Earth Science Glossary - Tarbuck & Lutgens
Hypocenter
Focus (earthquake).
Upslope fog
Fog created when air moves up a slope and cools adiabatically.
Frontal fog
Fog formed when rain evaporates as it falls through a layer of cool air.
Normal polarity
A magnetic field that is the same as that which exists at present.
Pipe
A vertical conduit through which magmatic materials have passed.
Annual mean temperature
An average of the 12 monthly temperature means.
Occultation
An eclipse of a star or planet by the Moon or a planet.
Lunar eclipse
An eclipse of the Moon.
Solar eclipse
An eclipse of the Sun.
Pelagic zone
Open ocean of any depth. Animals in this zone swim or float freely.
Humus
Organic matter in soil produced by the decomposition of plants and animals.
Barometric tendency
Pressure tendency.
External process
Process such as weathering, mass wasting, or erosion that is powered by the Sun and transforms solid rock into sediment.
Chemical sedimentary rock
Sedimentary rock consisting of material that was precipitated from water by either inorganic or organic means.
Stratified drift
Sediments deposited by glacial meltwater.
Andesitic composition
See Intermediate composition.
Surface waves
Seismic waves that travel along the outer layer of Earth.
Body waves
Seismic waves that travel through Earth's interior.
Climate-feedback mechanism
Several different possible outcomes that may result when one of the atmosphere's elements is altered.
Lunar highlands
Terrae.
Inner planets
Terrestrial planets.
Reverse polarity
A magnetic field opposite to that which exists at present.
Modified Mercalli intensity scale
A 12-point scale developed to evaluate earthquake intensity based on the amount of damage to various structures.
Zodiac
A band along the ecliptic containing the 12 constellations of the zodiac.
Seawall
A barrier constructed to prevent waves from reaching the area behind the wall. Its purpose is to defend property from the force of breaking waves.
Period
A basic unit of the geologic calendar that is a subdivision of an era. Periods may be divided into smaller units called epochs.
Equatorial low
A belt of low pressure lying near the equator and between the subtropical highs.
Wave-cut platform
A bench or shelf in the bedrock at sea level, cut by wave erosion.
Fold
A bent rock layer or series of layers that were originally horizontal and subsequently deformed.
Arctic (A) air mass
A bitterly cold air mass that forms over the frozen Arctic Ocean.
Magma
A body of molten rock found at depth, including any dissolved gases and crystals.
Convergent plate boundary
A boundary in which two plates move together, causing one of the slabs of lithosphere to be consumed into the mantle as it descends beneath on an overriding plate.
Transform plate boundary
A boundary in which two plates slide past one another without creating or destroying lithosphere.
Fault
A break in a rock mass along which movement has occurred.
Aurora
A bright display of ever-changing light caused by solar radiation interacting with the upper atmosphere in the region of the poles.
Shield volcano
A broad, gently sloping volcano built from fluid basaltic lavas.
High
A center of high pressure characterized by anticyclonic winds.
Low
A center of low pressure characterized by cyclonic winds.
Proton-proton chain
A chain of thermonuclear reactions by which nuclei of hydrogen are built up into nuclei of helium.
Volcanic island arc
A chain of volcanic islands generally located a few hundred kilometers from a trench where active subduction of one oceanic slab beneath another is occurring. Also known simply as island arc.
Hot spot track
A chain of volcanic structures produced as a lithospheric plate moves over a mantle plume.
Magnetic reversal
A change in Earth's magnetic field from normal to reverse or vice versa.
Ionic bond
A chemical bond between two oppositely charged ions formed by the transfer of valence electrons from one atom to the other.
Metallic bond
A chemical bond present in all metals that may be characterized as an extreme type of electron sharing in which the electrons move freely from atom to atom.
Covalent bond
A chemical bond produced by the sharing of electrons.
Pothole
A circular depression in a bedrock stream channel created by the abrasive action of particles swirling in fast- moving eddies.
Basin
A circular downfolded structure.
Country breeze
A circulation pattern characterized by a light wind blowing into a city from the surrounding countryside. It is best developed on clear and otherwise calm nights when the urban heat island is most pronounced.
Spectral class
A classification of a star according to the characteristics of its spectrum.
Fault scarp
A cliff created by movement along a fault. It represents the exposed surface of the fault prior to modification by weathering and erosion.
Tundra climate
A climate found almost exclusively in the Northern Hemisphere or at high altitudes in many mountainous regions. A treeless climatic realm of sedges, grasses, mosses, and lichens that is dominated by a long, bitterly cold winter.
Subarctic climate
A climate found north of the humid continental climate and south of the polar climate and characterized by bitterly cold winters and short, cool summers. Places within this climatic realm experience the highest annual temperature ranges on Earth.
Marine west coast climate
A climate found on windward coasts from latitudes 40° to 65° and dominated by maritime air masses. Winters are mild and summers are cool.
Humid subtropical climate
A climate generally located on the eastern side of a continent and characterized by hot, sultry summers and cool winters.
Dry climate
A climate in which yearly precipitation is not as great as the potential loss of water by evaporation. Also known as arid climate.
Dry-summer subtropical climate
A climate located on the west sides of continents between latitudes 30° and 45°. It is the only humid climate with a strong winter precipitation maximum.
Ice cap climate
A climate that has no monthly means above freezing and supports no vegetative cover except in a few scattered high mountain areas. This climate, with its perpetual ice and snow, is confined largely to the ice sheets of Greenland and Antarctica.
Tropical wet and dry
A climate that is transitional between the wet tropics and the subtropical steppes.
Middle cloud
A cloud occupying the height range from 2000 to 6000 meters.
Bright nebula
A cloud of glowing gas excited by ultraviolet radiation from hot stars.
Dark nebula
A cloud of interstellar dust that obscures the light of more distant stars and appears as an opaque curtain.
Nebula
A cloud of interstellar gas and/or dust.
Low cloud
A cloud that forms below a height of 2000 meters (1,200 miles).
High cloud
A cloud that normally has its base above 6000 meters (3,728 miles); the base may be lower in winter and at high-latitude locations.
Fog
A cloud with its base at or very near Earth's surface.
Emergent coast
A coast where land that was formerly below sea level has been exposed either because of crustal uplift or a drop in sea level or both.
Submergent coast
A coast with a form that is largely the result of the partial drowning of a former land surface either because of a rise of sea level or subsidence of the crust or both.
Glaze
A coating of ice on objects formed when supercooled rain freezes on contact.
Ventifact
A cobble or pebble polished and shaped by the sandblasting effect of wind.
Tectonic plate
A coherent unit of Earth's rigid outer layer that includes the crust and upper unit.
Lithospheric plate
A coherent unit of Earth's rigid outer layer that includes the crust and upper unit. Also known simply as a plate.
Polar (P) air mass
A cold air mass that forms in a high- latitude source region.
Protostar
A collapsing cloud of gas and dust destined to become a star.
Surf
A collective term for breakers; also, the wave activity in the area between the shoreline and the outer limit of breakers.
Stalagmite
A columnlike form that grows upward from the floor of a cavern.
Soil
A combination of mineral and organic matter, water, and air; the portion of the regolith that supports plant growth.
Interface
A common boundary where different parts of a system interact.
pH scale
A common measure of the degree of acidity or alkalinity of a solution, it is a logarithmic scale ranging from 0 to 14. A value of 7 denotes a neutral solution, values below 7 indicate greater acidity, and numbers above 7 indicate greater alkalinity.
Wash
A common term for a desert stream course that is typically dry except for brief periods immediately following a rain.
Highland climate
A complex pattern of climate conditions associated with mountains. Highland climates are characterized by large differences that occur over short distances.
Ionosphere
A complex zone of ionized gases that coincides with the lower portion of the thermosphere.
Basaltic composition
A compositional group of igneous rocks indicating that the rock contains substantial dark silicate minerals and calcium-rich plagioclase feldspar.
Granitic composition
A compositional group of igneous rocks that indicates a rock is composed almost entirely of light-colored silicates.
Economic mineral
A concentration of a mineral resource or reserve that can be profitably extracted from Earth.
Hot spot
A concentration of heat in the mantle capable of producing magma, which in turn extrudes onto Earth's surface. The intraplate volcanism that produced the Hawaiian islands is one example.
Prominence
A concentration of material above the solar surface that appears as a bright archlike structure.
Bowen's reaction series
A concept proposed by N. L. Bowen that illustrates the relationships between magma and the minerals crystallizing from it during the formation of igneous rocks.
Deep-sea fan
A cone-shaped deposit at the base of the continental slope. The sediment is transported to the fan by turbidity currents that follow submarine canyons.
Cone of depression
A cone-shaped depression in the water table immediately surrounding a well.
Volcanic cone
A cone-shaped structure built by successive eruptions of lava and/or pyroclastic materials.
Rock
A consolidated mixture of minerals.
Oceanic ridge system
A continuous elevated zone on the floor of all the major ocean basins and varying in width from 500 to 5000 kilometers (300-3000 miles). The rifts at the crests of ridges represent divergent plate boundaries.
Mid-ocean ridge
A continuous mountainous ridge on the floor of all the major ocean basins and varying in width from 500 to 5000 kilometers (300 to 3000 miles). The rifts at the crests of these ridges represent divergent plate boundaries.
Atoll
A continuous or broken ring of coral reef surrounding a central lagoon.
Absorption spectrum
A continuous spectrum with dark lines superimposed. Also known as dark-line spectrum.
Fissure
A crack in rock along which there is a distinct separation.
Point bar
A crescent-shaped accumulation of sand and gravel deposited on the inside of a meander.
Terrane
A crustal block bounded by faults, whose geologic history is distinct from the histories of adjoining crustal blocks.
Oxbow lake
A curved lake produced when a stream cuts off a meander.
Sunspot
A dark spot on the Sun, which is cool in contrast to the surrounding photosphere.
Numerical date
A date that specifies the actual number of years that have passed since an event occurred.
Crevasse
A deep crack in the brittle surface of a glacier.
Ocean basin
A deep submarine region that lies beyond the continental margins.
Tidal delta
A deltalike feature created when a rapidly moving tidal current emerges from a narrow inlet and slows, depositing its load of sediment.
Globule
A dense, dark nebula thought to be the birthplace of stars. Also known as glaze.
Placer
A deposit formed when heavy minerals are mechanically concentrated by currents, most commonly streams and waves. Placers are sources of gold, tin, platinum, diamonds, and other valuable minerals.
Blowout (deflation hollow)
A depression excavated by the wind in easily eroded deposits.
Sinkhole
A depression produced in a region where soluble rock has been removed by groundwater.
Wave base
A depth equal to one-half the wavelength measured from the still water level. Below this depth, water movement associated with a wave is negligible.
Climate
A description of aggregate weather conditions; the sum of all statistical weather information that helps describe a place or region.
Protoplanet
A developing planetary body that grows by the accumulation of planetesimals.
Psychrometer
A device consisting of two thermometers(wet bulb and dry bulb) that is rapidly whirled and, with the use of tables, yields the relative humidity and dew point.
Interior drainage
A discontinuous pattern of intermittent streams that do not flow to the ocean.
Photon
A discrete amount (quantum) of electromagnetic energy.
Turbidity current
A downslope movement of dense, sediment-laden water created when sand and mud on the continental shelf and slope are dislodged and thrown into suspension.
Rectangular pattern
A drainage pattern characterized by numerous right-angle bends that develops on jointed or fractured bedrock.
Rain shadow desert
A dry area on the lee side of a mountain range. Many middle-latitude deserts are of this type.
Alluvial fan
A fan-shaped deposit of sediment formed when a stream's slope is abruptly reduced.
Strike-slip fault
A fault along which the movement is horizontal.
Reverse fault
A fault in which the material above the fault plane moves up in relation to the material below.
Dip-slip fault
A fault in which the movement is parallel to the dip of the fault.
Normal fault
A fault in which the rock above the fault plane has moved down relative to the rock below.
Column
A feature found in caves that is formed when a stalactite and stalagmite join.
Positive-feedback mechanism
A feedback mechanism that enhances or drives change.
Negative-feedback mechanism
A feedback mechanism that tends to maintain a system as it is—that is, maintain the status quo.
Arkose
A feldspar-rich sandstone.
Black dwarf
A final state of evolution for a star, in which all of its energy sources are exhausted and it no longer emits radiation.
Granule
A fine structure visible on the solar surface caused by convective cells below.
Basalt
A fine-grained igneous rock of mafic composition.
Banded iron formations
A finely layered iron and silica- rich (chert) layer deposited mainly during the Precambrian.
Playa
A flat area on the floor of an undrained desert basin. Following heavy rain, the playa becomes a lake.
Terrace
A flat, benchlike structure produced by a stream, which was left elevated as the stream cut downward.
Spiral galaxy
A flattened, rotating galaxy with pinwheel- like arms of interstellar material and young stars winding out from its nucleus.
Spring
A flow of groundwater that emerges naturally at the ground surface.
Angiosperm
A flowering plant in which fruits contain the seeds.
Advection fog
A fog formed when warm, moist air is blown over a cool surface.
Anticline
A fold in sedimentary strata that resembles an arch; the opposite of syncline.
Cloud
A form of condensation best described as a dense concentration of suspended water droplets or tiny ice crystals.
Travertine
A form of limestone (CaCO3) that is deposited by hot springs or as a cave deposit.
Index fossil
A fossil that is associated with a particular span of geologic time.
Geyser
A fountain of hot water ejected periodically.
Joint
A fracture in rock along which there has been no movement.
Cold front
A front along which a cold air mass thrusts beneath a warmer air mass.
Warm front
A front along which a warm air mass overrides a retreating mass of cooler air.
Occluded front
A front formed when a cold front overtakes a warm front. It marks the beginning of the end of a middle-latitude cyclone.
Evolution (theory of)
A fundamental theory in biology and paleontology that sets forth the process by which members of a population of organisms come to differ from their ancestors. Organisms evolve by means of mutations, natural selection, and genetic factors. Modern species are descended from related but different species that lived in earlier times.
Barred spiral galaxy
A galaxy having straight arms extending from its nucleus.
Elliptical galaxy
A galaxy that is round or elliptical in outline. It contains little gas and dust, no disk or spiral arms, and few hot, bright stars.
Irregular galaxy
A galaxy that lacks symmetry.
Emission nebula
A gaseous nebula that derives its visible light from the fluorescence of ultraviolet light from a star in or near the nebula.
Humidity
A general term referring to water vapor in the air but not to liquid droplets of fog, cloud, or rain.
Mountain belt
A geographic area of roughly parallel and geologically connected mountain ranges developed as a result of plate tectonics.
Oil trap
A geologic structure that allows for significant amounts of oil and gas to accumulate.
Alpine glacier
A glacier confined to a mountain valley, which in most instances had previously been a stream valley. Also known as a valley glacier.
Piedmont glacier
A glacier that forms when one or more valley glaciers emerge from the confining walls of mountain valleys and spread out to create a broad sheet in the lowlands at the base of the mountains.
Food web
A group of interrelated food chains.
Gymnosperm
A group of seed-bearing plants that includes conifers and Ginkgo. The term means "naked seed," a reference to the unenclosed condition of the seeds.
Anthracite
A hard, metamorphic form of coal that burns clean and hot.
Anticyclone
A high-pressure center characterized by a clockwise flow of air in the Northern Hemisphere.
Pyroclastic flow
A highly heated mixture, largely of ash and pumice fragments, traveling down the flanks of a volcano or along the surface of the ground.
Dune
A hill or ridge of wind-deposited sand.
Proglacial lake
A lake created when a glacier acts as a dam, blocking the flow of a river or trapping glacial meltwater. The term refers to the position of such lakes just beyond the outer limits of a glacier.
Pluvial lake
A lake formed during a period of increased rainfall. During the Pleistocene epoch, this occurred in some nonglaciated regions during periods of ice advance elsewhere.
Air mass
A large body of air that is characterized by a sameness of temperature and humidity.
Midlatitude (middle-latitude) cyclone
A large center of low pressure with an associated cold front and often a warm front. Frequently accompanied by abundant precipitation.
Caldera
A large depression typically caused by collapse or ejection of the summit area of a volcano.
Cluster (star)
A large group of stars.
Supercontinent
A large landmass that contains all, or nearly all, of the existing continents.
Batholith
A large mass of igneous rock that formed when magma was emplaced at depth, crystallized, and subsequently exposed by erosion.
Heliosphere
A large region of space that extends far beyond Pluto's orbit, marked by solar winds and the Sun's magnetic field.
Accretionary wedge
A large wedge-shaped mass of sediment that accumulates in subduction zones. Here, sediment is scraped from the subducting oceanic plate and accreted to the overriding crustal block.
Red giant
A large, cool star of high luminosity; a star occupying the upper-right portion of the Hertzsprung- Russell diagram.
Exfoliation dome
A large, dome-shaped structure, usually composed of granite, formed by sheeting.
Shield
A large, relatively flat expanse of ancient metamorphic rock within the stable continental interior.
Ice shelf
A large, relatively flat mass of floating ice that forms where glacial ice flows into bays and extends seaward from the coast but remains attached to the land along one or more sides.
Pahoehoe flow
A lava flow with a smooth-to-ropey surface.
Hubble's law
A law that relates the distance to a galaxy and its velocity.
Outer core
A layer beneath the mantle about 2200 kilometers (1364 miles) thick that has the properties of a liquid.
Horizon
A layer in a soil profile. Also known as the soil horizon.
Temperature inversion
A layer in the atmosphere of limited depth where the temperature increases rather than decreases with height.
Desert pavement
A layer of coarse pebbles and gravel created when wind removed the finer material.
Soil horizon
A layer of soil that has identifiable characteristics produced by chemical weathering and other soil-forming processes.
Halocline
A layer of water in which there is a high rate of change in salinity in the vertical dimension.
Thermocline
A layer of water in which there is a rapid change in temperature in the vertical dimension.
Pycnocline
A layer of water in which there is a rapid change of density with depth.
Radiosonde
A lightweight package of weather instruments fitted with a radio transmitter and carried aloft by a balloon.
Isobar
A line drawn on a map connecting points of equal atmospheric pressure, usually corrected to sea level.
Syncline
A linear downfold in sedimentary strata; the opposite of anticline.
Continental rift
A linear zone along which continental lithosphere stretches and pulls apart. Its creation may mark the beginning of a new ocean basin.
Land breeze
A local wind blowing from land toward the water during the night in coastal areas.
Sea breeze
A local wind blowing from the sea during the afternoon in coastal areas.
Perched water table
A localized zone of saturation above the main water table created by an impermeable layer (aquiclude).
Rift valley
A long, narrow trough bounded by normal faults. It represents a region where divergence is taking place.
Subduction zone
A long, narrow zone where one lithospheric plate descends beneath another.
Meander
A looplike bend in the course of a stream.
Open cluster
A loosely formed group of stars of similar origin.
Barrier island
A low, elongate ridge of sand that parallels the coast.
Thrust fault
A low-angle reverse fault.
Cyclone
A low-pressure center characterized by a counterclockwise flow of air in the Northern Hemisphere.
Giant (star)
A luminous star of large radius.
Lunar breccia
A lunar rock formed when angular fragments and dust are welded together by the heat generated by the impact of a meteoroid.
Tropical rain forest
A luxuriant broadleaf evergreen forest; also, the name given the climate associated with this vegetation.
Era
A major division on the geologic calendar; eras are divided into shorter units called periods.
Transform fault
A major strike-slip fault that cuts through the lithosphere and accommodates motion between two plates.
Passive continental margin
A margin that consists of a continental shelf, continental slope, and continental rise. These margins are not associated with plate boundaries and therefore experience little volcanism and few earthquakes.
Tidal flat
A marshy or muddy area that is covered and uncovered by the rise and fall of the tide.
Iceberg
A mass of floating ice produced by a calving glacier. Usually 20 percent or less of the iceberg protrudes above the waterline.
Ice cap
A mass of glacial ice covering a high upland or plateau and spreading out radially.
Mantle plume
A mass of hotter-than-normal mantle material that ascends toward the surface, where it may lead to igneous activity. These plumes of solid yet mobile material may originate as deep as the core-mantle boundary.
Laccolith
A massive igneous body intruded between preexisting strata.
Black hole
A massive star that has collapsed to such a small volume that its gravity prevents the escape of all radiation.
Incised meander
A meandering channel that flows in a steep, narrow valley. Incised meanders form either when an area is uplifted or when base level drops.
Viscosity
A measure of a fluid's resistance to flow.
Permeability
A measure of a material's ability to transmit water.
Stellar parallax
A measure of stellar distance.
Intensity (earthquake)
A measure of the degree of earthquake shaking at a given locale, based on the amount of damage.
Temperature
A measure of the degree of hotness or coldness of a substance; a measure of the average kinetic energy of individual atoms or molecules in a substance.
Competence
A measure of the largest particle a stream can transport; a factor that is dependent on velocity.
Sheeting
A mechanical weathering process characterized by the splitting-off of slablike sheets of rock.
Slab pull
A mechanism that contributes to plate motion in which cool, dense oceanic crust sinks into the mantle and "pulls" the trailing lithosphere along.
Ridge push
A mechanism that may contribute to plate motion. It involves the oceanic lithosphere sliding down the oceanic ridge under the pull of gravity.
Mercury barometer
A mercury-filled glass tube in which the height of the mercury column is a measure of air pressure.
Equatorial system
A method of locating stellar objects much like the coordinate system used on Earth's surface.
Hydraulic fracturing
A method of opening up pore space in otherwise impermeable rocks, permitting natural gas to flow out into wells.
Vein deposit
A mineral filling a fracture or fault in a host rock. Such deposits have a sheetlike, or tabular, form.
Nonmetallic mineral resource
A mineral resource that is not a fuel or processed for the metals it contains.
Tenacity
A mineral's toughness or resistance to breaking or deforming.
Air
A mixture of many discrete gases, of which nitrogen and oxygen are most abundant, in which varying quantities of tiny solid and liquid particles are suspended.
Rock cycle
A model that illustrates the origin of the three basic rock types and the interrelatedness of Earth materials and processes.
Ozone
A molecule of oxygen that contains three oxygen atoms.
Moment magnitude
A more precise measure of earthquake magnitude than the Richter scale that is derived from the amount of displacement that occurs along a fault zone.
Fault-block mountain
A mountain formed by the displacement of rock along a fault.
Glacial trough
A mountain valley that has been widened, deepened, and straightened by a glacier.
Slide
A movement common to mass-wasting processes in which the material moving downslope remains fairly coherent and moves along a well-defined surface.
Spicule
A narrow jet of rising material in the solar chromosphere.
Arête
A narrow knifelike ridge separating two adjacent glaciated valleys.
Hogback
A narrow, sharp-crested ridge formed by the upturned edge of a steeply dipping bed of resistant rock.
Cavern
A naturally formed underground chamber or series of chambers most commonly produced by solution activity in limestone.
Ore deposit
A naturally occurring concentration of one or more metallic minerals that can be extracted economically.
Mineral
A naturally occurring, inorganic crystalline material with a unique chemical composition.
Detachment fault
A nearly horizontal fault that may extend hundreds of kilometers below the surface. Such a fault represents a boundary between rocks that exhibit ductile deformation and rocks that exhibit brittle deformation.
Globular cluster
A nearly spherically shaped group of densely packed stars.
Longshore current
A nearshore current that flows parallel to the shore.
Cap rock
A necessary part of an oil trap. The cap rock is impermeable and hence keeps upwardly mobile oil and gas from escaping at the surface.
Electron
A negatively charged subatomic particle that has a negligible mass and is found outside an atom's nucleus.
Trophic level
A nourishment level in a food chain. Plant and algae producers constitute the lowest level, followed by herbivores and a series of carnivores at progressively higher levels.
Magnitude (stellar)
A number given to a celestial object to express its relative brightness.
Monocline
A one-limbed flexure in strata. The strata are unusually flat-lying or very gently dipping on both sides of the monocline.
Jetties
A pair of structures extending into the ocean at the entrance to a harbor or river that are built for the purpose of protecting against storm waves and sediment deposition.
Rapids
A part of a stream channel in which the water suddenly begins flowing more swiftly and turbulently because of an abrupt steepening of the gradient.
Estuary
A partially enclosed coastal water body that is connected to the ocean. Salinity here is measurably reduced by the freshwater flow of rivers.
Col
A pass between mountain valleys where the headwalls of two cirques intersect.
Columnar joints
A pattern of cracks that form during cooling of molten rock to generate columns that are generally six sided.
Exotic stream
A permanent stream that traverses a desert and has its source in well-watered areas outside the desert.
Color
A phenomenon of light by which otherwise identical objects may be differentiated.
Liquefaction
A phenomenon, sometimes associated with earthquakes, in which soils and other unconsolidated materials containing abundant water are turned into a fluid- like mass that is not capable of supporting buildings.
Inclusion
A piece of one rock unit contained within another. Inclusions are used in relative dating. The rock mass adjacent to the one containing the inclusion must have been there first in order to provide the fragment.
Conduit
A pipelike opening through which magma moves toward Earth's surface. It terminates at a surface opening called a vent.
Andean-type plate margin
A plate boundary that generates continental volcanic arcs.
H-R diagram
A plot of stars according to their absolute magnitudes and spectral types. Stands for Hertzsprung- Russell diagram.
Stock
A pluton similar to but smaller than a batholith. Stony meteorite One of the three main categories of meteorites. Such meteorites are composed largely of silicate minerals with inclusions of other minerals.
Backswamp
A poorly drained area on a floodplain that results when natural levees are present.
Active continental margin
A portion of the seafloor adjacent to the continents that is usually narrow and consisting of highly deformed sediments. These margins occur where oceanic lithosphere is being subducted beneath the margin of a continent.
Proton
A positively charged subatomic particle found in the nucleus of an atom.
Fossil succession
A principle in which fossil organisms succeed one another in a definite and determinable order, so any time period can be recognized by its fossil content.
Cross-cutting
A principle of relative dating which says that a rock or fault is younger than any rock (or fault) through which it cuts.
Lateral continuity (principle of)
A principle which states that sedimentary beds originate as continuous layers that extend in all directions until they grade into a different type of sediment or thin out at the edge of a sedimentary basin.
Subduction erosion
A process in subduction zones in which sediment and rock are scraped off the bottom of the overriding plate and transported into the mantle.
Inertia
A property of matter that resists a change in its motion.
Horn
A pyramid-like peak formed by glacial action in three or more cirques surrounding a mountain summit.
Welded tuff
A pyroclastic rock composed of particles that have been fused together by the combination of heat still contained in the deposit after it has come to rest and by the weight of overlying material.
Cactolith
A quasi-horizontal chonolith composed of anastomosing ductoliths, whose distal ends curl like a harpolith, thin like a sphenolith, or bulge discordantly like an akmolith or ethmolith.
Seismic waves
A rapidly moving ocean wave generated by earthquake activity capable of inflicting heavy damage in coastal regions.
Cinder cone
A rather small volcano built primarily of pyroclastics ejected from a single vent. Also known as a scoria cone.
Barograph
A recording barometer.
Circular orbital motion
A reference to the movement of water in a wave. As a wave travels, energy is passed along by moving in a circle. The waveform advances but the water does not advance appreciably.
Rift zone
A region of Earth's crust along which divergence is taking place.
Kuiper belt
A region outside the orbit of Neptune where most short-period comets are thought to originate.
Divergent plate boundary
A region where the rigid plates are moving apart, typified by the mid-ocean ridges. Also known as a spreading center.
Reflection nebula
A relatively dense dust cloud in interstellar space that is illuminated by starlight.
Outwash plain
A relatively flat, gently sloping plain consisting of materials deposited by meltwater streams in front of the margin of an ice sheet.
Valley train
A relatively narrow body of stratified drift deposited on a valley floor by meltwater streams that issue from a valley glacier.
Debris flow
A relatively rapid type of mass wasting that involves a flow of soil and regolith containing a large amount of water. Also called mudflow.
Humid continental climate
A relatively severe climate characteristic of broad continents in the middle latitudes between approximately 40° and 50° north latitude. This climate is not found in the Southern Hemisphere, where the middle latitudes are dominated by the oceans.
Nonrenewable resource
A resource that forms or accumulates over such long time spans that it must be considered as fixed in total quantity.
Renewable resource
A resource that is virtually inexhaustible or that can be replenished over relatively short time spans.
Tombolo
A ridge of sand that connects an island to the mainland or to another island.
Lateral moraine
A ridge of till along the sides of an alpine glacier composed primarily of debris that fell to the glacier from the valley walls.
Medial moraine
A ridge of till formed when lateral moraines from two coalescing alpine glaciers join.
End moraine
A ridge of till marking a former position of the front of a glacier.
Igneous rock
A rock formed by the crystallization of molten magma.
Dome
A roughly circular upfolded structure similar to an anticline.
Octet rule
A rule which says that atoms combine in order that each may have the electron arrangement of a noble gas; that is, the outer energy level contains eight electrons.
Baymouth bar
A sandbar that completely crosses a bay, sealing it off from the open ocean.
Richter scale
A scale of earthquake magnitude based on the motion of a seismograph.
Enhanced Fujita intensity scale (EF-scale)
A scale originally developed by T. Theodore Fujita for classifying the severity of a tornado, based on the correlation of wind speed with the degree of destruction.
Magnetic time scale
A scale that shows the ages of magnetic reversals and is based on the polarity of lava flows of various ages.
Saffir-Simpson hurricane scale
A scale, from 1 to 5, used to rank the relative intensities of hurricanes.
Submarine canyon
A seaward extension of a valley that was cut on the continental shelf during a time when sea level was lower, or a canyon carved into the outer continental shelf, slope, and rise by turbidity currents.
Wave-cut cliff
A seaward-facing cliff along a steep shoreline formed by wave erosion at its base and mass wasting.
Distributary
A section of a stream that leaves the main flow.
Graded bed
A sediment layer that is characterized by a decrease in sediment size from bottom to top.
Breccia
A sedimentary rock composed of angular fragments that were lithified.
Conglomerate
A sedimentary rock composed of rounded, gravel-size particles.
Evaporite deposits
A sedimentary rock formed of material deposited from solution by evaporation of water.
Clastic rock
A sedimentary rock made of broken fragments of preexisting rock.
Seismic gap
A segment of an active fault zone that has not experienced a major earthquake over a span when most other segments have. Such segments are probable sites for future major earthquakes.
Secondary (S) wave
A seismic wave that involves oscillation perpendicular to the direction of propagation.
Magnetometer
A sensitive instrument used to measure the intensity of Earth's magnetic field at various points.
Bode's law
A sequence of numbers that approximates the mean distances of the planets from the Sun.
Main-sequence stars
A sequence of stars on the Hertzsprung-Russell diagram, containing the majority of stars, that runs diagonally from the upper left to the lower right.
Mohs scale
A series of 10 minerals used as a standard in determining hardness.
Transverse dunes
A series of long ridges oriented at right angles to the prevailing wind; these dunes form where vegetation is sparse and sand is very plentiful.
Planetary nebula
A shell of incandescent gas expanding from a star.
Cutoff
A short channel segment created when a river erodes through the narrow neck of land between meanders.
Groin
A short wall built at a right angle to the shore to trap moving sand.
Eyepiece
A short-focal-length lens used to enlarge the image in a telescope. The lens nearest the eye.
Dark silicate mineral
A silicate mineral that contains ions of iron and/or magnesium in its structure. Dark silicates are dark in color and have a higher specific gravity than nonferromagnesian silicates.
Light silicate mineral
A silicate mineral that lacks iron and/or magnesium. Light silicates are generally lighter in color and have lower specific gravities than dark silicates.
Esker
A sinuous ridge composed largely of sand and gravel deposited by a stream flowing in a tunnel beneath a glacier near its terminus.
Stationary front
A situation in which the surface position of a front does not move; the flow on either side of such a boundary is nearly parallel to the position of the front.
Axial precession
A slow motion of Earth's axis that traces out a cone over a period of 26,000 years. Also known simply as precession.
Solifluction
A slow, downslope flow of water-saturated materials common to permafrost areas.
Comet
A small body that generally revolves about the Sun in an elongated orbit.
Tarn
A small lake in a cirque.
Tornado
A small, very intense cyclonic storm with exceedingly high winds, most often produced along cold fronts in conjunction with severe thunderstorms.
Local wind
A small-scale wind produced by a locally generated pressure gradient. Examples include land and sea breezes and mountain and valley breezes.
Fumarole
A vent in a volcanic area from which fumes or gases escape.
Soil taxonomy
A soil classification system consisting of six hierarchical categories based on observable soil characteristics. The system recognizes 12 soil orders.
Immature soil
A soil lacking horizons.
Planetesimal
A solid celestial body that accumulated during the first stages of planetary formation. Planetesimals aggregated into increasingly larger bodies, ultimately forming the planets.
Snow
A solid form of precipitation produced by sublimination of water vapor.
Barchan dune
A solitary sand dune shaped like a crescent with its tips pointing downward.
Cenozoic era
A span on the geologic time scale beginning about 65 million years ago following the Mesozoic era.
Mesozoic era
A span on the geologic time scale between the Paleozoic and Cenozoic eras from about 248 million to 65 million years ago.
Paleozoic era
A span on the geologic time scale between the eons of the Precambrian and Mesozoic era from about 540 million to 248 million years ago.
Oort cloud
A spherical shell composed of comets that orbit the Sun at distances generally greater than 10,000 times the Earth-Sun distance.
Hot spring
A spring in which the water is 6-9°C (10-15°F) warmer than the mean annual air temperature of its locality.
Neutron star
A star of extremely high density, composed entirely of neutrons.
Nova
A star that explosively increases in brightness.
White dwarf
A star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size; believed to be near its final stage of evolution.
Eruptive variable
A star that varies in brightness.
Cepheid variable
A star whose brightness varies periodically because it expands and contracts. A type of pulsating star.
Kame
A steep-sided hill composed of sand and gravel that originates when sediment is collected in openings in stagnant glacial ice.
Fiord
A steep-sided inlet of the sea formed when a glacial trough was partially submerged.
Thunderstorm
A storm produced by a cumulonimbus cloud and always accompanied by lightning and thunder. It is of relatively short duration and usually accompanied by strong wind gusts, heavy rain, and sometimes hail.
Braided stream
A stream consisting of numerous intertwining channels.
Dendritic pattern
A stream system that resembles the pattern of a branching tree.
Ephemeral stream
A stream that is usually dry because it carries water only in response to specific episodes of rainfall. Most desert streams are of this type.
Drumlin
A streamlined asymmetrical hill composed of glacial till. The steep side of the hill faces the direction from which the ice advanced.
Volcanic bomb
A streamlined pyroclastic fragment ejected from a volcano while molten.
Coast
A strip of land that extends inland from the coastline as far as ocean-related features can be found.
Chemical bond
A strong attractive force that exists between atoms in a substance. It involves the transfer or sharing of electrons that allows each atom to attain a full valence shell.
Rip current
A strong narrow surface or near-surface current of short duration and high speed flowing seaward through the breaker zone at nearly right angles to the shore. It represents the return to the ocean of water that has been piled up on the shore by incoming waves.
Silicon-oxygen tetrahedron
A structure composed of four oxygen atoms surrounding a silicon atom that constitutes the basic building block of silicate minerals.
Coral reef
A structure formed in a warm, shallow, sunlit ocean environment that consists primarily of the calcite- rich remains of corals as well as the slimy secretions of algae and the hard parts of many other small organisms.
Cross-bedding
A structure in which relatively thin layers are inclined at an angle to the main bedding. Formed by currents of wind or water.
Breakwater
A structure protecting a nearshore area from breaking waves.
Pluton
A structure that results from the emplacement and crystallization of magma beneath the surface of Earth. Also known as an intrusion.
Neutron
A subatomic particle found in the nucleus of an atom. A neutron is electrically neutral and has a mass approximately that of a proton.
Abyssal zone
A subdivision of the benthic zone characterized by extremely high pressures, low temperatures, low oxygen, few nutrients, and no sunlight.
Asthenosphere
A subdivision of the mantle situated below the lithosphere. This zone of weak material exists below a depth of about 100 kilometers (60 miles) and in some regions extends as deep as 700 kilometers (430 miles). The rock within this zone is easily deformed. Also known as the low-velocity zone.
Guyot
A submerged flat-topped seamount.
Chemical compound
A substance formed by the chemical combination of two or more elements in definite proportions and usually having properties different from those of its constituent elements.
Compound
A substance formed by the chemical combination of two or more elements in definite proportions and usually having properties different from those of its constituent elements.
Element
A substance that cannot be decomposed into simpler substances by ordinary chemical or physical means.
Food chain
A succession of organisms in an ecological community through which food energy is transferred from producers through herbivores and on to one or more carnivores.
Solar flare
A sudden and tremendous eruption in the solar chromosphere.
Flare
A sudden brightening of an area on the Sun.
Lightning
A sudden flash of light generated by the flow of electrons between oppositely charged parts of a cumulonimbus cloud or between the cloud and the ground.
Unconformity
A surface that represents a break in the rock record, caused by erosion or nondeposition.
Köppen classification
A system for classifying climates devised by Wladimir Köppen that is based on mean monthly and annual values of temperature and precipitation.
Open system
A system in which both matter and energy flow into and out of the system. Most natural systems are of this type.
Trellis pattern
A system of streams in which nearly parallel tributaries occupy valleys cut in folded strata.
Radial pattern
A system of streams running in all directions away from a central elevated structure, such as a volcano.
Closed system
A system that is self-contained with regard to matter—that is, no matter enters or leaves.
Sill
A tabular igneous body that was intruded parallel to the layering of preexisting rock.
Dike
A tabular-shaped intrusive igneous feature that cuts through the surrounding rock.
Radio telescope
A telescope designed to make observations in radio wavelengths.
Reflecting telescope
A telescope that concentrates light from distant objects by using a concave mirror.
Refracting telescope
A telescope that uses a lens to bend and concentrate the light from distant objects.
Playa lake
A temporary lake in a playa.
Hypothesis
A tentative explanation that is tested to determine whether it is valid.
Vesicular texture
A term applied to igneous rocks that contain small cavities called vesicles, which are formed when gases escape from lava.
Subsoil
A term applied to the B horizon of a soil profile. Subtropical high Not a continuous belt of high pressure but rather several semipermanent, anticyclonic centers characterized by subsidence and divergence located roughly between latitudes 25° and 35°.
Hadean eon
A term found on some versions of the geologic time scale. It refers to the earliest interval (eon) of Earth history and ended 4 billion years ago.
Model
A term often used synonymously with hypothesis but that is less precise because it is sometimes used to describe a theory as well.
Concordant
A term used to describe intrusive igneous masses that form parallel to the bedding of the surrounding rock.
Discordant
A term used to describe plutons that cut across existing rock structures, such as bedding planes.
Glassy texture
A term used to describe the texture of certain igneous rocks, such as obsidian, that contain no crystals.
Fine-grained texture
A texture of igneous rocks in which the crystals are too small for individual minerals to be distinguished with the unaided eye.
Foliation
A texture of metamorphic rocks that gives the rock a layered appearance.
Astronomical theory
A theory of climatic change first developed by Yugoslavian astronomer Milutin Milankovitch. It is based on changes in the shape of Earth's orbit, variations in the obliquity of Earth's axis, and the wobbling of Earth's axis.
Collision-coalescence process
A theory of raindrop formation in warm clouds (above 0°C) in which large cloud droplets (giants) collide and join together with smaller droplets to form a raindrop. Opposite electrical charges may bind the cloud droplets together.
Paradigm
A theory that is held with a very high degree of confidence and is comprehensive in scope.
Continental drift
A theory that originally proposed that the continents are rafted about. It has essentially been replaced by the plate tectonics theory.
Bergeron process
A theory that relates the formation of precipitation to supercooled clouds, freezing nuclei, and the different saturation levels of ice and liquid water.
Glacier
A thick mass of ice originating on land from the compaction and recrystallization of snow that shows evidence of past or present flow.
Rime
A thin coating of ice on objects produced when supercooled fog droplets freeze on contact.
Lunar regolith
A thin, gray layer on the surface of the Moon, consisting of loosely compacted, fragmented material believed to have been formed by repeated meteoritic impacts.
Diurnal tidal pattern
A tidal pattern exhibiting one high tide and one low tide during a tidal day; a daily tide.
Semidiurnal tidal pattern
A tidal pattern exhibiting two high tides and two low tides per tidal day with small inequalities between successive highs and successive lows; a semi-daily tide.
Mixed tidal pattern
A tidal pattern exhibiting two high tides and two low tides per tidal day, with a large inequality in high water heights, low water heights, or both. Coastal locations that experience such a tidal pattern may also show alternating periods of diurnal and semidiurnal tidal patterns. Also called mixed semidiurnal.
Half graben
A tilted fault block in which the higher side is associated with mountainous topography and the lower side is a basin that fills with sediment.
Great Oxygenation Event
A time about 2.5 billion years ago, when a significant amount of oxygen appeared in the atmosphere.
Sidereal month
A time period based on the revolution of the Moon around Earth with respect to the stars.
Outlet glacier
A tongue of ice that normally flows rapidly outward from an ice cap or ice sheet, usually through mountainous terrain to the sea.
Karst
A topography consisting of numerous depressions called sinkholes.
Yazoo tributary
A tributary that flows parallel to the main stream because a natural levee is present.
Hanging valley
A tributary valley that enters a glacial trough at a considerable height above its floor.
Hurricane
A tropical cyclonic storm having winds in excess of 119 kilometers (74 miles) per hour.
Lava tube
A tunnel in hardened lava that acts as a horizontal conduit for lava flowing from a volcanic vent. Lava tubes allow fluid lavas to advance great distances.
Turbidite
A turbidity current deposit characterized by graded bedding.
Aa flow
A type of lava flow that has a jagged, blocky surface.
Compaction
A type of lithification in which the weight of overlying material compresses more deeply buried sediment. It is most important in the fine-grained sedimentary rocks such as shale.
Flow
A type of movement common to mass-wasting processes in which water-saturated material moves downslope as a viscous fluid.
Fall
A type of movement common to mass-wasting processes that refers to the free falling of detached individual pieces of any size.
Primary (P) wave
A type of seismic wave that involves alternating compression and expansion of the material through which it passes.
Ductile deformation
A type of solid state flow that produces a change in the size and shape of a rock body without fracturing. Occurs at depths where temperatures and confining pressures are high.
Disconformity
A type of unconformity in which the beds above and below are parallel.
Cryovolcanism
A type of volcanism that results from the eruption of magmas derived from the partial melting of ice.
Epoch
A unit of the geologic calendar that is a subdivision of a period.
Graben
A valley formed by the downward displacement of a fault-bounded block.
Pulsating variable
A variable star that pulsates in size and luminosity.
Mesocyclone
A vertical cylinder of cyclonically rotating air (3 to 10 kilometers in diameter) that develops in the updraft of a severe thunderstorm and that often precedes the development of damaging hail or tornadoes.
Soil profile
A vertical section through a soil, showing its succession of horizons and the underlying parent material.
Pegmatite
A very coarse-grained igneous rock (typically granite) commonly found as a dike associated with a large mass of plutonic rock that has smaller crystals. Crystallization in a water-rich environment is believed to be responsible for the very large crystals.
Supergiant
A very large star of high luminosity.
Ice sheet
A very large, thick mass of glacial ice flowing outward in all directions from one or more accumulation centers.
Abyssal plain
A very level area of the deep-ocean floor, usually lying at the foot of the continental rise.
Parasitic cone
A volcanic cone that forms on the flank of a larger volcano.
Obsidian
A volcanic glass of felsic composition.
Composite cone
A volcano composed of both lava flows and pyroclastic material. Also known as a stratovolcano.
Tornado watch
A warning issued for areas of about 65,000 square kilometers (25,000 square miles), indicating that conditions are such that tornadoes may develop; it is intended to alert people to the possibility of tornadoes.
Tornado warning
A warning issued when a tornado has actually been sighted in an area or is indicated by radar.
Wave of oscillation
A water wave in which the waveform advances as the water particles move in circular orbits.
Marine terrace
A wave-cut platform that has been exposed above sea level.
Carbonic acid
A weak acid formed when carbon dioxide is dissolved in water. It plays an important role in chemical weathering.
Artesian well
A well in which the water rises above the level where it was initially encountered.
Theory
A well-tested and widely accepted view that explains certain observable facts.
Cassini division
A wide gap in the ring system of Saturn between the A ring and the B ring.
Chinook
A wind blowing down the leeward side of a mountain and warming by compression.
Prevailing wind
A wind that consistently blows from one direction more than from another.
Geostrophic wind
A wind, usually above a height of 600 meters (2000 feet), that blows parallel to the isobars.
Suture
A zone along which two crustal fragments are jointed together. For example, following a continental collision, the two continental blocks are sutured together.
Eye
A zone of scattered clouds and calm averaging about 20 kilometers (12 miles) in diameter at the center of a hurricane.
Dark-line spectrum
Absorption spectrum.
Wind
Air flowing horizontally with respect to Earth's surface.
Unstable air
Air that does not resist vertical displacement. If it is lifted, its temperature will not cool as rapidly as the surrounding environment, so it will continue to rise on its own.
Absolute instability
Air that has a lapse rate greater than the dry adiabatic rate.
Stable air
Air that resists vertical displacement. If it is lifted, adiabatic cooling will cause its temperature to be lower than the surrounding environment; if it is allowed, it will sink to its original position.
Absolute stability
Air with a lapse rate less than the wet adiabatic rate.
Air pollutants
Airborne particles and gases that occur in concentrations that endanger the health and well-being of organisms or disrupt the orderly functioning of the environment.
Phytoplankton
Algal plankton, which are the most important community of primary producers in the ocean.
Mineral resource
All discovered and undiscovered deposits of a useful mineral that can be extracted now or at some time in the future.
Rock structure
All features created by the processes of deformation from minor fractures in bedrock to a major mountain chain. Also known as geologic structure.
Precambrian
All geologic time prior to the Paleozoic era.
Valley glacier
Alpine glacier.
Ptolemaic system
An Earth-centered system of the universe. Pulsar A variable radio source of small size that emits radio pulses in very regular periods.
Talus
An accumulation of rock debris at the base of a cliff.
Delta
An accumulation of sediment formed where a stream enters a lake or an ocean.
Beach
An accumulation of sediment found along the landward margin of the ocean or a lake.
Continental (c) air mass
An air mass that forms over land; it is normally relatively dry.
Maritime (m) air mass
An air mass that originates over the ocean. These air masses are relatively humid.
Glacial drift
An all-embracing term for sediments of glacial origin, no matter how, where, or in what shape they were deposited. Also known simply as drift.
Reserve
An already identified deposit from which minerals can be extracted profitably.
Cirque
An amphitheater-shaped basin at the head of a glaciated valley produced by frost wedging and plucking.
Right ascension
An angular distance measured eastward along the celestial equator from the vernal equinox. Used with declination in a coordinate system to describe the position of celestial bodies.
Constellation
An apparent group of stars originally named for mythical characters. The sky is presently divided into 88 constellations.
Bajada
An apron of sediment along a mountain front created by the coalescence of alluvial fans.
Confined aquifer
An aquifer that has impermeable layers (aquitards) both above and below.
Sea arch
An arch formed by wave erosion when caves on opposite sides of a headland unite.
Circum-Pacific belt
An area approximately 40,000 kilometers (24,000 miles) in length surrounding the basin of the Pacific Ocean where oceanic lithosphere is continually subducted beneath the surrounding continental plates causing most of Earth's largest earthquakes.
Snowfield
An area where snow persists year-round.
Ion
An atom or a molecule that possesses an electrical charge.
Trench
An elongated depression in the seafloor produced by bending of oceanic crust during subduction. Also known as deep-ocean trench.
Spit
An elongated ridge of sand that projects from the land into the mouth of an adjacent bay.
Horst
An elongated, uplifted block of crust bounded by faults.
Recessional moraine
An end moraine formed as the ice front stagnated during glacial retreat.
La Niña
An episode of strong trade winds and unusually low sea-surface temperatures in the central and eastern Pacific. The opposite of El Niño.
Pleistocene epoch
An epoch of the Quaternary period beginning about 1.8 million years ago and ending about 10,000 years ago. Best known as a time of extensive continental glaciation.
Fissure eruption
An eruption in which lava is extruded from narrow fractures or cracks in the crust.
Mass extinction
An event in which a large percentage of species become extinct.
Supernova
An exploding star that increases in brightness many thousands of times.
Oceanic plateau
An extensive region on the ocean floor composed of thick accumulations of pillow basalts and other mafic rocks that in some cases exceed 30 kilometers (20 miles) in thickness.
Glacial erratic
An ice-transported boulder that was not derived from bedrock near its present site.
Stalactite
An icicle-like structure that hangs from the ceiling of a cavern.
Massive
An igneous pluton that is not tabular in shape.
Peridotite
An igneous rock of ultramafic composition thought to be abundant in the upper mantle.
Coarse-grained texture
An igneous rock texture in which the crystals are roughly equal in size and large enough so that individual minerals can be identified with the unaided eye.
Pyroclastic texture
An igneous rock texture resulting from the consolidation of individual rock fragments that are ejected during a violent volcanic eruption. Also known as fragmental texture.
Porphyritic texture
An igneous texture consisting of large crystals embedded in a matrix of much smaller crystals.
Celestial sphere
An imaginary hollow sphere upon which the ancients believed the stars were hung and carried around Earth.
Divide
An imaginary line that separates the drainage of two streams; often found along a ridge.
Parcel
An imaginary volume of air enclosed in a thin elastic cover. Typically it is considered to be a few hundred cubic meters in volume and is assumed to act independently of the surrounding air.
Hygrometer
An instrument designed to measure relative humidity.
Spectroscope
An instrument for directly viewing the spectrum of a light source.
Aneroid barometer
An instrument for measuring air pressure that consists of evacuated metal chambers that are very sensitive to variations in air pressure.
Barometer
An instrument that measures atmospheric pressure.
Seismograph
An instrument that records earthquake waves. Also known as a seismometer.
Echo sounder
An instrument used to determine the depth of water by measuring the time interval between emission of a sound signal and the return of its echo from the bottom.
Wind vane
An instrument used to determine wind direction.
Anemometer
An instrument used to determine wind speed. Also known as a cup anemometer.
Earth system science
An interdisciplinary study that seeks to examine Earth as a system composed of numerous interacting parts or subsystems.
Strain
An irreversible change in the shape and size of a rock body that is caused by stress.
Star dune
An isolated hill of sand that exhibits a complex form and develops where wind directions are variable.
Sea stack
An isolated mass of rock standing just offshore, produced by wave erosion of a headland.
Inselberg
An isolated mountain remnant characteristic of the late stage of erosion in an arid region.
Seamount
An isolated volcanic peak that rises at least 1000 meters (3000 feet) above the deep-ocean floor.
Volcanic neck
An isolated, steep-sided, erosional remnant consisting of lava that once occupied the vent of a volcano. Also known as a plug.
Daughter product
An isotope that results from radioactive decay.
Well
An opening bored into the zone of saturation.
Crystal
An orderly arrangement of atoms.
Eukaryotes
An organism whose genetic material is enclosed in a nucleus; plants, animals, and fungi are eukaryotes.
Nonconformity
An unconformity in which older metamorphic or intrusive igneous rocks are overlain by younger sedimentary strata.
Angular unconformity
An unconformity in which the strata below dip at an angle different from that of the beds above.
Ground moraine
An undulating layer of till deposited as the ice front retreats.
Continuous spectrum
An uninterrupted band of light emitted by an incandescent solid, liquid, or gas under pressure.
Zooplankton
Animal plankton.
Polar high
Anticyclones that are assumed to occupy the inner polar regions and are believed to be thermally induced, at least in part.
Fracture zone
Any break or rupture in rock along which no appreciable movement has taken place.
Disseminated deposit
Any economic mineral deposit in which the desired mineral occurs as scattered particles in the rock but in sufficient quantity to make the deposit an ore.
Hard stabilization
Any form of artificial structure built to protect a coast or to prevent the movement of sand along a beach. Examples include groins, jetties, breakwaters, and seawalls.
Ray (lunar)
Any of a system of bright elongated streaks, sometimes associated with a crater on the Moon.
Terrestrial planets
Any of the Earth-like planets, including Mercury, Venus, Mars, and Earth. Also known as the inner planets.
Silicate
Any one of numerous minerals that have the oxygen and silicon tetrahedron as their basic structure.
Meteorite
Any portion of a meteoroid that survives its traverse through Earth's atmosphere and strikes Earth's surface.
System
Any size group of interacting parts that form a complex whole.
Spheroidal weathering
Any weathering process that tends to produce a spherical shape from an initially blocky shape.
Polar wandering
As a result of paleomagnetic studies in the 1950s, researchers proposed that either the magnetic poles migrated greatly through time or the continents had gradually shifted their positions.
Low-velocity zone
Asthenosphere.
Astronomical unit (AU)
Average distance from Earth to the Sun; 1.5×10⁸ kilometers (93×10⁶ miles).
Pillow lava
Basaltic lava that solidifies in an underwater environment and develops a structure that resembles a pile of pillows.
Eruption column
Buoyant plumes of hot, ash-laden gases that can extend thousands of meters into the atmosphere.
Tropical storm
By international agreement, a tropical cyclone with maximum winds between 61 and 119 kilometers (38 and 74 miles) per hour.
Tropical depression
By international agreement, a tropical cyclone with maximum winds that do not exceed 61 kilometers (38 miles) per hour.
Dwarf planets
Celestial bodies that orbit stars and are massive enough to be spherical but have not cleared their neighboring regions of planetesimals.
Prokaryotes
Cells or organisms such as bacteria whose genetic material is not enclosed in a nucleus.
Contact metamorphism
Changes in rock caused by the heat from a nearby magma body. Also known as thermal metamorphism.
Cosmological red shift
Changes in the spectra of galaxies which indicate that they are moving away from the Milky Way as a result of the expansion of space.
Scoria cone
Cinder cone.
Clouds of vertical development
Clouds that have their bases in the low-height range but extend upward into the middle or high altitudes.
Freezing rain
Glaze.
Bar
Common term for sand and gravel deposits in a stream channel.
Isostatic adjustment
Compensation of the lithosphere when weight is added or removed. When weight is added, the lithosphere responds by subsiding, and when weight is removed, there is uplift.
Stratovolcano
Composite cone.
Hygroscopic nuclei
Condensation nuclei having a high affinity for water, such as salt particles.
Thermal metamorphism
Contact metamorphism.
Adiabatic temperature change
Cooling or warming of air caused when air is allowed to expand or is compressed, not because heat is added or subtracted.
Filaments
Dark, thin streaks that appear across the bright solar disk.
Brittle deformation
Deformation that involves the fracturing of rock. Associated with rocks near the surface.
Loess
Deposits of windblown silt, lacking visible layers, generally buff-colored, and capable of maintaining a nearly vertical cliff.
Kettle holes
Depressions created when blocks of ice became lodged in glacial deposits and subsequently melted.
Impact craters
Depressions result from collisions with bodies such as asteroids and comets.
Compressional stress
Differential stress that shortens a rock body.
Fault creep
Displacement along a fault that is so slow and gradual that little seismic activity occurs.
Spreading center
Divergent plate boundary.
Rain
Drops of water that fall from clouds that have a diameter of at least 0.5 millimeter (0.02 inch).
Arid climate
Dry climate.
Barchanoid dune
Dunes forming scalloped rows of sand oriented at right angles to the wind. This form is intermediate between isolated barchans and extensive waves of transverse dunes.
Parabolic dunes
Dunes that resemble barchans, except that their tips point into the wind; they often form along coasts that have strong onshore winds, abundant sand, and vegetation that partly covers the sand.
Crystal settling
During the crystallization of magma, the settling of the earlier-formed minerals that are denser than the liquid portion to the bottom of the magma chamber.
Interstellar matter
Dust and gases found between stars. Intertidal zone The area where land and sea meet and overlap; the zone between high and low tides.
Correlation
Establishing the equivalence of rocks of similar age in different areas.
Environment
Everything that surrounds and influences an organism.
Degenerate matter
Extremely dense solar material created by electrons being displaced inward toward an atom's nucleus.
Flood basalts
Flows of basaltic lava that issue from numerous cracks or fissures and commonly cover extensive areas to thicknesses of hundreds of meters.
Precipitation fog
Fog formed when rain evaporates as it falls through a layer of cool air.
Steam fog
Fog having the appearance of steam, produced by evaporation from a warm water surface into the cool air above.
Radiation fog
Fog resulting from radiation heat loss by Earth.
Differential stress
Forces that are unequal in different directions.
Sleet
Frozen or semifrozen rain formed when raindrops freeze as they pass through a layer of cold air.
Sea ice
Frozen seawater that is associated with polar regions. The area covered by sea ice expands in winter and shrinks in summer.
Volatiles
Gaseous components of magma dissolved in melt. Volatiles readily vaporize (form a gas) at surface pressures.
Selective absorbers
Gases that absorb and emit radiation only in certain wavelengths.
Fossil fuel
General term for any hydrocarbon that may be used as a fuel, including coal, oil, natural gas, bitumen from tar sands, and shale oil.
Deformation
General term for the processes of folding, faulting, shearing, compression, or extension of rocks as the result of various natural forces.
Rock flour
Ground-up rock produced by the grinding effect of a glacier.
Galactic cluster
Groups of gravitationally bound galaxies that sometimes contain thousands of galaxies.
Scoria
Hardened lava that has retained the vesicles produced by escaping gases.
Advection
Horizontal convective motion, such as wind.
White frost
Ice crystals instead of dew that form on surfaces when the dew point is below freezing.
Extrusive
Igneous activity that occurs outside the crust.
Intraplate volcanism
Igneous activity that occurs within a tectonic plate away from plate boundaries.
Intrusive
Igneous rock that formed below Earth's surface.
Ultramafic composition
Igneous rocks composed mainly of iron and magnesium-rich minerals.
Mafic
Igneous rocks with a low silica content and a high iron-magnesium content.
Aquitard
Impermeable beds that hinder or prevent groundwater movement.
Doppler radar
In addition to performing the tasks of conventional radar, a new generation of weather radar that can detect motion directly and hence greatly improve tornado and severe storm warnings.
Phenocryst
In an igneous rock with a porphyritic texture, a conspicuously large crystal embedded in a matrix of finer- grained crystals called the groundmass.
Polar easterlies
In the global pattern of prevailing winds, winds that blow from the polar high toward the subpolar low. These winds, however, should not be thought of as persistent winds, such as the trade winds.
Nuée ardente
Incandescent volcanic debris buoyed up by hot gases that moves downslope in an avalanche fashion.
Outer planets
Jovian planets.
Continent
Large, continuous areas of land that include the adjacent continental shelf and islands that are structurally connected to the mainland.
Conformable
Layers of rock that were deposited without interruption.
Original horizontality
Layers of sediments are generally deposited in a horizontal or nearly horizontal position.
Frontal wedging
Lifting of air resulting when cool air acts as a barrier over which warmer, lighter air will rise.
Isotherms
Lines connecting points of equal temperature.
Longitudinal (seif) dunes
Long ridges of sand oriented parallel to the prevailing wind; these dunes form where sand supplies are limited.
Subpolar low
Low pressure located at about the latitudes of the Arctic and Antarctic Circles. In the Northern Hemisphere the low takes the form of individual oceanic cells; in the Southern Hemisphere there is a deep and continuous trough of low pressure.
Lava
Magma that reaches Earth's surface.
Meteor shower
Many meteors appearing in the sky, caused by Earth intercepting a swarm of meteoritic particles.
Density
Mass per unit volume of a substance, usually expressed as grams per cubic centimeter (g/cm3).
Organic matter
Material composed of organic compounds consisting of the remains of once-living plants and animals and their waste products in the environment.
Decompression melting
Melting that occurs as rock ascends due to a drop in confining pressure.
Nonfoliated texture
Metamorphic rocks that do not exhibit foliation.
Regional metamorphism
Metamorphism associated with large-scale mountain-building processes.
Carbonate group
Mineral group whose members contain the carbonate ion (CO22-) and one or more kinds of positive ions. Calcite is a common example.
Nonsilicates
Mineral groups that lack silicas in their structures and account for less than 10 percent of Earth's crust.
Conditional instability
Moist air with a lapse rate between the dry and wet adiabatic rates.
Orographic lifting
Mountains acting as barriers to the flow of air, forcing the air to ascend. The air cools adiabatically, and clouds and precipitation may result.
Continental volcanic arc
Mountains formed in part by igneous activity associated with the subduction of oceanic lithosphere beneath a continent.
Compressional mountains
Mountains in which great horizontal forces have shortened and thickened the crust. Most major mountain belts are of this type.
Thermohaline circulation
Movements of ocean water caused by density differences brought about by variations in temperature and salinity.
Lahar
Mudflows on the slopes of volcanoes that result when unstable layers of ash and debris become saturated and flow downslope, usually following stream channels.
Geothermal energy
Natural steam used for power generation.
Hail
Nearly spherical ice pellets having concentric layers and formed by the successive freezing of layers of water.
Iron meteorite
One of the three main categories of meteorites. This group is composed largely of iron with varying amounts of nickel (5-20 percent). Most meteorite finds are irons.
Stony-iron meteorite
One of the three main categories of meteorites. This group, as the name implies, is a mixture of iron and silicate minerals.
Steppe
One of the two types of dry climate. A marginal and more humid variant of the desert that separates it from bordering humid climates. Also known as semiarid.
Desert
One of the two types of dry climate; the driest of the dry climates. Also known as arid.
Cirrus
One of three basic cloud forms; also one of the three high cloud types. They are thin, delicate ice-crystal clouds often appearing as veil-like patches or thin, wispy fibers.
Cumulus
One of three basic cloud forms; also the name given one of the clouds of vertical development. Cumulus are billowy individual cloud masses that often have flat bases.
Stratus
One of three basic cloud forms; also, the name given one of the flow clouds. They are sheets or layers that cover much or all of the sky.
Cementation
One way in which sedimentary rocks are lithified. As material precipitates from water that percolates through the sediment, open spaces are filled, and particles are joined into a solid mass.
Fossil magnetism
Paleomagnetism.
Strata
Parallel layers of sedimentary rock.
Plankton
Passively drifting or weakly swimming organisms that cannot move independently of ocean currents. Includes microscopic algae, protozoa, jellyfish, and larval forms of many animals.
Nekton
Pelagic organisms that can move independently of ocean currents by swimming or other means of propulsion.
Tide
Periodic change in the elevation of the ocean surface.
Relative dating
Placing rocks in their proper sequence or order to determine the chronological order of events.
Primary pollutants
Pollutants emitted directly from identifiable sources.
Secondary pollutants
Pollutants that are produced in the atmosphere by chemical reactions that occur among primary pollutants.
Fragmental texture
Pyroclastic texture.
Elements of weather and climate
Quantities or properties of the atmosphere that are measured regularly and that are used to express the nature of weather and climate.
Ultraviolet
Radiation with a wavelength from 0.2 to 0.4 micrometer.
Visible light
Radiation with a wavelength from 0.4 to 0.7 micrometer.
Infrared
Radiation with a wavelength from 0.7 to 200 micrometers.
Acid precipitation
Rain or snow with a pH value that is less than the pH of unpolluted precipitation.
Jet stream
Swift (120-240 kilometers per hour), high- altitude winds.
Variable stars
Red giants that overshoot equilibrium and then alternately expand and contract.
Habit
Refers to the common or characteristic shape of a crystal, or aggregate of crystals. Also known as crystal form and crystal shape.
Microcontinents
Relatively small fragments of continental crust that may lie above sea level, such as the island of Madagascar, or may be submerged, as exemplified by the Campbell Plateau located near New Zealand.
Elastic deformation
Rock deformation in which the rock will return to nearly its original size and shape when the stress is removed.
Detrital sedimentary rock
Rock formed from the accumulation of material that originated and was transported in the form of solid particles derived from both mechanical and chemical weathering.
Sedimentary rock
Rock formed from the weathered products of preexisting rocks that have been transported, deposited, and lithified.
Aquifer
Rock or soil through which groundwater moves easily.
Metamorphic rock
Rocks formed by the alteration of preexisting rock deep within Earth (but still in the solid state) by heat, pressure, and/or chemically active fluids.
Manganese nodules
Rounded lumps of hydrogenous sediment scattered on the ocean floor, consisting mainly of manganese and iron and usually containing small amounts of copper, nickel, and cobalt.
Glacial striations
Scratches and grooves on bedrock caused by glacial abrasion.
Striations (glacial)
Scratches or grooves in a bedrock surface caused by the grinding action of a glacier and its load of sediment.
Ultimate base level
Sea level; the lowest level to which stream erosion could lower the land.
Biogenous sediment
Seafloor sediments consisting of material of marine-organic origin.
Hydrogenous sediment
Seafloor sediments consisting of minerals that crystallize from seawater. An important example is manganese nodules.
Terrigenous sediment
Seafloor sediments derived from terrestrial weathering and erosion.
Monsoon
Seasonal reversal of wind direction associated with large continents, especially Asia. In winter, the wind blows from land to sea; in summer, from sea to land.
Shore
Seaward of the coast, a zone that extends from the highest level of wave action during storms to the lowest tide level.
Biochemical sedimentary rock
Sediment that forms when material dissolved in water is precipitated by water- dwelling organisms. Shells are common examples.
Bed load
Sediment that is carried by a stream along the bottom of its channel.
Foreshocks
Small earthquakes that often precede a major earthquake.
Meteoroid
Small solid particles that have orbits in the solar system.
Aftershocks
Smaller earthquakes that follow the main earthquake.
Lake-effect snow
Snow showers associated with a cP air mass to which moisture and heat are added from below as the air mass traverses a large and relatively warm lake (such as one of the Great Lakes), rendering the air mass humid and unstable.
Residual soil
Soil developed directly from the weathering of the bedrock below.
Transported soil
Soils that form on unconsolidated deposits.
Diffused light
Solar energy scattered and reflected in the atmosphere that reaches Earth's surface in the form of diffuse blue light from the sky.
Small solar system bodies
Solar system objects not classified as planets or moons that include dwarf planets, asteroids, comets, and meteoroids.
Freezing nuclei
Solid particles that serve as cores for the formation of ice crystals.
Black carbon
Soot generated by combustion processes and fires.
Energy levels
Spherically shaped, negatively charged zones that surround the nucleus of an atom. Also known as principal shells.
Population II
Stars poor in atoms heavier than helium. Nearly always relatively old stars found in the halo, globular clusters, or nuclear bulge.
Population I
Stars rich in atoms heavier than helium. Nearly always relatively young stars found in the disk of the galaxy.
Shear
Stress that causes two adjacent parts of a body to slide past one another.
Confining pressure
Stress that is applied uniformly in all directions.
Stromatolite
Structures that are deposited by algae and consist of layered mounds of calcium carbonate.
Solar winds
Subatomic particles ejected at high speed from the solar corona.
Phanerozoic eon
That part of geologic time represented by rocks containing abundant fossil evidence. The eon extending from the end of the Proterozoic eon (about 540 million years ago) to the present.
Dissolved load
That portion of a stream's load that is carried in solution.
Foreshore
That portion of the shore lying between the normal high and low water marks; the intertidal zone.
Mantle
The 2900-kilometer- (1800-mile-) thick layer of Earth located below the crust.
Tsunami
The Japanese word for a seismic sea wave.
Jovian planets
The Jupiter-like planets: Jupiter, Saturn, Uranus, and Neptune. These planets have relatively low densities. Also known as the outer planets.
Maria
The Latin name for the smooth areas of the Moon formerly thought to be seas.
Solum
The O, A, and B horizons in a soil profile. Living roots and other plant and animal life are largely confined to this zone.
Resolving power
The ability of a telescope to separate objects that would otherwise appear as one.
Storm surge
The abnormal rise of the sea along a shore as a result of strong winds.
Fluorescence
The absorption of ultraviolet light, which is reemitted as visible light.
Tidal current
The alternating horizontal movement of water associated with the rise and fall of the tide.
Calorie
The amount of heat required to raise the temperature of 1 gram of water 1°C.
Primary productivity
The amount of organic matter synthesized by organisms from inorganic substances through photosynthesis or chemosynthesis within a given volume of water or habitat in a unit of time.
Pressure gradient
The amount of pressure change occurring over a given distance.
Temperature gradient
The amount of temperature change per unit of distance.
Obliquity
The angle between the planes of Earth's equator and orbit.
Altitude (of the Sun)
The angle of the Sun above the horizon.
Declination (stellar)
The angular distance north or south of the celestial equator denoting the position of a celestial body.
Absolute magnitude
The apparent brightness of a star if it were viewed from a distance of 10 parsecs (32.6 light-years). Used to compare the true brightness of stars.
Doppler effect
The apparent change in wavelength of radiation caused by the relative motions of the source and the observer.
Parallax
The apparent shift of an object when viewed from two different locations.
Retrograde motion
The apparent westward motion of the planets with respect to the stars.
Luster
The appearance or quality of light reflected from the surface of a mineral.
Unsaturated zone
The area above the water table where openings in soil, sediment, and rock are not saturated but filled mainly with air.
Cut bank
The area of active erosion on the outside of a meander.
Source region
The area where an air mass acquires its characteristic properties of temperature and moisture.
Lapse rate (normal)
The average drop in temperature (6.5°C per kilometer [3.5°F per 1000 feet]) with increased altitude in the troposphere.
Atomic weight
The average of the atomic masses of isotopes for a given element.
Mean solar day
The average time between two passages of the Sun across the local celestial meridian.
Glacial budget
The balance, or lack of balance, between ice formation at the upper end of a glacier and ice loss in the zone of wastage.
Nebular theory
The basic idea that the Sun and planets formed from the same cloud of gas and dust in interstellar space.
Mesopause
The boundary between the mesosphere and the thermosphere.
Stratopause
The boundary between the stratosphere and the mesosphere.
Tropopause
The boundary between the troposphere and the stratosphere.
Front
The boundary between two adjoining air masses having contrasting characteristics.
Mohorovičić discontinuity (Moho)
The boundary separating the crust from the mantle, discernible by an increase in seismic velocity.
Bright-line spectrum
The bright lines produced by an incandescent gas under low pressure. Also known as emission spectrum.
Apparent magnitude
The brightness of a star when viewed from Earth.
Luminosity
The brightness of a star. The amount of energy radiated by a star.
Basalt plateau
The broad and extensive accumulation of lava from a succession of flows emanating from fissure eruptions.
Energy
The capacity to do work.
Umbra
The central, completely dark part of a shadow produced during an eclipse.
Condensation
The change of state from a gas to a liquid.
Freezing
The change of state from a liquid to a solid.
Melting
The change of state from a solid to a liquid.
Metamorphism
The changes in mineral composition and texture of a rock subjected to high temperature and pressure within Earth.
Stream valley
The channel, valley floor, and sloping valley walls of a stream.
Solar nebula
The cloud of interstellar gas and/or dust from which the bodies of our solar system formed.
Local Group
The cluster of 20 or so galaxies to which our galaxy belongs.
Coastline
The coast's seaward edge. The landward limit of the effect of the highest storm waves on the shore.
Streak
The color of a mineral in powdered form.
Evapotranspiration
The combined effect of evaporation and transpiration.
Intermediate composition
The composition of igneous rocks lying between felsic and mafic. Also known as andesitic composition.
Secondary enrichment
The concentration of minor amounts of metals that are scattered through unweathered rock into economically valuable concentrations by weathering processes.
Geocentric
The concept of an Earth-centered universe.
Catastrophism
The concept that Earth was shaped by catastrophic events of a short-term nature.
Isostasy
The concept that Earth's crust is floating in gravitational balance on the material of the mantle.
Uniformitarianism
The concept that the processes that have shaped Earth in the geologic past are essentially the same as those operating today.
Supersaturation
The condition of being more highly concentrated than is normally possible under given temperature and pressure conditions. When describing humidity, it refers to a relative humidity that is greater than 100 percent.
Supercooled
The condition of water droplets that remain in the liquid state at temperatures well below 0°C (32°F).
Convergence
The condition that exists when the distribution of winds in a given area results in a net horizontal inflow of air into the area. Because convergence at lower levels is associated with an upward movement of air, areas of convergent winds are regions favorable to cloud formation and precipitation.
Divergence
The condition that exists when the distribution of winds in a given area results in a net horizontal outflow of air from the region. In divergence at lower levels, the resulting deficit is compensated for by a downward movement of air from aloft; hence, areas of divergent winds are unfavorable to cloud formation and precipitation.
Air-mass weather
The conditions experienced in an area as an air mass passes over it. Because air masses are large and fairly homogenous, air-mass weather will be fairly constant and may last for several days.
Sublimation
The conversion of a solid directly to a gas without passing through the liquid state.
Hydrogen burning
The conversion of hydrogen through fusion to form helium.
Eclipse
The cutting off of the light of one celestial body by another passing in front of it.
Valley breeze
The daily upslope winds commonly encountered in a mountain valley.
Coriolis force (effect)
The deflective force of Earth's rotation on all free-moving objects, including the atmosphere and oceans. Deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Leaching
The depletion of soluble materials from the upper soil by downward-percolating water.
Crater
The depression at the summit of a volcano, or that which is produced by a meteorite impact.
Annual temperature range
The difference between the highest and lowest monthly temperature means.
Daily temperature range
The difference between the maximum and minimum temperatures for a day.
Drawdown
The difference in height between the bottom of a cone of depression and the original height of the water table.
Parsec
The distance at which an object would have a parallax angle of 1 second of arc (3.26 light-years).
Focal length
The distance from a lens to the point where it focuses parallel rays of light.
Light-year
The distance light travels in a year; about 6 trillion miles.
Fetch
The distance that wind has traveled across open water. It is one of three factors that influence the height, length, and period of a wave.
Electromagnetic spectrum
The distribution of electromagnetic radiation by wavelength.
Geologic time scale
The division of Earth history into blocks of time—eons, eras, periods, and epochs. The time scale was created using relative dating principles.
Westerlies
The dominant west-to-east motion of the atmosphere that characterizes the regions on the poleward side of the subtropical highs.
Eye wall
The doughnut-shaped area of intense cumulonimbus development and very strong winds that surrounds the eye of a hurricane.
Mass wasting
The downslope movement of rock, regolith, and soil under the direct influence of gravity.
Earthflow
The downslope movement of water-saturated, clay- rich sediment. Most characteristic of humid regions.
Slump
The downward slipping of a mass of rock or unconsolidated material moving as a unit along a curved surface.
Berm
The dry, gently sloping zone on the backshore of a beach at the foot of the coastal cliffs or dunes.
Valence electron
The electrons involved in the bonding process; the electrons occupying the highest principal energy level of an atom.
Natural levees
The elevated landforms that parallel some streams and act to confine their waters, except during floodstage.
Terminal moraine
The end moraine marking the farthest advance of a glacier.
Latent heat
The energy absorbed or released during a change in state.
Proterozoic eon
The eon following the Archean and preceding the Phanerozoic. It extends between about 2500 million (2.5 billion) and 540 million years ago.
Spring equinox
The equinox that occurs on March 21-22 in the Northern Hemisphere and on September 21-23 in the Southern Hemisphere.
Autumnal equinox
The equinox that occurs on September 21-23 in the Northern Hemisphere and on March 21-22 in the Southern Hemisphere.
Outgassing
The escape of gases that had been dissolved in magma.
Climate system
The exchanges of energy and moisture that occur among the atmosphere, hydrosphere, solid Earth, biosphere, and cryosphere.
Terrae
The extensively cratered highland areas of the Moon. Also known as lunar highlands.
Suspended load
The fine sediment carried within the body of flowing water.
Chromosphere
The first layer of the solar atmosphere found directly above the photosphere.
Floodplain
The flat, low-lying portion of a stream valley subject to periodic inundation.
Radiation pressure
The force exerted by electromagnetic radiation from an object such as the Sun.
Air pressure
The force exerted by the weight of a column of air above a given point.
Stress
The force per unit area acting on any surface within a solid.
Crystallization
The formation and growth of a crystalline solid from a liquid or gas.
Benthos
The forms of marine life that live on or in the ocean bottom.
Coma
The fuzzy, gaseous component of a comet's head.
Atmosphere
The gaseous portion of a planet; the planet's envelope of air. One of the traditional subdivisions of Earth's physical environment.
Continental shelf
The gently sloping submerged portion of the continental margin, extending from the shoreline to the continental slope.
Continental rise
The gently sloping surface at the base of the continental slope.
Geothermal gradient
The gradual increase in temperature with depth in the crust. The average is 30°C per kilometer in the upper crust.
Gravitational collapse
The gradual subsidence of mountains caused by lateral spreading of weak material located deep within these structures.
Perturbation
The gravitational disturbance of the orbit of one celestial body by another.
Circle of illumination
The great circle that separates daylight from darkness.
Abrasion
The grinding and scraping of a rock surface by the friction and impact of rock particles carried by water, wind, or ice.
Felsic
The group of igneous rocks composed primarily of feldspar and quartz.
Saturation
The maximum quantity of water vapor that the air can hold at any given temperature and pressure.
Mixing depth
The height to which convectional movements extend above Earth's surface. The greater the mixing depth, the better the air quality.
Spring tide
The highest tidal range, which occurs near the times of the new and full moons.
Wavelength
The horizontal distance separating successive crests or troughs.
Hydrothermal solution
The hot,watery solution that escapes from a mass of magma during the later stages of crystallization. Such solutions may alter the surrounding country rock and are frequently the source of significant ore deposits.
Cambrian explosion
The huge expansion in biodiversity that occurred at the beginning of the Paleozoic era.
Supercontinent cycle
The idea that the rifting and dispersal of one supercontinent is followed by a long period during which the fragments gradually reassemble into a new supercontinent.
Plane of the ecliptic
The imaginary plane that connects Earth's orbit with the celestial sphere.
Erosion
The incorporation and transportation of material by a mobile agent, such as water, wind, or ice.
Thermal gradient
The increase in temperature with depth. It averages 1°C per 30 meters (1-2°F per 100 feet) in the crust.
Escape velocity
The initial velocity an object needs to escape from the surface of a celestial body.
Backshore
The inner portion of the shore, lying landward of the high-tide shoreline. It is usually dry, being affected by waves only during storms.
Core
The innermost layer of Earth, located beneath the mantle. The core is divided into an outer core and an inner core.
Heat
The kinetic energy of random molecular motion.
Drainage basin
The land area that contributes water to a stream.
Gyre
The large circular surface current pattern found in each ocean.
Eon
The largest time unit on the geologic time scale, next in order of magnitude above era.
Regolith
The layer of rock and mineral fragments that nearly everywhere covers Earth's surface.
Mesosphere
The layer of the atmosphere immediately above the stratosphere and characterized by decreasing temperatures with height.
Stratosphere
The layer of the atmosphere immediately above the troposphere, characterized by increasing temperatures with height, due to the concentration of ozone.
Base level
The level below which a stream cannot erode.
Temporary (local) base level
The level of a lake, resistant rock layer, or any other base level that stands above sea level.
Deflation
The lifting and removal of loose material by wind.
Shoreline
The line that marks the contact between land and sea. It migrates up and down as the tide rises and falls.
Melt
The liquid portion of magma, excluding the solid crystals.
Santa Ana
The local name given a chinook wind in southern California.
Epicenter
The location on Earth's surface that lies directly above the focus of an earthquake.
Snowline
The lower limit of perennial snow.
Troposphere
The lowermost layer of the atmosphere. It is generally characterized by a decrease in temperature with height.
Neap tide
The lowest tidal range, which occurs near the times of the first- and third-quarter phases of the Moon.
Meteor
The luminous phenomenon observed when a meteoroid enters Earth's atmosphere and burns up; popularly called a "shooting star."
Benthic zone
The marine life zone that includes any seafloor surface, regardless of its distance from shore.
Oceanic zone
The marine-life zone beyond the continental shelf.
Neritic zone
The marine-life zone that extends from the low tideline out to the shelf break.
Mixing ratio
The mass of water vapor in a unit mass of dry air; commonly expressed as grams of water vapor per kilogram of dry air.
Parent material
The material on which a soil develops.
Groundmass
The matrix of smaller crystals within an igneous rock that has porphyritic texture.
Daily mean temperature
The mean temperature for a day, which is determined by averaging the hourly readings or, more commonly, by averaging the maximum and minimum temperatures for a day.
Monthly mean temperature
The mean temperature for a month that is calculated by averaging the daily means.
Bathymetry
The measurement of ocean depths and the charting of the shape or topography of the ocean floor.
Frost wedging
The mechanical breakup of rock caused by the expansion of freezing water in cracks and crevices.
Rock-forming minerals
The minerals that make up most of the rocks of Earth's crust.
Bituminous coal
The most common form of coal, often called soft, black coal.
Quaternary period
The most recent period on the geologic time scale. It began about 2.6 million years ago and extends to the present.
Revolution
The motion of one body about another, as Earth about the Sun.
Ebb current
The movement of a tidal current away from the shore.
Infiltration
The movement of surface water into rock or soil through cracks and pore spaces.
Turbulent flow
The movement of water in an erratic fashion, often characterized by swirling, whirlpool-like eddies. Most streamflow is of this type.
Laminar flow
The movement of water particles in straight- line paths that are parallel to the channel. The water particles move downstream, without mixing.
Earth science
The name for all the sciences that collectively seek to understand Earth. It includes geology, oceanography, meteorology, and astronomy.
El Niño
The name given to the periodic warming of the ocean that occurs in the central and eastern Pacific. A major El Niño episode can cause extreme weather in many parts of the world.
Paleomagnetism
The natural remnant magnetism in rock bodies. The permanent magnetization acquired by rock that can be used to determine the location of the magnetic poles and the latitude of the rock at the time it became magnetized. Also known as fossil magnetism.
Pressure tendency
The nature of the change in atmospheric pressure over the past several hours. It can be a useful aid in short-range weather prediction. Also known as barometric tendency.
Mountain breeze
The nightly downslope winds commonly encountered in mountain valleys.
Laurasia
The northern portion of Pangaea, consisting of North America and Eurasia.
Hydrogen fusion
The nuclear reaction in which hydrogen nuclei are fused into helium nuclei.
Mass number
The number of neutrons and protons in the nucleus of an atom.
Atomic number
The number of protons in the nucleus of an atom.
Corona
The outer, tenuous layer of the solar atmosphere.
Fossil assemblage
The overlapping ranges of a group of fossils (assemblage) collected from a layer. By examining such an assemblage, the age of the sedimentary layer can be established.
Occlusion
The overtaking of one front by another.
Tropic of Cancer
The parallel of latitude, 23.5° north latitude, marking the northern limit of the Sun's vertical rays.
Tropic of Capricorn
The parallel of latitude, 23.5° south latitude, marking the southern limit of the Sun's vertical rays.
Stable platform
The part of a craton that is mantled by relatively undeformed sedimentary rocks and underlain by a basement complex of igneous and metamorphic rocks.
Zone of wastage
The part of a glacier beyond the zone of accumulation where all of the snow from the previous winter melts, as does some of the glacial ice.
Zone of accumulation
The part of a glacier characterized by snow accumulation and ice formation. Its outer limit is the snowline.
Craton
The part of the continental crust that has attained stability; that is, it has not been affected by significant tectonic activity during the Phanerozoic eon. It consists of the shield and stable platform.
Physical environment
The part of the environment that encompasses water, air, soil, and rock, as well as conditions such as temperature, humidity, and sunlight.
Lower mantle
The part of the mantle that extends from the core-mantle boundary to a depth of 660 kilometers (400 miles).
Vapor pressure
The part of the total atmospheric pressure attributable to water-vapor content.
Orbit
The path of a body in revolution around a center of mass.
Sidereal day
The period of Earth's rotation with respect to the stars.
Synodic month
The period of revolution of the Moon with respect to the Sun, or its cycle of phases.
Urban heat island
The phenomenon of temperatures within a city being generally higher than in surrounding rural areas.
Mechanical weathering
The physical disintegration of rock, resulting in smaller fragments.
Aphelion
The place in the orbit of a planet where the planet is farthest from the Sun.
Megathrust fault
The plate boundary separating a subducting slab of oceanic lithosphere and the overlying plate.
Perihelion
The point in the orbit of a planet where it is closest to the Sun.
Focus (light)
The point where a lens or mirror causes light rays to converge.
Shelf break
The point where a rapid steepening of the gradient occurs, marking the outer edge of the continental shelf and the beginning of the continental slope.
Reservoir rock
The porous, permeable portion of an oil trap that yields oil and gas.
Penumbra
The portion of a shadow from which only part of the light source is blocked by an opaque body.
Deep-ocean basin
The portion of seafloor that lies between the continental margin and the oceanic ridge system. This region comprises almost 30 percent of Earth's surface.
Aphotic zone
The portion of the ocean where there is no sunlight.
Euphotic zone
The portion of the photic zone near the surface where light is bright enough for photosynthesis to occur.
Continental margin
The portion of the seafloor adjacent to the continents. It may include the continental shelf, continental slope, and continental rise.
Superposition
The principle that in any undeformed sequence of sedimentary rocks, each bed is older than the layers above and younger than the layers below.
Radiometric dating
The procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes.
Beach nourishment
The process by which large quantities of sand are added to the beach system to offset losses caused by wave erosion.
Partial melting
The process by which most igneous rocks melt. Since individual minerals have different melting points, most igneous rocks melt over a temperature range of a few hundred degrees. If the liquid is squeezed out after some melting has occurred, a melt with a higher silica content results.
Plucking (quarrying)
The process by which pieces of bedrock are lifted out of place by a glacier.
Photosynthesis
The process by which plants and algae produce carbohydrates from carbon dioxide and water in the presence of chlorophyll, using light energy and releasing oxygen.
Sorting
The process by which solid particles of various sizes are separated by moving water or wind. Also, the degree of similarity in particle size in sediment or sedimentary rock.
Refraction
The process by which the portion of a wave in shallow water slows, causing the wave to bend and tend to align itself with the underwater contours. Also known as wave refraction.
Deposition
The process by which water vapor is changed directly to a solid without passing through the liquid state.
Evaporation
The process of converting a liquid to a gas.
Magmatic differentiation
The process of generating more than one rock type from a single magma.
Seafloor spreading
The process of producing new seafloor between two diverging plates.
Subduction
The process of thrusting oceanic lithosphere into the mantle along a convergent boundary.
Reflection
The process whereby light bounces back from an object at the same angle at which it encounters a surface and with the same intensity.
Lithification
The process, generally cementation and/or compaction, of converting sediments to solid rock.
Chemical weathering
The processes by which the internal structure of a mineral is altered by the removal and/or addition of elements.
Orogenesis
The processes that collectively result in the formation of mountains.
Law of conservation of angular momentum
The product of the velocity of an object around a center of rotation (axis), and the distance squared of the object from the axis is constant.
Phases of the Moon
The progression of changes in the Moon's appearance during the month.
Chromatic aberration
The property of a lens whereby light of different colors is focused at different places.
Salinity
The proportion of dissolved salts to pure water, usually expressed in parts per thousand (‰).
Pangaea
The proposed supercontinent that 200 million years ago began to break apart and form the present landmasses.
Discharge
The quantity of water in a stream that passes a given point in a period of time.
Radiocarbon (carbon-14)
The radioactive isotope of carbon, which is produced continuously in the atmosphere and is used in dating events from the very recent geologic past (the last few tens of thousands of years).
Rockslide
The rapid slide of a mass of rock downslope along planes of weakness.
Solar constant
The rate at which solar radiation is received outside Earth's atmosphere on a surface perpendicular to the Sun's rays when Earth is at an average distance from the Sun.
Dry adiabatic rate
The rate of adiabatic cooling or warming in unsaturated air. The rate of temperature change is 1°C per 100 meters.
Wet adiabatic rate
The rate of adiabatic temperature change in saturated air. The rate of temperature change is variable, but it is always less than the dry adiabatic rate.
Environmental lapse rate
The rate of temperature decrease with increasing height in the troposphere.
Specific gravity
The ratio of a substance's weight to the weight of an equal volume of water.
Relative humidity
The ratio of the air's water-vapor content to its water-vapor capacity.
Seismogram
The record made by a seismograph.
Scattering
The redirecting (in all directions) of light by small particles and gas molecules in the atmosphere. The result is diffused light.
Albedo
The reflectivity of a substance, usually expressed as a percentage of the incident radiation reflected.
Asteroid belt
The region in which most asteroids orbit the Sun between Mars and Jupiter.
Forearc basin
The region located between a volcanic arc and an accretionary wedge where shallow-water marine sediments typically accumulate.
Photosphere
The region of the Sun that radiates energy to space. The visible surface of the Sun.
Angle of repose
The steepest angle at which loose material remains stationary, without sliding downslope.
Thermosphere
The region of the atmosphere immediately above the mesosphere and characterized by increasing temperatures due to absorption of very shortwave solar energy by oxygen.
Soil texture
The relative proportions of clay, silt, and sand in a soil. A soil's texture strongly influences its ability to retain and transmit water and air.
Offshore zone
The relatively flat submerged zone that extends from the breaker line to the edge of the continental shelf.
Transpiration
The release of water vapor to the atmosphere by plants.
Fossil
The remains or traces of organisms preserved from the geologic past.
Desalination
The removal of salts and other chemicals from seawater.
Hardness
The resistance a mineral offers to scratching.
Lithosphere
The rigid outer layer of Earth, including the crust and upper mantle.
Upwelling
The rising of cold water from deeper layers to replace warmer surface water that has been moved away.
Footwall block
The rock surface below a fault.
Hanging wall block
The rock surface immediately above a fault.
Geology
The science that examines Earth, its form and composition, and the changes it has undergone and is undergoing.
Climatology
The scientific study of climate.
Meteorology
The scientific study of the atmosphere and atmospheric phenomena; the study of weather and climate.
Oceanography
The scientific study of the oceans and oceanic phenomena.
Astronomy
The scientific study of the universe; it includes the observation and interpretation of celestial bodies and phenomena.
Archean eon
The second eon of Precambrian time, following the Hadean and preceding the Proterozoic. It extends between 3.8 billion and 2.5 billion years before the present.
Texture
The size, shape, and distribution of the particles that collectively constitute a rock.
Gradient
The slope of a stream; generally measured in feet per mile.
Creep
The slow downhill movement of soil and regolith.
Nucleus
The small heavy core of an atom that contains all of its positive charge and most of its mass.
Atom
The smallest particle that exists as an element.
Geosphere
The solid Earth, the largest of Earth's four major spheres.
Inner core
The solid innermost layer of Earth, about 1300 kilometers (800 miles) in radius.
Winter solstice
The solstice that occurs on December 21-22 in the Northern Hemisphere and on June 21-22 in the Southern Hemisphere.
Summer solstice
The solstice that occurs on June 21-22 in the Northern Hemisphere and on December 21-22 in the Southern Hemisphere.
Thunder
The sound emitted by rapidly expanding gases along the channel of lightning discharge.
Nuclear fusion
The source of the Sun's energy.
Gondwanaland
The southern portion of Pangaea, consisting of South America, Africa, Australia, India, and Antarctica.
Geologic time
The span of time since the formation of Earth, about 4.6 billion years.
Settling velocity
The speed at which a particle falls through a still fluid. The size, shape, and specific gravity of particles influence settling velocity.
Rotation
The spinning of a body, such as Earth, about its axis.
Radioactive decay
The spontaneous decay of certain unstable atomic nuclei.
Radioactivity
The spontaneous emission of certain unstable atomic nuclei.
Weather
The state of the atmosphere at any given time. Weathering The disintegration and decomposition of rock at or near Earth's surface.
Continental slope
The steep gradient that leads to the deep- ocean floor and marks the seaward edge of the continental shelf.
Slip face
The steep, leeward slope of a sand dune; it maintains an angle of about 34 degrees.
Polar front
The stormy frontal zone separating air masses of polar origin from air masses of tropical origin.
Seismology
The study of earthquakes and seismic waves.
Mineralogy
The study of minerals.
Spectroscopy
The study of spectra.
Tectonics
The study of the large-scale processes that collectively deform Earth's crust.
Cosmology
The study of the universe.
Elastic rebound
The sudden release of stored strain in rocks that results in movement along a fault.
Vent
The surface opening of a conduit or pipe.
Paleontology
The systematic study of fossils and the history of life on Earth.
Periodic table
The tabular arrangement of the elements according to atomic number.
Curie point
The temperature above which a material loses its magnetization.
Dew-point temperature
The temperature to which air has to be cooled in order to reach saturation.
Cleavage
The tendency of a mineral to break along planes of weak bonding.
Nor'easter
The term used to describe the weather associated with an incursion of mP air from the North Atlantic into the Northeast and Mid-Atlantic regions; strong northeast winds, freezing or near-freezing temperatures, and the possibility of precipitation make this an unwelcome weather event.
Plate tectonics
The theory which proposes that Earth's outer shell consists of individual plates that interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself.
Big Bang theory
The theory which proposes that the universe originated as a single mass, which subsequently exploded.
Flood current
The tidal current associated with the increase in the height of the tide.
Inclination of the axis
The tilt of Earth's axis from the perpendicular to the plane of Earth's orbit.
Wave period
The time interval between the passage of successive crests at a stationary point.
Half-life
The time required for one-half of the atoms of a radioactive substance to decay.
Solstice
The time when the vertical rays of the Sun are striking either the Tropic of Cancer or the Tropic of Capricorn. Solstice represents the longest or shortest day (length of daylight) of the year.
Equinox
The time when the vertical rays of the Sun are striking the equator. The length of daylight and darkness is equal at all latitudes at equinox.
Magnitude (earthquake)
The total amount of energy released during an earthquake.
Capacity
The total amount of sediment a stream is able to transport.
Biomass
The total mass of a defined organism or group of organisms in a particular area or ecosystem.
Biosphere
The totality of life on Earth; the parts of the solid Earth, hydrosphere, and atmosphere in which living organisms can be found.
Radiation
The transfer of energy (heat) through space by electromagnetic waves. Also known as electromagnetic radiation.
Convection
The transfer of heat by the movement of a mass or substance. It can take place only in fluids.
Conduction
The transfer of heat through matter by molecular activity. Energy is transferred through collisions from one molecule to another.
Greenhouse effect
The transmission of short-wave solar radiation by the atmosphere, coupled with the selective absorption of longer-wavelength terrestrial radiation, especially by water vapor and carbon dioxide.
Beach drift
The transport of sediment in a zigzag pattern along a beach caused by the uprush of water from obliquely breaking waves.
Wave of translation
The turbulent advance of water created by breaking waves.
Tensional stress
The type of stress that tends to pull apart a body.
Water table
The upper level of the saturated zone of groundwater.
Photic zone
The upper part of the ocean into which any sunlight penetrates.
Zone of fracture
The upper portion of a glacier, consisting of brittle ice.
Surface soil
The uppermost layer in a soil profile: the A horizon.
Differential weathering
The variation in the rate and degree of weathering caused by such factors as mineral makeup, degree of jointing, and climate.
Eccentricity
The variation of an ellipse from a circle.
Wave height
The vertical distance between the trough and crest of a wave.
Crust
The very thin outermost layer of Earth.
Earthquake
The vibration of Earth produced by the rapid release of energy.
Heliocentric
The view that the Sun is at the center of the solar system.
Pyroclastic material
The volcanic rock ejected during an eruption, including ash, bombs, and blocks.
Porosity
The volume of open spaces in rock or soil.
Eluviation
The washing out of fine soil components from the horizon by downward-percolating water.
Hydrosphere
The water portion of our planet; one of the traditional subdivisions of Earth's physical environment.
Absolute humidity
The weight of water vapor in a given volume of air (usually expressed in grams/m3).
Beach face
The wet, sloping surface that extends from the berm to the shoreline.
Ecliptic
The yearly path of the Sun plotted against the background of stars.
Shadow zone
The zone between 104 and 143 degrees distance from an earthquake epicenter in which direct waves do not arrive because of refraction by Earth's core.
Ring of Fire
The zone of active volcanoes surrounding the Pacific Ocean.
Nearshore zone
The zone of beach that extends from the low-tide shoreline seaward to where waves break at low tide.
Intertropical convergence zone (ITCZ)
The zone of general convergence between the Northern and Southern Hemisphere trade winds.
Benioff zone
The zone of inclined seismic activity that extends from a trench downward into the asthenosphere.
Zone of saturation
The zone where all open spaces in sediment and rock are completely filled with water.
Focus (earthquake)
The zone within Earth where rock displacement produces an earthquake. Also known as the hypocenter.
Asteroids
Thousands of small planetlike bodies, ranging in size from a few hundred kilometers to less than a kilometer, whose orbits lie mainly between those of Mars and Jupiter.
Condensation nuclei
Tiny bits of particulate matter that serve as surfaces on which water vapor condenses.
Aerosols
Tiny solid and liquid particles suspended in the atmosphere.
Saltation
Transportation of sediment through a series of leaps or bounces.
Trade winds
Two belts of winds that blow almost constantly from easterly directions and are located on the equatorward sides of the subtropical highs.
Radio interferometer
Two or more radio telescopes that combine their signals to achieve the resolving power of a larger telescope.
Binary stars
Two stars revolving around a common center of mass under their mutual gravitational attraction.
Sediment
Unconsolidated particles created by the weathering and erosion of rock, by chemical precipitation from solution in water, or from the secretions of organisms and transported by water, wind, or glaciers.
Alluvium
Unconsolidated sediment deposited by a stream.
Dark matter
Undetected matter that is thought to exist in great quantities in the universe.
Localized convective lifting
Unequal surface heating that causes localized pockets of air (thermals) to rise because of their buoyancy.
Till
Unsorted sediment deposited directly by a glacier.
Ore
Usually a useful metallic mineral that can be mined at a profit. The term is also applied to certain nonmetallic minerals such as fluorite and sulfur.
Isotopes
Varieties of the same element that have different mass numbers; their nuclei contain the same number of protons but different numbers of neutrons.
Rock avalanche
Very rapid downslope movement of rock and debris. These rapid movements may be aided by a layer of air trapped beneath the debris, and they have been known to reach speeds of over 200 kilometers (125 miles) per hour.
Dwarf galaxy
Very small galaxies, usually elliptical and lacking spiral arms.
Plug
Volcanic neck.
Overrunning
Warm air gliding up a retreating cold air mass.
Calving
Wastage of a glacier that occurs when large pieces of ice break off into water.
Groundwater
Water in the zone of saturation.
Runoff
Water that flows over the land rather than infiltrating into the ground.
Swells
Wind-generated waves that have moved into an area of weaker winds or calm.