Glossary of Geological Terms

AA: A blocky and fragmented form of basaltic lava occurring in flows with fissured and angular surfaces.

ABLATION: The annual amount of ice and snow lost from a glacier by the processes of melting, sublimation, wind erosion, and iceberg calving.

ABSOLUTE AGE: The age in years of a particular geologic event or feature, generally obtained with radiometric dating techniques. (Compare Relative age.)

ABYSSAL PLAIN: A flat, sediment-covered province of the seafloor.

ACTIVE MARGIN: A continental margin characterized by earthquakes, igneous activity, and/or uplifted mountains resulting from convergent or transform plate motion.

A-HORIZON: The uppermost layer of a soil, containing organic material and leached minerals.

ALLUVIAL FAN: A low, cone-shaped deposit of terrigenous sediment formed where a stream undergoes an abrupt widening as it leaves a mountain front for an open valley.

AMPHIBOLITE: A mostly nonfoliated metamorphic rock consisting primarily of amphibole and plagioclase feldspar.

ANDESITE: A volcanic rock type intermediate in composition between rhyolite and basalt; the extrusive equivalent of diorite.

ANGLE OF REPOSE: The steepest slope angle at which a particular sediment will lie without cascading down.

ANGULAR UNCONFORMITY: An unconformity in which the bedding planes of the rocks above and below are not parallel.

ANION: Any negatively charged ion; the opposite of cation.

ANTECEDENT STREAM: A stream that existed before the present topography was created, thereby maintaining its original course despite changes in the structure of the underlying rocks and in topography.

ANTICLINE: A large upfold of strata, usually from 100 m to 300 km in width, whose limbs are lower than its center. (Compare Syncline.)

AQUICLUDE: A stratum with low permeability that acts as a barrier to the flow of groundwater. Also called confining layer.

AQUIFER: A permeable formation that stores and transmits groundwater in sufficient quantity to supply wells.

ARETE: The sharp, jagged crest along the divide between glacial cirques, resulting from the headward erosion of the walls of adjoining cirques.

ARGILLITE: A low-grade metamorphic rock made from a shaly sedimentary rock, characterized by irregular fracture and lack of foliation.

ARTESIAN FLOW: Flow in a confined aquifer, in which the groundwater is at a greater pressure than in an unconfined aquifer at similar depths, thereby causing water in a well that penetrates a confined aquifer (an artesian well) to rise above the level of the aquiclude.

ASEISMIC RIDGE: A submarine ridge of volcanic origin, far from present plate boundaries and therefore characterized by the absence of seismic activity. (Compare Mid-ocean ridge, which is seismically active.)

ASTHENOSPHERE: The weak layer below the lithosphere that is marked by low seismic wave velocities and high seismic wave attenuation. Movement in the asthenosphere occurs by plastic deformation.

ASYMMETRICAL FOLD: A fold of strata in which the dips of the two limbs are unequal.

ATOLL: A continuous or broken circle of coral reefs and low coral islands surrounding a central lagoon.

ATOM: The smallest unit of an element that retains the element's physical and chemical properties.

ATOMIC NUMBER: The number of protons in the nucleus of an atom.

ATOMIC WEIGHT: The sum of the masses of the protons and neutrons in the atomic nucleus of an element.

AXIAL PLANE: In folds, the plane that most nearly separates two symmetrical limbs. In a simple anticline, it is vertical; in complex folding, it is perpendicular to the direction of compression.

BACKSHORE: The upper, generally dry, zone of the shore, extending landward from the upper limit of wave wash at high tide to the upper limit of shore-zone processes.

BACKWASH: The return flow of water down a beach after a wave has broken.

BADLAND: Topography characterized by intricate patterns of stream erosion developed on surfaces with little or no vegetative cover overlying unconsolidated or poorly cemented clays, silts, or sands.

BARCHAN: A crescent-shaped eolian sand dune that moves across a flat surface with its convex face upwind and its concave slip face downwind.

BARRIER ISLAND: A long, narrow island parallel to the shore, composed of sand and built by wave action.

BASAL SLIP: The sliding of a glacier along its base.

BASALT: A fine-grained, dark, mafic igneous rock composed largely of plagioclase feldspar and pyroxene: the extrusive equivalent of gabbro.

BASE LEVEL: The level below which a stream cannot erode: usually sea level, sometimes locally the level of a lake or resistant formation.

BASEMENT: The oldest rocks recognized in a given area; a complex of metamorphic and igneous rocks that underlies all the sedimentary formations. Usually Precambrian or Paleozoic in age.

BASIN (TECTONIC): A circular, synclinelike depression of strata that dips radically toward a central point.

BATHOLITH: A great, irregular mass of rock cutting across the country rock, with an exposed surface of at least 100 km2; usually an intrusive igneous rock, but sometimes derived from the country rock through very high temperature and pressure metamorphism. (See also Discordant intrusion.)

BAUXITE: A rock composed primarily of hydrous aluminum oxides and formed by intense chemical weathering in tropical areas with good drainage; a major ore of aluminum.

BEDDING: A characteristic of sedimentary rocks in which parallel planar surfaces separate layers of different grain sizes or compositions deposited at different times.

BEDDING SEQUENCE: A pattern of interbedding of different sedimentary rock types or sedimentary rocks with different sedimentary structures that is characteristic of a certain sedimentary environment.

BED LOAD: The sediment that a stream moves along the bottom of its channel by rolling and bouncing (saltation).

BEDROCK: The solid rock underlying unconsolidated surface materials, such as soil.

B-HORIZON: The intermediate layer in a soil, below the A-horizon and above the C-horizon, consisting of clays and oxide materials.

BIOCHEMICAL SEDIMENT, ROCK: A sediment or rock containing the mineral remains of organisms, such as shells, or minerals precipitated as a result of biological processes, such as in iron formations.

BIOMASS: Organic carbon-containing material of biological origin, including living and dead animals and plants.

BIOSPHERE: The parts of the atmosphere, hydrosphere, and lithosphere occupied by living organisms.

BIOTURBATION: The reworking of existing sediments by organisms.

BLOWOUT: (1) A parabola-shaped eolian sand dune, typically one blown back from a beach, that has its convex slip face oriented downwind. (2) A shallow circular or elliptical depression in sand or dry soil formed by wind erosion. (See also Deflation.)

BLUESCHIST: A metamorphic rock formed under conditions of high pressure (in excess of 5000 bars) and relatively low temperature, often containing the blue minerals glaucophane (an amphibole) and kyanite.

BOLIDE-IMPACT HYPOTHESIS: The proposal that an extraterrestrial object slammed into Earth 65 million years ago, causing a global climate change that wiped out half of Earth's species, including the dinosaurs.

BOTTOMSET BED: A flat-lying bed of fine sediment deposited seaward of a delta and then buried by continued delta growth.

BOWEN REACTION SERIES: A simple schematic description of the order in which different minerals crystallize during the cooling and progressive crystallization of a magma.

BRAIDED STREAM: A stream so choked with sediment that it divides and recombines numerous times, forming many small and meandering channels.

BRECCIA: See Sedimentary breccia; Volcanic breccia.

BRITTLE MATERIAL: A material that breaks abruptly when its elastic limit is reached; the opposite of a ductile material.

BURIAL METAMORPHISM: A low-grade metamorphism in which buried sedimentary rocks are metamorphosed by the heat and pressure exerted by overlying sediments and sedimentary rocks; bedding and other sedimentary structures are preserved.

CALCIUM CYCLE: The set of processes that carry calcium through Earth's systems.

CALDERA: A large basin-shaped volcanic depression that can form after an eruption if the volcano collapses through the roof of the emptied magma chamber. Potentially catastrophic eruptions of a resurgent caldera can occur when fresh magma reenters the collapsed magma chamber.

CAPACITY (STREAM): The amount of sediment and detritus a stream can transport past any point in a given amount of time. (Compare Competence.)

CARBONATE COMPENSATION DEPTH: The ocean depth below which the solution rate of calcium carbonate (CaCO3) becomes so great that no carbonate organisms or sediments are preserved on the seafloor.

CARBONATE PLATFORM: Broad, shallow area where both biological and nonbiological carbonates are deposited.

CARBONATE SEDIMENT, ROCK: A sediment or sedimentary rock formed from the accumulation of carbonate minerals precipitated organically or inorganically. Rocks are chiefly limestone and dolostone.

CATACLASTIC METAMORPHISM: High-pressure metamorphism occurring primarily by the crushing, shearing, and recrystallization of rock during tectonic movements and resulting in the formation of very fine grained rock.

CATION: Any positively charged ion; the opposite of anion.

CEMENTATION: A lithification process in which minerals are precipitated in the pore space of sediments, often binding the grains.

CENTRAL VENT: The largest vent of a volcano, situated at the center of its cone.

CHANNEL: The trough through which water flows in a stream valley; sometimes reserved for the deepest part of the streambed, in which the main current flows.

CHEMICAL SEDIMENT, ROCK: A sediment or sedimentary rock that is formed at or near its place of deposition by chemical precipitation, usually from seawater.

CHEMICAL WEATHERING: The set of all chemical reactions that can act on rock exposed to water and the atmosphere and so dissolve the minerals or change them to more stable forms.

CHERT: A sedimentary rock made up of chemical or biochemically precipitated silica.

C-HORIZON: The lowest layer of a soil, consisting of fragments of rock and their chemically weathered products.

CINDER CONE: A steep, conical hill built up about a volcanic vent and composed of coarse pyroclastic rock fragments expelled from the vent by escaping gases.

CIRQUE: The head of a glacial valley, usually with the form of one half of an inverted cone. The upper edges have the steepest slopes, approaching the vertical, and the base may be flat or hollowed out. The base is commonly occupied by a small lake or pond after deglaciation.

CLASTIC SEDIMENT, ROCK: A sediment or sedimentary rock formed from particles (clasts) derived from the erosion of preexisting rocks and mechanically transported.

CLAY: Any of a number of hydrous aluminosilicate minerals with sheetlike crystal structure, formed by the weathering and hydration of other silicates. Also, any mineral fragments smaller than 0.0039 mm.

CLEAVAGE (MINERAL): The tendency of a crystal to break along certain preferred planes in the crystal lattice; also, the geometric pattern of such a breakage.

CLEAVAGE (ROCK): The tendency of a rock to break along certain planes induced during deformation or metamorphism, usually in the direction of preferred orientation of the minerals in the rock.

COAL: The metamorphic product of stratified plant remains. It contains more than 50 percent carbon compounds and burns readily.

COAST: The strip of land adjacent to an ocean or sea and extending from low tide landward to the point of major change in landscape features.

COMPACTION: The decrease in volume and porosity of a sediment caused by burial beneath other sediments.

COMPETENCE (STREAM): A measure of the largest particle a stream is able to transport, not the total volume. (Compare Capacity.)

COMPOSITE VOLCANO: A volcanic cone containing layers of both lava flows and pyroclastic rocks. Synonym for stratovolcano.

COMPRESSIVE FORCES: Forces that squeeze together or shorten a body. Compressive forces dominate at convergent plate boundaries.

CONCENTRATION FACTOR: The ratio of the abundance of an element in a mineral deposit to its average abundance in the crust.

CONCORDANT INTRUSION: An intrusive igneous rock having contacts with the country rock that are parallel to bedding or foliation planes. (Compare Discordant intrusion.)

CONDUCTION: See Heat conduction.

CONFINED AQUIFER: An aquifer overlain by relatively impermeable strata (aquicludes), thereby causing the water to be contained under pressure. (Compare Unconfined aquifer.)

CONGLOMERATE: A sedimentary rock, a significant fraction of which is composed of rounded pebbles, cobbles, and boulders. The lithified equivalent of gravel.

CONSOLIDATED MATERIAL: Sediment that is lithified; that is, compacted and bound together by mineral cements.

CONTACT METAMORPHISM: Changes in the mineralogy and texture of rock resulting from the heat and pressure of an igneous intrusion in the near vicinity.

CONTINENTAL DRIFT: The horizontal displacement or rotation of continents relative to one another.

CONTINENTAL GLACIER: A continuous, thick glacier covering more than 50,000 km2 and moving independently of minor topographic features. (Compare Valley glacier.)

CONTINENTAL MARGIN: The portion of the ocean floor extending from the shoreline to the landward edge of the abyssal plain and including the continental shelf, slope, and rise.

CONTINENTAL RISE: A broad and gently sloping ramp that rises from an abyssal plain to the continental slope.

CONTINENTAL SHELF: The gently sloping submerged edge of a continent, extending commonly to a depth of about 200 m to the edge of the continental slope.

CONTINENTAL SHELF DEPOSITS: Sediments laid down in a tectonically quiet syncline at a passive continental margin.

CONTINENTAL SLOPE: The region of steep slopes between the continental shelf and continental rise.

CONTINUOUS REACTION SERIES: A reaction series in which the same mineral crystallizes throughout the range of temperatures in question, but in which there is gradual change in the chemical composition of the mineral with changing temperature. (Compare Discontinuous reaction series.)

CONTOUR: A curve on a topographic map that connects points of equal elevation.

CONVECTION: A mechanism of heat transfer in a flowing material in which hot material from the bottom rises because of its lesser density, while cool surface material sinks. (Compare Heat conduction.)

CONVERGENT PLATE BOUNDARY: A boundary at which Earth's plates collide and area is lost either by shortening and crustal thickening or by subduction of one plate beneath the other. The site of volcanism, earthquakes, trenches, and mountain building. (See also Subduction zone.)

CORE: The central part of the Earth below a depth of 2900 km. It is thought to be composed of iron and nickel and to be molten on the outside with a central solid inner core.

COUNTRY ROCK: The rock into which an igneous rock intrudes or a mineral deposit is emplaced.

COVALENT BOND: A bond between atoms in which outer electrons are shared.

CRATER: A bowl-shaped pit at the summit of most volcanoes, around the central vent.

CRATON: A portion of a continent that has not been subjected to major deformation for a prolonged time, typically since Precambrian or early Paleozoic time.

CREEP: Slow, downhill mass movement of soil and regolith under gravitational force.

CREVASSE: Any large vertical crack in the surface of a glacier or snowfield.

CROSS-BEDDING: Inclined beds in a sedimentary rock that were formed at the time of deposition by currents of wind or water in the direction in which the bed slopes downward.

CRUST: The outermost layer of the lithosphere, consisting of relatively light, low-melting temperature materials. Continental crust consists largely of granite and granodiorite. Oceanic crust is mostly basalt.

CRYSTAL: A form of matter, characterized by flat surfaces, in which the atoms, ions, or molecules are arranged in all directions to form a regular, repeating network.

CRYSTAL HABIT: The general shape of a crystal; for example, cubic, prismatic, or fibrous.

CRYSTALLIZATION: The formation of crystalline solids from a gas or liquid, such as in the formation of crystalline minerals in magma.

CUESTA: An asymmetrical ridge with one steep and one gentle face formed where gently dipping beds of erosion-resistant rocks are undercut by erosion of a weaker bed underneath.

CYCLE OF EROSION: A proposed sequence of changes in a landscape that progresses from high, rugged, tectonically formed mountains to low, rounded hills and finally to worn-down, tectonically stable plains.

DACITE: Volcanic equivalent of granodiorite.

DEBRIS AVALANCHE: A fast downhill mass movement of soil and rock.

DEBRIS FLOW: A fluid mass movement of rock fragments supported by a muddy matrix. Debris flows differ from earthflows in that they generally contain coarser material and move faster than earthflows.

DEBRIS SLIDE: A mass movement of rock material and soil largely as one or more units along planes of weakness at the base of or within the rock material.

DEFLATION: The removal of clay and dust from dry soil by strong winds that gradually scoop out shallow depressions in the ground.

DELTA: A body of sediment deposited in an ocean or lake at the mouth of a stream.

DENDRITIC DRAINAGE: A stream system that branches irregularly, resembling a branching tree.

DENSITY: The mass per unit volume of a substance, commonly expressed in grams per cubic centimeter.

DEPOSITIONAL REMANENT MAGNETIZATION: A weak magnetization created in sedimentary rocks by the rotation of magnetic crystals into line with the ambient field during settling.

DESERT PAVEMENT: A residual deposit left when continued deflation removes the fine grains of a soil and leaves a surface covered with close-packed cobbles.

DESERT VARNISH: A dark coating commonly found on the surface of rock in the desert. It consists of clays, iron oxides, and magnesium oxides produced during weathering.

DIAGENESIS: The chemical and physical changes undergone by buried sediments during lithification and compaction into sedimentary rock.

DIATREME: A volcanic vent filled with volcanic breccia by the explosive escape of gases.

DIKE: A tabular igneous intrusion that cuts across structures of surrounding rock. (See also Discordant intrusion.)

DIORITE: A plutonic rock with composition intermediate between granite and gabbro; the intrusive equivalent of andesite.

DIP: The maximum angle by which a stratum or other planar feature deviates from the horizontal. The angle is measured in a plane perpendicular to the strike.

DISCHARGE: The rate of water movement through a stream, measured in units of volume per unit time; the exit of groundwater to the surface (opposite of recharge).

DISCONTINUOUS REACTION SERIES: A reaction series in which the end members have different crystal structures (are distinct mineral phases). (Compare Continuous reaction series.)

DISCORDANT INTRUSION: An intrusive igneous rock that has contacts with the country rock cutting across bedding or foliation planes. (Compare Concordant intrusion.)

DISTRIBUTARY: A smaller branch of a large stream that receives water from the main channel; the opposite of a tributary.

DIVERGENT PLATE BOUNDARY: A boundary at which Earth's plates move apart and new lithosphere is created; the site of mid-ocean ridges, shallow-focus earthquakes, and volcanism.

DIVIDE: A ridge of high ground separating two drainage basins emptied by different streams.

DOLOSTONE: A sedimentary rock composed primarily of dolomite, a carbonate mineral with the general formula CaMg(CO3)2.

DOME (TECTONIC): A round or elliptical anticlinal upwarp of strata in which strata dip away in all directions from the high point. (See also Volcanic dome.)

DRAAS: Extremely large (1 km or more long and over 100 m high) composite of sand dunes found in deserts.

DRAINAGE BASIN: A region of land surrounded by divides and crossed by streams that funnel all its water into the network of streams draining the area, usuually to converge eventually to one river or lake.

DRAINAGE NETWORK: The pattern of tributaries, large and small, of a stream system.

DRIFT (GLACIAL): A collective term for all the rock, sand, and clay that are deposited by a glacier either as till or as outwash.

DRUMLIN: A smooth, streamlined hill composed of till and, in many cases, bedrock.

DRY WASH: An intermittent streambed in a desert canyon that carries water only briefly after a rain; called wadi in the Near East.

DUCTILE MATERIAL: A material that can undergo considerable change in shape by plastic deformation before rupture occurs; the opposite of a brittle material.

DUNE: An elongated mound of sand formed by wind or water.

EARTHFLOW: A fluid mass movement of mainly fine-grained material, along with some broken rock, at slow or moderate speeds.

EARTHQUAKE: The violent motion of the ground caused by the passage of seismic waves radiating from a fault along which sudden movement has taken place.

EBB TIDE: The part of the tide cycle during which the water level is falling.

ECLOGITE: An extremely high grade metamorphic rock containing the minerals garnet and pyroxene.

EFFLUENT STREAM: A stream or portion of a stream that receives some water from groundwater discharge because the stream's elevation is below the groundwater table. (Compare Influent stream.)

ELASTIC REBOUND THEORY: A theory of fault movement and earthquake generation holding that faults remain locked while strain energy accumulates in the rock formations on both sides, temporarily deforming them until a sudden slip along the fault releases the energy.

ELECTRON: A negatively charged atomic particle with a negligible mass (9.1 3 10228 gram) and a charge of 21.6 3 10219 coulomb, commonly expressed as 21. The position of an electron about an atomic nucleus is not fixed but is described as a region where an electron is most likely to be found.

ELEVATION: The vertical height of one point on the Earth above a given plane, usually sea level.

EOLIAN: Pertaining to or deposited by wind.

EON: The largest division of geologic time, embracing several eras; for example, the Phanerozoic eon, from 542 million years ago to the present.

EPEIROGENY: Large-scale, primarily vertical, movement of the crust, characteristically so gradual that rocks are little folded and faulted.

EPICENTER: The point on the Earth's surface directly above the focus of an earthquake.

EPOCH: A subdivision of a geologic period, often chosen to correspond to a stratigraphic sequence. Also used for a division of time corresponding to a paleomagnetic interval.

ERA: A division of geologic time including several periods, but smaller than an eon. Commonly recognized eras are Precambrian, Paleozoic, Mesozoic, and Cenozoic.

ERG: Extensive region, or sea, of composite dunes formed by wind-transported sand and found in major deserts.

EROSION: The set of all processes by which soil and rock are loosened and moved downhill or downwind.

ERRATIC: Rock fragment (especially boulder-sized) carried by a glacier away from the outcrop from which it was derived, often into an area underlain by a rock type different from that of the rock fragment.

ESKER: A glacial deposit of sand and gravel in the form of a continuous, winding ridge, formed from the deposits of a stream flowing beneath the ice.

ESTUARY: A body of water along a coastline, open to the ocean but diluted by fresh water.

EXCLUSIVE ECONOMIC ZONE: The zone from a country's coast to 200 miles offshore in which the country has exclusive rights to mineral deposits.

EXFOLIATION: A physical weathering process in which sheets of rock are fractured and detached from an outcrop.

EXTRUSIVE IGNEOUS ROCK: An igneous rock formed from lava or from other products of volcanic material spewed out onto the surface of the Earth.

FAULT: A planar or gently curved fracture in the Earth's crust across which there has been relative displacement of the two blocks of rock parallel to the fracture.

FAULT-BLOCK MOUNTAINS: A mountain or range formed when the crust is broken into blocks of different elevations by normal faulting.

FAULTING: The processes by which crustal forces cause a rock formation to break and slip along a fault.

FELSIC: An adjective used to describe a light-colored igneous rock that is poor in iron and magnesium and contains abundant feldspars and quartz.

FERRIC IRON: Iron with a 13 charge (Fe31).

FERROUS IRON: Iron with a 12 charge (Fe 21). FIRN: Old, dense, compacted snow.

FISSURE ERUPTION: A volcanic eruption emanating from an elongate fissure rather than a central vent.

FJORD: A former glacial valley with steep walls and a U-shaped profile, now occupied by the sea.

FLOOD BASALT: A basltic plateau extending many kilometers in flat, layered flows originating from fissure eruptions.

FLOODPLAIN: A level plain of stratified, unconsolidated sediment on either side of a stream, submerged during floods and built up by silt and sand carried out of the main channel.

FLOOD TIDE: The part of the tide cycle during which the water is rising or leveling off at high water.

FLUX: The flow of Earth's chemicals from one reservoir to another. (See also Geochemical cycle.)

FOCUS (EARTHQUAKE): The point along a fault at which the rupture occurs. Also called the hypocenter. (See also Epicenter.)

FOLD: A bent or warped stratum or sequence of strata that was originally horizontal, or nearly so, and was subsequently deformed.

FOLD AXIS: Within each stratum involved in a fold, the axis connecting all the points in the center of the fold, from which both limbs bend.

FOLD BELT: Synonym for Orogenic belt.

FOLDING: The processes by which crustal forces deform an area of crust so that layers of rock are pushed into folds.

FOLIATION: A set of flat or wavy planes in a metamorphic rock, produced by structural deformation.

FORAMINIFER: A group of single-celled organisms whose secretions and calcite shells account for most of the ocean's carbonate sediments.

FORAMINIFERAL OOZE: A calcareous pelagic sediment composed of the shells of dead foraminifera.

FORESET BED: One of the inclined beds found in cross-bedding; also an inclined bed deposited on the outer front of a delta.

FORESHORE: The marine zone between the upper limit of wave wash at high tide and the low-tide mark.

FORMATION: The basic unit for the naming of rocks in stratigraphy; a set of rocks that are or once were horizontally continuous, that share some distinctive feature of lithology, and that are large enough to be mapped.

FOSSIL FUEL: A general term for combustible geologic deposits of hydrocarbons of biologic origin, including coal, oil, natural gas, oil shales, and tar sands.

FRACTIONAL CRYSTALLIZATION: The separation of a cooling magma into components by the successive formation and removal of crystals at progressively lower temperatures.

FRACTURE: The irregular breaking of a crystal along a surface not parallel to a crystal face; serves to identify minerals.

FRINGING REEF: A coral reef that is directly attached to a landmass not composed of coral.

FUMAROLE: Small volcanic vent that emits gas and steam from which minerals precipitate onto surrounding surfaces.

GABBRO: A black, coarse-grained, intrusive igneous rock, composed of calcic feldspars and pyroxene; the intrusive equivalent of basalt.

GEOCHEMICAL CYCLE: The set of processes that carry a particular chemical from reservoir to reservoir in Earth's systems.

GEOLOGIC TIME: The time from the formation of the Earth to the present, divided into periods of time during which known geological events have taken place.

GEOLOGIC TIME SCALE: The division of geologic history into eras, periods, and epochs accomplished through stratigraphy and paleontology.

GEOLOGY: The science of Earth-how it originated, how it evolved, how it works, and how human intervention can affect it.

GEOTHERM: A curve on a temperature-pressure or temperature-depth graph that describes how temperature in the Earth changes with depth. Different tectonic provinces are characterized by more or less rapid increases of temperature with depth.

GEOTHERMAL ENERGY: Energy generated by using the heat energy of the crust, especially in volcanic regions.

GLACIER: A mass of ice and surficial snow that persists throughout the year and flows downhill under its own weight. The size range is from 100 m to 10,000 km. (See also Continental glacier; Valley glacier.)

GLACIER SURGE: A period of unusually rapid movement of a glacier, sometimes lasting more than a year.

GLASS: A rock formed when magma or molten rock is cooled too rapidly to allow crystal growth.

GLASSY: Adjective indicating that a material does not have an orderly, repeating, three-dimensional array of atoms.

GLOBAL CHANGE: A change in climate that has worldwide effects on the environment, life, and other components of Earth.

GNEISS: A coarse-grained regional metamorphic rock that shows banding and parallel alignment of minerals.

GRADED BEDDING: A bed in which the coarsest particles are concentrated at the bottom and grade gradually upward into fine silt, the whole bed having been deposited by a waning current.

GRADED STREAM: A stream whose smooth longitudinal profile is unbroken by resistant ledges, lakes, or waterfalls and that exactly maintains the slope, velocity, and discharge required to carry its sediment load in equilibrium without erosion or sedimentation.

GRANITE: A felsic, coarse-grained, intrusive igneous rock composed of quartz, orthoclase feldspar, sodium-rich plagioclase feldspar, and micas. Also sometimes a metamorphic product. (Compare Rhyolite.)

GRANITIZATION: The formation of metamorphic granite from other rocks by recrystallization with or without complete melting.

GRANODIORITE: A plutonic rock similar to granite in composition, except that plagioclase feldspar is present in greater abundance than orthoclase feldspar; the intrusive equivalent of dacite.

GRANULITE: A nonfoliated regional metamorphic rock with coarse interlocking grains, generally formed under conditions of relatively high pressure and temperature.

GRAVEL: The coarsest clastic sediment, consisting mostly of particles larger than 2 mm and including cobbles and boulders.

GREENHOUSE EFFECT: A global warming effect in which carbon dioxide and water vapor absorb infrared radiation from Earth's surface and radiate it back to the surface.

GREENSCHIST: A schist containing chlorite and epidote (which are green) and formed by low-temperature, low-pressure metamorphism of mafic volcanic rocks.

GREENSTONE: A field term applied to any low-grade metamorphosed mafic igneous rock (for example, basalt, gabbro, or diabase). Chlorite accounts for their greenish cast.

GROUNDWATER: The mass of water in the ground (below the unsaturated zone) occupying the total pore space in the rock and moving slowly where permeability allows.

GROUNDWATER TABLE: The upper surface of the saturated zone of groundwater. Also called the water table.

GUYOT: A flat-topped submarine mountain or seamount.

HALF-LIFE: The time required for half of a sample of a given radioactive isotope to decay.

HANGING VALLEY: The valley left by a melted glacial tributary that enters a larger glacial valley above its base, high up on the valley wall.

HARDNESS (MINERAL): A measure of the ease with which the surface of a mineral can be scratched.

HEAT CONDUCTION: The transfer of the vibrational energy of atoms and molecules, which constitutes heat energy, by the mechanism of atomic or molecular impact. (Compare Convection.)

HEAT RESERVOIR: A subsurface region containing enough heat to be used for geothermal energy.

HOGBACK: A formation similar to a cuesta in that it is a ridge formed by slower erosion of hard strata, but having two steep, equally inclined slopes.

HORNFELS: A high-temperature metamorphic rock of uniform grain size showing no foliation. Usually formed by contact metamorphism.

HOT SPOT: The volcanic surface expression of a mantle plume.

HUMUS: The decayed part of the organic matter in a soil.

HYDRATION: The absorption of water by a mineral, usually in weathering.

HYDROLOGIC CYCLE: The cyclical movement of water from the ocean to the atmosphere, through rain to the surface, through runoff and groundwater to streams, and back to the sea.

HYDROLOGY: The science of that part of the hydrologic cycle between rain and return to the sea; the study of the movement and characteristics of water on and within the land.

HYDROTHERMAL ACTIVITY: Any process involving high-temperature groundwater, especially the alteration and emplacement of minerals and the formation of hot springs and geysers.

HYDROTHERMAL METAMORPHISM: A form of metamorphism, frequently associated with mid-ocean ridges, in which hot fluids percolate through the crust and metamorphose the invaded rocks.

HYDROTHERMAL VEIN: A cluster of minerals precipitated by hydrothermal activity in a rock cavity.

ICEBERG CALVING: The breaking off of blocks of ice from a glacier when it moves to a shoreline, forming icebergs.

IGNEOUS ROCK: A rock formed by the solidification of a magma.

INFILTRATION: The movement of groundwater or hydrothermal water into rock or soil through pores and joints.

INFLUENT STREAM: A stream or portion of a stream that recharges groundwater through the stream bottom because its elevation is above the groundwater table. (Compare Effluent stream.)

INTRUSIVE IGNEOUS ROCK: Igneous rock that forced its way in a molten state into the country rock. Also called an intrusion.

ION: An atom or group of atoms that has gained or lost electrons and so has a net electric charge.

IONIC BOND: A bond formed between atoms by electrostatic attraction between oppositely charged ions.

IRON FORMATION: A sedimentary rock containing much iron, usually more than 15 percent, as sulfide, oxide, hydroxide, or carbonate; a low-grade ore of iron.

ISLAND ARC: A linear or arc-shaped chain of volcanic islands formed at a convergent plate boundary. The island arc is formed in the overriding plate from rising melt derived from the subducted plate and from the asthenosphere above that plate.

ISOCHRON: A line connecting points of equal age.

ISOSTASY, PRINCIPLE OF: The mechanism whereby areas of the crust rise or subside until the mass of their topography is buoyantly supported or compensated by the thickness of crust below, which floats on the denser mantle. The theory of isostasy holds that continents and mountains are supported by low-density crustal roots.

ISOTOPE: One of several forms of one element, all having the same number of protons in the nucleus, but differing in their number of neutrons and thus in their atomic weight.

KAME: A ridgelike or hilly local glacial deposit of coarse clastic sediment formed as a delta at the glacier front by meltwater streams.

KARST TOPOGRAPHY: An irregular topography characterized by sinkholes, caverns, and lack of surface streams; formed in humid regions because an underlying carbonate formation has been riddled with underground drainage channels that capture the surface streams.

KETTLE: A hollow or depression formed in glacial deposits when outwash was deposited around a residual block of ice that later melted.

LAHAR: A mudflow of unconsolidated volcanic ash, dust, breccia, and boulders that occurs when pyroclastic or lava deposits mix with rain or the water of a lake, river, or melting glacier.

LAMINAR FLOW: A flow in which streamlines are straight or gently curved and parallel. (Compare Turbulent flow.)

LANDFORM: A characteristic landscape feature on the Earth's surface that attained its shape through the processes of erosion and sedimentation; for example, a hill or a valley.

LATERITE: A distinctive, deep-red soil formed in very humid regions, characterized by high alumina and iron oxide content, and produced by rapid chemical weathering of feldspar minerals.

LAVA: Magma that has reached the surface.

LEVEE: A ridge along a stream bank, formed by deposits left when floodwater slowed on leaving the channel; also, an artificial barrier to floods built in the same form.

LIMB (FOLD): The two relatively planar sides of a fold, one on either side of the axial plane.

LIMESTONE: A sedimentary rock composed mainly of calcium carbonate (CaCO3), usually as the mineral calcite.

LINEAR DUNE: A long, narrow eolian sand dune that is aligned parallel to the direction of the prevailing wind.

LITHIFICATION: The chemical and physical diagenetic processes that bind and harden a sediment into a sedimentary rock. (See also Compaction.)

LITHOSPHERE: The outer, rigid shell of the Earth, situated above the asthenosphere and containing the crust, the uppermost part of the mantle, the continents, and the plates.

LOESS: An unstratified, wind-deposited, dusty sediment rich in clay minerals.

LONGITUDINAL PROFILE: A cross section of a stream from its head to its mouth, showing elevation versus distance to the mouth.

LONGSHORE CURRENT: A current that flows parallel to the shoreline; the summed longshore components of water motion of waves that break obliquely with respect to the shore.

LONGSHORE DRIFT: The movement of sediment along a beach by swash and backwash of waves that approach the shore obliquely.

LUSTER: The general quality of the shine of a mineral surface, described by such subjective terms as dull, glassy, or metallic. (See Table 2.3.)

MAFIC: Adjective describing dark-colored minerals rich in iron and magnesium and relatively poor in silica (for example, pyroxene, amphibole, or olivine); also, describing rocks rich in mafic minerals.

MAGMA: Molten rock material that forms igneous rocks upon cooling. Magma that reaches the surface is lava.

MAGMA CHAMBER: A magma-filled cavity within the lithosphere.

MAGNETIC STRATIGRAPHY: The study and correlation of polarity epochs and events in the history of the Earth's magnetic field as contained in magnetic rocks.

MAGNITUDE: A measure of earthquake size, determined by taking the common logarithm (base 10) of the largest ground motion observed during the arrival of a P wave or seismic surface wave and applying a standard correction for distance to the epicenter.

MANTLE: The main bulk of the solid Earth, between the crust and the core, ranging from depths of about 40 km to 2900 km. It is composed of dense, mafic silicate minerals and divided into concentric layers by phase changes that are caused by the increase in pressure with depth.

MANTLE PLUME: Rising jet of hot, partially molten material, thought to emanate from the deep mantle and responsible for intraplate volcanism.

MARBLE: The metamorphosed equivalent of limestone or other carbonate rock.

MARINE EVAPORITE SEDIMENT, ROCK: A sediment or sedimentary rock consisting of minerals precipitated by evaporating seawater. Includes salt and gypsum.

MASS MOVEMENT: A downhill movement of soil or fractured rock under the force of gravity.

MASS WASTING: The sum of all mass movements and related erosional phenomena.

MEANDER: A broad, semicircular curve in a stream that develops as the stream erodes the outer bank of a bend and deposits sediment (as point bars) against the inner bank.

M�LANGE: A formation found at convergent plate boundaries consisting of a heterogeneous mixture of rock materials in which fragments of diverse composition, size, and texture were mixed and consolidated by tremendous deformational pressure.

MESA: A flat-topped, steep-sided upland topped by a resistant formation.

METALLIC BOND: A type of covalent bond in which freely mobile electrons are shared and dispersed among ions of metallic elements, which have the tendency to lose electrons and pack together as cations.

METAMORPHIC FACIES: Characteristic assemblages of minerals in metamorphic rocks that are indicative of the range of pressures and temperatures experienced during metamorphism.

METAMORPHIC ROCK: A rock whose original mineralogy, texture, or composition has been changed by the effects of pressure, temperature, or the gain or loss of chemical components.

METASOMATISM: A change in the bulk chemical composition of a rock by fluid transport of some chemical components into or out of the rock.

METEORIC WATER: Rainwater, snow, hail, and sleet.

MICROPLATE TERRANE: A block within an orogenic belt containing rock assemblages that contrast sharply with those in the surrounding areas, interpreted as small continents, seamounts, or island arcs that were accreted onto the larger continent at a convergent plate boundary.

MID-OCEAN RIDGE: A major elevated linear feature of the seafloor consisting of many small, slightly offset segments, with a total length of 200 to 20,000 km. A mid-ocean ridge occurs at a divergent plate boundary, a site where two plates are being pulled apart and new oceanic lithosphere is being created.

MIGMATITE: A rock with both igneous and metamorphic characteristics that shows large crystal and laminar flow structures. Probably formed metamorphically in the presence of water and without complete melting.

MINERAL: A naturally occurring, inorganic, crystalline solid with a specific chemical composition.

MINERALOGY: The study of mineral composition, structure, appearance, stability, occurrence, and associations. The mineralogy of a rock is the mineral assemblage contained within that rock.

MOHOROVICŠIC� DISCONTINUITY, MOHO: The boundary between crust and mantle, at a depth of 5 to 45 km, marked by a rapid increase in seismic wave velocity to more than 8 km per second.

MOHS SCALE OF HARDNESS: An empirical, ascending scale of mineral hardness. (See Table 2.2.)

MOMENT MAGNITUDE: A measure of earthquake size, determined by the slip of the fault, the area of the break, and the rigidity of rock. (See also Richter magnitude.)

MORAINE: A glacial deposit of till left at the margins of an ice sheet. Subdivided into ground moraine, lateral moraine, medial moraine, and end moraine.

MUDFLOW: A mass movement of material mostly finer than sand, along with some rock debris, lubricated with large amounts of water. The water tends to make mudflows move faster than earthflows or debris flows.

MUDSTONE: The lithified equivalent of mud; a fine-grained sedimentary rock similar to shale but less finely laminated.

MYLONITE: A very fine grained metamorphic rock commonly found in major thrust faults and produced by shearing and rolling during fault movement.

NATURAL LEVEE: See Levee.

NEAP TIDE: A tide cycle of unusually small amplitude that occurs twice monthly when the lunar and solar tides are opposed-that is, when the gravitational pull of the Sun is at right angles to that of the Moon. (Compare Spring tide.)

NEUTRON: An electrically neutral elementary particle in the atomic nucleus, having the mass of one proton.

NONFOLIATED METAMORPHIC ROCKS: Metamorphic rocks that have no preferred orientation of crystals, or very weak orientation, and thus show little or no slaty cleavage or schistosity.

OBSIDIAN: Dark volcanic glass, usually of felsic composition.

OFFSHORE: The marine zone extending from the breaker zone to the edge of the continental shelf.

OIL RESERVOIR: A bed of permeable and porous rock that contains commercially producible oil.

OIL SHALE: A dark-colored shale that contains organic material and that can be crushed and heated to liberate oil.

OIL TRAP: A tectonic or sedimentary structure that impedes upward movement of oil or gas and allows it to collect beneath the barrier.

OPHIOLITE SUITE: An assemblage of mafic and ultramafic igneous rocks with deep-sea sediments found on land, believed to be associated with divergent plate boundaries and the seafloor environment.

ORE DEPOSIT: A sedimentary, igneous, or metamorphic rock containing minerals, commonly metallic oxides or silicates, that can be commercially mined.

ORGANIC SEDIMENT, ROCK: A sediment or sedimentary rock consisting entirely or in part of organic carbon-rich deposits formed by the decay of once-living material after burial. Includes coal and organic carbon-rich shales.

ORIGINAL HORIZONTALITY, PRINCIPLE OF: The proposition that all sedimentary bedding is horizontal at the time of deposition.

OROGENIC BELT: A linear region that has been subjected to folding and other deformation in a mountain-building episode.

OROGENY: The tectonic process in which large areas are folded, thrust-faulted, metamorphosed, and subjected to plutonism. The cycle ends with uplift and the formation of mountains.

OUTCROP: A segment of bedrock exposed to the atmosphere.

OUTWASH: A sediment deposited by meltwater streams emanating from a glacier.

OVERTURNED FOLD: A fold in which a limb has tilted past vertical so that the older strata are uppermost.

OXBOW LAKE: A long, broad, crescent-shaped lake formed when a stream abandons a meander and takes a new course.

OXIDATION: A chemical reaction in which electrons are lost from an atom and its charge becomes more positive; a chemical combination of an element with oxygen.

OZONE: A molecule (O3) that absorbs ultraviolet radiation in the stratosphere but creates smog when it forms near Earth's surface.

PAHOEHOE: A basaltic lava flow with a glassy, smooth, and ropy surface.

PALEOMAGNETIC STRATIGRAPHY: A branch of stratigraphy in which the remanent magnetization recorded in a rock is used to place the rock on the magnetic time scale constructed from known temporal variations in the Earth's magnetic field.

PALEOMAGNETISM: The remanent magnetization recorded in ancient rocks; allows the reconstruction of Earth's ancient magnetic field and the positions of the continents.

PALEONTOLOGY: The science of fossils of ancient life forms and their evolution.

PANGAEA: Supercontinent that coalesced in the latest Paleozoic era and comprised all present continents. The breakup of Pangaea began in Mesozoic time, as inferred from paleomagnetic and other data.

PARTIAL MELTING: A process in which heating melts some of the minerals in a mass of rock while the rest remain solid. Partial melting occurs because the minerals that compose a rock melt at different temperatures.

PASSIVE MARGIN: A continental margin characterized by thick, flat-lying, shallow-water sediments with only limited tectonism related to divergent plate motion.

PEAT: A marsh or swamp deposit of water-soaked plant remains containing more than 50 percent carbon.

PEDALFER: A common soil type in temperate regions; characterized by an abundance of iron oxides and clay minerals deposited in the B-horizon by leaching.

PEDIMENT: A planar, sloping rock surface forming a ramp up to the front of a retreating mountain range in an arid region. It may be covered locally by thin alluvial deposits.

PEDOCAL: A common soil type of arid regions, characterized by accumulation of calcium carbonate in the B-horizon.

PEGMATITE: A vein of extremely coarse grained granite, often containing economic amounts of rare elements.

PELAGIC SEDIMENT: A deep-sea sediment composed of fine-grained detritus that slowly settles from surface waters. Common constituents are clay, foraminiferal ooze, and silica ooze.

PERCHED WATER TABLE: The upper surface of an isolated body of groundwater that is perched above and separated from the main body of groundwater by an aquiclude.

PERIDOTITE: A coarse-grained ultramafic igneous rock composed of olivine with small amounts of pyroxene and amphibole.

PERIOD (GEOLOGIC): The most commonly used unit of geologic time, representing one subdivision of an era.

PERIOD (WAVE): The time interval between the arrival of successive crests in a homogeneous wave train.

PERMAFROST: A permanently frozen aggregate of ice and soil occurring in very cold regions.

PERMEABILITY: The ability of a formation to transmit groundwater or other fluids through pores and cracks.

PHENOCRYST: A large crystal surrounded by a finer matrix in an igneous rock. An igneous rock that contains abundant phenocrysts is called a porphyry.

PHOSPHORITE: A sedimentary rock composed largely of calcium phosphate, usually as a variety of the mineral apatite and largely in the form of concretions and nodules. Primary ore of phosphate minerals and elemental phosphorus.

PHOTOSYNTHESIS: The process by which green organisms use chlorophyll and the energy from sunlight to make organic matter (carbohydrates) out of carbon dioxide and water. (See also Respiration.)

PHREATIC EXPLOSION: A volcanic eruption of steam, mud, and debris caused by the expansion of steam formed when magma comes in contact with groundwater or seawater.

PHYLLITE: A metamorphic rock that is intermediate in grade between slate and mica schist. Small crystals of micas give a silky sheen to the cleavage surfaces.

PHYSICAL WEATHERING: The set of all physical processes by which an outcrop is broken up into smaller particles.

PILLOW LAVA: A basaltic lava that forms under water when many small tongues of lava break through the chilled ocean floor and quickly solidify into a rock formation resembling a pile of sandbags.

PLACER: A clastic sedimentary deposit of a valuable mineral or native metal in unusually high concentration, usually segregated because of its greater density.

PLANETARY DIFFERENTIATION: The process by which heating, cooling, and gravitation sorted the materials of our planet so that it evolved into concentric layers that differ chemically and physically.

PLASTIC FLOW: Deformation of the shape or volume of a substance without fracturing.

PLATE: One of the dozen or more segments of the lithosphere that ride as distinct units over the asthenosphere.

PLATEAU: An extensive upland region at high elevation with respect to its surroundings.

PLATE TECTONICS: The theory and study of plate formation, movement, interactions, and destruction; the attempt to explain seismicity, volcanism, mountain building, and evidence of paleomagnetism in terms of plate motions.

PLATFORM: A sediment-covered, tectonically stable, almost level region of a continent.

PLAYA, PLAYA LAKE: The flat floor of a closed basin in an arid region, usually rich in evaporate minerals. It may be occupied by an intermittent lake.

PLUNGING FOLD: A fold whose axis is not horizontal but dips.

PLUTON: A large igneous intrusion, at least 1 km3, formed at depth in the crust.

PLUTONISM: Igneous activity at depth in the crust.

POINT BAR: A deposit of sediment on the inner bank of a meander that forms because the stream velocity is lower against the inner bank.

POLYMORPH: One of two or more alternative possible structures for a single chemical compound; for example, the minerals calcite and aragonite are polymorphs of calcium carbonate (CaCO3).

POROSITY: The percentage of the total volume of a rock that is pore space (not occupied by mineral grains).

PORPHYROBLAST: A large crystal in a finer-grained matrix in a metamorphic rock: analogous to a phenocryst in an igneous rock.

PORPHYRY: An igneous rock containing abundant phenocrysts suspended in a finely crystalline matrix.

POTABLE WATER: Water that is agreeable to the taste and not dangerous to the health.

POTHOLE: A hemispherical hole in the bedrock of a streambed, formed by abrasion of small pebbles and cobbles in a strong current.

PRECIPITATE: To drop out of a saturated solution as crystals; the crystals that drop out of a saturated solution.

PRESSURE: A force distributed over a surface divided by the area of the surface. Confining pressure is uniform in all directions, while directed pressure is exerted in a particular direction.

PROTON: With neutrons, one of the two types of particles in the atomic nucleus that account for most of the atom's mass. The mass of a proton is valued at 1 unit and is equivalent to the mass of 18 3 6 electrons. Each proton has a positive electrical charge of 1.6 3 10219 coulombs, expressed as 11.

PUMICE: A form of volcanic glass, usually of felsic composition, so filled with holes from the escape of gas during quenching that it resembles a sponge and has very low density. (Compare Obsidian.)

P WAVE: The primary or fastest wave traveling away from a seismic event through the solid rock and consisting of a train of compressions and dilations of the material.

PYROCLAST: Fragment of volcanic material ejected during an eruption.

PYROCLASTIC FLOW: A glowing cloud of volcanic ash, fragments of volcanic rock, and gases that moves rapidly downhill away from the eruptive center during a volcanic eruption.

PYROXENE GRANULITE: A coarse-grained regional metamorphic rock containing pyroxene; formed at high temperatures and pressures deep in the crust.

RADIAL DRAINAGE: A system of streams running in a radial pattern away from the center of a circular elevation, such as a volcano or dome.

RADIOACTIVITY: The emission of energetic particles and/or radiation during radioactive decay.

RADIOMETRIC DATING: The method of obtaining ages of geological materials by measuring the relative abundances of radioactive parent and daughter isotopes in them.

RAIN SHADOW: An area of low rainfall on the leeward slope of a mountain range.

REACTION SERIES: A series of chemical reactions occurring in a cooling magma by which a mineral formed at high temperature becomes unstable in the melt and reacts to form another mineral.

RECHARGE: In hydrology, the replenishment of groundwater, usually by infiltration of meteoric water through the soil.

RECTANGULAR DRAINAGE: A system of streams in which each straight segment of each stream takes one of two characteristic perpendicular directions, usually following sets of joints.

RECURRENCE INTERVAL: The average time interval between occurrences of a geologic event, such as a flood or earthquake, of a given or greater magnitude.

REEF: A mound or ridge-shaped organic structure that is built by calcareous organisms, is wave resistant, and stands in relief above the surrounding seafloor.

REGIONAL METAMORPHISM: Metamorphism caused by deep burial or strong tectonic forces that impose high temperatures and high pressures over large belts or regions of the crust.

REGOLITH: The layer of loose, heterogeneous material lying on top of bedrock; includes soil, unweathered fragments of parent rock, and rock fragments weathered from the bedrock.

REJUVENATION (OF MOUNTAINS): Renewed uplift in a mountain chain on the site of earlier uplifts, returning the area to a more youthful stage of the cycle of erosion.

RELATIVE AGE: The age of a geologic event or feature relative to other geologic events or features and expressed in terms of the geologic time scale. (Compare Absolute age.)

RELATIVE HUMIDITY: The amount of water vapor in the air, expressed as a percentage of the total amount of water vapor that the air could hold at that temperature if saturated.

RELIEF: The maximum regional difference in elevation.

RESERVES: Deposits of minerals, coal, or oil and gas that have been shown to be extractable profitably with existing technology. Proven reserves are those for which good estimates of the quantity and quality have been made. (See also Resources.)

RESERVOIR: A source or place of residence for elements in a chemical cycle or hydrologic cycle.

RESIDENCE TIME: The average length of time that an atom of a particular element spends in a reservoir before leaving.

RESOURCES: Discovered and undiscovered deposits of minerals, coal, or oil and gas that are or may become available for use in the future; includes reserves, plus discovered deposits not now commercially or technologically extractable, plus undiscovered deposits that may be inferred to exist. (See also Reserves.)

RESPIRATION: The process by which carbohydrates combine with oxygen to produce carbon dioxide and water and to release energy. (See also Photosynthesis.)

RETROGRADE METAMORPHISM: Metamorphism in which a rock that has been metamorphosed to a fairly high grade is later remetamorphosed at lower temperature and pressure to a lower grade.

RHYOLITE: The fine-grained volcanic or extrusive equivalent of granite, light-brown to gray and compact.

RICHTER MAGNITUDE: A measure of earthquake size, determined by taking the common logarithm (base 10) of the largest ground motion observed during the arrival of a P wave or seismic surface wave and applying a standard correction for distance to the epicenter. (See also Moment magnitude.)

RIFT VALLEY: A fault trough formed at a divergent plate boundary or other area of tension.

RIPPLE: A very small dune of sand or silt whose long dimension is formed at right angles to the current.

RIVER: A general term for a relatively large stream, or the main branches of a stream system.

ROCK AVALANCHE: The rapid, downhill-flowing mass movement of broken rock material, during which further breakage of the material may occur.

ROCK CYCLE: The set of geologic processes by which each of the three great groups of rocks is produced from the other two: sedimentary rocks are metamorphosed to metamorphic rocks or melted to create igneous rocks, and all rocks may be uplifted and eroded to make sediments, which lithify to sedimentary rocks.

ROCKFALL: The relatively free falling of a newly detached segment of bedrock from a cliff or other steep slope.

ROCK FLOUR: A glacial sediment of extremely fine (silt- and clay-size) ground rock formed by abrasion of rocks at the base of the glacier.

ROCKSLIDE: The mass movement of large blocks of detached bedrock sliding more or less as a unit.

RUNOFF: The amount of rainwater that does not infiltrate the ground but leaves an area in surface drainage.

SALTATION: The movement of sand or fine sediment by short jumps above the ground or streambed under the influence of a current too weak to keep it permanently suspended.

SANDBLASTING: A physical weathering process in which rock is eroded by the impact of sand grains carried by the wind, frequently leading to ventifact formation of pebbles and cobbles.

SANDSTONE: A clastic rock composed of grains from 0.0625 to 2 mm in diameter, usually quartz, feldspar, and rock fragments, bound together by a cement of quartz, carbonate, or other minerals, or by a matrix of clay minerals.

SATURATED ZONE: The zone of soil and rock in which pores are completely filled with groundwater.

SCHIST: A metamorphic rock characterized by strong foliation or schistosity.

SCHISTOSITY: The parallel arrangement of sheety or prismatic minerals like micas and amphiboles resulting from metamorphism.

SCIENTIFIC METHOD: A general research strategy, based on creative analyses of verifiable data, by which scientists propose and test hypotheses that explain some aspect of how the physical realm works.

SEAFLOOR SPREADING: The mechanism by which new seafloor is created at ridges at divergent plate boundaries as adjacent plates move apart. This process may continue at a few centimeters per year through many geologic periods.

SEAMOUNT: An isolated, tall mountain on the seafloor that may extend more than 1 km from base to peak.

SEDIMENT: Any of a number of materials deposited at Earth's surface by physical agents (such as wind, water, and ice), chemical agents (precipitation from oceans, lakes, and rivers), or biological agents (organisms, living and dead).

SEDIMENTARY BASIN: A region of considerable extent (at least 10,000 km2) that is the site of accumulation of a large thickness of sediments.

SEDIMENTARY BRECCIA: A clastic rock composed mainly of large angular fragments.

SEDIMENTARY ENVIRONMENT: A geographically limited area where sediments are preserved; characterized by its landforms, climate, relative energy of water and wind currents, biological activity, and the relative abundance of various chemical substances.

SEDIMENTARY ROCK: A rock formed by the accumulation and cementation of mineral grains by wind, water, or ice transportation to the site of deposition or by chemical precipitation at the site.

SEDIMENTARY STRUCTURE: Any structure of a sedimentary or weakly metamorphosed rock that was formed at the time of deposition; includes bedding, cross-bedding, graded bedding, ripples, scour marks, mudcracks.

SEISMIC GAP METHOD: A predictive model for earthquake occurrences along active fault zones based on the study of segments that have experienced little or no movement and are thought to be under high stress.

SEISMICITY: The worldwide or local distribution of earthquakes in space and time; a general term for the number of earthquakes in a unit of time.

SEISMIC SURFACE WAVE: A seismic wave that follows the Earth's surface only, with a speed less than that of S waves.

SEISMIC WAVE: A general term for the elastic waves produced by earthquakes or explosions. (See also P wave; S wave; Seismic surface wave.)

SEISMOGRAPH: An instrument for magnifying and recording the motions of the Earth's surface that are caused by seismic waves.

SHADOW ZONE: A zone 105� to 142� from the epicenter of an earthquake in which there is no penetration of seismic waves through the Earth because of wave refraction or because the waves are not transmitted upon entering the liquid core.

SHALE: A very fine grained clastic rock composed of silt and clay that tends to part along bedding planes. (See also Oil shale.)

SHEARING FORCES: Forces that deform a body so that parts of the body on opposite sides of a plane slide past one another; that is, forces acting tangentially to the plane. Shearing forces dominate at transform fault plate boundaries.

SHIELD: A large region of stable, ancient basement rocks within a continent.

SHIELD VOLCANO: A large, broad volcanic cone with very gentle slopes built up by nonviscous basaltic lavas.

SHOCK METAMORPHISM: Metamorphism that occurs when minerals are subjected to the high pressures and temperatures of shock waves generated by impacts.

SHORELINE: The straight or sinuous, smooth or irregular interface between land and sea.

SILICA OOZE: A pelagic sediment consisting of the remains of tiny organisms that have shells made of amorphous silica.

SILICATE ROCK: An igneous or metamorphic rock made up largely of silicate minerals, such as feldspar, mica, or garnet.

SILICEOUS SEDIMENTARY ROCK: Rock containing abundant free silica of either organic or inorganic origin, formed by biochemical, chemical, or physical deposition of silica.

SILL: A horizontal, tabular igenous intrusion running between parallel layers of bedded country rock. (See also Concordant intrusion.)

SILTSTONE: A clastic rock that contains mostly silt-sized material, from 0.0039 to 0.062 mm.

SINKHOLE: A small, steep depression caused in karst topography by the dissolution and collapse of subterranean caverns in carbonate formations.

SLATE: The lowest grade of foliated metamorphic rock, easily split into thin sheets; produced primarily by the metamorphism of shale.

SLIP FACE: The steep downwind face of a dune on which sand is deposited in cross-beds at the angle of repose.

SLIP (FAULT): The motion of one face of a fault relative to the other.

SLUMP: A slow mass movement of unconsolidated materials that slide as a unit.

SOIL: The surface accumulation of sand, clay, and humus that composes the regolith, but excluding the larger fragments of unweathered rock.

SOLAR CONSTANT: The amount of radiation from the Sun that reaches the top of the atmosphere; about 1370 watts per square meter.

SOLIFLUCTION: The creep of soil saturated with water and/or ice, caused by alternate freezing and thawing; most common in polar regions.

SOLUBILITY (MINERAL): The extent to which a mineral can dissolve in water; the amount of the mineral dissolved in water when the solution reaches the saturation point.

SORTING: A measure of the homogeneity of the sizes of particles in a sediment or sedimentary rock.

SOURCE BED: Organic sediment or rock that liberates oil or gas when heated during burial. Usually source beds are organic-rich black shales or limestones.

SPECIFIC GRAVITY: The ratio of the density of a given substance to the density of water.

SPHEROIDAL WEATHERING: A physical and chemical weathering process in which curved layers split off from a rounded boulder, leaving a spherical inner core.

SPIT: A long range of sand deposited by longshore currents and longshore drift where the coast takes an abrupt inward turn. It is attached to land at the upstream end.

SPRING TIDE: A tide cycle of unusually large amplitude that occurs twice monthly when the lunar and solar tides are in phase. (Compare Neap tide.)

STACKS: Isolated rocky prominences or pinnacles left standing above a marine platform as erosional remnants.

STALACTITE: An icicle-like or toothlike deposit of calcite or aragonite hanging from the roof of a cave. It is deposited by evaporation and precipitation from solutions seeping through limestone.

STALAGMITE: An inverted icicle-shaped deposit that builds up on a cave floor beneath a stalactite and is formed by the same process as a stalactite.

STOCK (VOLCANIC): An intrusion with the characteristics of a batholith but less than 100 km2 in area.

STRATIFICATION: The characteristic layering or bedding of sedimentary rocks.

STRATIGRAPHIC SEQUENCE: A set of deposited beds that reflects the changing conditions and sedimentary environments that define the geologic history of a region.

STRATIGRAPHY: The science of description, correlation, and classification of strata in sedimentary rocks, including the interpretation of the sedimentary environments of those strata.

STRATOSPHERE: The upper atmosphere, 10 to 50 km above the surface, where a protective ozone layer forms.

STRATOVOLCANO: See Composite volcano.

STREAK: The fine deposit of mineral dust left on an abrasive surface when a mineral is scraped across it; especially the characteristic color of the dust.

STREAM: A general term for any body of water, large or small, that moves under the force of gravity in a channel. (Compare River.)

STREAM PIRACY: The erosion of a divide between two streams by the more competent stream, leading to the capture of all or part of the drainage of the slower stream by the faster.

STRESS: The force exerted, in terms of force per unit area, when one body presses upon, pulls upon, or pushes tangentially against another body.

STRIATION (GLACIAL): Scratches left on bedrock and boulders by overriding ice, showing the direction of glacial motion.

STRIKE: The angle between true north and the horizontal line contained in any planar feature (inclined bed, dike, fault plane, and so forth); also the geographic direction of this horizontal line.

SUBDUCTION: The sinking of an oceanic plate beneath an overriding plate; occurs at convergent plate boundaries.

SUBDUCTION ZONE: The zone between a sinking oceanic plate and an overriding plate, descending away from a trench and characterized by high seismicity. (See also Convergent plate boundary.)

SUBLIMATION: A phase change in which a substance passes between the solid and gaseous states without passing through the liquid state. Glaciers can lose ice through sublimation.

SUBMARINE CANYON: An underwater canyon in the continental shelf.

SUBMARINE FAN: A terrigenous cone- or fan-shaped deposit located at the foot of a continental slope, usually seaward of large rivers and submarine canyons.

SUBSIDENCE: A gentle epeirogenic movement in which a broad area of the crust sinks without appreciable deformation.

SUPERPOSED STREAM: A stream that flows through resistant formations because its course was established at a higher level on uniform rocks before downcutting began.

SUPERPOSITION, PRINCIPLE OF: The principle that, except in extremely deformed strata, a bed that overlies another bed is always the younger.

SURF: The foamy, bubbly surface of water waves as they break close to shore.

SURFACE TENSION: The attractive force between molecules at a surface.

SURFACE WAVE: See Seismic surface wave.

SURF ZONE: An offshore belt along which the waves collapse into breakers as they approach the shore.

SUSPENDED LOAD: The fine sediment kept suspended in a stream because the settling velocity of the sediment is lower than the upward velocity of eddies.

SUSTAINABLE DEVELOPMENT: Growth in economies and in living standards that can last indefinitely without harming the environment.

SUTURE: A zone of intensely deformed rocks that marks the boundary where two continents collided.

SWASH: The landward rush of water from a breaking wave up the slope of the beach.

S WAVE: The secondary seismic wave, which travels more slowly than the P wave and consists of elastic vibrations transverse to the direction of travel. S waves cannot penetrate a liquid.

SWELL: An oceanic water wave with a wavelength on the order of 30 m or more and a wave height of approximately 2 m or less that may travel great distances from its source.

SYNCLINE: A large downfold, whose limbs are higher than its center. (Compare Anticline.)

SYNCRUDE: Synthetic oil produced from coal.

TALUS: A deposit of large angular fragments of physically weathered bedrock, usually at the base of a cliff or steep slope.

TAR SAND: A sandy deposit of organic matter impregnated with a tarry substance made up mostly of hydrocarbons, from which petroleum can be extracted.

TENSIONAL FORCES: Forces that stretch a body and pull it apart. Tensional forces dominate at divergent plate boundaries.

TERRACE (STREAM VALLEY): A flat, steplike surface above the floodplain in a stream valley, marking a former floodplain that existed at the higher level before regional uplift or an increase in discharge caused the stream to erode into the former floodplain.

TERRIGENOUS SEDIMENT: Sediment eroded from the land surface.

TEXTURE (ROCK): The rock characteristics of grain or crystal size, size variability, rounding or angularity, and preferred orientation.

THERMOREMANANT MAGNETIZATION: A permanent magnetization acquired by minerals in igneous rocks during crystallization.

TIDAL FLAT: A broad, flat region of muddy or sandy sediment, covered and uncovered in each tide cycle.

TIDAL SURGE: Waves that overrun a beach and batter sea cliffs when an intense storm passes near the shore during a spring tide.

TIDE: The rise and fall of the water level of the ocean that occurs twice a day and is caused by the gravitational attraction of the Moon and, to a lesser degree, the Sun, with greater force on the parts of the Earth facing and opposite the Moon (and Sun).

TILL: An unstratified and poorly sorted sediment containing all sizes of fragments from clay to boulders, deposited by glacial action.

TILLITE: The lithified equivalent of till.

TOPOGRAPHY: The shape of the Earth's surface, above and below sea level; the set of landforms in a region; the distribution of elevations.

TOPSET BED: A horizontal sedimentary bed formed at the top of a delta and overlying the foreset beds.

TRACE ELEMENT: An element that appears in a mineral in a concentration of less than 1 percent (often less than 0.001 percent).

TRANSFORM FAULT PLATE BOUNDARY: A boundary at which Earth's plates slide horizontally past each other, approximately at right angles to their divergent plate boundaries.

TRANSPIRATION: The release of water vapor by plants into the atmosphere.

TRANSVERSE DUNE: A dune that has its axis perpendicular (transverse) to the prevailing winds or to a current. The upwind or upcurrent side has a gentle slope, and the downwind or downcurrent side lies at the angle of repose.

TRELLIS DRAINAGE: A system of streams in which tributaries tend to lie in parallel valleys formed in steeply dipping beds in folded belts.

TRENCH: A long, narrow, deep trough in the seafloor; marks the line along which a plate bends down into a subduction zone.

TRIBUTARY: A stream that discharges water into a larger stream.

TSUNAMI: A large destructive wave caused by seafloor movements in an earthquake.

TURBIDITE: The sedimentary deposit of a turbidity current, typically showing graded bedding.

TURBIDITY CURRENT: A mass of mixed water and sediment that flows downhill along the bottom of an ocean or lake because it is denser than the surrounding water. It may reach high speeds and erode rapidly.

TURBULENT FLOW: A high-velocity flow in which streamlines are neither parallel nor straight but curled into small tight eddies. (Compare Laminar flow.)

ULTRAMAFIC ROCK: An igneous rock consisting mainly of mafic minerals and containing less than 10 percent feldspar. Includes peridotite, amphibolite, dunite, and pyroxenite.

UNCONFINED AQUIFER: An aquifer that is not overlain by an aquiclude, thereby causing the level of water in a well that penetrates the aquifer to be at the level of the surrounding groundwater table. (Compare Confined aquifer.)

UNCONFORMITY: A surface that separates two strata. It represents an interval of time in which deposition stopped, erosion removed some sediments and rock, and deposition resumed. (See also Angular unconformity.)

UNCONSOLIDATED MATERIAL: Unlithified sediment that has no mineral cement or matrix binding its grains.

UNIFORMITARIANISM, PRINCIPLE OF: The concept that the processes that have shaped Earth through geologic time are the same as those observable today.

UNSATURATED ZONE: The region in the ground between the surface and the groundwater table in which pores are not completely filled with water.

UPWARPED MOUNTAINS: Mountains elevated by uplift of broad regions without faulting.

U-SHAPED VALLEY: A deep valley with steep upper walls that grade into a flat floor; typical shape of a valley eroded by a glacier.

VALLEY (STREAM): The entire area between the top of the slopes on either side of a stream.

VALLEY GLACIER: A glacier that is smaller than a continental glacier or an icecap and that flows mainly along well-defined valleys in mountainous regions.

VARVE: A thin pair of sedimentary layers grading upward from coarse to fine and light to dark, found in a glacial lake and representing one year's deposition.

VEIN: A deposit of foreign minerals within a rock fracture or joint.

VENTIFACT: A rock that exhibits the effects of sandblasting or snowblasting on its surfaces, which become flat with sharp edges in between.

VISCOSITY: A measure of a liquid's resistance to flow.

VOLCANIC ASH: A volcanic sediment of rock fragments, usually glass, less than 2 mm in diameter, that is formed when escaping gases force out a fine spray of magma.

VOLCANIC ASH-FLOW DEPOSIT: A layer of volcanic ash and debris deposited during a pyroclastic flow.

VOLCANIC BRECCIA: A pyroclastic rock in which all fragments are more than 2 mm in diameter.

VOLCANIC DOME: A rounded accumulation around a volcanic vent of congealed lava too viscous to flow away quickly; hence usually rhyolite lava.

VOLCANIC TUFF: A consolidated rock composed of pyroclastic rock fragments and fine volcanic ash welded together by their own heat.

VOLCANISM: The processes that form volcanoes; the progress of magma as it rises up through the crust, emerges onto the surface as lava, and solidifies into volcanic rocks and landforms.

VOLCANO: A hill or mountain that forms from the accumulation of matter that erupts at the surface.

WAVE-CUT TERRACE: A level surface formed by wave erosion of coastal bedrock to the bottom of the turbulent breaker zone. May appear above sea level if uplifted or if sea level drops.

WAVE HEIGHT: The vertical distance from the trough to the crest of a wave.

WAVELENGTH: The distance between two successive peaks, or between troughs, of a wave.

WAVE REFRACTION: The bending of water waves as they encounter different depths and bottom conditions, or of other waves as they pass from one medium to another of different properties.

WEATHERING: The set of all processes that decay and break up rock by a combination of physical fracturing and chemical decomposition.

WORLD OCEAN: The combination of all the individual oceans (Atlantic, Pacific, and so on) considered as a single interconnected body of water.

Y 

YARDANG: A streamlined, sharp-crested ridge aligned with the direction of the prevailing wind in arid regions. Yardangs appear to have been carved by wind erosion and abrasion by silt and dust carried by the wind.

Z

ZEOLITE: A class of silicate minerals containing water in cavities within the crystal structure. Formed by alteration at low temperature and pressure of other silicates, often volcanic glass.

ZONED CRYSTAL: A single crystal of one mineral which has a different chemical composition in its inner and outer parts; formed in minerals that can have variation in abundance of some elements and caused by the changing concentration of elements in a cooling magma.