- A -

• a’a’ A lava flow with a rubbly surface.
• abandoned meander A meander that dries out after being cut off.
• ablation The removal of ice at the toe of a glacier by melting, sublimation (the evaporation of ice into water vapor), and/or calving.
• 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).
• absolute age Numerical age (the age specified in years).
• absolute plate velocity The movement of a plate relative to a fixed point in the mantle.
• abyssal plain A broad, relatively flat region of the ocean that lies at least 4.5 km below sea level.
• Acadian orogeny A convergent mountain-building event that occurred around 400 million years ago, during which continental slivers accreted to the eastern edge of the North American continent.
• accreted terrane A block of crust that collided with a continent at a convergent margin and stayed attached to the continent.
• accretionary coast A coastline that receives more sediment than erodes away.
• accretionary orogen An orogen formed by the attachment of numerous buoyant slivers of crust to an older, larger continental block.
• 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.
• acid mine runoff A dilute solution of sulfuric acid, produced when sulfur-bearing minerals in mines react with rainwater, that flows out of a mine.
• acid rain Precipitation in which air pollutants react with water to make a weak acid that then falls from the sky.
• active continental margin A continental margin that coincides with a plate boundary.
• active fault A fault that has moved recently or is likely to move in the future.
• active sand The top layer of beach sand, which moves daily because of wave action.
• active volcano A volcano that has erupted within the past few centuries and will likely erupt again.
• adiabatic cooling The cooling of a body of air or matter without the addition or subtraction of thermal energy (heat).
• adiabatic heating The warming of a body of air or matter without the addition or subtraction of heat.
• aerosols Tiny solid particles or liquid droplets that remain suspended in the atmosphere for a long time.
• aftershocks The series of smaller earthquakes that follow a major earthquake.
• agents of erosion Components of the Earth System (e.g., moving glaciers, rivers, wind) that cause erosion to occur.
• air The mixture of gases that make up the Earth’s atmosphere.
• air-fall tuff Tuff formed when ash settles gently from the air.
• air mass A body of air, about 1,500 km across, that has recognizable physical characteristics.
• air pressure The push that air exerts on its surroundings.
• albedo The reflectivity of a surface.
• Alleghenian orogeny The orogenic event that occurred about 270 million years ago when Africa collided with North America.
• alloy A metal containing more than one type of metal atom.
• alluvial fan A gently sloping apron of sediment dropped by an ephemeral stream at the base of a mountain in arid or semi-arid regions.
• alluvium Sorted sediment deposited by a stream.
• alluvium-filled valley A valley whose floor fills with sediment.
• amber Hardened (fossilized) ancient sap or resin.
• amphibolite facies A set of metamorphic mineral assemblages formed under intermediate pressures and temperatures.
• amplitude The height of a wave from crest to trough.
• Ancestral Rockies The late Paleozoic uplifts of the Rocky Mountain region; they eroded away long before the present Rocky Mountains formed.
• angiosperm A flowering plant.
• angle of repose The angle of the steepest slope that a pile of uncemented material can attain without collapsing from the pull of gravity.
• angularity The degree to which grains have sharp or rounded edges or corners.
• angular unconformity An unconformity in which the strata below were tilted or folded before the unconformity developed; strata below the unconformity therefore have a different tilt than strata above.
• anhedral grains Crystalline mineral grains without well-formed crystal faces.
• annual probability The likelihood, expressed as a percentage, that an event (e.g., a flood of a given size) will happen in a given year.
• Antarctic bottom water mass The mass of cold, dense water that sinks along the coast of Antarctica.
• antecedent stream A stream that cuts across an uplifted mountain range; the stream must have existed before the range uplifted and must then have been able to downcut as fast as the land was rising.
• anthracite coal Shiny black coal formed at temperatures between 200° and 300°C. A high-rank coal.
• anticline A fold with an arch-like shape in which the limbs dip away from the hinge.
• anticyclone The clockwise flow of air around a high-pressure mass.
• Antler orogeny The Late Devonian mountain-building event in which slices of deep-marine strata were pushed eastward, up and over the shallow-water strata on the western coast of North America.
• anvil cloud A large cumulonimbus cloud that spreads laterally at the tropopause to form a broad, flat top.
• aphanitic A textural term for fine-grained igneous rock.
• 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.
• aquiclude Sediment or rock that transmits no water.
• aquifer Sediment or rock that transmits water easily.
• aquitard Sediment or rock that does not transmit water easily and therefore retards the motion of the water.
• archaea A kingdom of “old bacteria,” now commonly found in extreme environments like hot springs. (Also called “archaeobacteria.”)
• Archean Eon The middle Precambrian Eon.
• Archimedes’ principle The mass of the water displaced by a block of material equals the mass of the whole block of material.
• arête A residual knife-edge ridge of rock that separates two adjacent cirques.
• argillaceous sedimentary rock Sedimentary rock that contains abundant clay.
• arkose A clastic sedimentary rock composed of sand-sized grains that include quartz and feldspar.
• arroyo The channel of an ephemeral stream; dry wash; wadi.
• artesian spring A place where groundwater gushes out of the ground under pressure.
• artesian well A well in which water rises on its own.
• ash fall Ash that falls to the ground out of an ash cloud.
• ash flow An avalanche of ash that tumbles down the side of an explosively erupting volcano.
• ash (volcanic) Very fine particles erupted by a volcano; they consist of glass shards formed by the rapid cooling of lava droplets erupted into the air, and/or of tiny rock particles blasted into the air by an explosion.
• assimilation The process of magma contamination in which blocks of wall rock fall into a magma chamber and dissolve.
• asteroid One of many millions of small, rocky, and/or metallic objects that orbit the Sun, consisting of fragments of once-larger planetesimals, or chunks of protoplanetary material; most lie in the region between Mars and Jupiter.
• 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.
• atm A unit of air pressure that approximates the pressure exerted by the atmosphere at sea level.
• atmosphere A layer of gases that surrounds a planet.
• atoll A coral reef that develops around a circular reef surrounding a lagoon.
• atomic number The number of protons in the nucleus of a given element.
• atomic weight The number of protons plus the number of neutrons in the nucleus of a given element. (Also known as atomic mass.)
• aurora australis The same phenomenon as the aurora borealis, but in the Southern Hemisphere.
• aurora borealis A ghostly curtain of varicolored light that appears across the night sky in the Northern Hemisphere when charged particles from the Sun interact with the ions in the ionosphere.
• avalanche A turbulent cloud of debris mixed with air that rushes down a steep hill slope at high velocity; the debris can be rock and/or snow.
• avalanche chute A downslope hillside pathway along which avalanches repeatedly fall, consequently clearing the pathway of mature trees.
• avulsion The process in which a river overflows a natural levee and begins to flow in a new direction.
• axial plane The imaginary surface that encompasses the hinges of successive layers of a fold.
• axial surface In the context of folds, this is the imaginary plane that contains the hinge lines of successive layers in the fold; it is the surface that divides a fold into its two separate limbs.
• axial trough A narrow depression that runs along a mid-ocean ridge axis.

- B -

• backscattered light Atmospheric scattered sunlight that returns back to space.
• backshore zone The zone of beach that extends from a small step cut by high-tide swash to the front of the dunes or cliffs that lie farther inshore.
• backswamp The low marshy region between the bluffs and the natural levees of a floodplain.
• backwash The gravity-driven flow of water back down the slope of a beach.
• bajada An elongate wedge of sediment formed by the overlap of several alluvial fans emerging from adjacent valleys.
• Baltica A Paleozoic continent that included crust that is now part of today’s Europe.
• banded-iron formation (BIF) Iron-rich sedimentary layers consisting of alternating gray beds of iron oxide and red beds of iron-rich chert.
• bar (1) A sheet or elongate lens or mound of alluvium; (2) a unit of air pressure measurement approximately equal to 1 atm.
• barchan dune A crescent-shaped dune whose tips point downwind.
• barrier island An offshore sand bar that rises above the mean high-water level, forming an island.
• barrier reef A coral reef that develops offshore, separated from the coast by a lagoon.
• basal sliding The phenomenon in which meltwater accumulates at the base of a glacier, so that the mass of the glacier slides on a layer of water or on a slurry of water and sediment.
• basalt A fine-grained mafic igneous rock.
• base level The lowest elevation a stream channel’s floor can reach at a given locality.
• basement Older igneous and metamorphic rocks making up the Earth’s crust beneath sedimentary cover.
• basement uplift Uplift of basement rock by faults that penetrate deep into the continental crust.
• base metals Metals that are mined but not considered precious. Examples include copper, lead, zinc, and tin.
• basin A fold or depression shaped like a right-side-up bowl.
• 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.
• batholith A vast composite, intrusive, igneous rock body up to several hundred kilometers long and 100 km wide, formed by the intrusion of numerous plutons in the same region.
• bathymetric map A map illustrating the shape of the ocean floor.
• bathymetric profile A cross section showing ocean depth plotted against location.
• bathymetry Variation in depth.
• bauxite A residual mineral deposit rich in aluminum.
• baymouth bar A sandspit that grows across the opening of a bay.
• beach A band of sand or gravel that lies along a coastline; wave action affects the profile and sediment size of a beach.
• beach drift The gradual migration of sand along a beach.
• beach erosion The removal of beach sand caused by wave action and long-shore currents.
• beach face A steeply concave part of the foreshore zone formed where the swash of the waves actively scours the sand.
• bed A distinct layer of sedimentary strata.
• bedding Layering or stratification in sedimentary rocks.
• bed load Large particles, such as sand, pebbles, or cobbles, that bounce or roll along a stream bed.
• bedrock Rock still attached to the Earth’s crust.
• Bergeron process Precipitation involving the growth of ice crystals in a cloud at the expense of water droplets.
• berm A horizontal or landward-sloping terrace in the backshore zone of a beach that receives sediment during a storm.
• big bang 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.
• biochemical sedimentary rock Sedimentary rock formed from material (such as shells) produced by living organisms.
• biodiversity The number of different species that exist at a given time.
• biofuel Gas or liquid fuel made from plant material (biomass). Examples of biofuel include alcohol (from fermented sugar), biodiesel from vegetable oil, and wood.
• biogeochemical cycle The exchange of chemicals between living and nonliving reservoirs in the Earth System.
• bioremediation The injection of oxygen and nutrients into a contaminated aquifer to foster the growth of bacteria that will ingest or break down contaminants.
• 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.
• bioturbation The mixing of sediment by burrowing animals such as clams and worms.
• bituminous coal Dull, black intermediate-rank coal formed at temperatures between 100° and 200°C.
• black-lung disease Lung disease contracted by miners from the inhalation of too much coal dust.
• 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.
• blind fault A fault that does not intersect the ground surface.
• block In the context of igneous materials, a block is a chunk of igneous rock blasted out of a volcano.
• blocking temperature The temperature below which isotopes in a mineral are no longer free to move, so the radiometric clock starts.
• blowout A deep, bowl-like depression scoured out of desert terrain by a turbulent vortex of wind.
• blue shift The phenomenon in which a source of light moving toward you appears to have a higher frequency.
• body waves Seismic waves that pass through the interior of the Earth.
• bog A wetland dominated by moss and shrubs.
• bolide A solid extraterrestrial object such as a meteorite, comet, or asteroid that explodes in the atmosphere.
• bomb A frozen blob of rock formed when magma ejected from a volcano freezes in flight; bombs are typically streamlined.
• bornhardt An inselberg with a loaf geometry, like that of Uluru (Ayers Rock) in central Australia.
• Bowen’s reaction series The sequence in which different silicate minerals crystallize during the progressive cooling of a melt.
• braided stream A sediment-choked stream consisting of entwined subchannels.
• breaker A water wave in which water at the top of the wave curves over the base of the wave.
• breakwater An offshore wall, built parallel or at an angle to the beach, that prevents the full force of waves from reaching a harbor.
• breccia Coarse sedimentary rock consisting of angular fragments; or rock broken into angular fragments by faulting.
• breeder reactor A nuclear reactor that produces its own fuel.
• brine Water that is not fresh but is less salty than seawater; brine may be found in estuaries.
• brittle deformation The cracking and fracturing of a material subjected to stress.
• brittle-ductile transition (brittle-plastic transition) The depth above which materials behave brittlely and below which materials behave ductilely (plastically); this transition typically lies between a depth of 10 and 15 km in continental crustal rock, and 60 m deep in glacial ice.
• buoyancy The upward force acting on a less dense object immersed or floating in denser material.
• burial metamorphism Metamorphism due to the increase in temperature and pressure in a rock when it has been buried to a depth of several kilometers.
• butte A medium-sized, flat-topped hill in an arid region.

- C -

• 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.
• caliche A solid mass created where calcite cements the soil together (also called calcrete).
• calving The breaking off of chunks of ice at the edge of a glacier.
• Cambrian explosion The remarkable diversification of life, indicated by the fossil record, that occurred at the beginning of the Cambrian Period.
• Canadian Shield A broad, low-lying region of exposed Precambrian rock in the Canadian interior.
• canyon A trough or valley with steeply sloping walls, cut into the land by a stream.
• capacity The total volume of sediment that a stream can carry.
• capillary fringe The thin subsurface layer in which water molecules seep up from the water table by capillary action to fill pores.
• carbonate rocks Rocks containing calcite and/or dolomite.
• carbon-14 (¹⁴C) dating A radiometric dating process that can tell us the age of organic material containing carbon originally extracted from the atmosphere.
• cast Sediment that preserves the shape of a shell it once filled before the shell dissolved or mechanically weathered away.
• catabatic winds Strong winds that form at the margin of a glacier where the warmer air above ice-free land rises and the cold, denser air from above the glaciers rushes in to take its place.
• catastrophic change Change that takes place either instantaneously or rapidly in geologic time.
• catchment Drainage network.
• cement Mineral material that precipitates from water and fills the spaces between grains, holding the grains together.
• 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.
• Cenozoic The most recent era of the Phanerozoic Eon, lasting from 65 Ma up until the present.
• chalk Very fine-grained limestone consisting of weakly cemented plankton shells.
• change of state The process in which a material changes from one phase (liquid, gas, or solid) to another.
• channel A trough dug into the ground surface by flowing water.
• channeled scablands A barren, soil-free landscape in eastern Washington, scoured clean by a flood unleashed when a large glacial lake drained.
• chatter marks Wedge-shaped indentations left on rock surfaces by glacial plucking.
• chemical sedimentary rock Sedimentary rock made up of minerals that precipitate directly from water solution.
• chemical weathering The process in which chemical reactions alter or destroy minerals when rock comes in contact with water solutions and/or air.
• chert A sedimentary rock composed of very fine-grained silica (cryptocrystalline quartz).
• Chicxulub crater A circular excavation buried beneath younger sediment on the Yucután peninsula; geologists suggest that a meteorite landed there 65 Ma.
• chimney (1) A conduit in a magma chamber in the shape of a long vertical pipe through which magma rises and erupts at the surface; (2) an isolated column of strata in an arid region.
• cinder cone A subaerial volcano consisting of a cone-shaped pile of tephra whose slope approaches the angle of repose for tephra.
• cinders Fragments of glassy rock ejected from a volcano.
• cirque A bowl-shaped depression carved by a glacier on the side of a mountain.
• cirrus cloud A wispy cloud that tapers into delicate, feather-like curls.
• clast A fragment of detritus (e.g., a sand grain or a pebble).
• clastic (detrital) sedimentary rock Sedimentary rock consisting of cemented-together detritus derived from the weathering of preexisting rock.
• cleavage (1) The tendency of a mineral to break along preferred planes; (2) a type of foliation in low-grade metamorphic rock.
• cleavage planes A series of surfaces on a crystal that form parallel to the weakest bonds holding the atoms of the crystal together.
• cliff (or scarp) retreat The change in the position of a cliff face caused by erosion.
• climate The average weather conditions, along with the range of conditions, of a region over a year.
• cloud A mist of tiny water droplets in the sky.
• coal An organic sedimentary rock formed from plant debris.
• coalbed methane Natural gas produced during the diagenesis of coal.
• coal gasification The transformation, by human activity, of coal into various gases.
• coal rank A measurement of the carbon content of coal; higher-rank coal forms at higher temperatures.
• coal reserve The quantities of discovered, but not yet mined, coal in sedimentary rock of the continents.
• coal swamp A swamp whose oxygen-poor water allows thick piles of woody debris to accumulate; this debris transforms into coal upon deep burial.
• coast The boundary region between land and the sea.
• coastal plain Low-relief regions of land adjacent to the coast.
• coastal wetland A vegetated area extending from the shore inland.
• cold front The boundary at which a cold air mass pushes underneath a warm air mass.
• collision The process of two buoyant pieces of lithosphere converging and squashing together.
• columnar jointing A type of fracturing that yields roughly hexagonal columns of basalt; columnar joints form when a dike, sill, or lava flow cools.
• comet A ball of ice and dust, probably remaining from the formation of the Solar System, that orbits the Sun.
• 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.
• competence The ability of flowing water to carry sediment, as represented by the largest clast size that the stream can transport.
• composite volcano Stratovolcano.
• compositional banding A type of metamorphic foliation, found in gneiss, defined by alternating bands of light and dark minerals.
• compressibility The degree to which a material’s volume changes in response to squashing.
• compression A push or squeezing felt by a body.
• compressional waves Waves in which particles of material move back and forth parallel to the direction in which the wave itself moves.
• conchoidal fractures Smoothly curving, clamshell-shaped surfaces along which materials with no cleavage planes tend to break.
• condensation The process of gas molecules linking together to form a liquid.
• condensation nuclei Preexisting solid or liquid particles, such as aerosols, onto which water condenses during cloud formation.
• 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.
• confined aquifer An aquifer that is separated from the Earth’s surface by an overlying aquitard.
• conglomerate Very coarse-grained sedimentary rock consisting of rounded clasts.
• consuming boundary Convergent plate boundary.
• contact The boundary surface between two rock bodies (as between two stratigraphic formations, between an igneous intrusion and adjacent rock, between two igneous rock bodies, or between rocks juxtaposed by a fault).
• contact metamorphism Thermal metamorphism.
• contaminant plume A cloud of contaminated groundwater that moves away from the source of the contamination.
• continental crust The crust beneath the continents.
• continental divide A highland separating drainage that flows into one ocean from drainage that flows into another.
• continental drift hypothesis The idea that continents have moved and are still moving slowly across the Earth’s surface.
• continental glacier A vast sheet of ice that spreads over thousands of square kilometers of continental crust.
• continental-interior desert An inland desert that develops because by the time air masses reach the continental interior, they have lost all of their moisture.
• continental lithosphere Lithosphere topped by continental crust; this lithosphere reaches a thickness of 150 km.
• continental margin A continent’s coastline.
• continental rift A linear belt along which continental lithosphere stretches and pulls apart.
• continental rifting 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.
• continental rise The sloping sea floor that extends from the lower part of the continental slope to the abyssal plain.
• continental shelf A broad, shallowly submerged region of a continent along a passive margin.
• continental slope The slope at the edge of a continental shelf, leading down to the deep sea floor.
• continental volcanic arc A long curving chain of subaerial volcanoes on the margin of a continent adjacent to a convergent plate boundary.
• contour interval The vertical elevation difference between two adjacent contour lines on a topographic map.
• contour lines Lines on a map along which a parameter has a constant value; for example, all points along a contour line on a topographic map are at the same elevation.
• control rod Rods that absorb neutrons in a nuclear reactor and thus decrease the number of collisions between neutrons and radioactive atoms.
• convection Heat transfer that results when warmer, less dense material rises while cooler, denser material sinks.
• convergence zone A place where two surface air flows meet so that air has to rise.
• convergent margin Convergent plate boundary.
• convergent plate 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.
• coral reef A mound of coral and coral debris forming a region of shallow water.
• core The dense, iron-rich center of the Earth.
• core-mantle boundary An interface 2,900 km below the Earth’s surface separating the mantle and core.
• Coriolis effect The deflection of objects, winds, and currents on the surface of the Earth owing to the planet’s rotation.
• cornice A huge, overhanging drift of snow built up by strong winds at the crest of a mountain ridge.
• correlation The process of defining the age relations between the strata at one locality and the strata at another.
• cosmic rays Nuclei of hydrogen and other elements that bombard the Earth from deep space.
• cosmology The study of the overall structure of the Universe.
• country rock (wall rock) The preexisting rock into which magma intrudes.
• crater (1) A circular depression at the top of a volcanic mound; (2) a depression formed by the impact of a meteorite.
• craton A long-lived block of durable continental crust commonly found in the stable interior of a continent.
• cratonic platform A province in the interior of a continent in which Phanerozoic strata bury most of the underlying Precambrian rock.
• creep The gradual downslope movement of regolith.
• crevasse A large crack that develops by brittle deformation in the top 60 m of a glacier.
• critical mass A sufficiently dense and large mass of radioactive atoms in which a chain reaction happens so quickly that the mass explodes.
• cross bed A lamination inclined to the main bedding; it represents the slip face of a layer deposited in a current.
• cross-cutting relation A means of determining the relative age of rock by looking at which rock or structure cuts another; the feature that has been cut is older.
• cross section A diagram depicting the geometry of materials underground as they would appear on an imaginary vertical slice through the Earth.
• crude oil Oil extracted directly from the ground.
• crust The rock that makes up the outermost layer of the Earth.
• crustal root Low-density crustal rock that protrudes downward beneath a mountain range.
• crystal A single, continuous piece of a mineral bounded by flat surfaces that formed naturally as the mineral grew.
• crystal face The flat surface of an euhedral mineral grain.
• crystal form The geometric shape of a crystal, defined by the arrangement of crystal faces.
• crystal habit The general shape of a crystal or cluster of crystals that grew unimpeded.
• crystal lattice The orderly framework within which the atoms or ions of a mineral are fixed.
• crystal structure The internal arrangement of atoms or ions within a crystal.
• crystalline Containing a crystal lattice.
• crystalline igneous rock A rock that solidifies from a melt and consists of interlocking crystals.
• cuesta An asymmetric ridge formed by tilted layers of rock, with a steep cliff on one side cutting across the layers and a gentle slope on the other side; the gentle slope is parallel to the layering.
• cumulonimbus cloud A rain-producing puffy cloud.
• cumulus cloud A puffy, cotton-ball-shaped cloud.
• current (1) A well-defined stream of ocean water; (2) the moving flow of water in a stream.
• cut bank The outside bank of the channel wall of a meander, which is continually undergoing erosion.
• cutoff A straight reach in a stream that develops when erosion eats through a meander neck.
• cyanobacteria Blue-green algae; a type of archaea.
• cycle A series of interrelated events or steps that occur in succession and can be repeated, perhaps indefinitely.
• cyclone (1) The counterclockwise flow of air around a low-pressure mass; (2) the equivalent of a hurricane in the Indian Ocean.
• cyclothem A repeated interval within a sedimentary sequence that contains a specific succession of sedimentary beds.

- D -

• 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.
• daughter isotope The decay product of radioactive decay.
• day The time it takes for the Earth to spin once on its axis.
• debris avalanche An avalanche in which the falling debris consists of rock fragments and dust.
• debris fall A sudden collapse of disaggregated material on a steep slope.
• debris flow A downslope movement of mud mixed with larger rock fragments.
• debris slide A sudden downslope movement of material consisting only of regolith.
• decompression melting The kind of melting that occurs when hot mantle rock rises to shallower depths in the Earth so that pressure decreases while the temperature remains unchanged.
• deep current An ocean current at a depth greater than 100 m.
• deep-focus earthquake An earthquake that occurs at a depth between 300 and 670 km; below 670 km, earthquakes do not happen.
• deflation The process of lowering the land surface by wind abrasion.
• deformation A change in the shape, position, or orientation of a material, by bending, breaking, or flowing.
• dehydration Loss of water.
• 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.
• delta plain The low, swampy land on the surface of a delta.
• delta-plain flood A flood in which water submerges a delta plain.
• dendritic network A drainage network whose interconnecting streams resemble the pattern of branches connecting to a deciduous tree.
• dendrochronologist A scientist who analyzes tree rings to determine the geologic age of features.
• density Mass per unit volume.
• denudation The removal of rock and regolith from the Earth’s surface.
• deposition The process by which sediment settles out of a transporting medium.
• depositional landform A landform resulting from the deposition of sediment where the medium carrying the sediment evaporates, slows down, or melts.
• desert A region so arid that it contains no permanent streams except for those that bring water in from elsewhere, and has very sparse vegetation cover.
• desertification The process of transforming nondesert areas into desert.
• desert pavement A mosaic-like stone surface forming the ground in a desert.
• desert varnish A dark, rusty-brown coating of iron oxide and magnesium oxide that accumulates on the surface of rock in an arid environment.
• detachment fault A nearly horizontal fault at the base of a fault system.
• detritus The chunks and smaller grains of rock broken off outcrops by physical weathering.
• dewpoint temperature The temperature at which air becomes saturated so that dew can form.
• diagenesis Changes that happen to sediment or sedimentary rock during and subsequent to lithification but at temperatures less than that of the lowest grade metamorphism.
• differential stress A condition causing a material to experience a push or pull in one direction of a greater magnitude than the push or pull in another direction; in some cases, differential stress can result in shearing.
• differential weathering What happens when different rocks in an outcrop undergo weathering at different rates.
• differentiation In the context of planet formation, the process by which a planet separates into a metallic core and a rocky mantle very early in its history.
• diffraction The splitting of light into many tiny beams that interfere with one another.
• digital elevation map (DEM) A map depicting the topography of an area, produced by a computer that uses 3-D data; the computer assigns a location (lat/long) and elevation to each point on a map.
• dike A tabular (wall-shaped) intrusion of rock that cuts across the layering of country rock.
• dimension stone An intact block of granite or marble to be used for architectural purposes.
• dip The angle at which a layer tilts, relative to horizontal; the angle is measured in an imaginary vertical plane that trends perpendicular to the strike.
• dipole A magnetic field with a north and south pole, like that of a bar magnet.
• dipole field (for Earth) The part of the Earth’s magnetic field, caused by the flow of liquid iron alloy in the outer core, that can be represented by an imaginary bar magnet with a north and south pole.
• dip-slip fault A fault in which sliding occurs up or down the slope (dip) of the fault.
• dip slope A hill slope underlain by bedding parallel to the slope.
• 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.
• discharge The volume of water in a conduit or channel passing a point in one second.
• discharge area A location where groundwater flows back up to the surface, and may emerge at springs.
• disconformity An unconformity parallel to the two sedimentary sequences it separates.
• displacement (or offset) The amount of movement or slip across a fault plane.
• disseminated deposit A hydrothermal ore deposit in which ore minerals are dispersed throughout a body of rock.
• dissolution A process during which materials dissolve in water.
• dissolved load Ions dissolved in a stream’s water.
• distillation column A vertical pipe in which crude oil is separated into several components.
• distributaries The fan of small streams formed where a river spreads out over its delta.
• divergence zone A place where sinking air separates into two flows that move in opposite directions.
• divergent plate boundary A boundary at which two lithosphere plates move apart from each other; they are marked by mid-ocean ridges.
• diversification The development of many different species.
• DNA (deoxyribonucleic acid) The complex molecule, shaped like a double helix, containing the code that guides the growth and development of an organism.
• doldrums A belt with very slow winds along the equator.
• dolostone A type of carbonate sedimentary rock that contains significant quantities of dolomite.
• dome Folded or arched layers with the shape of an overturned bowl.
• Doppler effect The phenomenon in which the frequency of wave energy appears to change when a moving source of wave energy passes an observer.
• 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.
• downcutting The process in which water flowing through a channel cuts into the substrate and deepens the channel relative to its surroundings.
• downdraft Downward-moving air.
• downgoing plate (or slab) A lithosphere plate that has been subducted at a convergent margin.
• downslope force The component of the force of gravity acting in the downslope direction.
• downslope movement The tumbling or sliding of rock and sediment from higher elevations to lower ones.
• downwelling zone A place where near-surface water sinks.
• drag fold A fold that develops in layers of rock adjacent to a fault during or just before slip.
• drainage divide A highland or ridge that separates one watershed from another.
• drainage network (or basin) An array of interconnecting streams that together drain an area.
• drawdown The phenomenon in which the water table around a well drops because the users are pumping water out of the well faster than it flows in from the surrounding aquifer.
• 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.
• dripstone Limestone (travertine in a cave) formed by the precipitation of calcium carbonate out of groundwater.
• drop stone A rock that drops to the sea floor once the iceberg that was carrying the rock melts.
• drumlin A streamlined, elongate hill formed when a glacier overrides glacial till.
• dry-bottom (polar) glacier A glacier so cold that its base remains frozen to the substrate.
• dry wash The channel of an ephemeral stream when empty of water.
• 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.
• ductile (plastic) deformation The bending and flowing of a material (without cracking and breaking) subjected to stress.
• dune A pile of sand generally formed by deposition from the wind.
• dust storm An event in which strong winds hit unvegetated land, strip off the topsoil, and send it skyward to form rolling dark clouds that block out the Sun.
• dynamic metamorphism Metamorphism that occurs as a consequence of shearing alone, with no change in temperature or pressure.
• dynamo A power plant generator in which water or wind power spins an electrical conductor around a permanent magnet.
• dynamothermal metamorphism Metamorphism that involves heat, pressure, and shearing.

- E -

• earthquake A vibration caused by the sudden breaking or frictional sliding of rock in the Earth.
• earthquake belt A relatively narrow, distinct belt of earthquakes that defines the position of a plate boundary.
• earthquake engineering The design of buildings that can withstand shaking.
• earthquake zoning The determination of where land is relatively stable and where it might collapse because of seismicity.
• Earth System The global interconnecting web of physical and biological phenomena involving the solid Earth, the hydrosphere, and the atmosphere.
• ebb tide The falling tide.
• eccentricity cycle The cycle of the gradual change of the Earth’s orbit from a more circular to a more elliptical shape; the cycle takes around 100,000 years.
• ecliptic The plane defined by a planet’s orbit.
• ecosystem An environment and its inhabitants.
• eddy An isolated, ring-shaped current of water.
• effusive eruption An eruption that yields mostly lava, not ash.
• Ekman spiral The change in flow direction of water with depth, caused by the Coriolis effect.
• Ekman transport The overall movement of a mass of water, resulting from the Eckman spiral, in a direction 90° to the wind direction.
• elastic-rebound theory The concept that earthquakes occur when rock elastically bends until it fractures; the fracturing generates earthquake energy and decreases the elastic energy stored in the rock.
• elastic strain A change in shape of a material that disappears instantly when stress is removed.
• electromagnet An electrical device that produces a magnetic field.
• electron microprobe A laboratory instrument that can focus a beam of electrons on a small part of a mineral grain in order to create a signal that defines its chemical composition.
• El Niño The flow of warm water eastward from the Pacific Ocean that reverses the upwelling of cold water along the western coast of South America and causes significant global changes in weather patterns.
• embayment A low area of coastal land.
• emergent coast A coast where the land is rising relative to sea level or sea level is falling relative to the land.
• end moraine (terminal moraine) A low, sinuous ridge of till that develops when the terminus (toe) of a glacier stalls in one position for a while.
• energy The capacity to do work.
• 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.
• eon The largest subdivision of geologic time.
• epeirogenic movement The gradual uplift or subsidence of a broad region of the Earth’s surface.
• epeirogeny An event of epeirogenic movement; the term is usually used in reference to the formation of broad mid-continent domes and basins.
• ephemeral (intermittent) 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.
• epicenter The point on the surface of the Earth directly above the focus of an earthquake.
• epicontinental sea A shallow sea overlying a continent.
• epoch An interval of geologic time representing the largest subdivision of a period.
• equant A term for a grain that has the same dimensions in all directions.
• equatorial low The area of low pressure that develops over the equator because of the intertropical convergence zone.
• equilibrium line The boundary between the zone of accumulation and the zone of ablation on a glacier.
• equinox One of two days out of the year (September 22 and March 21) in which the Sun is directly overhead at noon at the equator.
• era An interval of geologic time representing the largest subdivision of the Phanerozoic Eon.
• erg Sand seas formed by the accumulation of dunes in a desert.
• erosion The grinding away and removal of Earth’s surface materials by moving water, air, or ice.
• erosional coast A coastline where sediment is not accumulating and wave action grinds away at the shore.
• erosional landform A landform that results from the breakdown and removal of rock or sediment.
• erratic A boulder or cobble that was picked up by a glacier and deposited hundreds of kilometers away from the outcrop from which it detached.
• esker A ridge of sorted sand and gravel that snakes across a ground moraine; the sediment of an esker was deposited in subglacial meltwater tunnels.
• estuary An inlet in which seawater and river water mix, created when a coastal valley is flooded because of either rising sea level or land subsidence.
• Eubacteria The kingdom of “true bacteria.”
• euhedral crystal A crystal whose faces are well formed and whose shape reflects crystal form.
• eukaryotic cell A cell with a complex internal structure, capable of building multicellular organisms.
• eustatic sea-level change A global rising or falling of the ocean surface.
• evaporate The process of transforming a liquid into a gas.
• evapotranspiration The sum of evaporation from bodies of water and the ground surface and transpiration from plants and animals.
• evolution (biological) The change over time of populations of organisms, due to survival of the fittest.
• exfoliation The process by which an outcrop of rock splits apart into onion-like sheets along joints that lie parallel to the ground surface.
• exhumation The process (involving uplift and erosion) that returns deeply buried rocks to the surface.
• exotic terrane A block of land that collided with a continent along a convergent margin and attached to the continent; the term “exotic” implies that the land was not originally part of the continent to which it is now attached.
• expanding Universe theory The theory that the whole Universe must be expanding because galaxies in every direction seem to be moving away from us.
• explosive eruption Violent volcanic eruption that produces clouds and avalanches of pyroclastic debris.
• external energy (geology) Energy that comes to Earth from the Sun.
• external process A geomorphologic process—such as downslope movement, erosion, or deposition—that is the consequence of gravity or of the interaction between the solid Earth and its fluid envelope (air and water). Energy for these processes comes from gravity and sunlight.
• extinction The death of the last members of a species so that there are no parents to pass on their genetic traits to offspring.
• extinct volcano A volcano that was active in the past but has now shut off entirely and will not erupt in the future.
• extraordinary fossil A rare fossilized relict, or trace, of the soft part of an organism.
• 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.
• eye The relative calm in the center of a hurricane.
• eye wall A rotating vertical cylinder of clouds surrounding the eye of a hurricane.

- F -

• facet The flat surface of a cut gemstone; facets are produced by grinding.
• facies (1) Sedimentary: a group of rocks and primary structures indicative of a given depositional environment; (2) Metamorphic: a set of metamorphic mineral assemblages formed under a given range of pressures and temperatures.
• failure surface The detachment or sliding horizon on which downslope movement of rock or debris occurs.
• fault A fracture on which one body of rock slides past another.
• fault-block mountains An outdated term for a narrow, elongate range of mountains that develops in a continental-rift setting as normal faulting drops down blocks of crust, or tilts blocks.
• fault breccia Fragmented rock in which angular fragments were formed by brittle fault movement; fault breccia occurs along a fault.
• fault creep Gradual movement along a fault that occurs in the absence of an earthquake.
• fault gouge Pulverized rock consisting of fine powder that lies along fault surfaces; gouge forms by crushing and grinding.
• faulting Slip events along a fault.
• fault scarp A small step on the ground surface where one side of a fault has moved vertically with respect to the other.
• fault system A grouping of numerous related faults.
• fault trace (or line) The intersection between a fault and the ground surface.
• felsic An adjective used in reference to igneous rocks that are rich in elements forming feldspar and quartz.
• Ferrel cells The name given to the middle-latitude convection cells in the atmosphere.
• fetch The distance across a body of water along which a wind blows to build waves.
• fine-grained A textural term for rock consisting of many fine grains or clasts.
• firn Compacted granular ice (derived from snow) that forms where snow is deeply buried; if buried more deeply, firn turns into glacial ice.
• fission track A line of damage formed in the crystal lattice of a mineral by the impact of an atomic particle ejected during the decay of a radioactive isotope.
• fissure A conduit in a magma chamber in the shape of a long crack through which magma rises and erupts at the surface.
• fjord A deep, glacially carved, U-shaped valley flooded by rising sea level.
• flank eruption An eruption that occurs when a secondary chimney, or fissure, breaks through the flank of a volcano.
• 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.
• flexing The process of folding in which a succession of rock layers bends and slip occurs between the layers.
• flocculation The clumping together of clay suspended in river water into bunches that are large enough to settle out.
• 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.
• 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.
• floodplain The flat land on either side of a stream that becomes covered with water during a flood.
• floodplain flood A flood during which a floodplain is submerged.
• flood stage The stage when water reaches the top of a stream channel.
• flood tide The rising tide.
• floodway A mapped region likely to be flooded, in which people avoid constructing buildings.
• flow fold A fold that forms when the rock is so soft that it behaves like weak plastic.
• flowstone A sheet of limestone that forms along the wall of a cave when groundwater flows along the surface of the wall.
• fluvial deposit Sediment deposited in a stream channel, along a stream bank, or on a floodplain.
• flux Flow.
• focus The location where a fault slips during an earthquake (hypocenter).
• fog A cloud that forms at ground level.
• fold A bend or wrinkle of rock layers or foliation; folds form as a consequence of ductile deformation.
• fold axis An imaginary line that, when moved parallel to itself, can trace out the shape of a folded surface.
• fold-thrust belt An assemblage of folds and related thrust faults that develop above a detachment fault.
• foliation Layering formed as a consequence of the alignment of mineral grains, or of compositional banding in a metamorphic rock.
• foraminifera Microscopic plankton with calcitic shells, components of some limestones.
• foreland sedimentary basin A basin located under the plains adjacent to a mountain front, which develops as the weight of the mountains pushes the crust down, creating a depression that traps sediment.
• foreshocks The series of smaller earthquakes that precede a major earthquake.
• foreshore zone The zone of beach regularly covered and uncovered by rising and falling tides.
• formation Stratigraphic formation.
• fossil The remnant, or trace, of an ancient living organism that has been preserved in rock or sediment.
• fossil assemblage A group of fossil species found in a specific sequence of sedimentary rock.
• fossil correlation A determination of the stratigraphic relation between two sedimentary rock units, reached by studying fossils.
• 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.
• fossiliferous limestone Limestone consisting of abundant fossil shells and shell fragments.
• fossilization The process of forming a fossil.
• fossil succession The principle that the assemblage of fossil species in a given sequence of sedimentary strata differs from that found in older sequences or in younger sequences; a given species appears at a certain level and then disappears (goes extinct) at a higher level.
• fractional crystallization The process by which a magma becomes progressively more silicic as it cools, because early-formed crystals settle out.
• 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.
• fragmental igneous rock Fragments of igneous material that have been stuck together to form a coherent mass.
• fresh rock Rock whose mineral grains have their original composition and shape.
• friction Resistance to sliding on a surface.
• fringing reef A coral reef that forms directly along the coast.
• front The boundary between two air masses.
• frost wedging The process in which water trapped in a joint freezes, forces the joint open, and may cause the joint to grow.
• fuel rod A metal tube that holds the nuclear fuel in a nuclear reactor.
• Fujita scale A scale that distinguishes among tornadoes on the basis of wind speed, path dimensions, and possible damage.

- G -

• Ga Billions of years ago (abbreviation).
• gabbro A coarse-grained, intrusive mafic igneous rock.
• Gaia The term used for the Earth System, with the implication that it resembles a complex living entity.
• galaxy An immense system of hundreds of billions of stars.
• gas-giant planet The outer planets (Jupiter, Saturn, Uranus, Neptune) that are very large and consist mostly of volatile elements.
• gas hydrate An ice-like solid consisting of water and methane.
• gem A mineral or form of a mineral that is particularly beautiful and/or rare, and thus has value.
• gene An individual component of the DNA code that guides the growth and development of an organism.
• genetics The study of genes and how they transmit information.
• geocentric Universe concept An ancient Greek idea suggesting that the Earth sat motionless in the center of the Universe while stars and other planets and the Sun orbited around it.
• geochronology The science of dating geologic events in years.
• geode A cavity in which euhedral crystals precipitate out of water solutions passing through a rock.
• geographical pole The locations (north and south) where the Earth’s rotational axis intersects the planet’s surface.
• geoid A shape representing the pull of gravity as a function of location on the Earth; it is the shape that the surface of the Earth would have if the surface were entirely covered by the ocean, so that elevation represented the strength of the pull of gravity.
• geologic column A composite stratigraphic chart that represents the entirety of the Earth’s history.
• geologic history The sequence of geologic events that has taken place in a region.
• geologic map A map showing the distribution of rock units and structures across a region.
• geologic structures Bends, breaks, and fabrics in rocks that form as a result of deformation.
• geologic time The span of time since the formation of the Earth.
• geologic time scale A scale that describes the intervals of geologic time.
• geologists Scientists who study the Earth.
• geology The study of the Earth, including our planet’s composition, behavior, and history.
• geotherm The change in temperature with depth in the Earth.
• geothermal energy Heat and electricity produced by using the internal heat of the Earth.
• geothermal gradient The rate of change in temperature with depth.
• 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.
• geyser A fountain of steam and hot water that erupts periodically from a vent in the ground in a geothermal region.
• glacial abrasion The process by which clasts embedded in the base of a glacier grind away at the substrate as the glacier flows.
• glacial advance The forward movement of a glacier’s toe when the supply of snow exceeds the rate of ablation.
• glacial drift Sediment deposited in glacial environments.
• glacial incorporation The process by which flowing ice surrounds and incorporates debris.
• glacial marine Sediment consisting of ice-rafted clasts mixed with marine sediment.
• glacial outwash Coarse sediment deposited on a glacial outwash plain by meltwater streams.
• glacially polished surface A polished rock surface created by the glacial abrasion of the underlying substrate.
• glacial plucking (or quarrying) The process by which a glacier breaks off and carries away fragments of bedrock.
• glacial rebound The process by which the surface of a continent rises back up after an overlying continental ice sheet melts away and the weight of the ice is removed.
• glacial retreat The movement of a glacier’s toe back toward the glacier’s origin; glacial retreat occurs if the rate of ablation exceeds the rate of supply.
• glacial striations Scratches or troughs carved into rock by the sediment embedded in ice at the base of a flowing glacier.
• glacial subsidence The sinking of the surface of a continent caused by the weight of an overlying glacial ice sheet.
• glacial till Sediment transported by flowing ice and deposited beneath a glacier or at its toe.
• glaciation A period of time during which glaciers grew and covered substantial areas of the continents.
• glacier A river or sheet of ice that slowly flows across the land surface and lasts all year long.
• glass A solid in which atoms are not arranged in an orderly pattern.
• glassy igneous rock Igneous rock consisting entirely of glass, or of tiny crystals surrounded by a glass matrix.
• glide horizon The surface along which a slump slips.
• global change The transformations or modifications of both physical and biological components of the Earth System through time.
• global circulation The movement of volumes of air in paths that ultimately take it around the planet.
• global climate change Transformations or modifications in Earth’s climate over time.
• global cooling A fall in the average atmospheric temperature.
• 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.
• global warming A rise in the average atmospheric temperature.
• gneiss A compositionally banded metamorphic rock typically composed of alternating dark- and light-colored layers.
• Gondwana A supercontinent that consisted of today’s South America, Africa, Antarctica, India, and Australia. Also called Gondwanaland.
• graben A down-dropped crustal block bounded on either side by a normal fault dipping toward the basin.
• graded bed A layer of clastic sediment or sedimentary rock in which clast size progressively decreases from the base to the top of the bed; graded beds form by deposition from a turbidity current.
• gradualism The theory that evolution happens at a constant, slow rate.
• grain A fragment of a mineral crystal or of a rock.
• grain rotation The process by which rigid, inequant mineral grains distributed through a soft matrix may rotate into parallelism as the rock changes shape owing to differential stress.
• granite A coarse-grained intrusive silicic igneous rock.
• granulite facies A set of metamorphic mineral assemblages formed at very high pressures and temperatures.
• gravitational energy The force of attraction that one mass has on another.
• gravitational spreading A process of lateral spreading that occurs in a material because of the weakness of the material; gravitational spreading causes continental glaciers to grow and mountain belts to undergo orogenic collapse.
• graywacke An informal term used for sedimentary rock consisting of sand-size up to small-pebble-size grains of quartz and rock fragments all mixed together in a muddy matrix; typically, graywacke occurs at the base of a graded bed.
• greenhouse conditions (greenhouse period) Relatively warm global climate leading to the rising of sea level for an interval of geologic time.
• 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.
• greenhouse gases Atmospheric gases, such as carbon dioxide and methane, that regulate the Earth’s atmospheric temperature by absorbing infrared radiation.
• greenschist facies A set of metamorphic mineral assemblages formed under relatively low pressures and temperatures.
• greenstone A low-grade metamorphic rock formed from basalt; if foliated, the rock is called greenschist.
• Greenwich mean time (GMT) The time at the astronomical observatory in Greenwich, England; time in all other time zones is set in relation to GMT.
• Grenville orogeny The orogeny that occurred about 1 billion years ago and yielded the belt of deformed and metamorphosed rocks that underlie the eastern fifth of the North American continent.
• groin A concrete or stone wall built perpendicular to a shoreline in order to prevent beach drift from removing sand.
• ground moraine A thin, hummocky layer of till left behind on the land surface during a rapid glacial recession.
• groundwater Water that resides under the surface of the Earth, mostly in pores or cracks of rock or sediment.
• groundwater contamination The introduction of harmful and/or non-natural chemicals to groundwater.
• group A succession of stratigraphic formations that have been lumped together, making a single, thicker stratigraphic entity.
• growth ring A rhythmic layering that develops in trees, travertine deposits, and shelly organisms as a consequence of seasonal changes.
• gusher A fountain of oil formed when underground pressure causes the oil to rise on its own out of a drilled hole.
• guyot A seamount that had a coral reef growing on top of it, so that it is now flat-crested.
• gymnosperm A plant whose seeds are “naked,” not surrounded by a fruit.
• gyre A large circular flow pattern of ocean surface currents.

- H -

• 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.
• Hadley cells The name given to the low-latitude convection cells in the atmosphere.
• hail Falling ice balls from the sky, formed when ice crystallizes in turbulent storm clouds.
• hail streak An approximately 2-by-10-km stretch of ground, elongate in the direction of a storm, onto which hail has fallen.
• half-graben A wedge-shaped basin in cross section that develops as the hanging-wall block above a normal fault slides down and rotates; the basin develops between the fault surface and the top surface of the rotated block.
• half-life The time it takes for half of a group of a radioactive element’s isotopes to decay.
• halocline The boundary in the ocean between surface-water and deepwater salinities.
• hamada Barren rocky highlands in a desert.
• hand specimen (geology) A chunk of rock, about the size of a fist, typically collected to serve as a sample for further study.
• hanging valley A glacially carved tributary valley whose floor lies at a higher elevation than the floor of the trunk valley.
• hanging wall The rock or sediment above an inclined fault plane.
• hardness In mineralogy, hardness refers to the resistance of a mineral to scratching; a harder mineral can scratch a softer mineral.
• hard water Groundwater that contains dissolved calcium and magnesium, usually after passing through limestone or dolomite.
• head (1) The elevation of the water table above a reference horizon; (2) the edge of ice at the origin of a glacier.
• headland A place where a hill or cliff protrudes into the sea.
• head scarp The distinct step along the upslope edge of a slump where the regolith detached.
• headward erosion The process by which a stream channel lengthens up its slope as the flow of water increases.
• headwaters The beginning point of a stream.
• heat Thermal energy resulting from the movement of molecules.
• heat capacity A measure of the amount of heat that must be added to a material to change its temperature.
• heat flow The rate at which heat rises from the Earth’s interior up to the surface.
• heat-transfer melting Melting that results from the transfer of heat from a hotter magma to a cooler rock.
• heliocentric Universe concept An idea proposed by Greek philosophers around 250 B.C.E. suggesting that all heavenly objects including the Earth orbited the Sun.
• Hercynian orogen The late Paleozoic orogen that affected parts of Europe; a continuation of the Alleghenian orogen.
• heterosphere A term for the upper portion of the atmosphere in which gases separate into distinct layers on the basis of composition.
• hiatus The interval of time between deposition of the youngest rock below an unconformity and deposition of the oldest rock above the unconformity.
• high-altitude westerlies Westerly winds at the top of the troposphere.
• high-grade metamorphic rocks Rocks that metamorphose under relatively high temperatures.
• high-level waste Nuclear waste containing greater than 1 million times the safe level of radioactivity.
• hinge The portion of a fold where curvature is greatest.
• hogback A steep-sided ridge of steeply dipping strata.
• Holocene The period of geologic time since the last glaciation.
• Holocene climatic maximum The period from 5,000 to 6,000 years ago, when Holocene temperatures reached a peak.
• homosphere The lower part of the atmosphere, in which the gases have stirred into a homogenous mixture.
• hoodoo The local name for the brightly colored shale and sandstone chimneys found in Bryce Canyon National Park in Utah.
• horn A pointed mountain peak surrounded by at least three cirques.
• hornfels Rock that undergoes metamorphism simply because of a change in temperature, without being subjected to differential stress.
• horse latitudes The region of the subtropical high in which winds are weak.
• horst The high block between two grabens.
• hot spot A location at the base of the lithosphere, at the top of a mantle plume, where temperatures can cause melting.
• hot-spot track A chain of now-dead volcanoes transported off the hot spot by the movement of a lithosphere plate.
• hot-spot volcano An isolated volcano not caused by movement at a plate boundary, but rather by the melting of a mantle plume.
• hot spring A spring that emits water ranging in temperature from about 30°C to 104°C.
• hummocky surface An irregular and lumpy ground surface.
• hurricane A huge rotating storm, resembling a giant spiral in map view, in which sustained winds blow over 119 km per hour.
• hurricane track The path a hurricane follows.
• hyaloclastite A rubbly extrusive rock consisting of glassy debris formed in a submarine or sub-ice eruption.
• hydration The absorption of water into the crystal structure of minerals; a type of chemical weathering.
• hydraulic conductivity The coefficient K in Darcy’s law; hydraulic conductivity takes into account the permeability of the sediment or rock as well as the fluid’s viscosity.
• hydraulic gradient The slope of the water table.
• hydraulic head The elevation to which groundwater rises in a pipe drilled into the ground; in non-artesian systems, the head is the water table.
• hydrocarbon A chain-like or ring-like molecule made of hydrogen and carbon atoms; petroleum and natural gas are hydrocarbons.
• hydrocarbon reserve An accumulation of accessible oil and gas.
• hydrocarbon system The association of source rock, migration pathway, reservoir rock, seal, and trap geometry that leads to the occurrence of a hydrocarbon reserve.
• hydrologic cycle The continual passage of water from reservoir to reservoir in the Earth System.
• hydrolysis The process in which water chemically reacts with minerals and breaks them down.
• hydrosphere The Earth’s water, including surface water (lakes, rivers, and oceans), groundwater, and liquid water in the atmosphere.
• hydrothermal deposit An accumulation of ore minerals precipitated from hot-water solutions circulating through a magma or through the rocks surrounding an igneous intrusion.
• hydrothermal metamorphism The change that occurs in a rock due to interaction with high-temperature water solutions.
• hypocenter (or focus) The point below the Earth’s surface where the energy is produced during an earthquake.
• hypothesis A reasonable idea that has the possibility of being correct, but has not yet been proven.
• hypsometric curve A graph that plots surface elevation on the vertical axis and the percentage of the Earth’s surface on the horizontal axis.

- I -

• ice age An interval of time in which the climate was colder than it is today, glaciers occasionally advanced to cover large areas of the continents, and mountain glaciers grew; an ice age can include many glacials and interglacials.
• iceberg A large block of ice that calves off the front of a glacier and drops into the sea.
• icehouse conditions (icehouse period) A period of time when the Earth’s temperature was cooler than it is today and ice ages could occur.
• ice-margin lake A meltwater lake formed along the edge of a glacier.
• ice-rafted sediment Sediment carried out to sea by icebergs.
• ice sheet A vast glacier that covers the landscape.
• ice shelf A broad, flat region of ice along the edge of a continent formed where a continental glacier flowed into the sea.
• ice stream A portion of a glacier that travels much more quickly than adjacent portions of the glacier.
• ice tongue The portion of a valley glacier that has flowed out into the sea.
• igneous rock Rock that forms when hot molten rock (magma or lava) cools and freezes solid.
• ignimbrite Rock formed when deposits of pyroclactic flows solidify.
• inactive fault A fault that last moved in the distant past and probably won’t move again in the near future, yet is still recognizable because of displacement across the fault plane.
• inactive sand The sand along a coast that is buried beneath a layer of active sand and moves only during severe storms or not at all.
• incised meander A meander that lies at the bottom of a steep-walled canyon.
• index minerals Minerals that serve as good indicators of metamorphic grade.
• induced seismicity Seismic events caused by the actions of people (e.g., filling a reservoir, that lies over a fault, with water).
• industrial minerals Minerals that serve as the raw materials for manufacturing chemicals, concrete, and wallboard, among other products.
• inequant A term for a mineral grain whose length and width are not the same.
• inertia The tendency of an object at rest to remain at rest.
• infiltrate Seep down into.
• injection well A well in which a liquid is pumped down into the ground under pressure so that it passes from the well back into the pore space of the rock or regolith.
• inner core The inner section of the core 5,155 km deep to the Earth’s center at 6,371 km, and consisting of solid iron alloy.
• inselberg An isolated mountain or hill in a desert landscape created by progressive cliff retreat, so that the hill is surrounded by a pediment or an alluvial fan.
• insolation Exposure to the Sun’s rays.
• interglacial A period of time between two glaciations.
• interior basin A basin with no outlet to the sea.
• interlocking texture The texture of crystalline rocks in which mineral grains fit together like pieces of a jigsaw puzzle.
• intermediate magma A silicate melt that contains a moderate amount (~60%) of silica.
• internal energy (geology) Energy provided by heat sources within the Earth.
• internal process A process in the Earth System, such as plate motion, mountain building, or volcanism, ultimately caused by Earth’s internal heat.
• intertidal zone The area of coastal land across which the tide rises and falls.
• intertropical convergence zone The equatorial convergence zone in the atmosphere.
• intraplate earthquake Earthquake that occurs away from plate boundaries.
• intrusive contact The boundary between country rock and an intrusive igneous rock.
• intrusive igneous rock Rock formed by the freezing of magma underground.
• ionosphere The interval of Earth’s atmosphere, at an elevation between 50 and 400 km, containing abundant positive ions.
• iron catastrophe The proposed event very early in Earth history when the Earth partly melted and molten iron sank to the center to form the core.
• isobar A line on a map along which the air has a specified pressure.
• isograd (1) A line on a pressure-temperature graph along which all points are taken to be at the same metamorphic grade; (2) A line on a map making the first appearance of a metamorphic index mineral.
• isostasy (or isostatic equilibrium) The condition that exists when the buoyancy force pushing lithosphere up equals the gravitational force pulling lithosphere down.
• isostatic compensation The process in which the surface of the crust slowly rises or falls to reestablish isostatic equilibrium after a geologic event changes the density or thickness of the lithosphere.
• isotherm Lines on a map or cross section along which the temperature is constant.
• isotopes Different versions of a given element that have the same atomic number but different atomic weights.
• isotopic dating The science of dating geologic events in years by measuring the ratio of parent radioactive atoms to daughter product atoms.

- J -

• jet stream A fast-moving current of air that flows at high elevations.
• jetty A manmade wall that protects the entrance to a harbor.
• joints Naturally formed cracks in rocks.
• joint set A group of systematic joints.
• Jovian A term used to describe the outer gassy, Jupiter-like planets (gasgiant planets).

- K -

• kame A stratified sequence of lateral-moraine sediment that’s sorted by water flowing along the edge of a glacier.
• karst landscape 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.
• kerogen The waxy molecules into which the organic material in shale transforms on reaching about 100°C. At higher temperatures, kerogen transforms into oil.
• kettle hole A circular depression in the ground made when a block of ice calves off the toe of a glacier, becomes buried by till, and later melts.
• knob-and-kettle topography A land surface with many kettle holes separated by round hills of glacial till.
• K-T boundary event The mass extinction that happened at the end of the Cretaceous Period, 65 million years ago, possibly because of the collision of an asteroid with the Earth.

- L -

• laccolith A shallow igneous intrusion that has a blister-like shape.
• lag deposit A surface accumulation of coarser sediment that forms where wind has blown away finer sediment.
• lagoon A body of shallow seawater separated from the open ocean by a barrier island.
• 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.
• landform A distinct feature of a landscape, as defined by its shape and character.
• landscape A region of the land surface, with definable characteristics.
• landslide A sudden movement of rock and debris down a nonvertical slope.
• landslide-potential map A map on which regions are ranked according to the likelihood that a mass movement will occur.
• land subsidence Sinking elevation of the ground surface; the process may occur over an aquifer that is slowly draining and decreasing in volume because of pore collapse.
• La Niña Years in which the El Niño event is not strong.
• lapilli Marble-to-plum-sized fragments of pyroclastic debris.
• Laramide orogeny The mountain-building event that lasted from about 80 Ma to 40 Ma, in western North America; in the United States, it formed the Rocky Mountains as a result of basement uplift and the warping of the younger overlying strata into large monoclines.
• latent heat of condensation The heat released during condensation, which comes only from a change in state.
• lateral moraine A strip of debris along the side margins of a glacier.
• laterite soil Soil formed over iron-rich rock in a tropical environment, consisting primarily of a dark-red mass of insoluble iron and/or aluminum oxide.
• Laurentia A continent in the early Paleozoic Era composed of today’s North America and Greenland.
• Laurentide ice sheet An ice sheet that spread over northeastern Canada during the Pleistocene ice age(s).
• lava Molten rock that has flowed out onto the Earth’s surface.
• lava dome A dome-like mass of rhyolitic lava that accumulates above the eruption vent.
• lava flows Sheets or mounds of lava that flow onto the ground surface or sea floor in molten form and then solidify.
• lava lake A large pool of lava produced around a vent when lava fountains spew forth large amounts of lava in a short period of time.
• 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.
• leach To dissolve and carry away.
• leader A conductive path stretching from a cloud toward the ground, along which electrons leak from the base of the cloud, and which provides the start for a lightning flash to the ground.
• lightning flash A giant spark or pulse of current that jumps across a gap of charge separation.
• light year The distance that light travels in one Earth year (about 6 trillion miles or 9.5 trillion km).
• lignite Low-rank coal that consists of 50% carbon.
• limb The side of a fold, showing less curvature than at the hinge.
• limestone Sedimentary rock composed of calcite.
• liquefaction The transformation of seemingly solid sediment into a liquid-like slurry, in response to ground shaking.
• lithification The transformation of loose sediment into solid rock through compaction and cementation.
• lithologic correlation A correlation based on similarities in rock type.
• 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.
• lithosphere plate A portion of the outer, relatively rigid layer of the Earth; most seismic activity happens at the boundaries of plates, while the interior of a plate is relatively stable.
• little ice age A period of cooler temperatures, between 1500 and 1800 C.E., during which many glaciers advanced.
• local base level A base level upstream from a drainage network’s mouth.
• lodgment till A flat layer of till smeared out over the ground when a glacier overrides an end moraine as it advances.
• loess Layers of fine-grained sediments deposited from the wind; large deposits of loess formed from fine-grained glacial sediment blown off outwash plains.
• longitudinal (seif) dune A dune formed when there is abundant sand and a strong, steady wind, and whose axis lies parallel to the wind direction.
• longitudinal profile A cross-sectional image showing the variation in elevation along the length of a river.
• longshore current A current that flows parallel to a beach.
• lower mantle The deepest section of the mantle, stretching from 670 km down to the core-mantle boundary.
• low-grade metamorphic rocks Rocks that underwent metamorphism at relatively low temperatures.
• low-velocity zone The asthenosphere underlying oceanic lithosphere in which seismic waves travel more slowly, probably because rock has partially melted.
• luster The way a mineral surface scatters light.
• L-waves (love waves) Surface seismic waves that cause the ground to ripple back and forth, creating a snake-like movement.

- M -

• Ma Millions of years ago (abbreviation).
• macrofossil A fossil large enough to be seen with the naked eye.
• mafic A term used in reference to magmas or igneous rocks that are relatively poor in silica and rich in iron and magnesium.
• magma Molten rock beneath the Earth’s surface.
• magma chamber A space below ground filled with magma.
• magma contamination The process in which flowing magma incorporates components of the country rock through which it passes.
• magmatic deposit An ore deposit formed when sulfide ore minerals accumulate at the bottom of a magma chamber.
• 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.
• magnetic declination The angle between the direction a compass needle points at a given location and the direction of true north.
• magnetic dipole A magnetic entity that has a north and south end.
• magnetic field The region affected by the force emanating from a magnet.
• magnetic field lines The trajectories along which magnetic particles would align, or charged particles would flow, if placed in a magnetic field.
• magnetic force The push or pull exerted by a magnet.
• 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.
• magnetic pole The north or south ends of a magnet; field lines point straight down at the pole.
• 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.
• magnetic-reversal chronology The history of magnetic reversals through geologic time.
• magnetization The degree to which a material can exert a magnetic force.
• magnetometer An instrument that measures the strength of the Earth’s magnetic field.
• magnetosphere The region protected from the electrically charged particles of the solar winds by Earth’s magnetic field.
• magnetostratigraphy The comparison of the pattern of magnetic reversals in a sequence of strata, with a reference column showing the succession of reversals through time.
• magnitude Any numerical representation of the size of an earthquake as determined by measuring the amplitude of ground motion.
• manganese nodules Lumpy accumulations of manganese-oxide minerals precipitated onto the sea floor.
• mantle The thick layer of rock below the Earth’s crust and above the core.
• mantle plume A column of very hot rock rising up through the mantle.
• marble A metamorphic rock composed of calcite and transformed from a protolith of limestone.
• mare The broad darker areas on the Moon’s surface, which consist of flood basalts that erupted over 3 billion years ago and spread out across the Moon’s lowlands.
• marginal sea A small ocean basin created when sea-floor spreading occurs behind an island arc.
• marine magnetic anomaly An unusually strong or unusually weak magnetic field, as measured over the sea floor; in map view, they look like stripes that are parallel to the mid-ocean ridge.
• maritime tropical air mass A mass of air that originates over tropical or subtropical oceanic regions.
• marsh A wetland dominated by grasses.
• mass extinction event A time when vast numbers of species abruptly vanish.
• mass movement (or mass wasting) The gravitationally caused downslope transport of rock, regolith, snow, or ice.
• matrix Finer-grained material surrounding larger grains in a rock.
• meander A snake-like curve along a stream’s course.
• meandering stream A reach of stream containing many meanders (snakelike curves).
• meander neck A narrow isthmus of land separating two adjacent meanders.
• mean sea level The average level between the high and low tide over a year at a given point.
• mechanical weathering Physical weathering.
• medial moraine A strip of sediment in the interior of a glacier, parallel to the flow direction of the glacier, formed by the lateral moraines of two merging glaciers.
• Medieval warm period A period of high temperatures in the Middle Ages.
• melt Molten (liquid) rock.
• 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.
• melting curve The line defining the range of temperatures and pressures at which a rock melts.
• melting temperature The temperature at which the thermal vibration of the atoms or ions in the lattice of a mineral is sufficient to break the chemical bonds holding them to the lattice, so a material transforms into a liquid.
• meltwater lake A lake fed by glacial meltwater.
• Mercalli intensity scale An earthquake characterization scale based on the amount of damage that the earthquake causes.
• mesa A large, flat-topped hill (with a surface area of several square kilometers) in an arid region.
• mesopause The boundary that marks the top of the mesosphere of Earth’s atmosphere.
• mesosphere The cooler layer of atmosphere overlying the stratosphere.
• Mesozoic The middle of the three Phanerozoic eras; it lasted from 245 Ma to 65 Ma.
• metaconglomerate Conglomerate that has undergone metamorphism, but in which clasts are still recognizable; typically the clasts are stretched or flattened.
• metal A solid composed almost entirely of atoms of metallic elements; it is generally opaque, shiny, smooth, and malleable, and can conduct electricity.
• metallic bond A chemical bond in which the outer atoms are attached to each other in such a way that electrons flow easily from atom to atom.
• 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.
• metamorphic facies A set of metamorphic mineral assemblages indicative of metamorphism under a specific range of pressures and temperatures.
• 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.
• metamorphic grade An informal designation of the degree to which a rock has undergone metamorphism; high-grade rocks have endured higher temperatures than low-grade rocks.
• metamorphic mineral A mineral formed by solid-state transitions under metamorphic conditions.
• metamorphic mineral assemblage A group of minerals that form in a rock as a result of metamorphism.
• 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.
• metamorphic texture The arrangement of grains (e.g., preferred orientation) formed as a result of metamorphism.
• metamorphic zone The region between two metamorphic isograds, typically named after an index mineral found within the region.
• metamorphism The process by which one kind of rock transforms into a different kind of rock.
• 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.
• meteor An object that has entered a planet’s atmosphere and is glowing and evaporating as it streaks to the planet’s surface.
• meteoric water Water that falls to Earth from the atmosphere as either rain or snow.
• meteorite A piece of rock or metal alloy that fell from space and landed on Earth.
• micrite Limestone consisting of lime mud (i.e., very fine-grained limestone).
• microfossil A fossil that can be seen only with a microscope or an electron microscope.
• mid-latitude (wave) cyclone The circulation of air around large, low-pressure masses.
• mid-ocean ridge A 2-km-high submarine mountain belt that forms along a divergent oceanic plate boundary.
• 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.
• Milankovitch cycles Climate cycles that occur over tens to hundreds of thousands of years, because of changes in Earth’s orbit and tilt.
• mine A site at which ore is extracted from the ground.
• 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.
• mineral classes Groups of minerals distinguished from each other on the basis of chemical composition.
• mineralogy The study of minerals.
• mineral resources The minerals extracted from the Earth’s upper crust for practical purposes.
• Mississippi Valley–type (MVT) ore An ore deposit, typically in dolostone, containing lead- and zinc-bearing minerals that precipitated from groundwater that had moved up from several kilometers depth in the upper crust; such deposits occur in the upper Mississippi Valley.
• Moho The seismic-velocity discontinuity that defines the boundary between the Earth’s crust and mantle.
• Mohs hardness scale A list of ten minerals in a sequence of relative hardness, with which other minerals can be compared.
• mold A cavity in sedimentary rock left behind when a shell that once filled the space weathers out.
• moment-magnitude scale A numerical representation of the size of an earthquake that takes into account the area of the fault that slipped, the amount of slip, and the strength of the rock that broke.
• monocline A fold in the land surface whose shape resembles that of a carpet draped over a stair step.
• monsoon A seasonal reversal in wind direction that causes a shift from a very dry season to a very rainy season in some regions of the world.
• moon A solid object of ice and/or rock and metal that orbits a planet.
• moraine A sediment pile composed of till deposited by a glacier.
• morphology (geology) Study of shapes or form; the characteristic shape of a geologic feature.
• mountain belt A linear band of mountains.
• mountain front The boundary between a mountain range and adjacent plains.
• mountain (alpine) glacier A glacier that exists in or adjacent to a mountainous region.
• mountain ice cap A mound of ice that submerges peaks and ridges at the crest of a mountain range.
• mouth The outlet of a stream where it discharges into another stream, a lake, or a sea.
• mudflow A downslope movement of mud at slow to moderate speed.
• mud pot A viscuous slurry that forms in a geothermal region when hot water or steam rises into soils rich in volcanic ash and clay.
• mudstone Very fine-grained sedimentary rock that will not easily split into sheets.
• mylonite Rock formed during dynamic metamorphism and characterized by foliation that lies roughly parallel to the fault (shear zone) involved in the shearing process; mylonites have very fine grains formed by the nonbrittle subdivision of larger grains.

- N -

• native metal A naturally occurring pure mass of a single metal in an ore deposit.
• natural arch An arch that forms when erosion along joints leaves narrow walls of rock; when the lower part of the wall erodes while the upper part remains, an arch results.
• natural hazard A situation or setting caused by natural processes that can result in damage to society.
• natural levees 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.
• natural selection The process by which the fittest organisms survive to pass on their characteristics to the next generation.
• neap tide An especially low tide that occurs when the angle between the direction of the Moon and the direction of the Sun is 90°.
• nebula A cloud of gas or dust in space.
• nebula theory of planet formation The concept that planets grow out of rings of gas, dust, and ice surrounding a new-born star.
• negative anomaly An area where the magnetic field strength is less than expected.
• negative feedback Feedback that slows a process down or reverses it.
• neocrystallization The growth of new crystals, not in the protolith, during metamorphism.
• Nevadan orogeny A convergent-margin mountain-building event that took place in western North America during the Late Jurassic Period.
• nonconformity A type of unconformity at which sedimentary rocks overlie basement (older intrusive igneous rocks and/or metamorphic rocks).
• nonflowing artesian well An artesian well in which water rises on its own up to a level that lies below the ground surface.
• nonfoliated metamorphic rock Rock containing minerals that recrystallized during metamorphism, but which has no foliation.
• nonmetallic mineral resources Mineral resources that do not contain metals; examples include building stone, gravel, sand, gypsum, phosphate, and salt.
• nonplunging fold A fold with a horizontal hinge.
• nonrenewable resource A resource that nature will take a long time (hundreds to millions of years) to replenish or may never replenish.
• nonsystematic joints Short cracks in rocks that occur in a range of orientations and are randomly placed and oriented.
• nor’easter A large, mid-latitude North American cyclone; when it reaches the East Coast, it produces strong winds that come out of the northeast.
• normal fault A fault in which the hanging-wall block moves down the slope of the fault.
• normal force The component of the gravitational force acting perpendicular to a slope.
• normal polarity Polarity in which the paleomagnetic dipole has the same orientation as it does today.
• normal stress The push or pull that is perpendicular to a surface.
• North Atlantic deep-water mass The mass of cold, dense water that sinks in the north polar regions.
• northeast tradewinds Surface winds that come out of the northeast and occur in the region between the equator and 30°N.
• nuclear fuel Pellets of concentrated uranium oxide or a comparable radioactive material that can provide energy in a nuclear reactor.
• nuclear fusion The process by which the nuclei of atoms fuse together, thereby creating new, larger atoms.
• nuclear reactor The part of a nuclear power plant where the fission reactions occur.
• nuclear waste Radioactive materials produced in a nuclear reactor.
• nuée ardente Pyroclastic flow.
• numerical age The age of a rock or structure as specified in years; referred to as “absolute age” in older literature.

- O -

• oasis A verdant region surrounded by desert, occurring at a place where natural springs provide water at the surface.
• oblique-slip fault A fault in which sliding occurs diagonally along the fault plane.
• obsidian An igneous rock consisting of a solid mass of volcanic glass.
• occluded front A front that no longer intersects the ground surface.
• oceanic crust The crust beneath the oceans; composed of gabbro and basalt, overlain by sediment.
• oceanic lithosphere Lithosphere topped by oceanic crust; it reaches a thickness of 100 km.
• offshore bar An elongate deposit of sand separated from the land by water.
• Oil Age The period of human history, including our own, so named because the economy depends on oil.
• oil field A region containing a significant amount of accessible oil underground.
• oil reserve The known supply of oil held underground.
• oil shale Shale containing kerogen.
• oil trap A geologic configuration that keeps oil underground in the reservoir rock and prevents it from rising to the surface.
• oil window The narrow range of temperatures under which oil can form in a source rock.
• olistotrome A large, submarine slump block, buried and preserved.
• ophiolite A slice of oceanic crust that has been thrust onto continental crust.
• ordinary well A well whose base penetrates below the water table, and can thus provide water.
• ore Rock containing native metals or a concentrated accumulation of ore minerals.
• ore deposit An economically significant accumulation of ore.
• ore minerals Minerals that have metal in high concentrations and in a form that can be easily extracted.
• organic carbon Carbon that has been incorporated in an organism.
• organic chemical A carbon-containing compound that occurs in living organisms, or that resembles such compounds; it consists of carbon atoms bonded to hydrogen atoms along with varying amounts of oxygen, nitrogen, and other chemicals.
• organic coast A coast along which living organisms control landforms along the shore.
• organic sedimentary rock Sedimentary rock (such as coal) formed from carbon-rich relicts of organisms.
• organic shale Lithified, muddy, organic-rich ooze that contains the raw materials from which hydrocarbons eventually form.
• original horizontality The principle that sediments are deposited in nearly horizontal layers.
• orogen (or orogenic belt) A linear range of mountains.
• orogenic collapse The process in which mountains begin to collapse under their own weight and spread out laterally.
• orogeny A mountain-building event.
• orographic barrier A landform that diverts air flow upward or laterally.
• outcrop An exposure of bedrock.
• outer core The section of the core, between 2,900 and 5,150 km deep, that consists of liquid iron alloy.
• outwash plain A broad area of gravel and sandbars deposited by a braided stream network, fed by the meltwater of a glacier.
• overburden The weight of overlying rock on rock buried deeper in the Earth’s crust.
• overriding plate (or slab) The plate at a subduction zone that overrides the downgoing plate.
• oversaturated solution A solution that contains so much solute (dissolved ions) that precipitation begins.
• oversized stream valley A large valley with a small stream running through it; the valley formed earlier when the flow was greater.
• oxbow lake A meander that has been cut off yet remains filled with water.
• oxidation reaction A reaction in which an element loses electrons; an example is the reaction of iron with air to form rust.
• ozone O₃, an atmospheric gas that absorbs harmful ultraviolet radiation from the Sun.
• ozone hole An area of the atmosphere, over polar regions, from which ozone has been depleted.

- P -

• pahoehoe A lava flow with a surface texture of smooth, glassy, rope-like ridges.
• paleoclimate The past climate of the Earth.
• paleomagnetism The record of ancient magnetism preserved in rock.
• paleontologists Scientists who study fossils and life evolution, in the context of geologic time.
• paleontology The study of ancient life and life evolution.
• paleopole The supposed position of the Earth’s magnetic pole in the past, with respect to a particular continent.
• paleosol Ancient soil preserved in the stratigraphic record.
• Paleozoic The oldest era of the Phanerozoic Eon.
• Pangaea A supercontinent that assembled at the end of the Paleozoic Era.
• Pannotia A supercontinent that may have existed sometime between 800 Ma and 600 Ma.
• parabolic dunes Dunes formed when strong winds break through transverse dunes to make new dunes whose ends point upwind.
• parallax The apparent movement of an object seen from two different points not on a straight line from the object (e.g., from your two different eyes).
• parallax method A trigonometric method used to determine the distance from the Earth to a nearby star.
• parent isotope A radioactive isotope that undergoes decay.
• partial melting The melting in a rock of the minerals with the lowest melting temperatures, while other minerals remain solid.
• passive continental margin A continental margin that is not a plate boundary.
• passive-margin basin A thick accumulation of sediment along a tectonically inactive coast, formed over crust that stretched and thinned when the margin first began.
• patterned ground A polar landscape in which the ground splits into pentagonal or hexagonal shapes.
• pause An elevation in the atmosphere where temperature stops decreasing and starts increasing, or vice versa.
• peat Compacted and partially decayed vegetation accumulating beneath a swamp.
• pedalfer soil A temperate-climate soil characterized by well-defined soil horizons and an organic A-horizon.
• pediment The broad, nearly horizontal bedrock surface at the base of a retreating desert cliff.
• pedocal soil Thin soil, formed in arid climates. It contains very little organic matter, but significant precipitated calcite.
• pegmatite A coarse-grained igneous rock containing crystals of up to tens of centimeters across and occurring in dike-shaped intrusions.
• pelagic sediment Microscopic plankton shells and fine flakes of clay that settle out and accumulate on the deep-ocean floor.
• Pelé’s hair Droplets of basaltic lava that mold into long glassy strands as they fall.
• Pelé’s tears Droplets of basaltic lava that mold into tear-shaped glassy beads as they fall.
• peneplain A nearly flat surface that lies at an elevation close to sea level; thought to be the product of long-term erosion.
• 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.
• percolation The process by which groundwater meanders through tiny, crooked channels in the surrounding material.
• peridotite A coarse-grained ultramafic rock.
• periglacial environment A region with widespread permafrost but without a blanket of snow or ice.
• period An interval of geologic time representing a subdivision of a geologic era.
• permafrost Permanently frozen ground.
• permanent magnet A special material that behaves magnetically for a long time all by itself.
• 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.
• permeability The degree to which a material allows fluids to pass through it via an interconnected network of pores and cracks.
• permineralization The fossilization process in which plant material becomes transformed into rock by the precipitation of silica from groundwater.
• petrified A term used by geologists to describe plant material that has transformed into rock by permineralization.
• petroglyph Drawings formed by chipping into the desert varnish of rocks to reveal the lighter rock beneath.
• petroleum Oil.
• phaneritic A textural term used to describe coarse-grained igneous rock.
• Phanerozoic Eon The most recent eon, an interval of time from 542 Ma to the present.
• phenocryst A large crystal surrounded by a finer-grained matrix in an igneous rock.
• photochemical smog Brown haze that blankets a city when exhaust from cars and trucks reacts in the presence of sunlight.
• photomicrograph A photograph of a thin section, as viewed through a petrographic microscope.
• photosynthesis The process during which chlorophyll-containing plants remove carbon dioxide from the atmosphere, form tissues, and expel oxygen back to the atmosphere.
• phreatomagmatic eruption An explosive eruption that occurs when water enters the magma chamber and turns into steam.
• phyllite A fine-grained metamorphic rock with a foliation caused by the preferred orientation of very fine-grained mica.
• phyllitic luster A silk-like sheen characteristic of phyllite, a result of the rock’s fine-grained mica.
• phylogenetic tree A chart representing the ideas of paleontologists showing which groups of organisms radiated from which ancestors.
• physical weathering The process in which intact rock breaks into smaller grains or chunks.
• piedmont glacier A fan or lobe of ice that forms where a valley glacier emerges from a valley and spreads out into the adjacent plain.
• pillow basalt Glass-encrusted basalt blobs that form when magma extrudes on the sea floor and cools very quickly.
• placer deposit Concentrations of metal grains in stream sediment that develop when rocks containing native metals erode and create a mixture of sand grains and metal fragments; the moving water of the stream carries away lighter mineral grains.
• planet A relatively large, spherical object that orbits a star and has cleared its orbit of most debris.
• planetesimal Tiny, solid pieces of rock and metal that collect in a planetary nebula and eventually accumulate to form a planet.
• plankton Tiny plants and animals that float in sea or lake water.
• plastic deformation The deformational process in which mineral grains behave like plastic and, when compressed or sheared, become flattened or elongate without cracking or breaking.
• plate One of about twenty distinct pieces of the relatively rigid lithosphere.
• plate boundary The border between two adjacent lithosphere plates.
• plate-boundary earthquakes The earthquakes that occur along and define plate boundaries.
• plate-boundary volcano A volcanic arc or mid-ocean ridge volcano, formed as a consequence of movement along a plate boundary.
• plate interior A region away from the plate boundaries that consequently experiences few earthquakes.
• plate tectonics Theory of plate tectonics.
• playa The flat, typically salty lake bed that remains when all the water evaporates in drier times; forms in desert regions.
• Pleistocene ice age The period of time from about 2 Ma to 14,000 years ago, during which the Earth experienced an ice age.
• plunge pool A depression at the base of a waterfall scoured by the energy of the falling water.
• plunging fold A fold with a tilted hinge.
• pluton An irregular or blob-shaped intrusion; can range in size from tens of meters across to tens of kilometers across.
• pluvial lake A lake formed to the south of a continental glacier as a result of enhanced rainfall during an ice age.
• point bar A wedge-shaped deposit of sediment on the inside bank of a meander.
• polar cell A high-latitude convection cell in the atmosphere.
• polar easterlies Prevailing winds that come from the east and flow from the polar high to the subpolar low.
• polar front The convergence zone in the atmosphere at latitude 60°.
• polar glacier Dry-bottom glacier.
• polar high The zone of high pressure in polar regions created by the sinking of air in the polar cells.
• polarity The orientation of a magnetic dipole.
• polarity chron The time interval between polarity reversals of Earth’s magnetic field.
• polarity subchron The time interval between magnetic reversals if the interval is of short duration (less than 200,000 years long).
• polarized light A beam of filtered light waves that all vibrate in the same plane.
• polar wander The phenomenon of the progressive changing through time of the position of the Earth’s magnetic poles relative to a location on a continent; significant polar wander probably doesn’t occur—in fact, poles seem to remain fairly fixed, while continents move.
• polar-wander path The curving line representing the apparent progressive change in the position of the Earth’s magnetic pole, relative to a locality X, assuming that the position of X on Earth has been fixed through time (in fact, poles stay fixed while continents move).
• pollen Tiny grains involved in plant reproduction.
• pollution Contaminants that have been introduced into the Earth System by human activity.
• polymorphs Two minerals that have the same chemical composition but a different crystal lattice structure.
• pore A small open space within sediment or rock.
• pore collapse The closer packing of grains that occurs when groundwater is extracted from pores, thus eliminating the support holding the grains apart.
• porosity The total volume of empty space (pore space) in a material, usually expressed as a percentage.
• porphyritic A textural term for igneous rock that has phenocrysts distributed throughout a finer matrix.
• positive anomaly An area where the magnetic field strength is stronger than expected.
• positive-feedback mechanism A mechanism that enhances the process that causes the mechanism in the first place.
• potentiometric surface The elevation to which water in an artesian system would rise if unimpeded; where there are flowing artesian wells, the potentiometric surface lies above ground.
• pothole A bowl-shaped depression carved into the floor of a stream by a long-lived whirlpool carrying sand or gravel.
• Precambrian The interval of geologic time between Earth’s formation about 4.57 Ga and the beginning of the Phanerozoic Eon 542 Ma.
• precession The gradual conical path traced out by Earth’s spinning axis; simply put, it is the “wobble” of the axis.
• precious metals Metals (like gold, silver, and platinum) that have high value.
• precipitation (1) The process by which atoms dissolved in a solution come together and form a solid; (2) rainfall or snow.
• preferred mineral orientation The metamorphic texture that exists where platy grains lie parallel to one another and/or elongate grains align in the same direction.
• preservation potential The likelihood that an organism will be preserved in the fossil record.
• pressure Force per unit area, or the “push” acting on a material in cases where the push is the same in all directions.
• pressure gradient The rate of pressure change over a given horizontal distance.
• pressure solution The process of dissolution at points of contact, between grains, where compression is greatest, producing ions that then precipitate elsewhere, where compression is less.
• prevailing winds Surface winds that generally flow in the same direction for long time periods.
• primary porosity The space that remains between solid grains or crystals immediately after sediment accumulates or rock forms.
• principal aquifer The geologic unit that serves as the primary source of groundwater in a region.
• principle of baked contacts When an igneous intrusion “bakes” (metamorphoses) surrounding rock, the rock that has been baked must be older than the intrusion.
• principle of cross-cutting relations If one geologic feature cuts across another, the feature that has been cut is older.
• principle of fossil succession In a stratigraphic sequence, different species of fossil organisms appear in a definite order; once a fossil species disappears in a sequence of strata, it never reappears higher in the sequence.
• principle of inclusions If a rock contains fragments of another rock, the fragments must be older than the rock containing them.
• principle of original continuity Sedimentary layers, before erosion, formed fairly continuous sheets over a region.
• principle of original horizontality Layers of sediment, when originally deposited, are fairly horizontal.
• principle of superposition In a sequence of sedimentary rock layers, each layer must be younger than the one below, for a layer of sediment cannot accumulate unless there is already a substrate on which it can collect.
• principle of uniformitariansim The physical processes we observe today also operated in the past in the same way, and at comparable rates.
• prograde metamorphism Metamorphism that occurs as temperatures and pressures are increasing.
• Proterozoic Eon The most recent of the Precambrian eons.
• protocontinent A block of crust composed of volcanic arcs and hot-spot volcanoes sutured together.
• protolith The original rock from which a metamorphic rock formed.
• protoplanet A body that grows by the accumulation of planetesimals but has not yet become big enough to be called a planet.
• protoplanetary disk The flattened cloud of dust, gas, and ice that orbits a nascent star prior to the formation of planets.
• protoplanetary nebula A ring of gas and dust that surrounded the newborn Sun, from which the planets were formed.
• protostar A dense body of gas that is collapsing inward because of gravitational forces and that may eventually become a star.
• pumice A glassy igneous rock that forms from felsic frothy lava and contains abundant (over 50%) pore space.
• 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.
• P-waves Compressional seismic waves that move through the body of the Earth.
• P-wave shadow zone A band between 103° and 143° from an earthquake epicenter, as measured along the circumference of the Earth, inside which Pwaves do not arrive at seismograph stations.
• pycnocline The boundary between layers of water of different densities.
• pyroclastic debris Fragmented material that sprayed out of a volcano and landed on the ground or sea floor in solid form.
• 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.
• pyroclastic rock Rock made from fragments blown out of a volcano during an explosion that were then packed or welded together.

- Q -

• quarry A site at which stone is extracted from the ground.
• quartzite A metamorphic rock composed of quartz and transformed from a protolith of quartz sandstone.
• quenching A sudden cooling of molten material to form a solid.
• quick clay Clay that behaves like a solid when still (because of surface tension holding the water-coated clay flakes together), but that flows like a liquid when shaken.

- R -

• radial network A drainage network in which the streams flow outward from a cone-shaped mountain, and define a pattern resembling spokes on a wheel.
• radioactive decay The process by which a radioactive atom undergoes fission or releases particles, thereby transforming into a new element.
• radioactive isotope An unstable isotope of a given element.
• rain band A spiraling arm of a hurricane radiating outward from the eye.
• rain shadow The inland side of a mountain range, which is arid because the mountains block rain clouds from reaching the area.
• range (for fossils) The interval of a sequence of strata in which a specific fossil species appears.
• rapids A reach of a stream in which water becomes particularly turbulent; as a consequence, waves develop on the surface of the stream.
• reach A specified segment of a stream’s path.
• recessional moraine The end moraine that forms when a glacier stalls for a while as it recedes.
• recharge area A location where water enters the ground and infiltrates down to the water table.
• recrystallization The process in which ions or atoms in minerals rearrange to form new minerals.
• rectangular network A drainage network in which the streams join each other at right angles because of a rectangular grid of fractures that breaks up the ground and localizes channels.
• recurrence interval The average time between successive geologic events.
• red giant A huge red star that forms when Sun-sized stars start to die and expand.
• red shift The phenomenon in which a source of light moving away from you very rapidly shifts to a lower frequency; that is, toward the red end of the spectrum.
• reef bleaching The death and loss of color of a coral reef.
• reflected ray A ray that bounces off a boundary between two different materials.
• reflection When energy (e.g., light) bounces off a surface.
• refracted ray A ray that bends as it passes through a boundary between two different materials.
• refraction The bending of a ray as it passes through a boundary between two different materials.
• reg A vast stony plain in a desert.
• regional metamorphism Dynamothermal metamorphism; metamorphism of a broad region, usually the result of deep burial during an orogeny.
• regolith Any kind of unconsolidated debris that covers bedrock.
• regression The seaward migration of a shoreline caused by a lowering of sea level.
• relative age The age of one geologic feature with respect to another.
• relative humidity The ratio between the measured water content of air and the maximum possible amount of water the air can hold at a given condition.
• relative plate velocity The movement of one lithosphere plate with respect to another.
• relief The difference in elevation between adjacent high and low regions on the land surface.
• renewable resource A resource that can be replaced by nature within a short time span relative to a human life span.
• reservoir rock Rock with high porosity and permeability, so it can contain an abundant amount of easily accessible oil.
• residence time The average length of time that a substance stays in a particular reservoir.
• residual mineral deposit Soils in which the residuum left behind after leaching by rainwater is so concentrated in metals that the soil itself becomes an ore deposit.
• resource A supply of usable material.
• resurgent dome The new mound, or cone, of igneous rock that grows within a caldera as an eruption begins anew.
• retrograde metamorphism Metamorphism that occurs as pressures and temperatures are decreasing; for retrograde metamorphism to occur, water must be added.
• return stroke An upward-flowing electric current from the ground that carries positive charges up to a cloud during a lightning flash.
• reversed polarity Polarity in which the paleomagnetic dipole points north.
• reverse fault A steeply dipping fault on which the hanging-wall block slides up.
• Richter magnitude scale A scale that defines earthquakes on the basis of the amplitude of the largest ground motion recorded on a seismogram.
• ridge axis The crest of a mid-ocean ridge; the ridge axis defines the position of a divergent plate boundary.
• ridge-push force The force that drives plates away from a mid-ocean ridge; it is caused by the fact that the ridge is elevated relative to the regions of oceanic plate away from the ridge.
• rifting The process by which continental lithosphere stretches and breaks apart; rifting produces normal faults and, commonly, volcanism.
• right-lateral strike-slip fault A strike-slip fault in which the block on the opposite fault plane from a fixed spot moves to the right of that spot.
• rip current A strong, localized seaward flow of water perpendicular to a beach.
• ripple marks Wave-like ridges and troughs on the surface of a sedimentary layer formed during deposition in a current.
• riprap Loose boulders or concrete piled together along a beach to absorb wave energy before it strikes a cliff face.
• roche moutonnée A glacially eroded hill that becomes elongate in the direction of flow and asymmetric; glacial rasping smoothes the upstream part of the hill into a gentle slope, while glacial plucking erodes the downstream edge into a steep slope.
• rock A coherent, naturally occurring solid, consisting of an aggregate of minerals or a mass of glass.
• rock burst A sudden explosion of rock off the ceiling or wall of an underground mine.
• 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.
• rockfall A sudden collapse of bedrock down a steep slope.
• rock flour Fine-grained sediment produced by glacial abrasion of the substrate over which a glacier flows.
• rock glacier A slow-moving mixture of rock fragments and ice.
• rock slide A sudden downslope movement of rock.
• rocky coast An area of coast where bedrock rises directly from the sea, so beaches are absent.
• Rodinia A proposed Precambrian supercontinent that existed around 1 billion years ago.
• rotational axis The imaginary line through the center of the Earth around which the Earth spins.
• R-waves Surface seismic waves that cause the ground to ripple up and down, like water waves in a pond.

- S -

• sabkah A region of formerly flooded coastal desert in which stranded seawater has left a salt crust over a mire of mud that is rich in organic material.
• salinity The degree of concentration of salt in water.
• 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.
• salt dome A rising bulbous dome of salt that bends up the adjacent layers of sedimentary rock.
• salt wedging The process in arid climates by which dissolved salt in groundwater crystallizes and grows in open pore spaces in rocks and pushes apart the surrounding grains.
• sand spit An area where the beach stretches out into open water across the mouth of a bay or estuary.
• sandstone Coarse-grained sedimentary rock consisting almost entirely of quartz.
• sand volcano A small mound of sand produced when sand layers below the ground surface liquify as a result of seismic shaking, causing the sand to erupt onto the Earth’s surface through cracks or holes in overlying clay layers.
• saprolite A layer of rotten rock created by chemical weathering in warm, wet climates.
• Sargasso Sea The center of North Atlantic Gyre, named for the tropical seaweed sargassum, which accumulates in its relatively noncirculating waters.
• saturated solution Water that carries as many dissolved ions as possible under given environmental conditions.
• saturated zone The region below the water table where pore space is filled with water.
• scattering The dispersal of energy that occurs when light interacts with particles in the atmosphere.
• schist A medium-to-coarse-grained metamorphic rock that possesses schistosity.
• schistosity Foliation caused by the preferred orientation of large mica flakes.
• scientific laws Principals of science that must be true; if they were violated, the physical world would not behave as we see it.
• scientific method A sequence of steps for systematically analyzing scientific problems in a way that leads to verifiable results.
• scientists People who study the character of the natural world, and try to come up with ideas to explain how it operates, in the context of physical laws.
• scoria A glassy mafic igneous rock containing abundant air-filled holes.
• scouring A process by which running water removes loose fragments of sediment from a stream bed.
• sea arch An arch of land protruding into the sea and connected to the mainland by a narrow bridge.
• sea-floor 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.
• sea ice Ice formed by the freezing of the surface of the sea.
• seal A relatively impermeable rock, such as shale, salt, or unfractured limestone, that lies above a reservoir rock and stops the oil from rising further.
• seam A sedimentary bed of coal interlayered with other sedimentary rocks.
• seamount An isolated submarine mountain.
• seasonal floods Floods that appear almost every year during seasons when rainfall is heavy or when winter snows start to melt.
• seasonal well A well that provides water only during the rainy season when the water table rises below the base of the well.
• sea stack An isolated tower of land just offshore, disconnected from the mainland by the collapse of a sea arch.
• seawall A wall of riprap built on the landward side of a backshore zone in order to protect shore cliffs from erosion.
• second The basic unit of time measurement, now defined as the time it takes for the magnetic field of a cesium atom to flip polarity 9,192,631,770 times, as measured by an atomic clock.
• secondary enrichment The process by which a new ore deposit forms from metals that were dissolved and carried away from preexisting ore minerals.
• secondary porosity New pore space in rocks, created some time after a rock first forms.
• secondary recovery technique A process used to extract the quantities of oil that will not come out of a reservoir rock with just simple pumping.
• sediment An accumulation of loose mineral grains, such as boulders, pebbles, sand, silt, or mud, that are not cemented together.
• sedimentary basin A depression, created as a consequence of subsidence, that fills with sediment.
• 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.
• sedimentary sequence A grouping of sedimentary units bounded on top and bottom by regional unconformities.
• sedimentary structure A characteristic of sedimentary deposits that pertains to the character of bedding and/or the surface features of a bed.
• sediment budget The proportion of sand supplied to sand removed from a depositional setting.
• sediment load The total volume of sediment carried by a stream.
• sediment maturity The degree to which a sediment has evolved from a crushed-up version of the original rock into a sediment that has lost its easily weathered minerals and become well sorted and rounded.
• sediment sorting The segregation of sediment by size.
• seep A place where oil-filled reservoir rock intersects the ground surface, or where fractures connect a reservoir to the ground surface, so that oil flows out onto the ground on its own.
• seiche Rhythmic movement in a body of water caused by ground motion.
• seismic belts (or zones) The relatively narrow strips of crust on Earth under which most earthquakes occur.
• seismicity Earthquake activity.
• seismic-moment magnitude scale A scale that defines earthquake size on the basis of calculations involving the amount of slip, length of rupture, depth of rupture, and rock strength.
• seismic ray The changing position of an imaginary point on a wave front as the front moves through rock.
• 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.
• 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.
• seismic velocity The speed at which seismic waves travel.
• seismic-velocity discontinuity A boundary in the Earth at which seismic-wave velocity changes abruptly.
• seismic waves Waves of energy emitted at the focus of an earthquake.
• seismogram The record of an earthquake produced by a seismograph.
• seismograph (seismometer) An instrument that can record the ground motion from an earthquake.
• semipermanent pressure cell A somewhat elliptical zone of high or low atmospheric pressure that lasts much of the year; it forms because high-pressure zones tend to be narrower over land than over sea.
• Sevier orogeny A mountain-building event that affected western North America between about 150 Ma and 80 Ma, a result of convergent margin tectonism; a fold-thrust belt formed during this event.
• shale Very fine-grained sedimentary rock that breaks into thin sheets.
• shatter cones Small, cone-shaped fractures formed by the shock of a meteorite impact.
• shear strain A change in shape of an object that involves the movement of one part of a rock body sideways past another part so that angular relationships within the body change.
• shear stress A stress that moves one part of a material sideways past another part.
• shear waves Seismic waves in which particles of material move back and forth perpendicular to the direction in which the wave itself moves.
• shear zone A fault in which movement has occurred ductilely.
• sheetwash A film of water less than a few millimeters thick that covers the ground surface during heavy rains.
• shield An older, interior region of a continent.
• shield volcano A subaerial volcano with a broad, gentle dome, formed either from low-viscosity basaltic lava or from large pyroclastic sheets.
• shocked quartz Grains of quartz that have been subjected to intense pressure such as occurs during a meteorite impact.
• shock metamorphism Solid-state changes in rock that result from the extreme pressure accompanying a meteorite impact.
• shoreline The boundary between the water and land.
• shortening The process during which a body of rock or a region of crust becomes shorter.
• short-term climate change Climate change that takes place over hundreds to thousands of years.
• Sierran arc A large continental volcanic arc along western North America that initiated at the end of the Jurassic Period and lasted until about 80 million years ago.
• silica SiO₂.
• silicate minerals Minerals composed of silicon-oxygen tetrahedra linked in various arrangements; most contain other elements too.
• silicate rock Rock composed of silicate minerals.
• siliceous sedimentary rock Sedimentary rock that contains abundant quartz.
• silicic Rich in silica with relatively little iron and magnesium.
• silicon-oxygen tetrahedron The basic building block of silicate minerals; it consists of one silicon atom surrounded by four oxygen atoms.
• sill A nearly horizontal table-top-shaped tabular intrusion that occurs between the layers of country rock.
• siltstone Fine-grained sedimentary rock generally composed of very small quartz grains.
• sinkhole A circular depression in the land that forms when an underground cavern collapses.
• slab-pull force The force that downgoing plates (or slabs) apply to oceanic lithosphere at a convergent margin.
• slate Fine-grained, low-grade metamorphic rock, formed by the metamorphism of shale.
• slaty cleavage The foliation typical of slate, and reflective of the preferred orientation of slate’s clay minerals, that allows slate to be split into thin sheets.
• slickensides The polished surface of a fault caused by slip on the fault; lineated slickensides also have groves that indicate the direction of fault movement.
• slip face The lee side of a dune, which sand slides down.
• slip lineations Linear marks on a fault surface created during movement on the fault; some slip lineations are defined by grooves, some by aligned mineral fibers.
• slope failure The downslope movement of material on an unstable slope.
• slump Downslope movement in which a mass of regolith detaches from its substrate along a spoon-shaped sliding surface and slips downward semicoherently.
• smelting The heating of a metal-containing rock to high temperatures in a fire so that the rock will decompose to yield metal plus a nonmetallic residue (slag).
• snotite A long gob of bacteria that slowly drips from the ceiling of a cave.
• snowball Earth Our planet during periods in the Precambrian when its entire surface was ice covered.
• snow line The boundary above which snow remains all year.
• soda straw A hollow stalactite in which calcite precipitates around the outside of a drip.
• soil Sediment that has undergone changes at the surface of the Earth, including reaction with rainwater and the addition of organic material.
• soil erosion The removal of soil by wind and runoff.
• soil horizon Distinct zones within a soil, distinguished from each other by factors such as chemical composition and organic content.
• soil moisture Underground water that wets the surface of the mineral grains and organic material making up soil, but lies above the water table.
• soil profile A vertical sequence of distinct zones of soil.
• Solar System The Sun and all the objects that orbit it (including planets, moons, comets, and asteroids).
• solar wind A stream of particles with enough energy to escape from the Sun’s gravity and flow outward into space.
• solid-state diffusion The slow movement of atoms or ions through a solid.
• solifluction The type of creep characteristic of tundra regions; during the summer, the uppermost layer of permafrost melts, and the soggy, weak layer of ground then flows slowly downslope in overlapping sheets.
• solstice A day on which the polar ends of the terminator (the boundary between the day hemisphere and the night hemisphere) lie 23.5° away from the associated geographic poles.
• Sonoma orogeny A convergent-margin mountain-building event that took place on the western coast of North America in the Late Permian and Early Triassic periods.
• 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- size fractions.
• source rock A rock (organic-rich shale) containing the raw materials from which hydrocarbons eventually form.
• southeast tradewinds Tradewinds in the Southern Hemisphere, which start flowing northward, deflect to the west, and end up flowing from southeast to northwest.
• southern oscillation The oscillating of atmospheric pressure cells back and forth across the Pacific Ocean, in association with El Niño.
• 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.
• 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.
• sphericity The measure of the degree to which a clast approaches the shape of a sphere.
• spreading boundary Divergent plate boundary.
• spreading rate The rate at which sea floor moves away from a mid-ocean ridge axis, as measured with respect to the sea floor on the opposite side of the axis.
• spring A natural outlet from which groundwater flows up onto the ground surface.
• spring tide An especially high tide that occurs when the Sun is on the same side of the Earth as the Moon.
• stable air Air that does not have a tendency to rise rapidly.
• stable slope A slope on which downward sliding is unlikely.
• stalactite An icicle-like cone that grows from the ceiling of a cave as dripping water precipitates limestone.
• stalagmite An upward-pointing cone of limestone that grows when drips of water hit the floor of a cave.
• standing wave A wave whose crest and trough remain in place as water moves through the wave.
• star A large sphere, composed dominantly of hydrogen and helium, in which fusion reactions are producing energy.
• star dune A constantly changing dune formed by frequent shifts in wind direction; it consists of overlapping crescent dunes pointing in many different directions.
• stellar wind Particles that have been ejected from a star and are shooting through space.
• stick-slip behavior Stop-start movement along a fault plane caused by friction, which prevents movement until stress builds up sufficiently.
• stone rings Ridges of cobbles between adjacent bulges of permafrost ground.
• stoping A process by which magma intrudes; blocks of wall rock break off and then sink into the magma.
• storm An episode of severe weather in which winds, precipitation, and in some cases lightning become strong enough to be bothersome and even dangerous.
• storm-center velocity A storm’s (hurricane’s) velocity along its track.
• storm surge Excess seawater driven landward by wind during a storm; the low atmospheric pressure beneath the storm allows sea level to rise locally, increasing the surge.
• strain The change in shape of an object in response to deformation (i.e., as a result of the application of a stress).
• strata A succession of sedimentary beds.
• stratified drift Glacial sediment that has been redistributed and stratified by flowing water.
• stratigraphic column A cross-section diagram of a sequence of strata summarizing information about the sequence.
• 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.
• stratigraphic sequence An interval of strata deposited during periods of relatively high sea level, and bounded above and below by regional unconformities.
• stratopause The temperature pause that marks the top of the stratosphere.
• stratosphere The stable, stratified layer of atmosphere directly above the troposphere.
• stratovolcano A large, cone-shaped subaerial volcano consisting of alternating layers of lava and tephra.
• stratus cloud A thin, sheet-like, stable cloud.
• streak The color of the powder produced by pulverizing a mineral on an unglazed ceramic plate.
• stream A ribbon of water that flows in a channel.
• stream bed The floor of a stream.
• stream capacity The total quantity of sediment a stream carries.
• stream competence The maximum particle size that a stream can carry.
• stream gradient The slope of a stream’s channel in the downstream direction.
• stream piracy (or capture) The situation in which headward erosion causes one stream to intersect the course of another, previously independent stream, so that the intersected stream starts to flow down the channel of the first stream.
• stream rejuvenation The renewed downcutting of a stream into a floodplain or peneplain, caused by a relative drop of the base level.
• stream terrace A flat surface, underlain by alluvium, that borders a stream; terraces form when the stream cuts down into the alluvium that it had deposited previously.
• 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.
• stretching The process during which a layer of rock or a region of crust becomes longer.
• strike The compass trend of an imaginary horizontal line on a plane.
• 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.
• strip mining The scraping off of all soil and sedimentary rock above a coal seam in order to gain access to the seam.
• stromatolite Layered mounds of sediment formed by cyanobacteria; cyanobacteria secrete a mucuous-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.
• structural control The condition in which geologic structures, such as faults, affect the distribution and drainage of water or the shape of the land surface.
• subaerial Pertaining to land regions above sea level (i.e., under air).
• subduction The process by which one oceanic plate bends and sinks down into the asthenosphere beneath another plate.
• subduction zone The region along a convergent boundary where one plate sinks beneath another.
• sublimation The evaporation of ice directly into vapor without first forming a liquid.
• submarine canyon A narrow, steep canyon that dissects a continental shelf and slope.
• submarine fan A wedge-shaped accumulation of sediment at the base of a submarine slope; fans usually accumulate at the mouth of a submarine canyon.
• submarine slump The underwater downslope movement of a semicoherent block of sediment along a weak mud detachment.
• submergent coast A coast at which the land is sinking relative to sea level.
• subpolar low The rise of air where the surface flow of a polar cell converges with the surface flow of a Ferrel cell, creating a low-pressure zone in the atmosphere.
• subsidence The vertical sinking of the Earth’s surface in a region, relative to a reference plane.
• substrate A general term for material just below the ground surface.
• subtropical high (subtropical divergence zone) A belt of high pressure in the atmosphere at 30° latitude formed where the Hadley cell converges with the Ferrel cell, causing cool, dense air to sink.
• subtropics Desert climate regions that lie on either side of the equatorial tropics between the lines of 20° and 30° north or south of the equator.
• summit eruption An eruption that occurs in the summit crater of a volcano.
• sunspot cycle The cyclic appearance of large numbers of sunspots (black spots thought to be magnetic storms on the Sun’s surface) every 9 to 11.5 years.
• supercontinent cycle The process of change during which supercontinents develop and later break apart, forming pieces that may merge once again in geologic time to make yet another supercontinent.
• 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.
• superplume A huge mantle plume.
• superposed stream A stream whose geometry has been laid down on a rock structure and is not controlled by the structure.
• superposition The principle that younger layers of sediment are deposited on older layers of sediment; thus, in a sequence of strata, the oldest layer is at the base.
• surface current An ocean current in the top 100 m of water.
• surface load Sediment that rolls or bounces in wind along the ground surface.
• surface waves Seismic waves that travel along the Earth’s surface.
• surface westerlies The prevailing surface winds in North America and Europe, which come out of the west or southwest.
• surf zone A region of the shore in which breakers crash onto the shore.
• surge (glacial) A pulse of rapid flow in a glacier.
• suspended load Tiny solid grains carried along by a stream without settling to the floor of the channel.
• sustainable growth Enlargement of society and the economy in ways that can be supported indefinitely by renewable natural resources.
• swamp A wetland dominated by trees.
• swash The upward surge of water that flows up a beach slope when breakers crash onto the shore.
• S-waves Seismic shear waves that pass through the body of the Earth.
• 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.
• swelling clay Clay possessing a mineral structure that allows it to absorb water between its layers and thus swell to several times its original size.
• symmetry The condition in which the shape of one part of an object is a mirror image of the other part.
• syncline A trough-shaped fold whose limbs dip toward the hinge.
• systematic joints Long planar cracks that occur fairly regularly throughout a rock body.

- T -

• tabular intrusions Sheet intrusions that are planar and of roughly uniform thickness.
• Taconic orogeny A convergent mountain-building event that took place around 400 million years ago, in which a volcanic island arc collided with eastern North America.
• tailings pile A pile of waste rock from a mine.
• talus apron A wedge-shaped pile of rock fragments that accumulates at the base of a cliff.
• tar Hydrocarbons that exist in solid form at room temperature.
• tarn A lake that forms at the base of a cirque on a glacially eroded mountain.
• 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.
• taxonomy The study and classification of the relationships among different forms of life.
• temperate glacier A glacier that exists in regions where it is warm enough for liquid water to occur in films between the grains of ice.
• tension A stress that pulls on a material and could lead to stretching.
• tephra Unconsolidated accumulations of pyroclastic grains.
• terminal moraine The end moraine at the farthest limit of glaciation.
• terminator The boundary between the half of the Earth that has daylight and the half experiencing night.
• terrace The elevated surface of an older floodplain into which a younger floodplain had cut down.
• terrestrial A term used to describe the inner, Earth-like planets.
• thalweg The deepest part of a stream’s channel.
• theory A scientific idea supported by an abundance of evidence that has passed many tests and failed none.
• theory of evolution An idea that explains how the assemblage of species on Earth and the character of species on Earth have changed over time, as a consequence of the survival of the fittest; the idea is a “theory” because it has successfully explained many observations, can make testable predictions, and has not failed any test.
• 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.
• thermal metamorphism Metamorphism caused by heat conducted into country rock from an igneous intrusion.
• thermocline A boundary between layers of water with differing temperatures.
• thermohaline circulation The rising and sinking of water driven by contrasts in water density, which is due in turn to differences in temperature and salinity; this circulation involves both surface and deep-water currents in the ocean.
• thermosphere The outermost layer of the atmosphere containing very little gas.
• thin section A 3/100-mm-thick slice of rock that can be examined with a petrographic microscope.
• thin-skinned deformation A distinctive style of deformation characterized by displacement on faults that terminate at depth along a subhorizontal detachment fault.
• thrust fault A gently dipping reverse fault; the hanging-wall block moves up the slope of the fault.
• tidal bore A visible wall of water that moves toward shore with the rising tide in quiet waters.
• tidal flat A broad, nearly horizontal plain of mud and silt, exposed or nearly exposed at low tide but totally submerged at high tide.
• tidal reach The difference in sea level between high tide and low tide at a given point.
• tide The daily rising or falling of sea level at a given point on the Earth.
• tide-generating force The force, caused in part by the gravitational attraction of the Sun and Moon, and in part by the centrifugal force created by the Earth’s spin, that generates tides.
• tidewater glacier A glacier that has flowed into the sea.
• till A mixture of unsorted mud, sand, pebbles, and larger rocks deposited by glaciers.
• tillite A rock formed from hardened ancient glacial deposits and consisting of larger clasts distributed through a matrix of sandstone and mudstone.
• toe (terminus) The leading edge or margin of a glacier.
• tombolo A narrow ridge of sand that links a sea stack to the mainland.
• topographic map A map that uses contour lines to represent variations in elevation.
• topography Variations in elevation.
• topsoil The top soil horizons, which are typically dark and nutrient-rich.
• tornado A near-vertical, funnel-shaped cloud in which air rotates extremely rapidly around the axis of the funnel.
• tornado swarm Dozens of tornadoes produced by the same storm.
• tower karst A karst landscape in which steep-sided residual bedrock towers remain between sinkholes.
• transform fault A fault marking a transform plate boundary; along mid-ocean ridges, transform faults are the actively slipping segment of a fracture zone between two ridge segments.
• transform plate boundary A boundary at which one lithosphere plate slips laterally past another.
• transgression The inland migration of shoreline resulting from a rise in sea level.
• transition zone The middle portion of the mantle, from 400 to 670 km deep, in which there are several jumps in seismic velocity.
• transpiration The release of moisture as a metabolic by-product.
• transverse dune A simple, wave-like dune that appears when enough sand accumulates for the ground surface to be completely buried, but only moderate winds blow.
• trap In the context of hydrocarbons, a trap is a geologic configuration that accumulates and holds oil underground.
• 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.
• travertine A carbonate rock formed by precipitation of carbonate minerals from water at springs or on the surface of caves.
• trellis network A drainage system that develops across a landscape of parallel valleys and ridges so that major tributaries flow down the valleys and join a trunk stream that cuts through the ridge; the resulting map pattern resembles a garden trellis.
• trench A deep elongate trough bordering a volcanic arc; a trench defines the trace of a convergent plate boundary.
• triangulation The method for determining the map location of a point from knowing the distance between that point and three other points; this method is used to locate earthquake epicenters.
• tributary A smaller stream that flows into a larger stream.
• triple junction A point where three lithosphere plate boundaries intersect.
• tropical depression A tropical storm with winds reaching up to 61 km per hour; such storms develop from tropical disturbances, and may grow to become hurricanes.
• tropical disturbance Cyclonic winds that develop in the tropics.
• tropopause The temperature pause marking the top of the troposphere.
• troposphere The lowest layer of the atmosphere, where air undergoes convection and where most wind and clouds develop.
• truncated spur A spur (elongate ridge between two valleys) whose end was eroded off by a glacier.
• trunk stream The single larger stream into which an array of tributaries flow.
• tsunami A large wave along the sea surface triggered by an earthquake or large submarine slump.
• tuff A pyroclastic igneous rock composed of volcanic ash and fragmented pumice, formed when accumulations of the debris cement together.
• tundra A cold, treeless region of land at high latitudes, supporting only species of shrubs, moss, and lichen capable of living on permafrost.
• turbidite A graded bed of sediment built up at the base of a submarine slope and deposited by turbidity currents.
• turbidity current A submarine avalanche of sediment and water that speeds down a submarine slope.
• turbulence The chaotic twisting, swirling motion in flowing fluid.
• typhoon The equivalent of a hurricane in the western Pacific Ocean.

- U -

• ultimate base level Sea level; the level below which a trunk stream cannot cut.
• ultramafic A term used to describe igneous rocks or magmas that are rich in iron and magnesium and very poor in silica.
• unconfined aquifer An aquifer that intersects the surface of the Earth.
• unconformity A boundary between two different rock sequences representing an interval of time during which new strata were not deposited and/or were eroded.
• unconsolidated Consisting of unattached grains.
• undercutting Excavation at the base of a slope that results in the formation of an overhang.
• undersaturated A term used to describe a solution capable of holding more dissolved ions.
• uniformitarianism The principle that the same physical processes observed today are responsible for the formation of ancient geologic features; put concisely, “the present is the key to the past.”
• Universe The sum of all matter and energy making up the hundreds of billions of known galaxies.
• unsaturated zone The region of the subsurface above the water table.
• unstable air Air that is significantly warmer than air above and has a tendency to rise quickly.
• unstable ground Land capable of slumping or slipping down-slope in the near future.
• unstable slope A slope on which sliding will likely happen.
• updraft Upward-moving air.
• uplift The upward vertical movement of the ground surface, as occurs during mountain building.
• upper mantle The uppermost section of the mantle, reaching down to a depth of 400 km.
• upwelling zone A place where deep water rises in the ocean, or hot magma rises in the asthenosphere.
• U-shaped valley A steep-walled valley shaped by glacial erosion into the form of a U.

- V -

• vacuum Space that contains very little matter in a given volume (e.g., a region in which air has been removed).
• valley A trough with sloping walls, cut into the land by a stream.
• valley glacier A river of ice that flows down a mountain valley.
• Van Allen radiation belts Belts of solar wind particles and cosmic rays that surround the Earth, trapped by Earth’s magnetic field.
• varve A pair of thin layers of glacial lake-bed sediment, one consisting of silt brought in during the spring floods, and the other of clay deposited during the winter when the lake’s surface freezes over and the water is still.
• vascular plant A plant with woody tissue and seeds and veins for transporting water and food.
• vein A seam of minerals that forms when dissolved ions carried by water solutions precipitate in cracks.
• vein deposit A hydrothermal deposit in which the ore minerals occur in veins that fill cracks in preexisting rocks.
• velocity-versus-depth curve A graph that shows the variation in the velocity of seismic waves with increasing depth in the Earth.
• ventifact (faceted rock) A desert rock whose surface has been faceted by the wind.
• vesicles Open holes in igneous rock formed by the preservation of bubbles in magma as the magma cools into solid rock.
• viscosity The resistance of material to flow.
• volatiles Elements or compounds such as H₂O and CO₂ that evaporate easily and can exist in gaseous forms at the Earth’s surface.
• volatility A specification of the ease with which a material evaporates.
• volcanic arc A curving chain of active volcanoes formed adjacent to a convergent plate boundary.
• volcanic ash Tiny glass shards formed when a fine spray of exploded lava freezes instantly upon contact with the atmosphere.
• volcanic bomb A large piece of pyroclastic debris thrown into the atmosphere during a volcanic eruption.
• volcanic breccias Rocks composed of angular chunks of volcanic debris that have been cemented together.
• volcanic-danger-assessment map A map delineating areas that lie in the path of potential lava flows, lahars, debris flows, or pyroclastic flows of an active volcano.
• volcanic debris flow A mixture of water and clasts of volcanic material that moves down the slope of a volcano.
• volcanic gas Elements or compounds that bubble out of magma or lava in gaseous form.
• volcanic island arc The volcanic island chain that forms on the edge of the overriding plate where one oceanic plate subducts beneath another oceanic plate.
• volcaniclastic deposit A layer composed of fragments of igneous material (e.g., tephra, ash, bombs) erupted from volcanoes.
• 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.
• V-shaped valley A valley whose cross-sectional shape resembles a V; the valley probably has a river running down the point of the V.

- W -

• 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.
• wadi The name used in the Middle East and North Africa for a dry wash.
• warm front A front in which warm air rises slowly over cooler air in the atmosphere.
• waste rock Rock dislodged by mining activity yet containing no ore minerals.
• waterfall A place where water drops over an escarpment.
• water gap An opening in a resistant ridge where a trunk river has cut through the ridge.
• watershed The region that collects water that feeds into a given drainage network.
• 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.
• wave A means of transmitting energy from one location to another; waves can be vibrations that propagate through a material, or undulations of electromagnetic fields that can propagate either through a material or in a vacuum.
• wave base The depth, approximately equal in distance to half a wavelength in a body of water, beneath which there is no wave movement.
• wave-cut bench A platform of rock, cut by wave erosion, at the low-tide line that was left behind a retreating cliff.
• wave-cut notch A notch in a coastal cliff cut out by wave erosion.
• wave erosion The combined effects of the shattering, wedging, and abrading of a cliff face by waves and the sediment they carry.
• wave front The boundary between the region through which a wave has passed and the region through which it has not yet passed.
• wavelength The horizontal difference between two adjacent wave troughs or two adjacent crests.
• wave refraction (ocean) The bending of waves as they approach a shore so that their crests make no more than a 5° angle with the shoreline.
• weather Local-scale conditions as defined by temperature, air pressure, relative humidity, and wind speed.
• weathered rock Rock that has reacted with air and/or water at or near the Earth’s surface.
• weathering The processes that break up and corrode solid rock, eventually transforming it into sediment.
• weather system A specific set of weather conditions, reflecting the configuration of air movement in the atmosphere, that affects a region for a period of time.
• welded tuff Tuff formed by the welding together of hot volcanic glass shards at the base of pyroclastic flows.
• well A hole in the ground dug or drilled in order to obtain water.
• Western Interior Seaway A north-south-trending seaway that ran down the middle of North America during the Late Cretaceous Period.
• wet-bottom (temperate) glacier A glacier with a thin layer of water at its base, over which the glacier slides.
• wetted perimeter The area in which water touches a stream channel’s walls.
• wind abrasion The grinding away at surfaces in a desert by windblown sand and dust.
• wind gap An opening through a high ridge that developed earlier in geologic history by stream erosion, but that is now dry.

- X -

• xenolith A relict of wall rock surrounded by intrusive rock when the intrusive rock freezes.

- Y -

• yardang A mushroom-like column with a resistant rock perched on an eroding column of softer rock; created by wind abrasion in deserts where a resistant rock overlies softer layers of rock.
• yazoo stream A small tributary that runs parallel to the main river in a floodplain because the tributary is blocked from entering the main river by levees.
• Younger Dryas An interval of cooler temperatures that took place 4,500 years ago during a general warming/glacier-retreat period.

- Z -

• zeolite facies The metamorphic facies just above diagenetic conditions, under which zeolite minerals form.
• zone of ablation The area of a glacier in which ablation (melting, sublimation, calving) subtracts from the glacier.
• 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.
• zone of aeration Unsaturated zone.
• 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.