Geology Cumaltive Final Exam

Isotopic dating

(equivalent to radiometric dating) A method to determine a rock's numerical age by measuring the ratio of parent to daughter isotopes in minerals. (page 356)

Morphology

(fossil morphology) The shape of features visible on a fossil. (page 332)

Numerical age

(in older literature, 'absolute age') The age of a geologic feature given in years. (page 340)

Evolution

(life evolution) The change in populations of life that take place over time, leading to the modification of a given species, and/or to the appearance of new species or extinction of existing species. (page 327)

Plunge

(structural geology) The angle that a line makes with respect to horizontal, as measured in a vertical plane. (page 305)

Strike

(structural geology) The compass orientation of a horizontal line on a plane. (page 305)

Bearing

(structural geology) The compass orientation of a linear feature. (page 305)

Uplift

(structural geology) The vertical rise of a region of land (or the upward displacement of the Earth's surface). (page 429)

Glassy igneous rock

Igneous rock consisting entirely of glass, or of tiny crystals surrounded by a glass matrix. (page 127)

Asteroid

One of the fragments of solid material, left over from planet formation or produced by collision of planetesimals, that resides between the orbits of Mars and Jupiter. (page 14)

Vesicle

Open holes in igneous rock formed by the preservation of bubbles in magma as the magma cools into solid rock. (page 144)

Solar System

Our Sun and all the materials that orbit it (including planets, moons, asteroids, Kuiper Belt objects, and Oort Cloud objects). (page 13)

K-Pg boundary event

In recent years, due to changes in terminology used on the geologic time scale, geologists are alternatively referring to it as the K-Pg boundary event -- Pg stands for Paleogene. (page 380)

Soil order

In the United States, a country that includes many midlatitude climates, soil scientists frequently use the U.S. Comprehensive Soil Classification System, which distinguishes among 12 soil orders based on the physical characteristics and environment of soil formation. (page 181)

Cross bed

Internal laminations in a bed, inclined at an angle to the main bedding; cross beds are a relict of the slip face of dunes or ripples. (page 199)

Subsoil

Ions and clay accumulate in the B-horizon, or subsoil. (page 180)

Dissolved load

Ions dissolved in a stream's water. (page 470)

Banded iron formation (BIF)

Iron-rich sedimentary layers consisting of alternating gray beds of iron oxide and red beds of iron-rich chert. (page 372)

Bed load

Large particles, such as sand, pebbles, or cobbles, that bounce or roll along a streambed. (page 470)

Block

Large, angular pyroclastic fragments consisting of volcanic rock, broken up during the eruption. (page 142)

Bomb

Larger blobs of lava that squirt out of a vent and then solidify become streamlined bombs, whose surfaces are typically streaked and polished. (page 142)

Blocky lava

Lava that is so viscous that it breaks into boulder-like blocks as it moves; typically, such lavas are andesitic or rhyolitic. (page 139)

Stromatolite

Layered mounds of sediment formed by cyanobacteria; cyanobacteria secrete a mucous-like substance to which sediment sticks, and as each layer of cyanobacteria gets buried by sediment, it colonizes the surface of the new sediment, building a mound upward. (page 369)

Foliation

Layering formed as a consequence of the alignment of mineral grains, or of compositional banding in a metamorphic rock. (page 220)

Bedding

Layering or stratification in sedimentary rocks. (page 106)

Crustal root

Low-density crustal rock that protrudes downward beneath a mountain range. (page 319)

Contact metamorphism

Metamorphism caused by heat conducted into country rock from an igneous intrusion. (page 227)

Thermal metamorphism

Metamorphism caused by heat conducted into country rock from an igneous intrusion. (page 227)

Burial metamorphism

Metamorphism due only to the consequences of very deep burial. (page 227)

Regional metamorphism

Metamorphism of a broad region, usually the result of deep burial during an orogeny. (page 230)

Dynamothermal metamorphism

Metamorphism that involves heat, pressure, and shearing. (page 230)

Dynamic metamorphism

Metamorphism that occurs as a consequence of shearing alone, with no change in temperature or pressure. (page 227)

Cement

Mineral material that precipitates from water and fills the spaces between grains, holding the grains together. (page 423)

Silicate

Minerals built from silicon-oxygen tetrahedra arranged in chains, sheets, or 3-D networks; they make up most of the Earth's crust and mantle. (page 95)

Ore mineral

Minerals that have metal in high concentrations and in a form that can be easily extracted. (page 417)

Melt

Molten (liquid) rock. (page 31)

Magma

Molten rock beneath the Earth's surface. (page 115)

Lava

Molten rock that has flowed out onto the Earth's surface. (page 113)

Pollution

Natural and synthetic contaminant materials introduced to the Earth's environment by the activities of humans. (page 624)

Joint

Naturally formed cracks in rocks. (page 303)

Metamorphic mineral

New minerals that grow in place within a solid rock under metamorphic temperatures and pressures. (page 216)

Mid-ocean ridge

A 2-km-high submarine mountain belt that forms along a divergent oceanic plate boundary. (page 51)

Thin section

A 3/100-mm-thick slice of rock that can be examined with a petrographic microscope. (page 107)

Comet

A ball of ice and dust, probably remaining from the formation of the Solar System, that orbits the Sun. (page 14)

P-wave shadow zone

A band between 103° and 143° from an earthquake epicenter, as measured along the circumference of the Earth, inside which P-waves do not arrive at seismograph stations. (page 287)

S-wave shadow zone

A band between 103° and 180° from the epicenter of an earthquake inside of which S-waves do not arrive at seismograph stations. (page 287)

Tachylite

A basaltic volcanic glass. (page 129)

Fold

A bend or wrinkle of rock layers or foliation; folds form as a consequence of ductile deformation. (page 308)

Coal

A black, organic rock consisting of greater than 50% carbon; it forms from the buried and altered remains of plant material. (page 403)

Laccolith

A blister-shaped igneous intrusion that forms when magma injects between layers underground in a manner that pushes overlying layers upward to form a dome. (page 121)

Protoplanet

A body that grows by the accumulation of planetesimals but has not yet become big enough to be called a planet. (page 22)

Transform boundary

A boundary at which one lithosphere plate slips laterally past another. (page 61)

Divergent boundary

A boundary at which two lithosphere plates move apart from each other; they are marked by mid-ocean ridges. (page 61)

Convergent boundary

A boundary at which two plates move toward each other so that one plate sinks (subducts) beneath the other; only oceanic lithosphere can subduct. (page 61)

Unconformity

A boundary between two different rock sequences representing an interval of time during which new strata were not deposited and/or were eroded. (page 344)

Seismic-velocity discontinuity

A boundary in the Earth at which seismic velocity changes abruptly. (page 287)

Pothole

A bowl-shaped depression carved into the floor of a stream by a long-lived whirlpool carrying sand or gravel. (page 469)

Basin and Range Province

A broad, Cenozoic continental rift that has affected a portion of the western United States in Nevada, Utah, and Arizona; in this province, tilted fault blocks form ranges, and alluvium-filled valleys are basins. (page 383)

Big Bang theory

A cataclysmic explosion that scientists suggest represents the formation of the Universe; before this event, all matter and all energy were packed into one volumeless point. (page 16)

Mold

A cavity in sedimentary rock left behind when a shell that once filled the space weathers out. (page 329)

Hot-spot track

A chain of now-dead volcanoes transported off the hot spot by the movement of a lithosphere plate. (page 70)

Hydrocarbon

A chain-like or ring-like molecule made of hydrogen and carbon atoms; petroleum and natural gas are hydrocarbons. (page 393)

Deformation

A change in the shape, position, or orientation of a material, by bending, breaking, or flowing. (page 298)

Phylogenetic tree

A chart representing the ideas of paleontologists showing which groups of organisms radiated from which ancestors. (page 333)

Sinkhole

A circular depression in the land that forms when an underground cavern collapses. (page 523)

Arkose

A clastic sedimentary rock containing both quartz and feldspar grains. (page 191)

Contaminant plume

A cloud of contaminated groundwater that moves away from the source of the contamination. (page 537)

Nebula

A cloud of gas or dust in space. (page 18)

Rock

A coherent, naturally occurring solid, consisting of an aggregate of minerals or a mass of glass. (page 103)

Lava fountains

A column of molten lava spraying upward under pressure from a volcanic vent. (page 142)

Mantle plume

A column of very hot rock rising up through the mantle. (page 70)

Earthquake early warning system

A communications network that provides an alert within microseconds after the first earthquake waves arrive at a seismograph near the epicenter, but before damaging vibrations reach population centers. (page 277)

Geologic column

A composite stratigraphic chart that represents the entirety of the Earth's history. (page 349)

Gneiss

A compositionally banded metamorphic rock typically composed of alternating dark- and light-colored layers. (page 221)

Feedback mechanism

A condition that arises when the consequence of a phenomenon influences the phenomenon itself. (page 613)

Fissure

A conduit in a magma chamber in the shape of a long crack through which magma rises and erupts at the surface. (page 145)

Laurentia

A continent in the early Paleozoic Era composed of today's North America and Greenland. (page 374)

Passive margin

A continental margin that is not a plate boundary. (page 61)

Stratigraphic column

A cross-section diagram of a sequence of strata summarizing information about the sequence. (page 346)

Longitudinal profile

A cross-sectional image showing the variation in elevation along the length of a river. (page 472)

Seismic-reflection profile

A cross-sectional view of the crust made by measuring the reflection of artificial seismic waves off boundaries between different layers of rock in the crust. (page 398)

Volcanic arc

A curving chain of active volcanoes formed adjacent to a convergent plate boundary. (page 130)

Blowout

A deep, bowl-like depression scoured out of desert terrain by a turbulent vortex of wind. (page 416)

Trench

A deep, elongate trough bordering a volcanic arc; a trench defines the trace of a convergent plate boundary. (page 53)

Protostar

A dense body of gas that is collapsing inward because of gravitational forces and that may eventually become a star. (page 18)

Geologic contact

A depositional contact is a type of geologic contact (contact, for short), a broader term used for any boundary between two rock bodies. (page 346)

Sedimentary basin

A depression, created as a consequence of subsidence, that fills with sediment. (page 209)

Gravity anomaly

A deviation between the observed pull of gravity and the geoid at a particular location. (page 291)

Solar cell

A device that can produce electricity directly from incoming solar energy. (page 412)

Bed

A distinct layer of sediment or sedimentary rock, typically reflecting continuous deposition during a relatively short time interval. (page 197)

Biomarker

A distinctive chemical derived from an organism and preserved in rock is called a chemical fossil, a molecular fossil, or a biomarker. (page 330)

Sedimentary structure

A distinctive shape or form (examples -- bed, ripple mark, cross bed) formed during deposition of sediment. (page 196)

Lava dome

A dome-like mass of rhyolitic lava that accumulates above the eruption vent. (page 139)

Metamorphic foliation

A fabric defined by parallel surfaces or layers that develop in a rock as a result of metamorphism; schistocity and gneissic layering are examples. (page 216)

Global cooling

A fall in the average atmospheric temperature. (page 611)

Pyroclastic flow

A fast-moving avalanche that occurs when hot volcanic ash and debris mix with air and flow down the side of a volcano. (page 142)

Strike-slip fault

A fault in which one block slides horizontally past another (and therefore parallel to the strike line), so there is no relative vertical motion. (page 306)

Oblique-slip fault

A fault in which sliding occurs diagonally along the fault plane. (page 306)

Dip-slip fault

A fault in which sliding occurs up or down the slope (dip) of the fault. (page 306)

Normal fault

A fault in which the hanging-wall block moves down the slope of the fault. (page 306)

Sheetwash

A film of water less than a few mm thick that covers the ground surface during heavy rains. (page 465)

Phyllite

A fine-grained metamorphic rock with a foliation caused by the preferred orientation of very fine-grained mica. (page 221)

Gem

A finished (cut and polished) gemstone ready to be set in jewelry. (page 97)

Flash flood

A flood that occurs during unusually intense rainfall or as the result of a dam collapse, during which the floodwaters rise very fast. (page 483)

Monocline

A fold in the land surface whose shape resembles that of a carpet draped over a stair step. (page 308)

Basin

A fold or depression shaped like a right-side-up bowl. (page 308)

Anticline

A fold with an arch-like shape in which the limbs dip away from the hinge. (page 308)

Speleothem

A formation that grows in a limestone cave by the accumulation of travertine precipitated from water solutions dripping in a cave or flowing down the wall of a cave. (page 540)

Macrofossil

A fossil large enough to be seen with the naked eye. (page 330)

Index fossil

A fossil species that is widespread and survived for a relatively short duration, and can therefore be used to correlate strata. (page 343)

Microfossil

A fossil that can be seen only with a microscope or an electron microscope. (page 330)

Geyser

A fountain of steam and hot water that erupts periodically from a vent in the ground in a geothermal region. (page 534)

Fault

A fracture on which one body of rock slides past another. (page 304)

Clast

A fragment or grain produced by the physical or chemical weathering of a pre-existing rock. (page 172)

Accretion

A general term for the attachment of a block of crust to the margin of a larger block during orogeny. (page 315)

Thrust fault

A gently dipping reverse fault; the hanging-wall block moves up the slope of the fault. (page 306)

Alluvial fan

A gently sloping apron of sediment dropped by an ephemeral stream at the base of a mountain in arid or semiarid regions. (page 474)

Uniformitarianism

A geologic principle stating that processes that can be observed today also happened in the past, at comparable rates, and can explain features preserved in the geologic record. Put simply, the present is the key to the past. (page 340)

Paleontologist

A geologist or biologist who studies the fossil record to reconstruct the history of life on Earth. (page 327)

Seismologist

A geophysicist who specializes in studying earthquakes. (page 248)

Pumice

A glassy igneous rock that forms from felsic frothy lava and contains abundant (over 50%) pore space. (page 129)

Scoria

A glassy, mafic, igneous rock containing abundant air-filled holes. (page 129)

Turbidite

A graded bed of sediment built up at the base of a submarine slope and deposited by turbidity currents. (page 200)

Travel-time curve

A graph that plots the time since an earthquake began on the vertical axis and the distance to the epicenter on the horizontal axis. (page 257)

Fossil assemblage

A group of fossil species found in a specific sequence of sedimentary rock. (page 341)

Kingdom

A high-level taxonomic division of life. Eukaryota are divided into six kingdoms, including Plantae, Animalia, and Fungi. (page 332)

Drainage divide

A highland or ridge that separates one watershed from another. (page 467)

Well

A hole in the ground dug or drilled in order to obtain water. (page 531)

Mineral

A homogenous, naturally occurring, solid inorganic substance with a definable chemical composition and an internal structure characterized by an orderly arrangement of atoms, ions, or molecules in a lattice. Most minerals are inorganic. (page 84)

Superplume

A huge mantle plume. (page 379)

Conventional reserve

A hydrocarbon reserve that can be extracted simply by pumping from a reservoir rock. (page 394)

Hydrocarbon reserve

A known supply of oil and gas held underground. (page 394)

Depositional landform

A landform resulting from the deposition of sediment where the medium carrying the sediment evaporates, slows down, or melts. (page 430)

Erosional landform

A landform that results from the breakdown and removal of rock or sediment. (page 430)

Caldera

A large circular depression with steep walls and a fairly flat floor, formed after an eruption as the center of the volcano collapses into the drained magma chamber below. (page 145)

Tsunami

A large wave along the sea surface triggered by an earthquake or large submarine slump. (page 271)

Stratovolcano

A large, cone-shaped subaerial volcano consisting of alternating layers of lava and tephra. (page 145)

A'a'

A lava flow with a rubbly surface. (page 138)

Pahoehoe

A lava flow with a surface texture of smooth, glassy, ropelike ridges. (page 138)

Atmosphere

A layer of gases that surrounds a planet. (page 28)

Graded bed

A layer of sediment, deposited by a turbidity current, in which grain size varies from coarse at the bottom to fine at the top. (page 200)

Rift

A linear belt along which continental lithosphere stretches and pulls apart. (page 72)

Orogen

A linear range of mountains. (page 298)

Mohs hardness scale

A list of ten minerals in a sequence of relative hardness, with which other minerals can be compared. (page 91)

Hot spot

A location at the base of the lithosphere, at the top of a mantle plume, where temperatures can cause melting. (page 70)

Oil seep

A location at the surface of the Earth where hydrocarbons are being released from underground naturally. (page 395)

Discharge area

A location where groundwater flows back up to the surface and may emerge at springs. (page 529)

Artesian spring

A location where the ground surface intersects a natural fracture (joint) that taps a confined aquifer in which the pressure can drive the water to the surface. (page 530)

Recharge area

A location where water enters the ground and infiltrates down to the water table. (page 529)

Craton

A long-lived block of durable continental crust commonly found in the stable interior of a continent. (page 368)

Dipole

A magnetic field with a north and south pole, like that of a bar magnet. (page 26)

Artificial levee

A man-made retaining wall to hold back a river from flooding. (page 484)

Geologic map

A map showing the distribution of rock units and structures across a region. (page 349)

Floodway

A mapped region likely to be flooded, in which people avoid constructing buildings. (page 485)

Darcy's law

A mathematical equation stating that a volume of water, passing through a specified area of material at a given time, depends on the material's permeability and hydraulic gradient. (page 530)

Oxbow lake

A meander that has been cut off yet remains filled with water. (page 476)

Temperature

A measure of the hotness or coldness of a material. (page 20)

Hardness

A measure of the relative ability of a mineral to resist scratching; it represents the resistance of bonds in the crystal structure from being broken. (page 91)

Intensity

A measure of the relative size of an earthquake (the severity of ground shaking) at a location, as determined by examining the amount of damage caused. (page 259)

Coal rank

A measurement of the carbon content of coal; higher-rank coal forms at higher temperatures. (page 406)

Schist

A medium-to-coarse-grained metamorphic rock that possesses schistosity. (page 221)

Marble

A metamorphic rock composed of calcite and transformed from a protolith of limestone. (page 223)

Quartzite

A metamorphic rock composed of quartz and transformed from a protolith of quartz sandstone. (page 223)

Metaconglomerate

A metamorphic rock produced by metamorphism of a conglomerate; typically, it contains flattened pebbles and cobbles. (page 221)

Strategic mineral

A mineral containing elements of importance to technology (particularly to the military). (page 424)

Gemstone

A mineral that has special value because it is rare and people consider it beautiful. (page 97)

Volcanic debris flow

A mixture of water and pyroclastic debris that moves downslope like wet concrete. (page 144)

Snowball Earth

A model proposing that, at times during Earth history, glaciers covered all land, and the entire ocean surface froze. (page 372)

Moment magnitude

A modern measure of earthquake size that reflects the amount of energy an earthquake produces, the size of rupture, and amount of displacement. (page 261)

Collisional orogen

A mountain belt formed when two relatively buoyant blocks of crust converge. (page 314)

Orogeny

A mountain-building event. (page 298)

Dolostone

A name sometimes used for a carbonate rock containing a high proportion of dolomite. (page 195)

Fracture zone

A narrow band of vertical fractures in the ocean floor; fracture zones lie roughly at right angles to a mid-ocean ridge, and the actively slipping part of a fracture zone is a transform fault. (page 53), 67

Spring

A natural outlet from which groundwater flows up onto the ground surface. (page 529)

Sill

A nearly horizontal tabletop-shaped tabular intrusion that occurs between the layers of country rock. (page 121)

Specific gravity

A number representing the density of a mineral, as specified by the ratio between the weight of a volume of the mineral and the weight of an equal volume of water. (page 92)

General circulation model (GCM)

A numerical calculation that simulates the flow of the atmosphere and resulting phenomena, due to changes in atmospheric temperature and other parameters. (page 620)

Natural levee

A pair of low ridges that appear on either side of a stream and develop as a result of the accumulation of sediment deposited naturally during flooding. (page 477)

Apparent polar-wander path

A path on the globe along which a magnetic pole appears to have wandered over time; in fact, the continents drift, while the magnetic pole stays fairly fixed. (page 50)

Icehouse period

A period of time when the Earth's temperature was cooler than it is today and ice ages could occur. (page 611)

Photomicrograph

A photograph of a thin section of rock, as seen through a microscope. (page 108)

Agent of erosion

A physical entity (wind, moving water, flowing ice) that grinds and removes material from the Earth's surface. (page 430)

Meteorite

A piece of rock or metal alloy that fell from space and landed on Earth. (page 35)

Hand specimen

A piece of rock, roughly the size of a fist, collected in order to examine. (page 107)

Dune

A pile of sand generally formed by deposition from the wind. (page 189)

Waterfall

A place where water drops over an escarpment. (page 474)

Triple junction

A point where three lithosphere plate boundaries intersect. (page 59)

Hypothesis

A possible explanation, involving only natural processes, that can explain a set of observations.

Dynamo

A power plant generator in which water or wind power spins an electrical conductor around a permanent magnet. (page 293)

Hydrofracturing

A process by which drillers generate new fractures or open preexisting ones underground, by pumping a high-pressure fluid into a portion of the drill hole, in order to increase the permeability of surrounding hydrocarbon-bearing rocks. (page 400)

Scouring

A process by which running water removes loose fragments of sediment from a streambed. (page 469)

Differentiation

A process early in a planet's history during which dense iron alloy melted and sank downward to form the core, leaving less-dense mantle behind. (page 366)

Hydrocarbon generation

A process in which oil shale warms to temperatures of greater than about 90°C so kerogen molecules transform into oil and natural gas molecules. (page 394)

Stream piracy

A process that happens when headward erosion by one stream causes the stream to intersect the course of another stream and capture its flow. (page 479)

EarthScope

A project, funded by the U.S. National Science Foundation, to study the Earth's interior, primarily using a large array of seismometers. (page 289)

Cratonic platform

A province in the interior of a continent in which Phanerozoic strata bury most of the underlying Precambrian rock. (page 322)

Field force

A push or pull that applies across a distance (i.e., without contact between objects); examples are gravity and magnetism. (page 289)

Compression

A push or squeezing felt by a body. (page 303)

Tuff

A pyroclastic igneous rock composed of volcanic ash and fragmented pumice, formed when accumulations of the debris cement together. (page 142)

Perched water table

A quantity of groundwater that lies above the regional water table because an underlying lens of impermeable rock or sediment prevents the water from sinking down to the regional water table. (page 527)

Radioactive element

A radioactive element is one that undergoes radioactive decay reactions that yield a different element. (page 352)

Nuclear fission

A radioactive element is one whose nuclei spontaneously emits subatomic particles, or undergoes nuclear fission by breaking into two smaller nuclei. (page 23)

Extraordinary fossil

A rare fossilized relict, or trace, of the soft part of an organism (page 331)

Rapids

A reach of a stream in which water becomes particularly turbulent; as a consequence, waves develop on the surface of the stream. (page 474)

Meandering stream

A reach of stream containing many meanders (snake-like curves). (page 476)

Stratigraphic formation

A recognizable layer of a specific sedimentary rock type or set of rock types, deposited during a certain time interval, that can be traced over a broad region. (page 346)

Large igneous province (LIP)

A region in which huge volumes of lava and/or ash erupted over a relatively short interval of geologic time. (page 156)

Geothermal region

A region of current or recent volcanism in which magma or very hot rock heats up groundwater, which may discharge at the surface in the form of hot springs and/or geysers. (page 534)

Karst

A region underlain by caves in limestone bedrock; the collapse of the caves creates a landscape of sinkholes separated by higher topography, or of limestone spires separated by low areas. (page 541)

Seal rock

A relatively impermeable rock, such as shale, salt, or unfractured limestone, that lies above a reservoir rock and stops the oil from rising further. (page 395)

Xenolith

A relict of wall rock surrounded by intrusive rock when the intrusive rock freezes. (page 125)

Flood-hazard map

A representation of a portion of the Earth's surface that is designed to show how the danger of flooding varies with location. (page 488)

Geologic cross section

A representation of geologic features underground as they would appear crossing a vertical slice through the Earth down to a specified depth. (page 431)

Phylogeny

A representation of life evolution showing how populations of the present relate to populations of the past. (page 333)

Metamorphic grade

A representation of the intensity of metamorphism, meaning the amount or degree of metamorphic change. (page 223)

Stream

A ribbon of water that flows in a channel. (page 464)

Global warming

A rise in the average atmospheric temperature. (page 611)

Salt dome

A rising bulbous dome of salt that bends up the adjacent layers of sedimentary rock. (page 397)

Source rock

A rock (organic-rich shale) containing the raw materials from which hydrocarbons eventually form. (page 393)

Travertine

A rock composed of crystalline calcium carbonate (CaCO3) formed by chemical precipitation from ground waterthat has seeped out at the ground surface. (page 194)

Fragmental igneous rock

A rock consisting of igneous chunks and/or shards that are packed together, welded together, or cemented together after having solidified. (page 127)

Migmatite

A rock formed when gneiss is heated high enough so that it begins to partially melt, creating layers, or lenses, of new igneous rock that mix with layers of the relict gneiss. (page 223)

Crystalline rock

A rock that consists of minerals that grew when a melt solidified, and eventually interlock like pieces of a jigsaw puzzle. (page 103)

Crystalline igneous rock

A rock that consists of minerals that grew when a melt solidified, and eventually interlock like pieces of a jigsaw puzzle. (page 127)

Global positioning system (GPS)

A satellite system people can use to measure rates of movement of the Earth's crust relative to one another, or simply to locate their position on the Earth's surface. (page 318)

Richter scale

A scale that defines earthquakes on the basis of the amplitude of the largest ground motion recorded on a seismogram. (page 260)

Geological Time Scale

A scale that describes the intervals of geologic time

Geologic time scale

A scale that describes the intervals of geologic time. (page 359)

Theory

A scientific idea supported by an abundance of evidence that has passed many tests and failed none.

Vein

A seam of minerals that forms when dissolved ions carried by water solutions precipitate in cracks. (page 304)

Braided stream

A sediment-choked stream consisting of entwined subchannels. (page 474)

Chain reaction

A self-perpetuating process in a nuclear reaction, whereby neutrons released during the fission trigger more fission. (page 409)

Scientific Method

A sequence of steps for systematically analyzing scientific problems in a way that leads to verifiable results.

Metamorphic facies

A set of metamorphic mineral assemblages indicative of metamorphism under a specific range of pressures and temperatures. (page 226)

Depositional environment

A setting in which sediments accumulate; its character (fluvial, deltaic, reef, glacial, etc.) reflects local conditions. (page 201)

Supernova

A short-lived, very bright object in space that results from the cataclysmic explosion marking the death of a very large star; the explosion ejects large quantities of matter into space to form new nebulae. (page 19)

Crystal

A single, continuous piece of a mineral bounded by flat surfaces that formed naturally as the mineral grew. (page 30)

Moon

A sizable solid body locked in orbit around a planet. (page 14)

Wadati-Benioff zone

A sloping band of seismicity defined by intermediate- and deep-focus earthquakes that occur in the downgoing slab of a convergent plate boundary. (page 263)

Drilling mud

A slurry of water mixed with clay that oil drillers use to cool a drill bit and flush rock cuttings up and out of the hole. (page 399)

Fault scarp

A small step on the ground surface where one side of a fault has moved vertically with respect to the other. (page 306)

Pore

A small, open space within sediment or rock. (page 525)

Tributary

A smaller stream that flows into a larger stream. (page 466)

Meander

A snake-like curve along a stream's course. (page 475)

Metal

A solid composed almost entirely of atoms of metallic elements; it is generally opaque, shiny, smooth, malleable, and can conduct electricity. (page 417)

Glass

A solid in which atoms are not arranged in an orderly pattern. (page 84)

Magma chamber

A space below ground filled with magma. (page 145)

Hot spring

A spring that emits water ranging in temperature from about 30°C to 104°C. (page 534)

Reverse fault

A steeply dipping fault on which the hanging-wall block slides up. (page 306)

Meteor

A streak of bright, glowing gas created as a meteoroid vaporizes in the atmosphere due to friction. (page 35)

Permanent stream

A stream that flows year-round because its bed lies below the water table, or because more water is supplied from upstream than can infiltrate the ground. (page 467)

Disappearing stream

A stream that intersects a crack or sinkhole leading to an underground cavern, so that the water disappears into the subsurface and becomes an underground stream. (page 544)

Ephemeral stream

A stream whose bed lies above the water table, so that the stream flows only when the rate at which water enters the stream from rainfall or meltwater exceeds the rate at which water infiltrates the ground below. (page 467)

Shear stress

A stress that moves one part of a material sideways past another part. (page 303)

Tension

A stress that pulls on a material and could lead to stretching. (page 303)

Cinder cone

A subaerial volcano consisting of a cone-shaped pile of tephra whose slope approaches the angle of repose for tephra. (page 145)

Shield volcano

A subaerial volcano with a broad, gentle dome, formed either from low-viscosity basaltic lava or from large pyroclastic sheets. (page 145)

Turbidity current

A submarine avalanche of sediment and water that speeds down a submarine slope. (page 200)

Trap

A subsurface configuration of seal rocks and structures that keep oil and/or gas underground, so it doesn't seep out at the surface. (page 395)

Strata

A succession of several layers or beds together. (page 197)

Landslide

A sudden movement of rock and debris down a nonvertical slope. (page 268)

Pangaea

A supercontinent that assembled at the end of the Paleozoic Era. (page 376)

Gondwana

A supercontinent that consisted of today's South America, Africa, Antarctica, India, and Australia. (Also called Gondwanaland.) (page 374)

Dike

A tabular (wall-shaped) intrusion of rock that cuts across the layering of country rock. (page 121)

Equant

A term for a grain that has the same dimensions in all directions. (page 105)

Inequant

A term for a mineral grain whose length and width are not the same. (page 105)

Lahar

A thick slurry formed when volcanic ash and debris mix with water, either in rivers or from rain or melting snow and ice on the flank of a volcano. (page 144)

Mass extinction event

A time when vast numbers of species abruptly vanish. (page 336)

Mass-extinction event

A time when vast numbers of species abruptly vanish. (page 605)

Fuel

A transportable substance that can serve a supply of energy. (page 392)

Channel

A trough dug into the ground surface by flowing water. (page 464)

Canyon

A trough or valley with steeply sloping walls, cut into the land by a stream. (page 473)

Valley

A trough with sloping walls, cut into the land by a stream. (page 473)

Syncline

A trough-shaped fold whose limbs dip toward the hinge. (page 308)

Columnar jointing

A type of fracturing that yields roughly hexagonal columns of basalt; columnar joints form when a dike, sill, or lava flow cools. (page 139)

Batholith

A vast composite, intrusive, igneous rock body up to several hundred km long and 100 km wide, formed by the intrusion of numerous plutons in the same region. (page 121)

Oasis

A verdant region surrounded by desert, occurring at a place where natural springs provide water at the surface. (page 530)

Distillation column

A vertical pipe in which crude oil is separated into several components. (page 400)

Soil profile

A vertical sequence of distinct zones of soil. (page 180)

Red giant

A very large, red star, formed when a star the size of our Sun starts to die and undergoes an immense expansion. (page 627)

Earthquake

A vibration caused by the sudden breaking or frictional sliding of rock in the Earth. (page 246)

Supervolcano

A volcano that erupts a vast amount (more than 1,000 cubic km) of volcanic material during a single event; none have erupted during recorded human history. (page 148)

Active volcano

A volcano that has erupted within the past few centuries and will likely erupt again. (page 161)

Dormant volcano

A volcano that has not erupted for hundreds to thousands of years but does have the potential to erupt again in the future. (page 161)

Extinct volcano

A volcano that was active in the past but has now shut off entirely and will not erupt in the future. (page 161)

Delta

A wedge of sediment formed at a river mouth when the running water of the stream enters standing water, the current slows, the stream loses competence, and sediment settles out. (pages 471, 478)

Point bar

A wedge-shaped deposit of sediment on the inside bank of a meander. (page 471)

Accretionary prism

A wedge-shaped mass of sediment and rock scraped off the top of a downgoing plate and accreted onto the overriding plate at a convergent plate margin. (page 66)

Artesian well

A well in which water rises on its own. (page 532)

Ordinary well

A well whose base penetrates below the water table and can thus provide water. (page 532)

Unconventional reserve

An accumulation of hydrocarbons that are too viscous to flow, and/or that occur in impermeable rock, so that they cannot be pumped simply by drilling a well (examples -- tar sand, oil shale, shale oil/gas). (page 394)

Sediment

An accumulation of loose mineral grains, such as boulders, pebbles, sand, silt, or mud, that are not cemented together. (page 171)

Drainage network

An array of interconnecting streams that together drain an area. (page 466)

Fold-thrust belt

An assemblage of folds and related thrust faults that develop above a detachment fault. (page 313)

Modified Mercalli Intensity scale (MMI)

An earthquake characterization scale based on the amount of damage that the earthquake causes. (page 259)

Ore deposit

An economically significant accumulation of ore. (page 418)

Fossil fuel

An energy resource such as oil or coal that comes from organisms that lived long ago and thus stores solar energy that reached the Earth then. (page 392)

Effusive eruption

An eruption that yields mostly lava, not ash. (page 146)

Flood

An event during which the volume of water in a stream becomes so great that it covers areas outside the stream's normal channel. (page 464)

Epeirogeny

An event of epeirogenic movement; the term is usually used in reference to the formation of broad mid continent domes and basins. (page 323)

Outcrop

An exposure of bedrock. (page 104)

Stalactite

An icicle-like cone that grows from the ceiling of a cave as dripping water precipitates limestone. (page 540)

Heliocentric model

An idea proposed by Greek philosophers around 250 B.C.E. suggesting that all heavenly objects including the Earth orbited the Sun. (page 12)

Obsidian

An igneous rock consisting of a solid mass of volcanic glass. (page 129)

Equipotential surface

An imaginary surface on which all points have the same potential energy. (page 290)

Galaxy

An immense system of hundreds of billions of stars. (page 13)

Landform

An individual feature of the landscape, characterized by a distinctive shape. (page 429)

Anthropocene

An informal name for the most recent interval of geologic time, during which the significant impact of human societyon the Earth System will likely be preserved in the geologic record. (page 385)

Seismometer

An instrument that can record the ground motion from an earthquake. (page 255)

Dimension stone

An intact block of granite or marble to be used for architectural purposes. (page 422)

Core-mantle boundary

An interface 2,900 km below the Earth's surface separating the mantle and core. (page 287)

Period

An interval of geologic time representing a subdivision of a geologic era. (page 349)

Epoch

An interval of geologic time representing the largest subdivision of a period. (page 349)

Era

An interval of geologic time representing the largest subdivision of the Phanerozoic Eon. (page 349)

Pluton

An irregular or blob-shaped intrusion; can range in size from tens of m across to tens of km across. (page 121)

Seamount

An isolated submarine mountain. (page 53)

Star

An object in the Universe in which fusion reactions occur pervasively, producing vast amounts of energy; our Sun is a star. (page 12)

Planet

An object that orbits a star, is roughly spherical, and has cleared its neighborhood of other objects. (page 13)

Shield

An older, interior region of a continent. (page 322)

Stalagmite

An upward-pointing cone of limestone that grows when drips of water hit the floor of a cave. (page 540)

Seismic tomography

Analysis by sophisticated computers of global seismic data in order to create a three-dimensional image of variations in seismic-wave velocities within the Earth. (page 288)

Earth materials

Any of the substances (minerals, rocks, metals, sediments, soils) composing the solid Earth. (page 30)

Lapilli

Any pyroclastic particle that is 2 to 64 mm in diameter (i.e., marble-sized); the particles can consist of frozen lava clots, pumice fragments, or ash clumps. (page 142)

Greenhouse gas

Atmospheric gases, such as carbon dioxide and methane, that regulate the Earth's atmospheric temperature by absorbing infrared radiation. (page 613)

Portland cement

Cement made by mechanically mixing limestone, sandstone, and shale in just the right proportions, before heating in a kiln, to provide the correct chemical makeup of cement. (page 423)

Mountain belt

Chains or ranges of mountains (page 298)

Breccia

Coarse sedimentary rock consisting of angular fragments; or rock broken into angular fragments by faulting. (page 191)

Sandstone

Coarse-grained sedimentary rock consisting almost entirely of quartz. (page 189)

Peat

Compacted and partially decayed vegetation accumulating beneath a swamp. (page 405)

Crystalline solid

Containing a crystal lattice. (page 84)

Isotope

Different versions of a given element that have the same atomic number but different atomic weights. (page 356)

Soil horizon

Distinct zones within a soil, distinguished from each other by factors such as chemical composition and organic content. (page 180)

Seismicity

Earthquake activity. (page 248)

Intraplate earthquake

Earthquakes that occur away from plate boundaries. (page 265)

Volatile materials

Elements or compounds such as H2O and CO2 that evaporate at relatively low temperatures and can exist in gaseous forms at the Earth's surface. (pages 21, 31)

External energy

Energy from the Sun that drives components of the Earth System; it contributes to the circulation and flow of the atmosphere and hydrosphere, and serves a major role in driving erosion. (page 33)

Tidal power

Energy produced by the daily rise and fall of the tides; people can utilize this energy, for example, by damming a bay or estuary, so that water passes through turbines when the tide changes. (page 411)

Internal energy

Energy that comes from the Earth's internal heat; it drives plate tectonics, and therefore, earthquakes, volcanoes, and mountain building. (page 32)

Reflection

Energy waves bouncing off of a boundary. (page 285)

Eruption

Episodes when volcanoes extrude lava and pyroclastic debris. (page 137)

Geologic structure

Features formed by the deformation of rock. (page 298)

Siltstone

Fine-grained sedimentary rock generally composed of very small quartz grains. (page 191)

Slate

Fine-grained, low-grade metamorphic rock, formed by the metamorphism of shale. (page 221)

Seasonal flood

Floods that appear almost every year during seasons when rainfall is heavy or when winter snows start to melt. (page 482)

Dome

Folded or arched layers with the shape of an overturned bowl. (page 308)

Pressure

Force per unit area, or the 'push' acting on a material in cases where the push (compressional stretch) is the same in all directions. (page 303)

Pyroclastic debris

Fragmented material that sprayed out of a volcano and landed on the ground or sea floor in solid form. (page 141)

Biofuel

Gas or liquid fuel made from plant material (biomass). Examples of biofuel include alcohol (from fermented sugar), biodiesel from vegetable oil, and wood. (page 411)

Shale gas

Gas that comes directly from a source rock (organic shale). (page 401)

Stratigraphic correlation

Geologists determine such relations by a process called stratigraphic correlation (correlation, for short). (page 348)

Isostasy

Geologists refer to the condition that exists when balance has been achieved as isostasy, or isostatic equilibrium. (page 319)

Topographic profile

Geologists represent variations in elevation along a given traverse by means of a topographic profile, the trace of the ground surface as it would appear on a vertical plane that sliced into the ground. (page 431)

Runoff

Geologists use the term runoff for all water flowing on the surface of the Earth -- runoff includes sheetwash plus the water in streams. (page 465)

Mineral classes

Groups of minerals distinguished from each other on the basis of chemical composition. (page 95)

Shale oil

Oil that is still residing in shale, a source rock. (page 401)

Plate

One of about 20 distinct pieces of the relatively rigid lithosphere. (page 44)

Groundwater

Water that resides under the surface of the Earth, mostly in pores or cracks of rock or sediment. (page 523)

Compressional wave

Waves in which particles of material move back and forth parallel to the direction in which the wave itself moves. (page 254)

Seismic wave

Waves of energy emitted at the focus of an earthquake. (page 248)

Stream terrace

When a stream downcuts through the alluvium of a floodplain so that a new, lower floodplain develops and the original floodplain becomes a step-like platform. (page 474)

Sediment liquefaction

When pressure in the water in the pores push sediment grains apart so that they become surrounded by water and no longer rest against each other, and the sediment becomes able to flow like a liquid. (page 269)

Drainage reversal

When the overall direction of flow in a drainage network becomes the opposite of what it once had been. (page 479)

Hydrothermal metamorphism

When very hot water passes through the crust and causes metamorphism of rock. (page 231)

Petrified wood

Petrified wood forms when wood is permineralized -- in fact, the word petrified literally means turned to stone. (page 330)

Terrestrial planet

Planets that are of comparable size and character to the Earth and consist of a metallic core surrounded by a rock mantle. (page 13)

Active margin

Plate boundaries. (page 61)

Acid rain

Precipitation in which air pollutants react with water to make a weak acid that then falls from the sky. (page 625)

Igneous rock

Rock that forms when hot molten rock (magma or lava) cools and freezes solid. (page 114)

Metamorphic rock

Rock that forms when preexisting rock changes into new rock as a result of an increase in pressure and temperature and/or shearing under elevated temperatures; metamorphism occurs without the rock first becoming a melt or a sediment. (page 215)

Hornfels

Rock that undergoes metamorphism simply because of a change in temperature, without being subjected to differential stress. (page 223)

Reservoir rock

Rock with high porosity and permeability, so it can contain an abundant amount of easily accessible oil. (page 395)

Tar sand

Sandstone reservoir rock in which less viscous oil and gas molecules have either escaped or been eaten by microbes, so that only tar remains. (page 402)

Science

Science is simply the use of observation, experiment, and calculation to explain how nature operates, and scientists are people who study and try to understand natural phenomena.

Scientific Law

Scientific laws are not the same as scientific theories, as the former do not provide an explanation for a phenomenon, while the latter do.

Geologist

Scientists who study the Earth

Aquitard

Sediment or rock that does not transmit water easily and therefore retards the motion of the water. (page 525)

Aquifer

Sediment or rock that transmits water easily. (page 525)

Soil

Sediment that has undergone changes at the surface of the Earth, including reaction with rainwater and the addition of organic material. (page 178)

Cast

Sediment that preserves the shape of a shell it once filled before the shell dissolved or mechanically weathered away. (page 329)

Organic sedimentary rock

Sedimentary rock (such as coal) formed from carbon-rich relicts of organisms. (page 188)

Clastic rock

Sedimentary rock consisting of cemented-together detritus derived from the weathering of preexisting rock. (page 103)

Clastic sedimentary rock

Sedimentary rock consisting of cemented-together detritus derived from the weathering of preexisting rock. (page 188)

Biochemical sedimentary rock

Sedimentary rock formed from material (such as shells) produced by living organisms. (page 188)

Chemical sedimentary rock

Sedimentary rocks made up of minerals that precipitate directly from water solution. (page 188)

Shear wave

Seismic waves in which particles of material move back and forth perpendicular to the direction in which the wave itself moves. (page 254)

Body wave

Seismic waves that pass through the interior of the Earth. (page 254)

Surface wave

Seismic waves that travel along the Earth's surface. (page 254)

Stratigraphic group

Several adjacent stratigraphic formations in a succession. (page 346)

Oil shale

Shale containing kerogen. (pages 394, 402)

Lava flow

Sheets or mounds of lava that flow onto the ground surface or sea floor in molten form and then solidify. (page 138)

Shatter Cone

Small, cone-shaped fractures formed by the shock of a meteorite impact.

Energy resource

Something that can be used to produce work; in a geologic context, a material (such as oil, coal, wind, flowing water) that can be used to produce energy. (page 391)

Alluvium

Sorted sediment deposited by a stream. (page 471)

Vacuum

Space that contains very little matter in a given volume (e.g., a region in which air has been removed). (page 18)

Stick-slip behavior

Stop-start movement along a fault planecaused by friction, which prevents movement until stress builds up sufficiently. (page 252)

Refractory materials

Substances that have a relatively high melting point and tend to exist in solid form. (page 22)

Hydrosphere

The Earth's water, including surface water (lakes, rivers, and oceans), groundwater, and liquid water in the atmosphere. (page 432)

Silicon-oxygen tetrahedron

The SiO44 - anionic group, in which four oxygen atoms surround a single silicon atom, thereby defining the corners of a tetrahedron. (page 95)

Relative age

The age of one geologic feature with respect to another. (page 340)

Energy density

The amount of energy that can be produced per unit mass of a fuel. (page 393)

Displacement

The amount of movement or slip across a fault plane. (page 304)

Biomass

The amount of organic material in a specified volume. (page 392)

Magnetic inclination

The angle between a magnetic needle free to pivot on a horizontal axis and a horizontal plane parallel to the Earth's surface. (page 48)

Magnetic declination

The angle between the direction a compass needle points at a given location and the direction of true north. (page 48)

Dip

The angle of a plane's slope as measured in a vertical plane perpendicular to the strike. (page 305)

Low-velocity zone (LVZ)

The asthenosphere underlying oceanic lithosphere in which seismic waves travel more slowly, probably because rock has partially melted. (page 286)

Gravity

The attractive force that one mass exerts on another; the magnitude depends on the size of the objects and the distance between them.

Residence time

The average length of time that a substance stays in a particular reservoir. (page 436)

Recurrence interval

The average time between successive geologic events. (page 485)

Climate

The average weather conditions, along with the range of conditions, of a region over a year. (page 611)

Headwaters

The beginning point of a stream. (page 464)

Ductile deformation

The bending and flowing of a material (without cracking and breaking) subjected to stress. (page 301)

Refraction

The bending of a ray as it passes through a boundary between two different materials. (page 285)

Plate boundary

The border between two adjacent lithosphere plates. (page 61)

Wave front

The boundary between the region through which a wave has passed and the region through which it has not yet passed. (page 283)

Water table

The boundary, approximately parallel to the Earth's surface, that separates substrate in which groundwater fills the pores from substrate in which air fills the pores. (page 525)

Domain

The broadest divisions of life (Archaea, Bacteria, Eukaryota). (page 332)

Energy

The capacity to do work. (page 392)

Strain

The change in shape of an object in response to deformation (i.e., as a result of the application of a stress) (page 299)

Magnetic reversal

The change of the Earth's magnetic polarity; when a reversal occurs, the field flips from normal to reversed polarity, or vice versa. (page 57)

Shock metamorphism

The changes that can occur in a rock due to the passage of a shock wave, generally resulting from a meteorite impact. (page 232)

Seismic ray

The changing position of an imaginary point on a wave front as the front moves through rock. (page 283)

Eruptive style

The character of a particular volcanic eruption; geologists name styles based on typical examples (e.g., Hawaiian, Strombolian). (page 145)

Color

The characteristic of a material due to the spectrum of light emitted or reflected by the material, as perceived by eyes or instruments. (page 91)

Rock composition

The chemical makeup of rock, typically represented by the proportions of different minerals. (page 105)

Black smoker

The cloud of suspended minerals formed where hot water spews out of a vent along a mid-ocean ridge; the dissolved sulfide components of the hot water instantly precipitate when the water mixes with seawater and cools. (page 63)

Streak

The color of the powder produced by pulverizing a mineral on an unglazed ceramic plate. (page 91)

Grade

The concentration of a useful metal in an ore -- the higher the concentration, the higher the grade. (page 418)

Elastic-rebound theory

The concept that earthquakes happen because stress builds up, causing rock adjacent to a fault to bend elastically until breaking and slip on a fault occurs; the slip relaxes the elastic bending and decreases stress. (page 252)

Nebular theory

The concept that planets grow out of rings of gas, dust, and ice surrounding a newborn star. (page 18)

Steady-state condition

The condition when proportions of a chemical in different reservoirs remain fairly constant even though there is a constant flux (flow) of the chemical among the reservoirs. (page 607)

Weather

The conditions of temperature, wind speed, humidity, air pressure, and storminess at a location at a given time. (page 611)

Energy grid

The configuration of power-generating sources, power lines, and transformers that distributes electrical power nationally. (page 415)

Hydrologic cycle

The continual passage of water from reservoir to reservoir in the Earth System. (page 433)

Brittle deformation

The cracking and fracturing of a material subjected to stress. (page 301)

Extinction

The death of the last members of a species so that there are no parents to pass on their genetic traits to offspring. (page 336)

Lower mantle

The deepest section of the mantle, stretching from 670 km down to the core-mantle boundary. (page 287)

Permeability

The degree to which a material allows fluids to pass through it via an interconnected network of pores and cracks. (page 525)

Core

The dense, iron-rich center of the Earth.

Magnetic anomaly

The difference between the expected strength of the Earth's magnetic field at a certain location and the actual measured strength of the field at that location. (page 294)

Marine magnetic anomaly

The difference between the expected strength of the Earth's main dipole field at a certain location on the sea floor and the actual measured strength of the magnetic field at that location. (page 55)

Relief

The difference in elevation between adjacent high and low regions on the land surface. (page 429)

Accretionary Disk

The disk-shaped body of gas, ice, and dust onto which matter falls; it ultimately evolves into a solar system.

Cone of depression

The downward-pointing, cone-shaped surface of the water table in a location where the water table is experiencing drawdown because of pumping at a well. (page 532)

Lava tube

The empty space left when a lava tunnel drains; this happens when the surface of a lava flow solidifies while the inner part of the flow continues to stream downslope. (page 138)

Gravitational potential energy

The energy stored in an object because of its position in a gravitational field; the energy is released when the object falls to a lower elevation. (page 289)

Biogeochemical cycle

The exchange of chemicals between living and nonliving reservoirs in the Earth System. (page 607)

Underground mining

The extraction of ore or coal by digging shafts and tunnels underground. (page 407)

Geocentric model

The first school advocated a geocentric model, in which the Earth sits motionless at the center of the Universe while the Moon and the planets whirl around it, and everything lies inside a revolving globe of stars. (page 12)

Floodplain

The flat land on either side of a stream that becomes covered with water during a flood. (pages 471, 476)

Crystal face

The flat surfaces of a crystal, formed during the crystal's growth. (page 85)

Slab-pull force

The force that downgoing plates (or slabs) apply to oceanic lithosphere at a convergent margin. (page 75)

Permineralization

The fossilization process in which plant material becomes transformed into rock by the precipitation of silica from groundwater. (page 330)

Giant planet

The four outer, or Jovian, planets of our Solar System, which are significantly larger than the rest of the planets and consist largely of gas and/or ice. (page 13)

Crystal habit

The general shape of a crystal or cluster of crystals that grew unimpeded. (page 93)

Earth System

The global interconnecting web of physical and biological phenomena involving the solid Earth, the hydrosphere, and the atmosphere. (page 28)

Seafloor spreading

The gradual widening of an ocean basin as new oceanic crust forms at a mid-ocean ridge axis and then moves away from the axis. (page 54)

Erosion

The grinding away and removal of Earth's surface materials by moving water, air, or ice. (page 429)

Facet

The ground and polished surface of a gem, produced by a gem cutter using a grinding lap. (page 99)

Magnetic-reversal chronology

The history of magnetic reversals through geologic time. (page 57)

Punctuated equilibrium

The hypothesis that evolution takes place in fits and starts; evolution occurs very slowly for quite a while and then, during a relatively short period, takes place very rapidly. (page 335)

Cryosphere

The ice components of the Earth System, including glaciers, snow, sea ice, and permafrost. (page 29)

Geoid

The imaginary shape a global ocean surface would take if impacted by only the Earth's gravitational pull and rotation. (page 291)

Axial surface

The imaginary surface that encompasses the hinges of successive layers of a fold. (page 308)

Bioremediation

The injection of oxygen and nutrients into a contaminated aquifer to foster the growth of bacteria that will ingest or break down contaminants. (page 539)

Transgression

The inland migration of shoreline resulting from a rise in sea level. (page 210)

Precambrian

The interval of geologic time between Earth's formation about 4.57 Ga and the beginning of the Phanerozoic Eon 542 Ma. (page 349)

Alpine-Himalayan chain

The largest orogenic belt on Earth today, formed by collisions of the former Gondwana continents with the southern margins of Europe and Asia. (page 382)

Eon

The largest subdivision of geologic time. (page 349)

Zone of leaching

The layer of regolith in which water dissolves ions and picks up very fine clay; these materials are then carried downward by infiltrating water. (page 179)

Asthenosphere

The layer of the mantle that lies between 100 - 150 km and 350 km deep; the asthenosphere is relatively soft and can flow when acted on by force. (page 38)

Annual probability

The likelihood that a flood of a given size or larger will happen at a specified locality during any given year. (page 485)

Preservation potential

The likelihood that an organism will be preserved in the fossil record. (page 331)

Focus

The location where a fault slips during an earthquake (hypocenter). (page 253)

Base level

The lowest elevation a stream channel's floor can reach at a given locality. (page 472)

Main shock

The major earthquake. (page 252)

Archimedes' principle

The mass of the water displaced by a block of material equals the mass of the whole block of material. (page 291)

Competence

The maximum particle size a stream can carry. (page 470)

Partial melting

The melting in a rock of the minerals with the lowest melting temperatures, while other minerals remain solid. (page 118)

Meltdown

The melting of the fuel rods in a nuclear reactor that occurs if the rate of fission becomes too fast and the fuel rods become too hot. (page 409)

Preferred orientation

The metamorphic texture that exists where platy grains lie parallel to one another and/or elongate grains align in the same direction. (page 218)

Archean Eon

The middle Precambrian eon (4.0 - 2.5 Ga). (page 367)

Transition zone

The middle portion of the mantle, from 400 to 670 km deep, in which there are several jumps in seismic velocity. (page 287)

Mineral resource

The minerals extracted from the Earth's upper crust for practical purposes. (page 391)

Climate change

The modification of average atmospheric temperature, oceanic temperature, and other weather characteristics in a given region over time. (page 611)

Phanerozoic Eon

The most recent eon, an interval of time from 542 Ma to the present. (page 372)

Pleistocene Ice Age

The most recent ice age in Earth history. It began at about 2.6 Ma, and has included several advances and retreats. (page 383)

Proterozoic Eon

The most recent of the Precambrian eons (2,500 - 541 Ma). (page 369)

Absolute plate velocity

The movement of a plate relative to a fixed point in the mantle. (page 77)

Saltation

The movement of a sediment in which grains bounce along their substrate, knocking other grains into the water column (or air) in the process. (page 470)

Carbon cycle

The movement of carbon among various reservoirs in the Earth System; this carbon can occur in the form of CO2, CaCO3, as pure carbon, or in a great variety of organic molecules. (page 610)

Migrate

The movement of hydrocarbons from source rocks to reservoir rocks. (page 395)

Relative plate velocity

The movement of one lithosphere plate with respect to another. (page 75)

Oil window

The narrow range of temperatures under which oil can form in a source rock. (page 394)

Magnitude

The number that represents the maximum amplitude of ground motion that would be measured by a seismometer placed at a specified distance from the epicenter (page 259)

Hadean Eon

The oldest of the Precambrian eons; the time between Earth's origin and the formation of the first rocks that have been preserved. (page 366)

Crystal lattice

The orderly framework within which the atoms or ions of a mineral are fixed. (page 84)

Protolith

The original rock from which a metamorphic rock formed. (page 215)

Mouth

The outlet of a stream where it discharges into another stream, a lake, or a sea. (page 464)

Landscape

The overall shape of the Earth's surface in a region. (page 429)

Nuclear reactor

The part of a nuclear power plant where the fission reactions occur. (page 409)

Paleoclimate

The past climate of the Earth. (page 611)

Holocene Epoch

The period of geologic time since the last glaciation. (page 384)

Oil Age

The period of human history, including our own, so named because the economy depends on oil. (page 413)

Cementation

The phase of lithification in which cement, consisting of minerals that precipitate from groundwater, partially or completely fills the spaces between clasts and attaches each grain to its neighbor. (page 189)

Compaction

The phase of lithification in which the pressure of the overburden on the buried rock squeezes out water and air that was trapped between clasts, and the clasts press tightly together. (page 189)

Ecliptic

The plane defined by a planet's orbit. (page 13)

Hubbert's Peak

The point on a production graph showing a resource plotted against time, at which the rate of production levels off and starts to decrease. (page 414)

Epicenter

The point on the surface of the Earth directly above the focus of an earthquake. (page 254)

Slickensides

The polished surface of a fault caused by slip on the fault; lineated slickensides also have grooves that indicate the direction of fault movement. (page 306)

Hinge

The portion of a fold where curvature is greatest. (page 308)

Hydraulic head

The potential energy available to drive the flow of a given volume of groundwater at a location; it can be measured as an elevation above a reference. (page 528)

Exhumation

The process (involving uplift and erosion) that returns deeply buried rocks to the surface. (page 321)

Continental rift

The process by which a continent stretches and splits along a belt; if it is successful, rifting separates a larger continent into two smaller continents separated by a divergent boundary. (page 72)

Fractional crystallization

The process by which a magma becomes progressively more silicic as it cools, because early-formed crystals settle out. (page 120)

Radioactive decay

The process by which a radioactive atom undergoes fission or releases particles, thereby being transformed into a new element. (page 356)

Metasomatism

The process by which a rock's overall chemical composition changes during metamorphism because of reactions with hot water that bring in or remove elements. (page 219)

Headward erosion

The process by which a stream channel lengthens up its slope as the flow of water increases. (page 465)

Metamorphism

The process by which one kind of rock transforms into a different kind of rock. (page 215)

Subduction

The process by which one oceanic plate bends and sinks down into the asthenosphere beneath another plate. (page 65)

Deposition

The process by which sediment settles out of a transporting medium. (page 429)

Natural selection

The process by which the fittest organisms survive to pass on their characteristics to the next generation. (page 335)

Nuclear fusion

The process by which the nuclei of atoms fuse together, thereby creating new, larger atoms. (page 18)

Chemical weathering

The process in which chemical reactions alter or destroy minerals when rock comes in contact with water solutions and/or air. (page 174)

Physical weathering

The process in which intact rock breaks into smaller grains or chunks. (page 172)

Orogenic collapse

The process in which mountains begin to collapse under their own weight and spread out laterally. (page 321)

Abrasion

The process in which one material (such as sand-laden water) grinds away at another (such as a stream channel's floor and walls). (page 469)

Downcutting

The process in which water flowing through a channel cuts into the substrate and deepens the channel relative to its surroundings. (page 465)

Mountain building

The process of causing a belt of land to undergo significant uplift, usually in association with deformation. (page 298)

Directional drilling

The process of controlling the trajectory of a drill bit to make sure that the drill hole goes exactly where desired. (page 399)

Fossilization

The process of forming a fossil. (page 327)

Assimilation

The process of magma contamination in which blocks of wall rock fall into a magma chamber and dissolve. (page 118)

Coal gasification

The process of producing relatively clean-burning gases from solid coal. (page 407)

Collision

The process of two buoyant pieces of lithosphere converging and squashing together. (page 72)

Weathering

The processes that break up and corrode solid rock, eventually transforming it into sediment. (page 172)

Stellar nucleosynthesis

The production of new, larger atoms by fusion reactions in stars; the process generates more massive elements that were not produced by the Big Bang. (page 19)

Stress

The push, pull, or shear that a material feels when subjected to a force; formally, the force applied per unit area over which the force acts. (page 303)

Coal reserve

The quantities of discovered, but not yet mined, coal in sedimentary rock of the continents. (page 406)

Geothermal gradient

The rate of change in temperature with depth. (page 36)

Seismogram

The record of an earthquake produced by a seismograph. (page 256)

Paleomagnetism

The record of ancient magnetism preserved in rock. (page 49)

Albedo

The reflectivity of a surface. (page 616)

Magnetic field

The region affected by the force emanating from a magnet. (page 26)

Metamorphic aureole

The region around a pluton, stretching tens to hundreds of meters out, in which heat transferred into the country rock and metamorphosed the country rock. (page 227)

Metamorphic zone

The region between two metamorphic isograds, typically named after an index mineral found within the region. (page 224)

Biosphere

The region of the Earth and atmosphere inhabited by life; this region stretches from a few km below the Earth's surface to a few km above. (page 31)

Magnetosphere

The region protected from the electrically charged particles of the solar winds by Earth's magnetic field. (page 26)

Watershed

The region that collects water that feeds into a given drainage network. (page 467)

Seismic belt

The relatively narrow strips of crust on Earth under which most earthquakes occur. (page 262)

Lithosphere

The relatively rigid, nonflowable, outer 100- to 150-km-thick layer of the Earth, constituting the crust and the top part of the mantle. (page 59)

Cambrian explosion

The remarkable diversification of life, indicated by the fossil record, that occurred at the beginning of the Cambrian Period. (page 375)

Fossil

The remnant, or trace, of an ancient living organism that has been preserved in rock or sediment. (page 327)

Groundwater depletion

The removal of groundwater at a rate exceeding the natural resupply of groundwater in a region. (page 536)

Soil erosion

The removal of soil by wind and runoff. (page 183)

Stream rejuvenation

The renewed downcutting of a stream into a floodplain or peneplain, caused by a relative drop of the base level. (page 479)

Viscosity

The resistance of material to flow. (page 138)

Pillow lava

The rind of a pillow momentarily stops the flow's advance, but within minutes the pressure of the lava squeezing into the pillow breaks the rind, and a new blob of lava squirts out, freezes, and produces another pillow. A flow made of these blobs is called pillow lava. (page 139)

Crust

The rock that makes up the outermost layer of the Earth. (page 36)

Radiometric dating

The science of dating geologic events in years by measuring the ratio of parent radioactive atoms to daughter product atoms. (page 356)

Geochronology

The science of dating geologic events in years. (page 356)

Regression

The seaward migration of a shoreline caused by a lowering of sea level. (page 210)

Moho

The seismic-velocity discontinuity that defines the boundary between the Earth's crust and mantle. Named for Andrija Mohorovičić. (page 285)

Bowen's reaction series

The sequence in which different silicate minerals crystallize during the progressive cooling of a melt. (page 122)

Fossil succession

The sequence of assemblages of fossil species preserved in the stratigraphic record. (page 341)

Geologic history

The sequence of geologic events that has taken place in a region. (page 341)

Aftershock

The series of smaller earthquakes that follow a major earthquake. (page 252)

Foreshock

The series of smaller earthquakes that precede a major earthquake. (page 252)

Limb (of fold)

The side of a fold, showing less curvature than at the hinge. (page 308)

Stream gradient

The slope of a stream's channel in the downstream direction. (page 472)

Geosphere

The solid Earth, from the surface to the center, constitutes the geosphere. (page 29)

Geologic time

The span of time since the formation of the Earth. (page 339)

Stellar wind

The stream of atoms emitted from a star into space. (page 19)

Taxonomy

The study and classification of the relationships among different forms of life. (page 332)

Paleontology

The study of life history through the examination of the fossil record and its relation to the stratigraphic record. (page 327)

Geophysics

The study of the Earth using quantitative analysis of earthquake waves, magnetic fields, and gravity, and/or the production of mathematical models of the behavior of the Earth's interior. (page 283)

Geology

The study of the Earth, including our planet's composition, behavior, and history.

Cosmology

The study of the overall structure of the Universe. (page 11)

Rock cycle

The succession of events that results in the transformation of Earth materials from one rock type to another, then another, and so on. (page 237)

Evapotranspiration

The sum of evaporation from bodies of water and the ground surface and transpiration from plants and animals. (page 436)

Paleopole

The supposed position of the Earth's magnetic pole in the past, with respect to a particular continent. (page 50)

Suture

The surface across which two separate blocks of crust areattached, after ocean floor between them has been subducted. (page 314)

Closure temperature

The temperature at which parent and daughter isotopes can no longer escape from a mineral, so the ratio of parents to daughters can be used for isotopic dating. (page 357)

Gradualism

The theory that evolution happens at a constant, slow rate. (page 335)

The Theory of Plate Tectonics

The theory that the outer layer of the Earth (the lithosphere) consists of separate plates that move with respect to one another.

Plate tectonics

The theory that the outer layer of the Earth (the lithosphere) consists of separate plates that move with respect to one another. (page 44)

Expanding Universe theory

The theory that the whole Universe must be expanding because galaxies in every direction seem to be moving away from us. (page 15)

Mantle

The thick layer of rock below the Earth's crust and above the core.

Capillary fringe

The thin subsurface layer in which water molecules seep up from the water table by capillary action to fill pores. (page 526)

Travel time

The time for seismic waves to pass through the Earth from the focus of the earthquake to a seismometer in that rock. (page 283)

Great oxygenation event

The time in Earth's history, about 2.4 Ga, when the concentration of oxygen in the atmosphere increased dramatically. (page 371)

Half-life

The time it takes for half of a group of a radioactive element's isotopes to decay. (page 356)

Topsoil

The top soil horizons, which are typically dark and nutrient-rich. (page 180)

Capacity

The total quantity of sediment a stream can carry. (page 471)

Porosity

The total volume of empty space (pore space) in a material, usually expressed as a percentage. (page 525)

Lithification

The transformation of loose sediment into solid rock through compaction and cementation. (page 189)

Global change

The transformations or modifications of both physical and biological components of the Earth System through time. (page 604)

Greenhouse effect

The trapping of heat in the Earth's atmosphere by carbon dioxide and other greenhouse gases, which absorb infrared radiation; somewhat analogous to the effect of glass in a greenhouse. (page 613)

Downslope movement

The tumbling or sliding of rock and sediment from higher elevations to lower ones. (page 429)

Upper mantle

The uppermost section of the mantle, reaching down to a depth of 400 km. (page 287)

Subsidence

The vertical sinking of the Earth's surface in a region, relative to a reference plane. (page 429)

Discharge

The volume of water in a conduit or channel passing a point in 1 second. (page 478)

Kerogen

The waxy molecules into which the organic material in shale transforms on reaching about 100°C. At higher temperatures, kerogen transforms into oil. (page 394)

Luster

The way a mineral surface scatters light. (page 91)

Heat

Thermal energy resulting from the movement of molecules. (page 20)

Evaporite

Thick salt deposits that form as a consequence of precipitation from saline water. (page 191)

Volcanic ash

Tiny glass shards formed when a fine spray of exploded lava freezes instantly upon contact with the atmosphere. (page 115)

Ash

Tiny glass shards formed when a fine spray of exploded lava freezes instantly upon contact with the atmosphere. (page 142)

Suspended load

Tiny solid grains carried along by a stream without settling to the floor of the channel. (page 470)

Aerosol

Tiny solid particles or liquid droplets that remain suspended in the atmosphere for a long time. (page 144)

Planetesimal

Tiny, solid pieces of rock and metal that collect in a planetary nebula and eventually accumulate to form a planet. (page 22)

Polymorph

Two minerals that have the same chemical composition but a different crystal lattice structure. (page 88)

Tephra

Unconsolidated accumulations of pyroclastic grains. (page 141)

Soil moisture

Underground water that wets the surface of the mineral grains and organic material making up soil, but lies above the water table. (page 525)

Bathymetry

Variation in depth. (page 30)

Topography

Variations in elevation. (page 431)

Flood basalt

Vast sheets of basalt that spread from a volcanic vent over an extensive surface of land; they may form where a rift develops above a continental hot spot, and where lava is particularly hot and has low viscosity. (page 156)

Conglomerate

Very coarse-grained sedimentary rock consisting of rounded clasts. (page 191)

Shale

Very fine-grained sedimentary rock that breaks into thin sheets. (page 191)

Mudstone

Very fine-grained sedimentary rock that will not easily split into sheets. (page 191)

Explosive eruption

Violent volcanic eruptions that produce clouds and avalanches of pyroclastic debris. (page 146)

Tectonic foliation

planar fabric, such as cleavage, schistocity, or gneissic banding, that develops in rocks; caused by compression or shearing during deformation (e.g., during mountain building). (page 312)

Crater

(1) A circular depression at the top of a volcanic mound; (2) a depression formed by the impact of a meteorite. (page 145)

Bar

(1) A sheet or elongate lens or mound of alluvium; (2) a unit of air-pressure measurement approximately equal to 1 atm. (page 471)

Volcano

(1) A vent from which melt from inside the Earth spews out onto the planet's surface; (2) a mountain formed by the accumulation of extrusive volcanic rock. (page 137)

Dry well

(1) A well that does not supply water because the well has been drilled into an aquitard or into rock that lies above the water table; (2) a well that does not yield oil, even though it has been drilled into an anticipated reservoir. (page 532)

Zone of accumulation

(1) The layer of regolith in which new minerals precipitate out of water passing through, thus leaving behind a load of fine clay; (2) the area of a glacier in which snowfall adds to the glacier. (page 179)

Sorting

(1) The range of clast sizes in a collection of sediment; (2) the degree to which sediment has been separated by flowing currents into different-sized fractions. (page 190)

Groundwater contamination

Addition of chemicals or microbes (e.g., from agricultural and industrial activities, and landfills or septic tanks) to the groundwater supply. (page 537)

Universe

All of space and all the matter and energy within it. (page 12)

Diagenesis

All of the physical, chemical, and biological processes that transform sediment into sedimentary rock and that alter the rock after the rock has formed. (page 210)

Ripple mark

Relatively small elongated ridges that form on a sedimentary bed surface at right angles to the direction of current flow. (page 199)

Greenhouse (hothouse) period

Relatively warm global climate leading to the rising of sea level for an interval of geologic time. (page 611)

Friction

Resistance to sliding on a surface (page 249)

Spire

Rhyolitic lava freezes while still in the vent and then pushes upward as a column-like spire up to 100 m above the vent. (page 139)

Silicate rock

Rock composed of silicate minerals. (page 31)

Ore

Rock containing native metals or a concentrated accumulation of ore minerals. (page 418)

Intrusive igneous rock

Rock formed by the freezing of magma underground. (page 115)

Pyroclastic rock

Rock made from fragments that were blown out of a volcano during an explosion and were then packed or welded together. (page 129)

Bedrock

Rock still attached to the Earth's crust. (page 104)

Extrusive igneous rock

Rock that forms by the freezing of lava above ground, after it flows or explodes out (extrudes) onto the surface and comes into contact with the atmosphere or ocean. (page 115)

Sedimentary rock

Rock that forms either by the cementing together of fragments broken off preexisting rock or by the precipitation of mineral crystals out of water solutions at or near the Earth's surface. (page 188)