This creates anaerobic conditions, leading to a dominant process of reduction of ferric iron to its ferrous form, giving the soil a blue-gray color.
mechanical weathering
the physical disintegration of rock into smaller pieces. Types include freeze-thaw weathering, exfoliation, granular disintegration, etc. Mechanical weathering breaks the rock but does not change the composition.
erosion
the physical removal of rock by an agent such as running water, glacial ice, or wind.
mouth
the place where a stream enters the ocean, a lake, or a larger stream.
stream mouth
the place where a stream enters the sea, a large lake, or a larger stream.
epicenter
the point on Earth's surface directly above the focus of an earthquake.
focus
the point within Earth from which seismic waves originate in an earthquake. The focus is also sometimes called the hypocenter.
magmatic underplating
the pooling of mafic magma at the base of the continental crust, which supplies the extra heat necessary to partially melt the overlying, silica-rich crustal rocks, producing silicic magma. This silicic magma collects and forms diapirs, which rise to a higher level in the crust and solidify as granitic plutons (or reach the surface and eruptive as rhyolite).
underplating
the pooling of magmas at the base of the continental crust.
country rock
the preexisting envelope of rocks into which an igneous magma is intruded. Country rock is the rock surrounding any igneous intrusion. The country rock often shows the marked thermal effects of contact metamorphism. Xenoliths contained in the solidified magma are usually derived from the country rock.
Holocene Epoch
the present epoch of the Quaternary Period, covering the last 10,000 years of geologic time from the end of the Pleistocene epoch. It is also sometimes called the Recent Epoch.
Recent (Holocene) Epoch
the present epoch of the Quaternary period.
flocculation
the process by which colloidal (i.e. clay-size) particles join together forming groups called floccules. Colloidal material tends to adopt this habit in the presence of neutral salts, notably salts of calcium. In the presence of alkaline salts, especially sodium, the particles adopt the opposite habit and separate as independent units. Calcium therefore improves the structure of the soil by helping aggregation.
differentiation
the process by which different ingredients separate from an originally homogenous mixture. Differentiation in magmas takes place mainly through crystal settling, the downward movement of minerals that are denser than the magma from which they crystallized. If crystal settling takes place in a mafic magma chamber, olivine and, perhaps, pyroxene crystallize and settle to the bottom of the magma chamber. This makes the remaining magma more silicic. Calcium-rich plagioclase also separates as it forms. The remaining magma is, therefore, the pleated of calcium, iron, and magnesium. Because these minerals were economical in using the relatively abundant silica, the remaining magma becomes richer and silica as well as in sodium and potassium.
orogenesis
the process by which mountains are formed, i.e. the deformational processes such as thrusting, folding, and faulting, which result from the collision of two continental plates.
percolation
the process by which water moves downward, under the influence of gravity, through pores in soil or cracks and joints in rock.
weathering
the process of breakdown and alteration of rock on the earth's surface in response to the changes in environmental conditions since the time of their formation.
corrosion
the process of erosion by chemical solution. All common rock-forming minerals are to some extent soluble in water (even quartz).
subduction
the process of one lithospheric plate descending beneath another overriding plate. The overriding plate may be of oceanic or continental material. The oceanic plate that is overridden is pushed down into the mantle and destroyed, its position being marked by a series of earthquake foci. Such subduction zones are marked by oceanic trenches.
sedimentation
the process of sediment deposition. It occurs when the velocity of flow in the transporting medium falls below its terminal, or settling, velocity.
carbonization
the process that leads to the preservation of fossils as thin films of carbon in sedimentary rock. As sediments are laid down, organic matter decomposes with the release of hydrogen, oxygen, and nitrogen, leaving only the carbon. The Burgess Shale has many such fossils.
basal sliding (basal slip)
the process whereby a glacier moves downslope due to a thin film of water generated by the pressure from its weight. The tremendous weight of the glacier causes the ice at the base of a glacier to melt sooner than the ice at its surface, generating the water which acts as a lubricant between the rock below the glacier and the glacial ice. This mechanism of motion dominates in warmer, temperate regions, whereas plastic flow dominates in very cold regions.
calving
the production of icebergs by the splitting off of large slabs of ice from a glacier or ice sheet at the edge of the sea.
alluvium
the products of sedimentation by rivers. There is a marked decrease in the size of alluvial matter down-valley, with finer material in the lower reaches.
throughflow
the rain water that flows down a hillside through the soil. It occurs when more rain falls on the surface of the ground and can be absorbed quickly downward by the soil.
aggradation
the raising of the level of the land surface due to the inability of a river to transport its load, leading to deposition.
geochronology
the science of age determination of parts of the Earth. It has two branches: absolute dating and relative dating. Absolute dating is more modern, and involves radiometric dating methods to put actual aes in years before present on organic remains, rocks, or sediments. Relative dating includes fossil correlations, pollen analysis, and correlation to help establish stratigraphy and place events into order. Absolute dating tries to give precise dates of occurrence.
speleology
the scientific study of caves and caverns, including their exploration, geology, and mineralogy.
seismology
the scientific study of earthquakes, including their origins and manifestations. A seismologist is a person who studies earthquakes.
glaciology
the scientific study of ice in all of its forms, including atmospheric, land, lake, river, and ocean ice.
limnology
the scientific study of lakes and other bodies of freshwater.
petrology
the scientific study of rocks.
geography
the scientific study of the features of the Earth's surface. Physical geography includes climatology, geomorphology, meteorology, and pedology. Human activity is covered by political or socioeconomic geography.
volcanology (vulcanology)
the scientific study of volcanoes.
Eocene Epoch
the second epoch of the Tertiary Period, which extended from the end of the Paleocene, 57.8 mya, to the beginning of the Oligocene Epoch, about 36.6 mya. During this period mammals were abundant and horses (perissodactyls) and artiodactyls appeared. The modern carnivore families also became established.
B horizon
the second highest layer of soil, immediately below the A horizon. It is characterized by the accumulation of material leached downward from the A horizon above; also called the zone of accumulation. The B horizon contains less humus and less weathered material than the A horizon. This layer is often quite clayey and stained red or brown by hematite and limonite. Calcite may also build up in the B horizon.
beach face
the section of the beach exposed to wave action. It is the steepest part of a beach. Gravel beaches have a steeper beach face than sand beaches.
relative time
the sequence in which events took place, rather than the number of years involved (not measured in time units).
Bowen's reaction series
the sequence in which minerals crystallize from a cooling basaltic magma. In a cooling magma, certain minerals are stable at higher melting temperatures and crystallize before those stable at lower temperatures. Bowen's reaction series is split into two branches, a discontinuous branch, which contains only ferromagnesian minerals, and a continuous branch, which contains only plagioclase feldspar. Crystallization in the discontinuous and continuous branches takes place at the same time. Looking at the discontinuous branch, one can see that olivine crystallizes before pyroxene and pyroxene crystallizes before amphibole. A complication is that early formed crystals react with the remaining melt and re-crystallize as cooling proceeds. For instance, early formed olivine crystals react with the melt and recrystallize to pyroxene when pyroxene's temperature of crystallization is reached. Upon further cooling, pyroxene continues to crystallize until all the melt is used up or the melting temperature of amphibole is reached. At this point, pyroxene reacts with the remaining melt and amphibole forms at its expense. If all of the iron and magnesium in the melt is used up before all of the pyroxene recrystallizes to amphibole, then the ferromagnesian minerals in the solid rock would be amphibole and pyroxene (the rock would not contain olivine or biotite). Looking at the continuous branch (contains only plagioclase feldspar) the composition of plagioclase changes as the magma cools and earlier forms crystals react with the melt. The first plagioclase crystals to form as the hot melt cools contain calcium but little or no sodium. As cooling continues, the early formed crystals grow and incorporate progressively more sodium into their crystal structures. Any magma left after the crystallization is completed along the two branches is richer in silicon than the original magma and also contains abundant potassium and aluminum. The potassium and aluminum combined with silicon to form orthoclase feldspar (also called potassium feldspar). If the water pressure is high, muscovite, the lighter colored form of mica, may also form at this stage. Excess SiO2 crystallizes as quartz. You should know everything shown in Figure 4 at the back of this glossary.
deposition
the settling or coming to rest of transported material. Deposition occurs after erosion and transport, and can be described as the creative part of the geomorphological system.
lee
the side of a hill or other prominence that is sheltered from the prevailing wind.
stoss
the side of a hill that faces the direction from which a glacier is coming. It is exposed to the abrasive action of the moving glacier, and as a result usually has gentle slopes and rounded features. The opposite side of the hill that faces the direction in which a glacier is going is called the lee side, and is somewhat steeper, consisting of plucked slopes.
headward erosion
the slow uphill growth of a valley above its original source through gullying, mass wasting, and sheet erosion. This process causes a stream to lengthen its Valley.
Chandler wobble
the small measurable wobble of the Earth with respect to its axis of rotation, discovered in 1891 and found to have a period of 14 months and an amplitude of 0 degrees, 5 minutes. Its origin is unknown, but it is assumed to be connected with movement of materials inside the Earth.
bed (stratum)
the smallest division of stratified sedimentary rocks, consisting of a single distinct sheetlike layer of sedimentary material. Beds are separated from other beds by planar surfaces called bedding planes.
chron
the smallest interval of geologic time in the hierarchy of the Chronomeric Standard terms used in chronostratigraphy. Chrons may be grouped together to form an age.
cement
the solid material that precipitates in the pore space between sediment particles, binding the grains together to form solid rock.
ice
the solid state of water. It melts at 0 C, requiring 80 calories (340,000 joules) per gram of ice (latent heat of fusion).
transgression
the spread of the sea over land areas. The transgression results in the shortening of rivers by the drowning of former valleys, leading to the creation of fjords, rias, and estuaries.
flood
the state of a river when the volume of water flowing in it exceeds bankfull, and water commences to spread away from the channel over the floodplain.
slip face
the steep, downwind slope of a dune formed from loose, cascading sand that generally keeps the slope at the angle of repose (about 34 degrees for loose, dry sand). Sand sliding down an oversteepened slip face forms high-angle cross-bedding within the dune.
exfoliation
the stripping of concentric rock slabs from the outer surface of a rock mass by a number of different weathering processes.
paleontology
the study of all aspects of ancient organisms, including their taxonomy, anatomy, ecology, and evolution. The evidence for this comes from fossils preserved in rocks.
palynology
the study of fossilized pollen and spores.
paleomagnetism
the study of natural remnant magnetization in order to determine the intensity and direction of the Earth's magnetic field in the geologic past.
paleobotany
the study of plant life of the geologic past.
petrochemistry
the study of the chemical composition of rocks.
paleoclimatology
the study of the climates of earlier geologic periods. The evidence for the former climate is obtained from sediments, fossils, and ice cores. When in doubt of your answer during an Oceans Bowl team challenge, drop this term and you will be assured of much success.
geomorphology
the study of the evolution of landforms.
paleogeography
the study of the physical geography of all or a part of the Earth's surface at some time in the geologic past.
taphonomy
the study of the processes affecting an organism from its death to its possible fossilization. Taphonomy elucidates the many differences that exist between a fossil assemblage and the community or communities of living plants or animals from which it came.
hydrology
the study of the properties, distribution, and circulation of water.
geology
the study of the structure and composition of the Earth.
hydrography
the study of water bodies on Earth and the strips of land bordering them; it involves description and measurement, with the subsequent presentation of this information on hydrographic charts.
landslide
the sudden downslope movement of rock and debris due to failure along a shear plane. The movement is started when the stress in the free face exceeds the resisting power of the potential shear plane. The result is a convex scar left upslope with a mass of jumbled debris at the base in a tongue-like form.
adret
the sunny or, in the Northern Hemisphere, south-facing slope of a valley. It is the side of a slope favored for farming.
Rodinia
the supercontinent that existed between 1100 and 750 million years ago, before Pangaea existed. It formed by the accretion and collision of fragments produced by the breakup of the older supercontinent Columbia. The hub (largest landmass) of Rodinia was Laurentia (the modern North America), which was situated approximately along the equator (and rotated 90° on its side). Other cratons that made up Rodinia included Baltica (the current Scandinavian and Baltic countries) and Siberia, as well as several others. Around 750 mya, Rodinia began to break apart, with the rift zone that formed between Laurentia and Baltica forming the ancestral North Atlantic Ocean (which has been called the Iapetus Ocean). These cratons eventually came back together to form Pangaea. The name Rodinia is from a Russian word meaning 'motherland'.
fault plane
the surface along which a fault forms.
footwall
the surface of rock beneath a fault plane or ore body. Compare to hanging wall.
substrate
the surface to which a fixed organism is attached.
Curie point
the temperature below which a material becomes magnetized. Above this temperature, permanent magnetism disappears. The Curie point for iron is 760 Celsius, and the Curie point for magnetite is 580°C.
differential weathering
the tendency for different types of rocks to weather at different rates. For example, shale (composed of soft clay minerals) tends to weather much faster than sandstone (composed of hard quartz mineral).
catastrophism
the theory, now generally thought to be false, that past geologic changes have occurred as a result of a number of sudden catastrophes (opposite of uniformitarianism).
1. Contact metamorphism
the thermal metamorphism developed in rocks that are intruded by hot magma (the rocks are in close proximity to magma).
continental crust
the thick, granitic crust under continents. It reaches a thickness in excess of 50 km under mountain ranges, although it is generally only 33 km thick. Continental crust is thicker and less dense than oceanic crust, and it is made of lighter colored materials (felsic rocks).
oceanic crust
the thin, basaltic crust under oceans. It is about 5-10 km thick and has a density of 3.0 g/cm3 (as opposed to 2.7 g/cm3 for continental crust). It is thinner, denser, darker, and younger than continental crust
lacuna
the time interval missing between beds above and below an unconformity.
half-life
the time taken for one half of a sample of radioactive element to decay to another element. It has a constant value for any particular radioisotope, and its measurement is the basis of radiometric data. For example, the half-life of carbon-14 is 5730 years.
K/T boundary
the time when the Cretaceous period changed into the Tertiary, when large-scale animal extinctions took place. Dinosaurs and ammonites ceased to exist. The cause is thought to be a general and rapid rise in world temperatures. High concentrations of iridium in rocks formed at this time lead some geologists to think that the climatic change resulted from a major meteorite impact, which filled the Earth's atmosphere with debris, cutting off most of the sunlight (iridium is relatively abundant in meteoroids). Many believe that the Chicxulub crater, off the coast of Mexico's Yucatan Peninsula, was the sight of this impact.
perched water table
the top of a body of groundwater separated from the main water table beneath it by a zone that is not saturated. It may form as groundwater collects above a lens of less permeable shale within a more permeable rock, such as sandstone.
resource
the total amount of any given geologic material of potential economic interest, whether discovered or not. The size of a nonrenewable resource does not change over time.
accumulation
the total amount of precipitation that gathers on a snowfield or glacier, or, the overall result of all processes that add mass to a snowfield, glacier, or ice floe.
pore space
the total amount of space taken up by openings between sediment grains.
drainage basin
the total area drained by a stream and its tributaries.
capacity (stream)
the total load that a stream can carry as bedload.
metamorphism
the transformation of preexisting rock into texturally or mineralogically distinct new rock as a result of high temperature, high pressure, or both, but without the rock melting in the process. The 2 most common types of metamorphism:
till
the unsorted and unstratified (unlayered) material deposited by glaciers and ice sheets. It contains angular material ranging from clay-sized particles to huge boulders and is deposited directly by a glacier without reworking by meltwater.
rigid zone
the upper part of a glacier in which there is no plastic flow.
overburden
the upper part of a sedimentary deposit. Its weight causes compaction of the lower part. It is composed of loose or consolidated material that must be removed prior to mining.
headwaters
the upper part of a stream near its source in the mountains.
stream headwaters
the upper part of a stream, near the source of the stream.
zone of accumulation
the upper portion of a glacier with perennial snow cover. The boundary line between the zone of accumulation and the zone of ablation is called the equilibrium line (or firn line), which marks the highest point at which the glacier's winter snow cover is lost during a melt season.
asthenosphere
the upper portion of the mantle, just beneath the lithosphere. The asthenosphere is of indeterminate thickness, and behaves plastically, allowing the lithospheric plates to ride on top of it. The velocity of seismic wave is considerably reduced as they enter this zone. The asthenosphere is thought to be composed of partly molten peridotite, with a liquid fraction having the composition of basalt.
water table
the upper surface of the zone of saturation. The water level at the surface of most lakes and rivers coincides with the water table. The slope of the water table is controlled largely by topography - the water table roughly parallels the land surface (particularly in the humid regions).
O horizon
the uppermost layer in a soil profile that consists entirely of nondecomposed and highly decomposed organic material. For example, fallen leaves and needles, along with ground vegetation would constitute the O horizon in a forested area.
A horizon
the uppermost, dark-colored layer of soil. It contains humus and other organic material. This layer is sometimes also called the zone of leaching, because some soluble material will have been dissolved out and passed to the B horizon underneath. The zone of leaching is located in the E horizon in older soils.
biostratigraphy
the use of information from fossils in the calibration of sequences of rock. Stratigraphy is the description and classification of bodies of rock and their correlation with one another.
solution weathering
the usually slow but effective process of weathering and erosion in which rocks are dissolved by water or other agents of chemical weathering. For example, calcite goes into solution when exposed to carbon dioxide and water. Caves conform underground when flowing groundwater dissolves the sedimentary rock limestone, which is mostly calcite.
Precambrian
the vast amount of time that preceded the Phanerozoic Eon and Paleozoic Era.
angle of dip
the vertical angle of an inclined bed as measured from the horizontal. The dip angle is always measured at a right angle to the strike - that is, perpendicular to the strike line.
throw
the vertical change in level of a previously continuous bed of rock as a result of faulting.
pipe
the vertical conduit below a volcano through which the magmatic materials passed. It is usually filled with breccia and may be mineralized.
relief
the vertical distance between points on Earth's surface.
wave height
the vertical distance between the crest and the trough of a wave.
stream discharge
the volume of water that flows past a given point in a unit of time.
epilimnion
the warmer upper layer of water in a lake or shallow sea. Photosynthesis may occur and green plants grow here because light can penetrate this layer.
uranium-235 dating
uranium-235 decays to lead-207 with a half-life of 713 million years, and uranium-235 dating is effective for dating things that are between 10 million and 4.6 billion years old.
uranium-238 dating
uranium-238 decays to lead-206 with a half-life of 4.5 billion years, and uranium-238 dating is effective for dating things that are between 10 million and 4.6 billion years old.
Diesel fuels
used as a transportation fuel for trucks, trains, and ships.
Kerosene and heating oils
used in aviation fuel and home heating.
plastics, polyethylene
used in computer frames, shopping bags, toys, CDs, etc.
Octane (C8H18)
used in gasoline - isooctane, a form of octane, is the best kind of gasoline for internal combustion engines.
Heptane (C7H16)
used in gasoline.
Heavy crude oils (C17H36-C22H46)
used in lubricating and engine oils.
Asphalt, waxes, greases, paraffins
used in paving and lubrication of machinery.
flood basalt (plateau basalt)
very fluid basaltic lava that flows over large areas. The largest known area of flood basalt covers 250,000 km2 of the Deccan plateau on the Indian subcontinent. Smaller examples occur in southern Africa and the Columbia River Plateau in the northwest U.S.
creep
very slow, continuous downslope movement of soil or rock debris down gentle slopes under the influence of gravity. It is the slowest of all the types of mass movement. Solifluction is a type of creep important in maritime, periglacial areas.
obsidian
volcanic glass. It is normally black or brown, exhibits conchoidal fracture, and is derived from rapidly cooled rhyolitic lava. Obsidian is one of the few rocks not composed of minerals. As is the case in most glasses, the arrangement of atoms is disordered in obsidian.
Mount Thera
volcanic island in the Aegean Sea that was the site of a large eruption during the time of the early ancient Greek civilization. The remnant of that eruption is the island of Santorini. Some archaeologists consider Thera the original lost continent of Atlantis.
Mount Mazama
volcano in Oregon that exploded about 6,600 years ago, forming the caldera that is now occupied by Crater Lake.
Mount St. Helens
volcano in southern Washington that erupted in 1980. A huge explosion destroyed the summit and north flank, and caused a giant landslide known as a debris avalanche.
Mount Vesuvius
volcano near Naples, Italy, that famously erupted in 79 A.D., burying the Roman city of Pompeii and at least four other towns under 5 to 8 meters of hot ash. The town was rediscovered in 1748 and was excavated.
Mount Pelée
volcano on the Caribbean island of Martinique whose eruption destroyed the port city of St. Pierre in 1902. About 29,000 people died, due to the fact that authorities claimed there was no danger and preventive evacuation.
Soufriére Hills volcano
volcano on the island of Montserrat in the Caribbean Sea that underwent a series of large eruptions between 1995 and 1997. Only 19 people were killed due to these eruptions because nearly all of the people in the southern part of the island were evacuated.
acid mine drainage
water from coal and metal mines that contain sulfuric acid formed by the oxidation of the sulfur in pyrite and other sulfide minerals when they are exposed to air my mining activity. Acid mine drainage can contaminate both surface and ground water.
capillary water
water held in the small pores within the soil, existing as a film around soil particles. It is this water that is mostly taken up by plant roots for growth, as opposed to the gravitational water that rapidly flows through the soil and removes plant nutrients during wet periods, and hygroscopic water, which is the water that remains in the soil even after air drying and is unavailable to plants.
groundwater
water that lies beneath the ground surface, filling the cracks, crevices, and pore spaces of rocks.
intrusive/extrusive equivalents
you should know the chart of intrusive equivalents shown in Figure 3 at the end of this glossary. Everything on this chart is important, including which minerals make up each rock, relative amounts of each mineral, and all of the trends (including numbers) shown at the bottom of the chart.
Benioff zones
zones of inclined seismic activity that are associated with oceanic trenches, where one plate is subducting beneath another.
