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Glossary of Terms for Geology

(From The Earth's Dynamic Systems, Fourth Edition by W. Kenneth Hamblin. Macmillan Publishing Company, New York, NY. Copyright © 1985)

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[ A ]
Aa flow A lava flow with a surface typified by angular, jagged blocks. Contrast with pahoehoe flow.
Ablation Reduction of a glacier by melting, evaporation, iceberg calfing, or deflation.
Abrasion The mechanical wearing away of a rock by friction, rubbing, scraping, or grinding. absolute time Geologic time measured in a specific duration of years (in contrast to relative time, which involves only the chronologic order of events).
Abyssal Pertaining to the great depths of the oceans, generally 2000 m (1000 fathoms) or more below sea level.
Abyssal floor The deep, relatively flat surface of the ocean floor located on both sides of the oceanic ridge. It includes the abyssal plains and the abyssal hills.
Abyssal hills The part of the ocean floor consisting of hills rising as much as 1000 m above the surrounding floor. They are found seaward of most abyssal plains and occur in profusion in basins isolated from continents by trenches, ridges, or rises.
Abyssal plains Flat areas of the ocean floor, having a slope of less than 1:1000. Most abyssal plains lie at the base of a continental rise and are simply areas where abyssal hills are completely covered with sediment.
Active plate margin (plate tectonics) The leading edge of a lithospheric plate bordered by a trench.
Aftershock An earthquake that follows a larger earthquake. Generally, many aftershocks occur over a period of days or even months after a major earthquake.
Agate A variety of cryptocrystalline quartz in which colors occur in bands. It is commonly deposited in cavities in rocks.
Aggradation The process of building up a surface by deposition of sediment.
A horizon The topsoil layer in a soil profile.
Alcove A large niche or recession formed in a steep cliff.
Alluvial fan A fan-shaped deposit of sediment built by a stream where it emerges from an upland or a mountain range into a broad valley or plain (see diagram). Alluvial fans are common in arid and semiarid climates but are not restricted to them.
Alluvium A general term for any sedimentary accumulations deposited by comparatively recent action of rivers. It thus includes sediment laid down in river beds, flood plains, and alluvial fans.
Alpine glacier A glacier occupying a valley. Synonymous with mountain glacier, valley glacier.
Amorphous solid A solid in which atoms or ions are not arranged in a definite crystal structure. Examples: glass, amber, obsidian.
Amphibole An important rockforming mineral group of ferromagnesian silicates. Amphibole crystals are constructed from double chains of silicon-oxygen tetrahedra. Example: hornblende.
Amphibolite A metamorphic rock consisting mostly of amphibole and plagioclase feldspar.
Andesite A fine-grained igneous rock composed mostly of plagioclase feldspar and from 25% to 40% amphibole and biotite, but no quartz or K-feldspar. It is abundant in mountains bordering the Pacific Ocean, such as the Andes Mountains of South America, from which the name was derived. Andesitic magma is believed to originate from fractionation of partially melted basalt.
Andesite line The boundary in the Pacific Ocean separating volcanoes of the inner Pacific basin, which discharge only basalt, from those near the continental margins, which discharge both andesite and basalt.
Angular unconformity An unconformity in which the older strata dip at a different angle (generally steeper) than the younger strata.
Anomaly A deviation from the norm or average.
Anorthosite A coarse-grained intrusive igneous rock composed primarily of calcium-rich plagioclase feldspar.
Anticline A fold in which the limbs dip away from the axis. After erosion, the oldest rocks are exposed in the central core of the fold.
Aphanitic texture A rock texture in which individual crystals are too small to be identified without the aid of a microscope. In hand specimens, aphanitic rocks appear to be dense and structureless.
Aquifer A permeable stratum or zone below the earth's surface through which ground water moves.
Arc-trench gap The geographic area in an island arc deep-sea trench system that separates the arc of volcanoes from the trench. In most cases, the gap is about 100 km wide.
Arete A narrow, sharp ridge separating two adjacent glacial valleys.
Arkose A sandstone containing at least 25% feldspar.
Artesian basin A geologic structural feature in which ground water is confined and is under artesian pressure.
Artesian-pressure surface The level to which water in an artesian system would rise in a pipe high enough to stop the flow.
Artesian waterGround water confined in an aquifer and under pressure great enough to cause the water to rise above the top of the aquifer when it is tapped by a well.
Ash Volcanic fragments the size of dust particles.
Ash flow A turbulent blend of unsorted pyroclastic material (mostly fine-grained) mixed with high-temperature gases ejected explosively from a fissure or crater.
Ash-flow tuff A rock composed of volcanic ash and dust, formed by deposition and consolidation of ash flows.
Asteroid A small, rocky planetary body orbiting the sun. Asteroids are numbered in the tens of thousands. Most are located between the orbit of Mars and the orbit of Jupiter. Their diameters range downward from 770 km.
Asthenosphere The zone in the earth directly below the lithosphere, from 70 to 200 km below the surface. Seismic velocities are distinctly lower in the asthenosphere than in adjacent parts of the earth's interior. The material in the asthenosphere is therefore believed to be soft and yielding to plastic flow.
Astrogeology The study of extraterrestrial bodies by the application of geologic methods and knowledge.
Asymmetric fold A fold (anticline or syncline) in which one limb dips more steeply than the other.
Atmosphere The mixture of gases surrounding a planet. The earth's atmosphere consists chiefly of oxygen and nitrogen, with minor amounts of other gases. Synonymous with air.
Atoll A ring of low coral islands surrounding a lagoon.
Atom The smallest unit of an element. Atoms are composed of protons, neutrons, and electrons. attitude The three-dimensional orientation of a bed, fault, dike, or other geologic structure. It is determined by the combined measurements of the dip and the strike of a structure.
Axial plane With reference to folds, an imaginary plane that intersects the crest or trough of a fold so as to divide the fold as symmetrically as possible.
Axis 1 (crystallography) An imaginary line passing through a crystal around which the parts of the crystal are symmetrically arranged. 2 (fold) The line where folded beds show maximum curvature. The line formed by the intersection of the axial plane with the bedding surface.

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[ B ]
Backswamp The marshy area of a flood plain at some distance beyond and lower than the natural levees that confine the river.
Backwash The return sheet flow down a beach after a wave is spent.
Badlands An area nearly devoid of vegetation and dissected by stream erosion into an intricate system of closely spaced, narrow ravines.
Bajada The surface of a system of coalesced alluvial fans.
Bar An offshore, submerged, elongate ridge of sand or gravel built on the sea floor by waves and currents.
Barchan dune A crescent. shaped dune, the tips or horns of which point downwind. Barchan dunes form in desert areas where sand is scarce.
Barrier island An elongate island of sand or gravel formed parallel to a coast.
Barrier reef An elongate coral reef that trends parallel to the shore of an island or a continent, separated from it by a lagoon.
Basalt A dark colored, aphanitic (fine-grained) igneous rock composed of plagioclase (over 50%) and pyroxene. Olivine may or may not be present. Basalt and andesite represent 98% of all volcanic rocks.
Base level The level below which a stream cannot effectively erode. Sea level is the ultimate base level, but lakes form temporary base levels for inland drainage systems.
Basement complex A series of igneous and metamorphic rocks lying beneath the oldest stratified rocks of a region. In shields, the basement complex is exposed over large areas.
Basin 1 (structural geology) A circular or elliptical downwarp. After erosion, the youngest beds are exposed in the central part of the structure. 2 (topography) A depression into which the surrounding area drains.
Batholith A large body of intrusive igneous rock exposed over an area of at least 100 km2.
Bathymetric chart A topographic map of the earth's surface underlying a body of water (such as the ocean floor).
Bathymetry The measurement of ocean depths and the charting (mapping) of the topography of the ocean floor.
Bauxite A mixture of various amorphous or crystalline hydrous aluminum oxides and aluminum hydroxides, commonly found as a residual clay deposit in tropical and subtropical regions. Bauxite is the principal commercial source of aluminum.
Bay (coast) A wide, curving recess or inlet between two capes or headlands.
Baymouth bar A narrow, usually submerged ridge of sand or gravel deposited across the mouth of a bay by longshore drift. Baymouth bars commonly are formed by extension of spits along embayed coasts.
Beach A deposit of wave-washed sediment along a coast between the landward limit of wave action and the outermost breakers.
Beach drift The migration of sediment along a beach caused by the impact of waves striking the shore at an oblique angle.
Bed A layer of sediment 1 cm or more in thickness.
Bedding plane A surface separating layers of sedimentary rock.
Bed load Material transported by currents along the bottom of a stream or river by rolling or sliding, in contrast to material carried in suspension or in solution.
Bedrock The continuous solid rock that underlies the regolith everywhere and is exposed locally at the surface. An exposure of bedrock is called an outcrop.
Benioff zone A zone of deep-focus earthquakes that dips away from a deep-sea trench and slopes beneath the adjacent continent or island arc.
Berm A nearly horizontal portion of a beach or backshore formed by storm waves. Some beaches have no berms; others have several.
B horizon The solid zone of accumulation underlying the A horizon of a soil profile. Some of the material dissolved by leaching in the A horizon is deposited in the B horizon.
Biologic material A general term for material originating from organisms. Examples: fossils (shells, bones, leaves), peat, coal. biosphere The totality of life on or near the earth's surface.
Biotite "Black mica." An important rock-forming ferromagnesian silicate with silicon-oxygen tetrahedra arranged in sheets.
Bird-foot delta A delta with distributaries extending seaward and in map view resembling the claws of a bird. Example: the Mississippi Delta.
Block faulting A type of normal faulting in which segments of the crust are broken and displaced to different elevations and orientations.
Blowout A basin excavated by wind erosion.
Blueschist A fine-grained schistose rock characterized by high pressure, low-temperature mineral assemblages, and typically blue in color.
Boulder A rock fragment with a diameter of more than 256 mm (about the size of a volleyball). A boulder is one size larger than a cobble.
Bracketed intrusion An intrusive rock that was once exposed at the surface by erosion and was subsequently covered by younger sediment. The relative age of the intrusion thus falls between, or is bracketed by, the ages of the younger and older sedimentary deposits.
Braided stream A stream with a complex of converging and diverging channels separated by bars or islands. Braided streams form where more sediment is available than can be removed by the discharge of the stream.
Breaker A collapsing water wave.
Breccia A general term for sediment consisting of angular fragments in a matrix of finer particles. Examples: sedimentary breccias, volcanic breccias, fault breccias, impact breccias.
Butte A somewhat isolated hill, usually capped with a resistant layer of rock and bordered by talus. A butte is an erosion remnant of a formerly more extensive slope.

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[ C ]
Calcite A mineral composed of calcium carbonate (CaCO3).
Caldera A large, more or less circular depression or basin associated with a volcanic vent. Its diameter is many times greater than that of the included vents. Calderas are believed to result from subsidence, or collapse, and mayor may not be related to explosive eruptions.
Calving The breaking off of large blocks of ice from a glacier that terminates in a body of water. capacity The maximum quantity of sediment a given stream, glacier, or wind can carry under a given set of conditions.
Capillary A small, tubular opening with a diameter about that of a human hair.
Capillary action The action by which a fluid (such as water) is drawn up into small openings (such as pore spaces in rocks) due to surface tension.
Capillary fringe A zone above the water table in which water is lifted by surface tension into openings of capillary size.
Carbonaceous Containing carbon.
Carbonate mineral A mineral formed by the bonding of carbonate ions (CO32-) with positive ions. Examples: calcite (CaCO3), dolomite [CaMg(CO3)2]
Carbonate rock A rock composed mostly of carbonate minerals. Examples: limestone, dolomite.
Carbon 14 A radioisotope of carbon. Its half-life is 5730 years.
Catastrophism The belief that geologic history consists of major catastrophic events involving processes that were far more intense than any we observe now. Contrast with uniformitarianism.
Cave A naturally formed subterranean open area, or chamber, or series of chambers, commonly produced in limestone by solution activity.
CementMinerals precipitated from ground water in the pore spaces of a sedimentary rock and binding the rock's particles together.
Cenozoic The era of geologic time from the end of the Mesozoic era (65 million years ago) to the present.
Chalcedony A general term for fibrous cryptocrystalline quartz.
Chalk A variety of limestone composed of shells of microscopic oceanic organisms.
Chemical decomposition Synonymous with chemical weathering.
Chemical weathering Chemical reactions that act on rocks exposed to water and the atmosphere so as to change their unstable mineral components to more stable forms. Oxidation, hydrolysis, carbonation, and direct solution are the most common reactions. Synonymous with decomposition.
Chert A sedimentary rock composed of granular cryptocrystalline silica.
C horizon The zone of soil consisting of partly decomposed bedrock underlying the B horizon. It grades downward into fresh, unweathered bedrock.
Cinder A fragment of volcanic ejecta from 0.5 to 2.5 cm in diameter.
Cinder cone A cone-shaped hill composed of loose volcanic fragments.
Cirque An amphitheater-shaped depression at the head of a glacial valley, excavated mainly by ice plucking and frost wedging.
Clastic 1 Pertaining to fragments (such as mud, sand, and gravel) produced by the mechanical breakdown of rocks. 2 A sedimentary rock composed chiefly of consolidated clastic material.
Clastic texture The texture of sedimentary rocks consisting of fragmentary particles of minerals, rocks, and organic skeletal remains.
Clay Sedimentary material composed of fragments with a diameter of less than 1/256 mm. Clay particles are smaller than silt particles.
Clay minerals A group of fine-grained crystalline hydrous silicates formed by weathering of minerals such as feldspar, pyroxene, or amphibole.
Cleavage The tendency of a mineral to break in a preferred direction along smooth planes.
Cobble A rock fragment with a diameter between 64 mm (about the size of a tennis ball) and 2567 mm (about the size of a volleyball). Cobbles are larger than pebbles but smaller than boulders.
Columnar jointing A system of fractures that splits a rock body into long prisms, or columns. It is characteristic of lava flows and shallow intrusive igneous flows.
Competence The maximum size of particles that a given stream, glacier, or wind can move at a given velocity.
Composite volcano A large volcanic cone built by extrusion of alternating layers of ash and lava. Synonymous with stratovolcano.
Compression A system of stresses that tends to reduce the volume of or shorten a substance.
Conchoidal fracture A type of fracture that produces a smooth, curved surface. It is characteristic of quartz and obsidian.
Concretion A spherical or ellipsoidal nodule formed byaccumulation of mineral matter after deposition of sediment.
Cone of depression A conical depression of the water table surrounding a well after heavy pumping.
Conglomerate A coarse-grained sedimentary rock composed of rounded fragments of pebbles, cobbles, or boulders.
Consequent stream A stream that has a course determined by! or directly resulting from, the original slope on which it developed. contact The surface separating two different rock bodies.
Contact metamorphismMetamorphism of a rock near its contact with a magma.
Continent A large landmass, from 20 to 60 km thick, composed mostly of granitic rock. Continents rise abruptly above the deep-ocean floor and include the marginal areas submerged beneath sea level. Examples: the African continent, the South American continent.
Continental accretion The theory that the continents have grown by incorporation of deformed sediments along their margins.
Continental crust The type of crust underlying the continents, including the continental shelves. The continental crust is commonly about 35 km thick. Its maximum thickness is 60 km, beneath mountain ranges. Its density is 2.7 g/cm3, and the velocities of primary seismic waves traveling through the crust are less than 6.2 km/sec. Synonymous with sial. Contrast with oceanic crust.
Continental drift The theory that the continents have moved in relation to one another.
Continental glacier A thick ice sheet covering large parts of a continent. Present-day examples are found in Greenland and Antarctica.
Continental margin The zone of transition from a continental mass to the adjacent ocean basin. It generally includes a continental shelf, continental slope, and continental rise.
Continental rise The gently sloping surface located at the base of a continental slope.
Continental shelf The submerged margin of a continental mass extending from the shore to the first prominent break in slope, which usually occurs at a depth of about 120 m.
Continental slope The slope that extends from a continental shelf down to the ocean deep. In some areas, such as off eastern North America, the continental slope grades into the more gently sloping continental rise.
Convection Movement of portions of a fluid as a result of density differences produced by heating.
Convection cell The space occupied by a single convection current.
Convection current A closed system in which material is transported as a result of thermal convection. Convection currents are characteristic of the atmosphere and of bodies of water. They are believed also to be generated in the interior of the earth. In the plate tectonic theory, convection within the mantle is thought to be responsible for the movement of tectonic plates.
Convergent plate boundary The zone where the leading edges of converging plates meet. Convergent plate boundaries are sites of considerable geologic activity and are characterized by volcanism, earthquakes, and crustal deformation. See also subduction zone.
Copernican period The period of lunar history during which rayed craters, such as Copernicus, and their associated rim deposits were formed (from 2 billion years ago to the present).
Copernican system The youngest system of rocks on the Moon, formed during the Copernican period.
Coquina A limestone composed of an aggregate of shells and shell fragments.
Coral A bottom-dwelling marine invertebrate organism of the class Anthozoa.
Core The central part of the earth below a depth of 2900 km.
Coriolis effect The effect produced by a Coriolis force, namely, the tendency of all particles of matter in motion on the earth's surface to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Country rock A general term for rock surrounding an igneous intrusion.
Covalent bond A chemical bond in which electrons are shared between different atoms so that none of the atoms has a net charge.
Crater An abrupt circular depression formed by extrusion of volcanic material, by collapse, or by the impact of a meteorite.
Craton The stable continental crust, including the shield and stable platform areas, most of which have not been affected by significant tectonic activity since the close of the Precambrian era.
Creep The imperceptibly slow downslope movement of material.
Crevasse 1 (glacial geology) A deep crack in the upper surface of a glacier. 2 (natural levee) A break in a natural levee.
Cross-beddingStratification inclined to the original horizontal surface upon which the sediment accumulated. It is produced by deposition on the slope of a dune or sand wave.
Crosscutting relations, principle of The principle that a rock is younger than any rock across which it cuts.
Crust (planetary structure) The outermost layer, or shell, of the earth (or any other differentiated planet). The earth's crust is generally defined as the part of the earth above the Mohorovicic discontinuity. It represents less than 1 % of the earth's total volume. See also continental crust, oceanic crust.
Crustal warping Gentle bending (upwarping or downwarping) of sedimentary strata.
Cryptocrystalline texture The texture of rocks composed of crystals too small to be identified with an ordinary microscope.
Crystal A solid, polyhedral form bounded by naturally formed plane surfaces resulting from growth of a crystal lattice.
Crystal face A naturally formed smooth plane surface of a crystal.
Crystal form The geometric shape of a crystal.
Crystal lattice A systematic, symmetrical network of atoms within a crystal.
Crystalline texture The rock texture resulting from simultaneous growth of crystals.
Crystallization The process of crystal growth. It occurs as a result of condensation from a gaseous state, precipitation from a solution, or cooling of a melt.
Crystal structure The orderly arrangement of atoms in a crystal.
Cuesta An elongate ridge formed on the tilted and eroded edges of gently dipping strata.

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[ D ]
Daughter isotope An isotope produced by radioactive decay of its parent isotope. The quantity of a daughter isotope continually increases with time.
Debris flow The rapid downslope movement of debris (rock, soil, and mud).
Debris slide A type of landslide in which comparatively dry rock fragments and soil move downslope at speeds ranging from slow to fast. The mass of debris does not show backward rotation (which occurs in a slump) but slides and rolls forward.
Declination, magnetic The horizontal angle between true north and magnetic north at a given point on the earth's surface.
DecompositionWeathering by chemical processes. Synonymous with chemical weathering.
Deep-focus earthquake An earthquake that originates at a depth greater than 300 km.
Deep-marine environment The sedimentary environment of the abyssal plains.
Deep-sea fan A cone-shaped or fan-shaped deposit of land-derived sediment located seaward of large rivers or submarine canyons. Synonymous with abyssal cone, abyssal fan, submarine cone.
Deep-sea trench See trench.
DeflationErosion of loose rock particles by wind.
Deflation basin A shallow depression formed by wind erosion where ground-water solution activity has left unconsolidated sediment exposed at the surface.
Degradation The general lowering of the surface of the land by processes of erosion.
Delta A large, roughly triangular body of sediment deposited at the mouth of a river.
Dendritic drainage pattern A branching stream pattern, resembling the branching of certain trees, such as oaks and maples.
Density The measure of concentration of matter in a substance; mass per unit volume, expressed in grams per cubic centimeter (g/cm3).
Density current A current that flows as a result of differences in density. In oceans, density currents are produced by differences in temperature, salinity, and turbidity (the concentration of material held in suspension).
Denudation The combined action of all of the various processes that cause the wearing away and lowering of the land, including weathering, mass wasting, stream action, and ground-water activity.
Deranged drainage A distinctively disordered drainage pattern formed in a recently glaciated area. It is characterized by irregular direction of stream flow, few short tributaries, swampy areas, and many lakes.
Desert pavement A veneer of pebbles left in place where wind has removed the finer material. desiccation The process of drying out. With reference to sedimentation, the loss of water from pore spaces by evaporation or compaction.
Detrital 1 Pertaining to detritus. 2 A rock formed from detritus.
Detritus A general term for loose rock fragments produced by mechanical weathering.
Diastrophism Large-scale deformation involving mountain building and metamorphism.
Differential erosion Variation in the rate of erosion on different rock masses. As a result of differential erosion, resistant rocks form steep cliffs, whereas nonresistant rocks form gentle slopes.
Differentiated planet A planetary body in which various elements and minerals are separated according to density and concentrated at different levels. The earth, for example, is differentiated, with heavy metals (iron and nickel) concentrated in the core, lighter minerals in the mantle, and still lighter materials in the crust, hydrosphere, and atmosphere.
Differentiation See magmatic differentiation, planetary differentiation, sedimentary differentiation.
Dike A tabular intrusive rock that occurs across strata or other structural features of the surrounding rock.
Dike swarm A group of associated dikes.
Diorite A phaneritic intrusive igneous rock consisting mostly of intermediate plagioclase feldspar and pyroxene, with some amphibole and biotite.
Dip The angle between the horizontal plane and a structural surface (such as a bedding plane, a joint, a fault, foliation, or other planar features).
Disappearing stream A stream that disappears into an underground channel and does not reappear in the same, or even in an adjacent, drainage basin. In karst regions, streams commonly disappear into sinkholes and follow channels through caves.
Discharge Rate of flow; the volume of water moving through a given cross section of a stream in a given unit of time.
Disconformity An unconformity in which beds above and below are parallel.
Discontinuity A sudden or rapid change in physical properties of rocks within the earth. Discontinuities are recognized by seismic data. See also Mohorovicic discontinuity.
DisintegrationWeathering by mechanical processes. Synonymous with mechanical weathering.
Dissolution The process by which materials are dissolved.
Dissolved load The part of a stream's load that is carried in solution.
Distributary Any of the numerous stream branches into which a river divides where it reaches its delta.
Divergent plate boundary A plate margin formed where the lithosphere splits into plates that drift apart from one another. Divergent plate boundaries are areas subject to tension, where new crust is generated by igneous activity. Synonymous with spreading center. See also oceanic ridge.
Divide A ridge separating two adjacent drainage basins.
Dolomite 1 A mineral composed of CaMg(CO3)2. A sedimentary rock composed primarily of the mineral dolomite.
Dolostone A sedimentary rock composed mostly of the mineral dolomite. Sometimes referred to simply as dolomite.
Dome 1 (structural geology) An uplift that is circular or elliptical in map view, with beds dipping away in all directions from a central area. 2 (topography) A general term for any dome-shaped landform.
Downwarp A downward bend or subsidence of a part of the earth's crust.
Drainage basin The total area that contributes water to a single drainage system.
Drainage system An integrated system of tributaries and a trunk stream, which collect and funnel surface water to the sea, a lake, or some other body of water.
Drift (glacial geology) A general term for sediment deposited directly on land by glacial ice or deposited in lakes, oceans, or streams as a result of glaciation. drip curtain A thin sheet of dripstone hanging from the ceiling or wall of a cave.
Dripstone A cave deposit formed by precipitation of calcium carbonate from ground water entering an underground cavern.
Drumlin A smooth, glacially streamlined hill that is elongate in the direction of ice movement. Drumlins are generally composed of till.
Dune A low mound of fine-grained material that accumulates as a result of sediment transport in a current system. Dunes have characteristic geometric forms that are maintained as they migrate. Sand dunes are commonly classified according to shape. See also barchan dune, longitudinal dune, parabolic dune, self dune, star dune, and transverse dune.

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[ E ]
Earthquake A series of elastic waves propagated in the earth, initiated where stress along a fault exceeds the elastic limit of the rock so that sudden movement occurs along the fault.
Ecology The study of relationships between organisms and their environments.
Ejecta Rock fragments, glass, and other material thrown out of an impact crater or a volcano.
Ejecta blanket Rock material (crushed rock, large blocks, breccia, and dust) ejected from an impact crater or explosion crater and deposited over the surrounding area.
Elastic deformation Temporary deformation of a substance, after which the material returns to its original size and shape. Example: the bending of mica flakes.
Elastic limit The maximum stress that a given substance can withstand without undergoing permanent deformation either by solid flow or by rupture.
Elastic-rebound theory The theory that earthquakes result from energy released by faulting; the sudden release of stored strain creates earthquake waves.
End moraine A ridge of till that accumulates at the margin of a glacier.
Entrenched meander A meander cut into the underlying rock as a result of regional uplift or lowering of the regional base level.
Environment of sedimentation See sedimentary environment.
Eolian Pertaining to wind.
Eolian environment The sedimentary environment of deserts, where sediment is transported and deposited primarily by wind. epicenter The area on the earth's surface that lies directly above the focus of an earthquake.
Epoch A division of geologic time; a subdivision of a period. Example: Pleistocene epoch.
Eratosthenian period The period of lunar when large craters, the rays of which are no longer visible, such as Eratosthenes, were formed (from 3.1 billion to 0.8 billion years ago).
Eratosthenian system The system of lunar rocks formed during the Eratosthenian period. It is older than the Copernican system but younger than the Imbrian system.
Erosion The processes that loosen sediment and move it from one place to another on the earth's surface. Agents of erosion include water, ice, wind, and gravity.
Erratic A large boulder carried by glacial ice to an area far removed from its point of origin.
Escarpment A cliff or very steep slope.
Esker A long, narrow, sinuous ridge of stratified glacial drift deposited by a stream flowing beneath a glacier in a tunnel or in a subglacial stream bed.
Estuary A bay at the mouth of a river formed by subsidence of the sand or by a rise in sea level. Fresh water from the river mixes with and dilutes seawater in an estuary.
Eugeocline (plate tectonics) A geocline in which volcanism is associated with clastic sedimentation. Eugeoclines are usually associated with an island arc.
Eugeosyncline A geosyncline situated seaward from a continent and characterized by sediments deposited by turbidity currents and derived in part from a volcanic arc.
Eustatic change of sea level A worldwide rise or fall in sea level resulting from a change in the volume of water or the capacity of ocean basins.
Evaporite A rock composed of minerals derived from evaporation of mineralized water. Examples: rock salt, gypsum.
Exfoliation A weathering process by which concentric shells, slabs, sheets, or flakes are successively broken loose and stripped away from a rock mass.
ExposureBedrock not covered with soil or regolith; outcrop. extrusive rock A rock formed from a mass of magma that flowed out on the surface of the earth. Example: basalt.

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[ F ]
Faceted spur A spur or ridge that has been beveled or truncated by faulting, erosion, or glaciation.
Facies A distinctive group of characteristics within part of a rock body (such as composition, grain size, or fossil assemblages) that differ as a group from those found elsewhere in the same rock unit. Examples: conglomerate facies, shale facies, brachiopod facies.
Fan A fan-shaped deposit of sediment. See also alluvial fan, deepsea fan.
Fault A surface along which a rock body has broken and been displaced.
Fault block A rock mass bounded by faults on at least two sides.
Fault scarp A cliff produced by faulting.
Faunal succession, principle of The principle that fossils in a stratigraphic sequence succeed one another in a definite, recognizable order.
Feldspar A mineral group consisting of silicates of aluminum and one or more of the metals potassium, sodium, or calcium. Examples: K-feldspar, Ca-plagioclase, Na-plagioclase.
Felsite A general term for lightcolored aphanitic (fine-grained) igneous rocks. Example: rhyolite.
Ferromagnesian minerals A variety of silicate minerals containing abundant iron and magnesium. Examples: olivine, pyroxene, amphibole.
Fiord A glaciated valley flooded by the sea to form a long, narrow, steep-walled inlet.
Firn Granular ice formed by recrystallization of snow. It is intermediate between snow and glacial ice. Sometimes referred to as neve.
Fissure An open fracture in a rock.
Fissure eruption Extrusion of lava along a fissure.
Flint A popular name for darkcolored chert (cryptocrystalline quartz).
Flood basalt An extensive flow of basalt erupted chiefly along fissures. Synonymous with plateau basalt.
Flood plain The flat, occasionally flooded area bordering a stream.
Fluvial Pertaining to a river or rivers.
Fluvial environment The sedimentary environment of river systems.
Focus The area within the earth where an earthquake originates.
Fold A bend, or flexure, in a rock.
Folded mountain belt A long, linear zone of the earth's crust where rocks have been intensely deformed by horizontal stresses and generally intruded by igneous rocks. The great folded mountains of the world (such as the Appalachians, the Himalayas, the Rockies, and the Alps) are believed to have been formed at convergent plate margins.
Foliation A planar feature in metamorphic rocks, produced by the secondary growth of minerals. Three major types are recognized: slaty cleavage, schistosity, and gneissic layering.
Footwall The block beneath a dipping fault surface.
Foreshore The seaward part of the shore or beach lying between high tide and low tide.
Formation A distinctive body of rock that serves as a convenient unit for study and mapping.
Fossil Naturally preserved reo mains or evidence of past life, such as bones, shells, casts, impressions, and trails.
Fossil fuel A fuel containing solar energy that was absorbed by plants and animals in the geologic past and thus is preserved in organic compounds in their remains. Fossil fuels include petroleum, natural gas, and coal.
Fracture zone 1 (field geology) A zone where the bedrock is cracked and fractured. 2 (oceanography) A zone of long, linear fractures on the ocean floor, expressed topographically by ridges and troughs. Fracture zones are the topographic expression of transform faults.
Fringing reef A reef that lies alongside the shore of a landmass.
Frost heaving The lifting of unconsolidated material by the freezing of subsurface water.
Frost wedging The forcing apart of rocks by the expansion of water as it freezes in fractures and pore spaces.

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[ G ]
Gabbro A dark-colored, coarse-grained rock composed of Ca-plagioclase, pyroxene, and possibly olivine, but no quartz.
Gas The state of matter in which a substance has neither independent shape nor independent volume. Gases can readily be compressed and tend to expand indefinitely.
Geocline An elongate prism of sedimentary rock deposited in a subsided part of the continental margins and adjacent oceanic crust. A modern example is the continental margins of the eastern United States. See also eugeocline, miogeocline.
Geode A hollow nodule of rock lined with crystals; when separated from the rock body by weathering, it appears as a hollow, rounded shell partly filled with crystals.
Geologic column A diagram representing divisions of geologic time and the rock units formed during each major period.
Geologic cross section A diagram showing the structure and arrangement of rocks as they would appear in a vertical plane below the earth's surface.
Geologic map A map showing the distribution of rocks at the earth's surface.
Geologic time scale The time scale determined by the geologic column and by radiometric dating of rocks.
Geosyncline A subsiding part of the lithosphere in which thousands of meters of sediment accumulate. See also eugeosyncline, miogeosyncline.
Geothermal Pertaining to the heat of the interior of the earth.
Geothermal energy Energy useful to human beings that can be extracted from steam and hot water found within the earth's crust.
Geothermal gradient The rate at which temperature increases with depth.
Geyser A thermal spring that intermittently erupts steam and boiling water.
Glacier A mass of ice formed from compacted recrystallized snow that is thick enough to flow plastically.
Glacial environment The sedimentary environment of glaciers and their meltwaters.
Glass 1 A state of matter in which a substance displays many properties of a solid but lacks crystal structure. 2 An amorphous igneous rock formed from a rapidly cooling magma.
Glassy texture The texture of igneous rocks in which the material is in the form of natural glass rather than crystal.
Global tectonics The study of the characteristics and origin of structural features of the earth that have regional or global significance.
Glossopteris flora An assemblage of late Paleozoic fossil plants named for the seed fern Glossopteris, one of the plants in the assemblage. These flora are widespread in South America, Africa, Australia, India, and Antarctica, and provide important evidence for the theory of continental drift.
Gneiss A coarse-grained metamorphic rock with a characteristic type of foliation (gneissic layering), resulting from alternating layers of light-colored and darkcolored minerals. Its composition is generally similar to that of granite.
Gneissic layering The type of foliation characterizing gneiss, resulting from alternating layers of the constituent silicic and mafic minerals.
Gondwanaland The ancient continental landmass that is thought to have split apart during Mesozoic time to form the present day continents of South America, Africa, India, Australia, and Antarctica.
Graben An elongate fault block that has been lowered in relation to the blocks on either side.
Gradation Leveling of the land due to erosion by such agents as river systems ground water, glaciers, wind, and waves.
Graded bedding A type of bedding in which each layer is characterized by a progressive decrease in grain size from the bottom of the bed to the top.
Graded stream A stream that has attained a state of equilibrium, or balance, between erosion and deposition, so that the velocity of the water is just great enough to transport the sediment load supplied from the drainage basin, and neither erosion nor deposition occurs.
Gradient (stream) The slope of a stream channel measured along the course of the stream.
Grain A particle of a mineral or rock, generally lacking well-developed crystal faces.
Granite A coarse-grained igneous rock composed of K-feldspar, plagioclase, and quartz, with small amounts of ferromagnesian minerals.
GranitizationFormation of granitic rock by metamorphism without complete melting.
Gravity anomaly An area where gravitational attraction is greater or less than its normal value.
Graywacke An impure sandstone consisting of rock fragments and grains of quartz and feldspar in a matrix of clay-size particles.
Groundmass The matrix of relatively fine-grained material between the phenocrysts in a porphyritic rock.
Ground moraine Glacial deposits that cover an area formerly occupied by a glacier; they typically produce a landscape of low, gently rolling hills.
Ground water Water below the earth's surface. It generally occurs in pore spaces of rocks and soil.
Guyot Aseamount with a flat top.
Gypsum An evaporite mineral composed of calcium sulfate with water (CaSO4· 2H2O).

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[ H ]
Half-Life The time required for half of a given sample of a radioactive isotope to decay to its daughter isotope.
Halite An evaporite mineral composed of sodium chloride (NaCl).
Hanging valley A tributary valley with the floor lying ("hanging") above the valley floor of the main stream or shore to which it flows (see diagram). Hanging valleys commonly are created by deepening of the main valley by glaciation, but they can also be produced by faulting or rapid retreat of a sea cliff.
Hanging wall The surface or block of rock that lies above an inclined fault plane.
Hardness 1 (mineralogy) The measure of the resistance of a mineral to scratching or abrasion. 2 (water) A property of water resulting from the presence of calcium carbonate and magnesium carbonate in solution.
Headland An extension of land seaward from the general trend of the coast; a promontory, cape, or peninsula.
Headward erosion Extension of a stream headward, up the regional slope of erosion.
Heat flow The flow of heat from the interior of the earth.
High-grade metamorphismMetamorphism that occurs under high temperature and high pressure.
Hogback A narrow, sharp ridge formed on steeply inclined, resistant rock.
Horizon 1 (geologic) A plane of stratification assumed to have been originally horizontal. 2 (soil) A layer of soil distinguished by characteristic physical properties. Soil horizons generally are designated by letters (for example, A horizon, B horizon, C horizon).
Horn A sharp peak formed at the intersection of the headwalls of three or more cirques.
Hornblende A variety of the amphibole mineral group. hornfels A nonfoliated metamorphic rock of uniform grain size, formed by high-temperature metamorphism. Hornfelses typically are formed by contact metamorphism around igneous intrusions.
Horst An elongate fault block that has been uplifted in relation to the adjacent rocks.
Hot spot The expression at the earth's surface of a mantle plume, or column of hot, buoyant rock rising in the mantle beneath a lithospheric plate.
Hummock A small, rounded or cone-shaped, low hill or a surface of other small, irregular shapes. A surface that is not equidimensional or ridgelike.
Hydraulic Pertaining to a fluid in motion.
Hydraulic head The pressure exerted by a fluid at a given depth beneath its surface. It is proportional to the 'height of the fluid's surface above the area where the pressure is measured.
Hydrologic system. The system of moving water at the earth's surface.
Hydrolysis Chemical combination of water with other substances. hydrosphere The waters of the earth, as distinguished from the rocks (lithosphere), the air (atmosphere), and (biosphere).
Hydrostatic pressure The pressure within a fluid (such as water) at rest, exerted on a given point within the body of the fluid.
Hydrothermal deposit A mineral deposit formed by high-temperature ground water. The high temperature commonly is associated with emplacement of a magma.

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[ I ]
Ice sheet A thick, extensive body of glacial ice that is not confined to valleys. Localized ice sheets are sometimes called ice caps.
Ice wedging A type of mechanical weathering in which rocks are broken by the expansion of water as it freezes in joints, pores, or bedding planes. Synonymous with frost wedging.
Igneous rock Rock formed by cooling and solidification of molten silicate minerals (magma). Igneous rocks include volcanic and plutonic rocks.
Imbrian period The period of lunar history during which the large multiringed basins, such as Mare Imbrium, were formed and the mare basalts extruded (from 3.9 billion to 3.1 billion years ago).
Imbrian system The system of rocks formed on the Moon during the Imbrian period.
Inclination, magnetic The angle between the horizontal plane and a magnetic line of force.
Inclusion A rock fragment incorporated into a younger igneous rock.
Intermediate-focus earthquake An earthquake with a focus located at a depth between 70 and 300 km.
Intermittent stream A stream through which water flows only part of the time.
Internal drainage A drainage system that does not extend to the ocean.
Interstitial Pertaining to material in the pore spaces of a rock. Petroleum and ground water are interstitial fluids. Minerals deposited by ground-water in a sandstone are interstitial minerals.
Intrusion 1 Injection of a magma into a preexisting rock. 2 A body of rock resulting from the process of intrusion.
Intrusive rockIgneous rock that, while it was fluid, penetrated into or between other rocks and solidified. It can later be exposed at the earth's surface after erosion of the overlying rock.
Inverted valley A valley that has been filled with lava or other resistant material and has subsequently been eroded into an elongate ridge.
Ion An atom or combination of atoms that has gained or lost one or more electrons and thus has a net electrical charge.
Ionic bond A chemical bond formed by electrostatic attraction betWeen oppositely charged ions.
Ionic substitution The replacement of one kind of ion in a crysta1line lattice by another kind that is of similar size and electrical charge.
Island A landform smaller than a continent and completely surrounded by water.
Island arc A chain of volcanic islands. Island arcs are generally convex toward the open ocean. Example: the Aleutian Islands.
Isostasy A state of equilibrium, resembling flotation, in which segments of the earth's crust stand at levels determined by their thickness and density. Isostatic equilibrium is attained by flow of material in the mantle.
Isotope One of the several forms of a chemical element that have the same number of protons in the nucleus but differ in the number of neutrons and thus differ in atomic weight.

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[ J ]
Joint A fracture in a rock along which no appreciable displacement has occurred.

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[ K ]
Kame A body of stratified glacial sediment. A mound or an irregular ridge deposited by a subglacial stream as an alluvial fan or a delta.
Karst topography A landscape characterized by sinks, solution valleys, and other features produced by ground-water activity.
Kettle A closed depression in a deposit of glacial drift formed where a block of ice was buried or partly buried and then melted.

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[ L ]
Laccolith A concordant igneous intrusion that has arched up the strata into which it was injected, so that it forms a pod-shaped or lensshaped body with a generally horizontal floor.
Lag deposit A residual accumulation of coarse fragments that remain on the surface after finer material has been removed by wind.
Lagoon A shallow body of seawater separated from the open ocean by a barrier island or reef.
Lamina (pl. laminae) A layer of sediment less than 1 cm thick.
Laminae Plural of lamina.
Laminar flow A type of flow in which the fluid moves in parallel lines. Contrast with turbulent flow.
Landform Any feature of the earth's surface having a distinct shape and origin. Landforms include major features (such as continents, ocean basins, plains, plateaus, and mountain ranges) and minor features (such as hills, valleys, slopes, drumlins, and dunes). Collectively, the landforms of the earth constitute the entire surface configuration of the planet.
Landslide A general term for relatively rapid types of mass movement, such as debris flows, debris slides, rockslides, and slumps.
Lateral moraine An accumulation of till deposited along the side margins of a valley glacier. It accumulates as a result of mass movement of debris on the sides of the glacier.
Laterite A soil that is rich in oxides of iron and aluminum formed by deep weathering in tropical and subtropical areas.
Laurasia The ancient continental landmass that is thought to have split apart to form Europe, Asia, North America, and Greenland.
LavaMagma that reaches the earth's surface.
Leach To dissolve and remove the soluble constituents of a rock or soil.
Leachate A solution produced by leaching. Example: water that has seeped through a waste disposal site and thus contains in solution various substances derived from the waste material.
Leaching The process by which ground water dissolves and transports soluble components of a rock or soil.
Leading edge (plate tectonics) A convergent or active plate margin. Contrast with trailing edge.
Lee slope The part of a hill, dune, or rock that is sheltered or turned away from the wind. Synonymous with slip face.
Levee, natural A broad, low embankment built up along the banks of a river channel during floods.
Limb The flank, or side, of a fold.
Limestone A sedimentary rock composed mostly of calcium carbonate (CaCO3).
Lineament A topographic feature or group of features having a linear configuration. Lineaments commonly are expressed as ridges or depressions or as an alignment of features such as stream beds, volcanoes, or vegetation.
Liquid The state of matter in which a substance flows freely and lacks crystal structure. Unlike a gas, a liquid retains the same volume independent of the shape of its container.
Lithification The processes by which sediment is converted into sedimentary rock. These processes include cementation and compaction.
Lithosphere The relatively rigid outer zone of the earth, which includes the continental crust, the oceanic crust, and the part of the mantle lying above the softer asthenosphere.
Load The total amount of sediment carried at a given time by a stream, glacier, or wind.
Loess Unconsolidated, deposited silt and dust.
Longitudinal dune An elongate sand dune oriented in the direction of the prevailing wind.
Longitudinal profile The profile of a stream or valley drawn along its length, from source to mouth.
Longitudinal wave A seismic body wave in which particles oscillate along lines in the direction in which the wave travels. Synonymous with P wave.
Longshore current A current in the surf zone moving parallel to the shore. Longshore currents occur where waves strike the shore at an angle. The waves push water and sediment obliquely up the beach, and the backwash returns straight down the beach face, so the water' and sediment follow a zigzag pattern, with net movement parallel to the shore.
Longshore drift The process in which sediment is moved in a zigzag pattern along a beach by the swash and backwash of waves that approach the shore obliquely.
Low-grade (low-rank) metamorphismMetamorphism that is accomplished under low or moderate temperature and low or moderate pressure.

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[ M ]
Mafic rock An igneous rock containing more than 50% ferromagnesian minerals.
Magma A mobile silicate melt, which can contain suspended crystals and dissolved gases as well as liquid.
Magmatic differentiation A general term for the various processes by which early-formed crystals or early-formed liquids are separated and removed from a magma to produce a rock with composition different from that of the original magma. Early-crystallized ferromagnesian minerals commonly are separated by gravitational settling, so that the parent magma is left enriched in silica, sodium, and potassium.
Magmatic segregation Separation of crystals of certain minerals from a magma as it cools. For example, some minerals (including certain valuable metals) crystallize while other components of the magma are still liquid. These earlyformed crystals can settle to the bottom of a magma chamber and thus become concentrated there, forming an ore deposit.
Magnetic anomaly A deviation of observed magnetic inclination or intensity (as measured by a magnetometer) from a constant normal value.
Magnetic reversal A complete 180-degree reversal of the polarity of the earth's magnetic field.
Mantle The zone of the earth's interior between the base of the crust (the Moho discontinuity) and the core.
Mantle plume A buoyant mass of hot mantle material that rises to the base of the lithosphere. Mantle plumes commonly produce volcanic activity and structural deformation in the central part of lithospheric plates.
Marble A metamorphic rock consisting mostly of metamorphosed limestone or dolomite.
Mare (pl. maria) Any of the relatively smooth, low, dark areas of the Moon. The lunar maria were formed by extrusion of lava.
Maria Plural of mare.
Mascon A concentration of mass in a local area beneath a lunar mare. The term is derived from mass concentration.
Mass movement The transfer of rock and soil downslope by direct action of gravity without a flowing medium (such as a river or glacial ice). Synonymous with mass wasting.
Matrix The relatively finegrained rock material occupying the space between larger particles in a rock. See also groundmass.
Meander A broad, looping bend in a river.
Mechanical weathering The breakdown of rock into smaller fragments by physical processes such as frost wedging. Synonymous with disintegration.
Medial moraine A ridge of till formed in the middle of a valley glacier by the junction of two lateral moraines where two valley valley glaciers converge.
Melt A substance altered from the solid state to the liquid state.
Mesa A flat-topped, steep-sided highland capped with a resistant rock formation. A mesa is smaller than a plateau but larger than a butte.
Mesozoic The era of geologic time from the end of the Paleozoic era (225 million years ago) to the beginning of the Cenozoic era (65 million years ago).
Metaconglomerate A metamorphosed conglomerate.
Metallic bond A chemical bond in which shared electrons move freely among the atoms.
Metamorphic Pertaining to the processes or products of metamorphism.
Metamorphic rock Any rock formed from preexisting rocks within the earth's crust by changes in temperature and pressure and by chemical action of fluids.
Metamorphism Alteration of the minerals and textures of a rock by changes in temperature and pressure and by a gain or loss of chemical components.
Meteorite Any particle of solid matter that has fallen to the earth, the Moon, or another planet from space.
Mica A group of silicate minerals exhibiting perfect cleavage in one direction.
Microcontinent A relatively small, isolated fragment of continental crust. Example: Madagascar.
Mid-Atlantic ridge The mountain range extending from north to south down the central part of the Atlantic Ocean floor.
Migmatite A mixture of igneous and metamorphic rocks in which thin dikes and stringers of granitic material interfinger with metamorphic rocks.
Mineral A naturally occurring inorganic solid having a definite internal structure and a definite chemical composition that varies only within strict limits. Chemical composition and internal structure determine its physical properties, including the tendency to assume a particular geometric form (crystal form).
Miogeocline (plate tectonics) A geocline situated near a continental margin containing a thick sequence of well-sorted clastic and chemical sediments derived from the continent.
Miogeosyncline A geosyncline situated near a continental margin and receiving well-sorted clastic and chemical sediments from the continent, not associated with volcanism.
Mobile belts Long, narrow belts in the continents that have been subjected to mountain building processes.
Mohorovicic discontinuity The first global seismic discontinuity below the surface of the earth. It lies at a depth varying from about 5 to 10 km beneath the ocean floor to about 35 km beneath the continents. Commonly referred to as the Moho.
Monadnock An erosion remnant rising above the peneplain.
Monocline A bend or fold in gently dipping horizontal strata.
Moraine A general term for a landform composed of till.
Mountain A general term for any landmass that stands above its surroundings. In the stricter geological sense, a mountain belt is a highly deformed part of the earth's crust that has been injected with igneous intrusions and the deeper parts of which have been metamorphosed. The topography of young mountains is high, but erosion can reduce old mountains to flat lowlands.
Mud crack A crack in a deposit of mud or silt resulting from the contraction that accomp"anies drying.
Mudflow A flowing mixture of mud and water.
Multiringed basin A large crater (more than 300 km in diameter) containing a series of concentric ridges and depressions. Example: the Orientale basin on the Moon.

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[ N ]
Nappe Faulted and overturned folds.
Natural arch An arch-shaped landform produced by weathering and differential erosion.
Neve Granular ice formed by recrystallization of snow. Synonymous with firn.
Nodule A small, irregular, knobby, or rounded rock that is generally harder than the surrounding rock.
Nonconformity An unconformity in which stratified rocks rest on eroded granitic or metamorphic rocks.
Normal fault A steeply inclined fault in which the hanging wall has moved downward in relation to the footwall. Synonymous with gravity fault.
Nuee ardente A hot cloud of volcanic fragments and superheated gases that flows as a mass, because it is denser than air. Upon cooling, it forms a type of rock called tuff, including ash-flow tuff and welded tuff.

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[ O ]
Obsidian A glassy igneous rock with a composition equivalent to that of granite.
Ocean basin A low part of the lithosphere lying between continental masses. The rocks of an ocean basin are mostly basalt with a veneer of oceanic sediment.
Oceanic crust The type of crust that underlies the ocean basins. It is about 5 km thick, composed predominantly of basalt. Its density is 3.0 g/cm3. The velocities of compressional seismic waves traveling through it exceed 6.2 km/sec. Compare with continental crust.
Oceanic ridge The continuous ridge, or broad, fractured topographic swell, that extends through the central part of the Arctic, Atlantic, Indian, and South Pacific oceans. It is several hundred kilometers wide, and its elevation above the ocean floor is 600 m or more. It thus constitutes a major structural and topographic feature of the earth.
Offshore The area from low tide seaward.
Oil shaleShale that is rich in hydrocarbon derivatives. In the United States, the chief oil shale is the Green River Formation, in the Rocky Mountain region.
Olivine A silicate mineral with magnesium and iron but no aluminum [(Mg,Fe)2SiO4].
Oolite A limestone consisting largely of spherical grains of calcium carbonate in concentric spherical layers.
Ooze (marine geology) Marine sediment consisting of more than 30% shell fragments of microscopic organisms.
Ophiolite A sequence of rocks characterized by ultramafic rocks at the base and Cin ascending order) gabbro, sheeted dikes, pillow lavas, and deep-sea sediments. The typical sequence of rocks constituting the oceanic crust.
Orogenesis The processes of mountain building.
Orogenic Pertaining to deformation of a continental margin to the extent that a mountain range is formed.
Orogenic belt A mountain belt. orogeny A major episode of mountain building.
Outcrop An exposure of bedrock.
Outlet glacier A tonguelike stream of ice, resembling a valley glacier, that forms where a continental glacier encounters a mountain system and is forced to move through a mountain pass in large streams.
Outwash Stratified sediment washed out from a glacier by meltwater streams and deposited in front of the end moraine.
Outwash plain The area beyond the margins of a glacier where meltwater deposits sand, gravel, and mud washed out from the glacier.
Overturned fold A fold in which at least one limb has been rotated through an angle greater than 90 degrees.
Oxbow lake A lake formed in the channel of an abandoned meander. oxidation Chemical combination of oxygen with another substance.

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[ P ]
Pahoehoe flow A lava flow with a billowy or ropy surface. Contrast with aa flow.
Paleocurrent An ancient current, which existed in the geologic past, with a direction of flow that can be inferred from cross-bedding, ripple marks, and other sedimentary structures.
Paleogeography The study of geography in the geologic past, including the patterns of the earth's surface, the distribution of land and ocean, and ancient mountains and other landforms.
Paleomagnetism The study of ancient magnetic fields, as preserved in the magnetic properties of rocks. It includes studies of changes in the position of the magnetic poles and reversals of the magnetic poles in the geologic past.
Paleontology The study of ancient life.
Paleowind An ancient wind, existing in the geologic past, the direction of which can be inferred from patterns of ancient ash falls, orientation of cross-bedding, and growth rates of colonial corals.
Paleozoic The era of geologic time from the end of the Precambrian (600 million years ago) to the beginning of the Mesozoic era (225 million years ago).
Pangaea A hypothetical continent from which the present continent originated by plate movement from the Mesozoic era to the present.
Parabolic dune A dune shaped like a parabola with the concave side toward the wind.
Partial melting The process by which minerals with low melting points liquefy within a rock body as a result of an increase in temperature or a decrease in pressure (or both) while other minerals in the rock are still solid. If the liquid (magma) is removed before other components of the parent rock have melted, the composition of the magma can be quite different from that of the parent rock. Partial melting is believed to be important in the generation of basaltic magma from peridotite at spreading centers and in the generation of granitic magma from basaltic crust at subduction zones.
Passive plate margin (plate tectonics) A lithospheric plate margin at which crust is neither created nor destroyed. Passive plate margins generally are marked by transform faults.
Peat An accumulation of partly carbonized plant material containing approximately 60% carbon and 30% oxygen. It is considered an early stage, or rank, in the development of coal.
Pebble A rock fragment with a diameter between 2 mm (about the size of a match head) and 64 mm (about the size of a tennis ball).
Pediment A gently sloping erosion surface formed at the base of a receding mountain front or cliff. It cuts across bedrock and can be covered with a veneer of sediment. Pediments characteristically form in arid and semiarid climates.
Pelagic sediment Deep-sea sediment composed of fine-grained detritus that slowly settles from surface waters. Common constituents are clay, radiolarian ooze, and foraminiferal ooze.
Peneplain An extensive erosion surface worn down almost to sea level. Subsequent tectonic activity can lift a peneplain to higher elevations.
Peninsula An elongate body of land extending into a body of water.
Perched water table The upper surface of a local zone of saturation that lies above the regional water table.
Peridotite A dark-colored igneous rock of coarse-grained texture, composed of olivine, pyroxene, and some other ferromagnesian minerals, but with essentially no feldspar and no quartz.
Permafrost Permanently frozen ground.
Permanent stream A stream or reach of a stream that flows continuously throughout the year. Synonymous with perennial stream.
Permeability The ability of a material to transmit fluids.
Phaneritic texture The texture of igneous rocks in which the interlocking crystals are large enough to be seen without magnification.
Phenocryst A crystal that is significantly larger than the crystals surrounding it. Phenocrysts form during an early phase in the cooling of a magma when the magma cools relatively slowly.
Physiographic map A map showing surface features of the earth.
Physiography The study of the surface features and landforms of the earth.
Pillar A landform shaped like a pillar.
Pillow lava An ellipsoidal mass of igneous rock formed by extrusion of lava underwater.
Pinnacle A tall, tower-shaped or spire-shaped pillar of rock.
Placer A mineral deposit formed by the sorting or washing action of water. Placers are usually deposits of heavy minerals, such as gold.
Plagioclase A group of feldspar minerals with a composition range from NaAlSi3O8 to CaAl2Si2O8.
Planetary differentiation The processes by which the materials in a planetary body are separated according to density, so that the originally homogeneous body is converted into a zoned or layered (shelled) body with a dense core, a mantle, and a crust.
Plastic deformation A permanent change in a substance's shape or volume that does not involve failure by rupture.
Plate (tectonics) A broad segment of the lithosphere (including the rigid upper mantle, plus oceanic and continental crust) that floats on the underlying asthenosphere and moves independently of other plates.
Plateau An extensive upland region.
Plateau basaltBasalt extruded in extensive, nearly horizontal layers, which, after uplift, tend to erode into great plateaus. Synonymous with flood basalt.
Plate tectonics The theory of global dynamics in which the lithosphere is believed to be broken into individual plates that move in response to convection in the upper mantle. The margins of the plates are sites of considerable geologic activity.
Platform reef An organic reef with a flat upper surface developed on submerged segments of a continental platform.
Playa A depression in the center of a desert basin, the site of occasional temporary lakes.
Playa lake A shallow temporary lake formed in a desert basin after rain.
Pleistocene The epoch of geologic time from the end of the Pliocene epoch of the Tertiary period (about 2 million years ago) to the beginning of the Holocene epoch of the Quaternary period (about 10,000 years ago). The major event during the Pleistocene was the expansion of continental glaciers in the Northern Hemisphere. Synonymous with glacial epoch, ice age.
Plucking (glacial geology) The process of glacial erosion by which large rock fragments are loosened by ice wedging, become frozen to the bottom surface of the glacier, and are torn out of the bedrock and transported by the glacier as it moves. The process involves the freezing of subglacial meltwater that seeps into fractures and bedding planes in the rock.
Plunge The inclination, with respect to the horizontal plane, of any linear structural element of a rock. The plunge of a fold is the inclination of the axis of the fold.
Plunging fold A fold with its axis inclined from the horizontal.
Plutonic rockIgneous rock formed deep beneath the earth's surface.
Pluvial lake A lake that was created under former climatic conditions, at a time when rainfall in the region was more abundant than it is now. Pluvial lakes were common in arid regions during the Pleistocene.
Point bar A crescent -shaped accumulation of sand and gravel deposited on the inside of a meander bend.
Polarity epoch A relatively long period of time during which the earth's magnetic field is oriented in either the normal direction or the reverse direction.
Polarity event A relatively brief interval of time within a polarity epoch; during a polarity event, the polarity of the earth's magnetic field is reversed with respect to the prevailing polarity of the epoch.
Polar wandering The apparent movement of the magnetic poles with respect to the continents.
Pore fluid A fluid, such as ground water or liquid rock material resulting from partial melting, that occupies pore spaces of a rock.
Pore space The spaces within a rock body that are unoccupied by solid material. Pore spaces include spaces between grains, fractures, vesicles, and voids formed by dissolution.
Porosity The percentage of the total volume of a rock or sediment that consists of pore space.
Porphyritic texture The texture of igneous rocks in which some crystals are distinctly larger than others.
Porphyry copper Deposits of copper disseminated throughout a porphyritic granitic rock.
Pothole A hole formed in a stream bed by sand and gravel swirled around in one spot by eddies.
Precambrian The division of geologic time from the formation of the earth (about 4.5 billion years ago) to the beginning of the Cambrian period of the Paleozoic era (about 600 million years ago). Also, the rocks formed during that time. Precambrian time constitutes about 90% of the earth's history.
Pre-Imbrian period The earliest period of lunar history, extending from the formation of the planet (about 4.5 billion years ago) to the formation of the multiringed basins (about 3.9 billion years ago). Pre-Imbrian system The system of rocks formed on the Moon during the pre-Imbrian period. It includes most of the material on the lunar highlands.
Pressure ridge An elongate uplift of the congealing crust of a lava flow, resulting from the pressure of underlying and still fluid lava.
Primary coast A coast shaped by subaerial erosion, deposition, volcanism, or tectonic activity. primary sedimentary structure A structure of sedimentary rocks (such as cross-bedding, ripple marks, or mud cracks) that originates contemporaneously with the deposition of the sediment (in contrast to a secondary structure, such as a joint or fault, which originates after the rock has been formed).
Primary wave See P wave.
Pumice A rock consisting of frothy natural glass.
P wave (primary seismic wave) A type of seismic wave, propagated like a sound wave, in which the material involved in the wave motion is alternately compressed and expanded.
Pyroclastic Pertaining to fragmental rock material formed by volcanic explosions.
Pyroclastic texture The rock texture of igneous rocks consisting of fragments of ash, rock, and glass produced by volcanic explosions.
Pyroxene A group of rock-forming silicate minerals composed of single chains of silicon-oxygen tetrahedra. Compare with amphibole, which is composed of double chains.

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[ Q ]
Quartz An important rock-forming silicate mineral composed of silicon-oxygen tetrahedra joined in a three-dimensional network. It is distinguished by its hardness, glassy luster, and conchoidal fracture.
Quartzite A sandstone recrystallized by metamorphism.

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[ R ]
Radioactivity The spontaneous disintegration of an atomic nucleus with the emission of energy.
Radiocarbon A radioactive isotope of carbon, C14, which is formed in the atmosphere and is absorbed by living organisms.
Radiogenic heat Heat generated by radioactivity.
Radiometric dating Determination of the age in years of a rock or mineral by measuring the proportions of an original radioactive material and its decay product. Synonymous with radioactive dating.
Rayed crater A meteorite crater that has a system of rays extending like splash marks from the crater rim.
Recessional moraine A ridge of till deposited at the margin of a glacier during a period of temporary stability in its general recession.
Recharge Replenishment of the ground-water reservoir by the addition of water.
Recrystallization Reorganization of elements of the original minerals in a rock resulting from changes in temperature and pressure and from the activity of pore fluids.
Reef A solid structure built of shells and other secretions of marine organisms, particularly coral.
Regolith The blanket of soil and loose rock fragments overlying the bedrock.
Rejuvenated stream A stream that has had its erosive power renewed by uplift or by a lowering of the base level or by climatic changes.
Relative age The age of a rock or an event as compared with some other rock or event.
Relative dating Determination of the chronologie order of a sequence of events in relation to one another without reference to their ages measured in years. Relative geologic dating is based primarily on superposition, faunal succession, and crosscutting relations.
Relative time Geologic time as determined by relative dating, that is, by placing events in chronologic order without reference to their ages measured in years.
Relief The difference in altitude between the high and the low parts of an area.
Reverse fault A fault in which the hanging wall has moved upward in relation to the footwall; a high angle thrust fault.
Rhyolite A fine-grained volcanic rock composed of quartz, K-feldspar, and plagioclase. It is the extrusive equivalent of a granite.
Rift system A system of faults resulting from extension.
Rift valley 1 A valley of regional extent formed by block faulting in which tensional stresses tend to pull the crust apart. Synonymous with graben. 2 The downdropped block along divergent plate margins.
Rill A very small stream.
Rille An elongate trench of cracklike valleys on the moon's surface. Rilles can be sinuous and meandering or relatively linear structural depressions.
Rip current A current formed on the surface of a body of water by the convergence of currents flowing in opposite directions. Rip currents are common along coasts where longshore currents move in opposite directions.
Ripple marks Small waves produced on a surface of sand or mud by the drag of wind or water moving over it.
River system A river with all of its tributaries.
Roche moutonnee An abraded knob of bedrock formed by an overriding glacier. It typically is striated and has a gentle slope facing the upstream direction of ice movement.
Roche moutonnee rock An aggregate of minerals that forms an appreciable part of the lithosphere.
Rockfall The most rapid type of mass movement, in which rocks ranging from large masses to small fragments are loosened from the face of a cliff.
Rock flour Fine-grained rock particles pulverized by glacial erosion.
Rock glacier A mass of poorly sorted, angular boulders cemented with interstitial ice. It moves slowly by the action of gravity.
Rockslide A landslide in which a newly detached segment of bedrock suddenly slides over an inclined surface of weakness (such as a joint or bedding plane).
Runoff Water that flows over the land surface.

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[ S ]
Sag pond A small lake that forms in a depression, or sag, where active or recent movement along a fault has impounded a stream.
Saltation The transportation of particles in a current of wind or water by a series of bouncing movements.
Saltation salt dome A dome produced in sedimentary rock by the upward movement of a body of salt.
Saltwater encroachment Displacement of fresh ground water by salt water in coastal areas, due to the greater density of salt water.
Sand Sedimentary material composed of fragments ranging in diameter from 0.0625 to 2 mm. Sand particles are larger than silt particles but smaller than pebbles. Much sand is composed of quartz grains, because quartz is abundant and resists chemical and mechanical disintegration, but other materials, such as shell fragments and rock fragments, can also form sand.
Sand wave A wave produced on a surface of sand by the drag of air or water moving over it. Sand waves include dunes and ripple marks.
Sandstone A sedimentary rock composed mostly of sand-size particles, usually cemented by calcite, silica, or iron oxide.
Scarp A cliff produced by faulting or erosion.
Schist A medium-grained or coarse-grained metamorphic rock with strong foliation (schistosity) resulting from parallel orientation of platy minerals, such as mica, chlorite, and talc.
Schistosity The type of foliation that characterizes schist, resulting from the parallel arrangement of coarse-grained platy minerals, such as mica, chlorite, and talc.
Scoria An igneous rock containing abundant vesicles.
Sea arch An arch cut by wave erosion through a headland.
Sea cave A cave formed by wave erosion.
Sea cliff A cliff produced by wave erosion.
Sea-floor spreading The theory that the sea floor spreads laterally away from the oceanic ridge as new lithosphere is created along the crest of the ridge by igneous activity.
Sea stack A small, pillar-shaped, rocky island formed by wave erosion through a headland near a sea cliff.
Seamount An isolated, conical mound rising more than 1000 m above the ocean floor. Seamounts are probably submerged shield volcanoes.
Secondary coast A coast formed by marine processes or the growth of marine organisms.
Secondary wave See S wave.
Sediment Material (such as gravel, sand, mud, and lime) that is transported and deposited by wind, water, ice, or gravity; material that is precipitated from solution; deposits of organic origin (such as coal and coral reefs).
Sedimentary differentiation The process in which distinctive sedimentary products (such as sand, shale, and lime) are generated and progressively separated from a rock mass by means of weathering, erosion, transportation, and deposition.
Sedimentary environment A place where sediment is deposited and the physical, chemical, and biological conditions that exist there. Examples: rivers, deltas, lakes, shallow-marine shelves.
Sedimentary rock Rock formed by the accumulation and consolidation of sediment.
Seep A spot where ground water or other fluids (such as oil) are discharged at the earth's surface.
Seif dune A longitudinal dune of great height and length.
Seismic Pertaining to earthquakes or to waves produced by natural or artificial earthquakes.
Seismic discontinuity A surface within the earth at which seismic wave velocities abruptly change.
Seismic ray The path along which a seismic wave travels. Seismic rays are perpendicular to the wave crest.
Seismic reflection profile A profile of the configuration of the ocean floor and shallow sediments on the floor obtained by reflection of artificially produced seismic waves.
Seismic wave A wave or vibration produced within the earth by an seismograph An instrument that records seismic waves.
Settling velocity The rate at which suspended solid material subsides and is deposited.
Shadow zone (seismology) An area where there is very little or no direct reception of seismic waves from a given earthquake because of refraction of the waves in the earth's core. The shadow zone for P waves is between about 103 and 143 degrees from the epicenter.
Shale A fine-grained clastic sedimentary rock formed by consolidation of clay and mud.
Shallow-focus earthquake An earthquake with a focus less than 70 km below the earth's surface.
Shallow-marine environment The sedimentary environment of the continental shelves, where the water is usually less than 200 m deep.
Sheeting A set of joints formed essentially parallel to the surface. It allows layers of rock to spall off as the weight of overlying rock is removed by erosion. It is especially well developed in granitic rock.
Shield An extensive area of a continent where igneous and metamorphic rocks are exposed and have approached equilibrium with respect to erosion and isostasy. Rocks of the shield are usually very old (that is, more than 600 million years old).
Shield volcano A large volcano shaped like a flattened dome and built up almost entirely of numerous flows of fluid basaltic lava. The slopes of shield volcanoes seldom exceed 10 degrees, so that in profile they resemble a shield or broad dome.
Shore The zone between the waterline at high tide and the waterline at low tide. A narrow strip of land immediately bordering a body of water, especially a lake or an ocean.
Sial A general term for the silicarich rocks that form the continental masses.
Silicate A mineral containing silicon-oxygen tetrahedra, in which four oxygen atoms surround each silicon atom.
Silicon-oxygen tetrahedron The structure of the ion SiO4-2, in which four oxygen atoms surround a silicon atom to form a four-sided pyramid, or tetrahedron.
Sill A tabular body of intrusive rock injected between layers of the enclosing rock.
Silt Sedimentary material composed of fragments ranging in diameter from 1/265 to 1/16 mm. Silt particles are larger than clay particles but smaller than sand particles.
Siltstone A fine-grained clastic sedimentary rock composed mostly of silt-size particles.
Sima A general term for the magnesium-rich igneous rocks (basalt, gabbro, and peridotite) of the ocean basins.
Sinkhole A depression formed by the collapse of a cavern roof.
Slate A fine-grained metamorphic rock with a characteristic type of foliation (slaty cleavage), resulting from the parallel arrangement of microscopic platy minerals, such as mica and chlorite.
Slaty cleavage The type of foliation that characterizes slate, resulting from the parallel arrangement of microscopic platy minerals, such as mica and chlorite. Slaty cleavage forms distinct zones of weakness within a rock, along which it splits into slabs.
Slip face See lee slope.
Slope retreat Progressive recession of a scarp or the side of a hill or mountain by mass movement and stream erosion.
Slump A type of mass movement in which material moves along a curved surface of rupture.
Snowline The line on a glacier separating the area where snow remains from year to year from the area where snow from the previous season melts.
Soil The surface material of the continents, produced by disintegration of rock. Regolith that has undergone chemical weathering in place.
Soil profile A vertical section of soil showing the soil horizons and parent material.
Solid The state of matter in which a substance has a definite shape and volume and some fundamental strength.
Solifluction A type of mass movement in which material moves slowly downslope in areas where the soil is saturated with water. It commonly occurs in permafrost areas.
Solution valley A valley produced by solution activity, either by dissolution of surface materials or by removal of subsurface materials such as limestone, gypsum, or salt.
Sorting The separation of particles according to size, shape, or weight. It occurs during transportation by running water or wind.
Spatter cone A low, steep-sided volcanic cone built by accumulation of splashes and spatters of lava (usually basaltic) around a fissure or vent.
Specific gravity The ratio of the weight of a substance to the weight of an equal volume of water.
Spheroidal weathering The process by which corners and edges of a rock body become rounded as a result of exposure to weathering on all sides, so that the rock acquires a spheroidal or ellipsoidal shape.
Spit A sandy bar projecting from the mainland into open water. Spits are formed by deposition of sediment moved by longshore drift.
Splay A small deltaic deposit formed on a flood plain where water and sediment are diverted from the main stream through a crevasse in a levee.
Spreading axis The imaginary axis through the earth about which a set of tectonic plates moves. The motion of a diverging plate can be described as rotation around a spreading axis.
Spreading center A plate boundary formed by tensional stress along the oceanic ridge. Synonymous with divergent plate boundary, spreading edge.
Spreading pole A pole of the imaginary axis about which a set of tectonic plates moves. The spreading poles are the two points at which a spreading axis intersects the earth's surface.
Spring A place where ground water flows or seeps naturally to the surface.
Stable platform The part of a continent that is covered with flatlying or gently tilted sedimentary strata and underlain by a basement complex of igneous and metamorphic rocks. The stable platform has not been extensively affected by crustal deformation.
Stack See sea stack.
Stalactite An icicle-shaped deposit of drips tone hanging from the roof of a cave.
Stalagmite A conical deposit of drips tone built up from a cave floor.
Star dune A mound of sand with a high central point and arms radiating in various directions.
Stock A small, roughly circular intrusive body, usually less than 100 km' in surface exposure.
Strata Plural of stratum.
Stratification The layered structure of sedimentary rock.
Stratovolcano A volcano built up of alternating layers of ash and lava flows. Synonymous with composite volcano.
Stratum (pl. strata) A layer of sedimentary rock.
Streak The color of a powdered mineral.
Stream load The total amount of sediment carried by a stream at a given time.
Stream order The hierarchical number of a stream segment. The smallest tributary has the order number of 1, and successively larger tributaries have progressively higher numbers.
Stream piracy Diversion of the headwaters of one stream into another stream. The process occurs by headward erosion of a stream having greater erosive power than the stream it captures.
Stream terrace One of a series of level surfaces in a stream valley representing the dissected remnants of an abandoned flood plain, stream bed, or valley floor produced in a previous stage of erosion or deposition.
Stress Force applied to a material that tends to change its dimensions or volume; force per unit area.
Striation A scratch or groove produced on the surface of a rock by a geologic agent, such as a glacier or stream.
Strike The bearing (compass direction) of a horizontal line on a bedding plane, a fault plane, -or some other planar structural feature.
Strike-slip fault A fault in which movement has occurred parallel to the strike of the fault.
Strike valley A valley that is eroded parallel to the strike of the underlying nonresistant strata.
Strip mining A method of mining in which soil and rock cover are removed to obtain the sought-after material.
Subaerial Occurring beneath the atmosphere or in the open air, with reference to conditions or processes (such as erosion) that occur on the land. Contrast with submarine and subterranean.
Subaqueous sand flow A type of mass movement in which saturated sand or silt flows beneath the surface of a lake or an ocean.
SubductionSubsidence of the leading edge of a lithospheric plate into the mantle.
Subduction zone An elongate zone in which one lithospheric plate descends beneath another. A subduction zone is typically marked by an oceanic trench, lines of volcanoes, and crustal deformation associated with mountain building. See also convergent plate boundary.
Submarine canyon A V-shaped trench or valley with steep sides cut into a continental shelf or continental slope.
Subsequent stream A tributary stream that is eroded along an underlying belt of nonresistant rock after the main drainage pattern has been established.
Subsidence A sinking or settling of a part of the earth's crust with respect to the surrounding parts.
Superposed stream A stream with a course originally established on a cover of rock now removed by erosion, so that the stream or drainage system is independent of the newly exposed rocks and structures. The stream pattern is thus superposed on, or placed upon, ridges or other structural features that were previously buried.
Superposition, principle of The principle that, in a series of sedimentary strata that has not been overturned, the oldest rocks are at the base and the youngest are at the top.
Surface creep Slow downwind movement of large sand grains by rolling or sliding along the surface due to the impact of smaller, saltating grains.
Surface wave (seismology) A seismic wave that travels along the earth's surface. Contrast with P waves and S waves, which travel through the earth.
Suspended load The part of a stream's load that is carried in suspension for a considerable period of time, without contact with the stream bed. It consists mainly of mud, silt, and sand. Contrast with bed load and dissolved load.
Swash The rush of water up onto a beach after a wave breaks.
S wave (secondary seismic wave) A seismic wave in which particles vibrate at right angles to the direction in which the wave travels. Contrast with P wave.
Symmetrical fold A fold in which the two limbs are essentially mirror images of each other.
Syncline A fold in which the limbs dip toward the axis. After erosion, the youngest beds are exposed in the central core of the fold.

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[ T ]
Talus Rock fragments that accumulate in a pile at the base of a ridge or cliff.
Tectonic creep Slow, apparently continuous movement along a fault (as opposed to the sudden rupture that occurs during an earthquake).
Tectonics The branch of geology that deals with regional or global structures and deformational features of the earth.
TensionStress that tends to pull materials apart.
Tephra A general term for pyroclastic material ejected from a volcano. It includes ash, dust, bombs, and other types of fragments.
Terminal moraine A ridge of material deposited by a glacier at the line of maximum advance of the glacier.
Terra (pl. terrae) A densely cratered highland on the Moon.
Terrace A nearly level surface bordering a steeper slope, such as a stream terrace or wave-cut terrace.
Terrae Plural of terra.
Texture The size, shape, and arrangement of the particles that make up a rock.
Thin section A slice of rock mounted on a glass slide and ground to a thickness of about 0.03 mm.
Thrust fault A low-angle fault (45 degrees or less) in which the hanging wall has moved upward in relation to the footwall. Thrust faults are characterized by horizontal compression rather than by vertical displacement.
Tidal bore A violent rush of tidal water.
Tidal flat A large, nearly horizontal area of land covered with water at high tide and exposed to the air at low tide. Tidal flats consist of fine-grained sediment (mostly mud, silt, and sand).
Till Unsorted and unstratified glacial deposit.
Tillite A rock formed by lithification of glacial till (unsorted, unstratified glacial sediment).
Tombolo A beach or bar connecting an island to the mainland.
Topography The shape and form of the earth's surface.
Trailing edge (plate tectonics) A passive or divergent plate margin. Contrast with leading edge.
Transform fault A special type of strike-slip fault forming the boundary between two moving lithospheric plates, usually along an offset segment of the oceanic ridge. See also passive plate margin.
Transform fault transpiration The process by which water vapor is released into the atmosphere by plants.
Transverse dune An asymmetrical dune ridge that forms at right angles to the direction of prevailing winds.
Travertine terrace A terrace formed from calcium carbonate deposited by water on a cave floor.
Trellis drainage pattern A drainage pattern in which tributaries are arranged in a pattern similar to that of a garden trellis.
Trench (marine geology) A narrow, elongate depression of the deep-ocean floor oriented parallel to the trend of a continent or an island arc.
Tributary A stream flowing into or joining a larger stream.
Tsunami A seismic sea wave; a long, low wave in the ocean caused by an earthquake, faulting, or a landslide on the sea floor. Its velocity can reach 800 km per hour. Tsunamis are commonly and incorrectly called "tidal waves."
Tuff A fine-grained rock composed of volcanic ash.
Turbidity current A current in air, water, or any other fluid caused by differences in the amount of suspended matter (such as mud, silt, or volcanic dust). Marine turbidity currents, laden with suspended sediment, move rapidly down continental slopes and spread out over the abyssal floor.
Turbulent flow A type of flow in which the path of motion is very irregular, with eddies and swirls.

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[ U ]
Ultimate base level The lowest possible level to which a stream can erode the earth's surface; sea level.
Ultramafic rock An igneous rock composed entirely of ferromagnesian minerals.
Unconformity A discontinuity in the succession of rocks, containing a gap in the geologic record. A buried erosion surface. See also angular unconformity, nonconformity.
Uniformitarianism The theory that geologic events are caused by natural processes, many of which are operating at the present time.
Upwarp An arched or uplifted segment of the crust.

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[ V ]
Valley glacier A glacier that is confined to a stream valley. Synonymous with alpine glacier, mountain glacier.
Varve A pair of thin sedimentary layers, one relatively coarse-grained and light-colored, and the other relatively fine-grained and dark-colored, formed by deposition on a lake bottom during a period of one year (see diagram). The coarse-grained layer is formed during spring runoff, and the fine-grained layer is formed during the winter when the surface of the lake is frozen.
Ventifact A pebble or cobble shaped and polished by wind abrasion.
Vesicle A small hole formed in a volcanic rock by a gas bubble that became trapped as the lava solidified.
Viscosity The tendency within a body to resist flow. An increase in viscosity implies a decrease in fluidity, or ability to flow.
Volatile 1 Capable of being readily vaporized. 2 A substance that can readily be vaporized, such as water or carbon dioxide.
Volcanic ash Dust-size particles ejected from a volcano.
Volcanic bomb A hard fragment of lava that was liquid or plastic at the time of ejection and acquired its form and surface markings during flight through the air. Volcanic bombs range from a few millimeters to more than a meter in diameter.
Volcanic front The line in a volcanic arc system (parallel to a trench) along which volcanism abruptly begins.
Volcanic neck The solidified magma that originally filled the vent or neck of an ancient volcano and has subsequently been exposed by erosion.
Volcanism The processes by which magma and gases are transferred from the earth's interior to the surface.

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[ W ]
Wash A dry stream bed.
Water gap A pass in a ridge through which a stream flows.
Water table The upper surface of the zone of saturation.
Wave base The lower limit of wave transportation and erosion, equal to half the wavelength.
Wave crest The highest part of a wave.
Wave height The vertical distance between a wave crest and the preceding trough.
Wave period The interval of time required for a wave crest to travel a distance equal to one wavelength; the interval of time required for two successive wave crests to pass a fixed point.
Wave refraction The process by which a wave is bent or turned from its original direction. In sea waves, as a wave approaches a shore obliquely, part of it reaches the shallow water near the shore while the rest is still advancing in deeper water; the part of the wave in the shallower water moves more slowly than the part in the deeper water. In seismic waves, refraction results from the wave encountering material with a different density or composition.
Wave trough The lowest part of a wave, between successive crests.
Wave-built terrace A terrace built up from wave-washed sediments. Wave-built terraces usually lie seaward of a wave-cut terrace.
Wave-cut cliff A cliff formed along a coast by the undercutting action of waves and currents.
Wave-cut platform A terrace cut across bedrock by wave erosion. Synonymous with wave-cut terrace.
Wave-cut terrace See wave-cut platform.
Wavelength The horizontal distance between similar points on two successive waves, measured perpendicular to the crest.
Weathering The processes by which rocks are chemically altered or physically broken into fragments as a result of exposure to atmospheric agents and the pressures and temperatures at or near the earth's surface, with little or no transportation of the loosened or altered materials.
Welded tuff A rock formed from particles of volcanic ash that were hot enough to become fused together.
Wind gap A gap in a ridge through which a stream, nowabandoned as a result of stream piracy, once flowed.
Wind shadow The area behind an obstacle where air movement is not capable of moving material.
Wrinkle ridge A sinuous, irregular segmented ridge on the surface of a lunar mare, believed to be a result of deformation of the lava.

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[ X ]
X-ray diffraction In mineralogy, the process of identifying mineral structures by exposing crystals to a beam of X-rays and studying the resulting diffraction patterns.

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[ Y ]
Yardang An elongate ridge carved by wind erosion.
Yazoo stream A tributary stream that flows parallel to the main stream for a considerable distance before joining it. Such a tributary is forced to flow along the base of a natural levee formed by the main stream.

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[ Z ]
Zone of aeration The zone below the earth's surface and above the water table, in which pore spaces are usually filled with air.
Zone of saturation The zone in the subsurface in which all pore spaces are filled with water.


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