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Glossary of Geophysical Sciences


14C method A method for determining the age in years of organic matter by calculating the amount of radioactive carbon still remaining, as compared to the stable isotope, 12C.

40K/40Ar method A method used for the dating of potassium-bearing rocks by using the ratio of radioactive 40K to its daughter, 40Ar.

aa A Hawaiian term for a lava flow that has a rough, jagged surface. compare pahoehoe .

ablation As applied to glacier ice, the process by which ice below the snow line is wasted by evaporation and melting.

absolute time Geologic time expressed in years before the present.

abundant metal Iron, aluminum, magnesium, manganese, and titanium. Ores of the abundant metals only need to be 3 - 5 times as metal-rich as average rock.

abyssal plain Large area of extremely flat ocean floor lying near a continent and generally over 4 km in depth.

acceleration The rate at which velocity changes, either by increasing or decreasing.

accretion The process by which the terrestrial planets grew, increasing their mass by gradually accumulating smaller bodies, called planetesimals.

acid mine drainage Water contamination by sulfuric acid produced by seepage through sulfur-bearing spoil and tailings from coal and metal mining

acid rain The acidity in rain due to gases from internal combustion engines and coal- and oil-burning power plants.

active layer The seasonally thawed zone above permafrost .

actualism see uniformitarianism .

aftershock An earthquake that follows and has its epicenter near a larger earthquake.

agate A siliceous rock with alternating bands of chalcedony and variously colored chert .

alluvial fan Land counterpart of a delta . An assemblage of sediments marking place where a stream moves from a steep gradient to a flatter gradient and suddenly loses transporting power. Typical of arid and semiarid climates but not confined to them.

alpha decay The process of radioactive decay in which the nucleus of an atom emits an alpha particle. The new atom's atomic number is lower by two and its atomic mass number is reduced by four.

alpha particle A particle consisting of two protons and two neutrons, produced during alpha decay. Identical to the nucleus of a 4He atom.

alpine glacier see valley glacier .

amygdule A gas cavity (vesicle ) in volcanic rock that has been filled with mineral matter such as calcite, chalcedony, or quartz.

amygdaloidal A textural term describing volcanic rocks that contain numerous amygdules.

andesite A fine-grained volcanic rock of intermediate composition, consisting largely of plagioclase and one or more mafic minerals.

andesite line The geographic boundary between rocks of the Pacific Basin, which are basaltic, and those around the rim of the basin, which are in part andesitic.

angle of incidence The angle at which a ray of energy approaches a surface.

angle of reflection The angle at which a reflected ray of energy leaves a surface.

angle of refraction The angle at which a refracted ray of energy leaves a surface after passing through it.

angle of repose The maximum angle at which loose material will come to rest when added to a pile of similar material.

angular unconformity  An unconformity in which the beds below the unconformity dip at a different angle than the beds above it.

anion An ion with a negative electrical charge. That is, an atom that has gained one or more electrons.

anticline A fold that is convex upward, or that had such an attitude at some stage of its development. compare syncline .

aphanitic A textural term meaning "fine-grained" that applies to igneous rocks.

aquifer A permeable region of rock or soil through which ground water can move.

aquitard A material of low permeability that greatly slows the movement of ground water.

arch Forms along a coast as wave erosion cuts through a headland.

Archean An eon of geologic time extending from about 3.9 billion years to 2.5 billion years ago.

arête A narrow, saw-toothed mountain ridge developed by glacier erosion in adjacent cirques .

arkose A sedimentary rock formed by the cementation of sand-sized grains of feldspar and quartz.

artesian well A well in which the water in the aquifer is under pressure that raises the water above the point that the well first encounters it.

assemblage The collection of minerals that characterize a rock or a facies.

asthenosphere The weak or "soft" zone in the upper mantle just below the lithosphere , involved in plate movement and isostatic adjustments. It lies 70 to 100 km below the surface and may extend to a depth of 400 km. Corresponds to the seismic low-velocity zone .

astronomic theory of glaciation A theory based on the changing position of the Earth in its orbit around the Sun.

asymmetric rock knob or hill Bedrock forms with a gentle slope on one side created by glacial abrasion and a steep slope on the opposite side created by glacial plucking.

atoll  A roughly circular reef with an occasional small, low, coral sand island surrounding a shallow lagoon.

atom A building block of matter, the smallest particle that has the chemical characteristics of a particular chemical element. It contains a nucleus of protons and neutrons surrounded by a cloud of electrons.

atomic mass number The sum of the number of protons and the number of neutrons in an atom. Approximately equal to the mass of the atom.

atomic number The number of protons in an atom, a quantity that determines which element the atom represents. Example: all atoms of oxygen have 8 protons.

aureole A zone surrounding an igneous intrusion, in which contact metamorphism has taken place.

authigenesis The process by which new minerals form in a sediment or sedimentary rock during or after deposition.

axial plane A geometric plane that intersects the trough or crest of a fold in such a way that the limbs of the fold are more or less symmetrically arranged with reference to it.

axis The line formed by the intersection of the axial plane of a fold with a bedding plane, marking where the bed shows its maximum curvature.

back-arc basin The region between an island arc and the continental mainland, commonly with at least some oceanic crust on its floor.

back swamp A swamp that forms in the low lying flood plain behind a levee.

backshore Lies between high tide mark and the foot of the beach dune or the limit of effective wave action.

banded iron formation (BIF) A sedimentary mineral deposit dominated by iron oxides, carbonates, or silicates that were deposited chemically from seawater. Most BIFs were formed between 2.5 and 3.5 billion years ago. Their formation is related to the rise of oxygen in the atmosphere.

bankfull stage A stream discharge that just fills the stream channel.

bar 1. A mass of sand, gravel, or alluvium deposited on the bed of a stream, sea, or lake, or at the mouth of a stream 2. A unit of pressure, approximately equal to atmospheric pressure at sea level.

barchan A crescent-shaped sand dune with horns pointing downwind.

barrier beaches or islands Long narrow beaches separated in many places from the mainland by lagoons.

barrier reef  A coral reef separated from the mainland by a lagoon.

basalt A dark colored extrusive igneous rock composed chiefly of calcium plagioclase and pyroxene. Extrusive equivalent of gabbro, underlies the ocean basins and comprises oceanic crust.

base flow Ground water that enters a stream channel, maintaining stream flow at times when it is not raining.

base level Of a stream is the point below which the stream cannot cut. A temporary base level along a stream, such as a lake may be removed by stream action . Ultimate base level is the ocean.

basin A synclinal structure, roughly circular in its outcrop pattern, in which beds dip gently toward the center from all directions.

batholith A large, discordant, intrusive body of igneous rock.

bauxite A rock composed of aluminum hydroxides and impurities in the form of silica, clay, silt, and iron hydroxides. A residual weathering product, exploited as the primary ore for aluminum.

bay barrier A beach that cuts off a bay from the sea.

beach replenishment Rebuilding a beach by adding sand to it.

beach Temporary accumulations of sediments that collect between low and high tide marks.

bed load Material in motion along a stream bed.

bedding A collective term used to signify presence of beds, or layers, in sedimentary rocks and deposits.

bedding plane Surface separating layers of sedimentary rocks and deposits. Each bedding plane marks termination of one deposit and beginning of another of different character, such as a surface separating a sandstone bed from an overlying mudstone bed. Rock tends to breaks or separate, readily along bedding planes.

bedrock Any solid rock exposed at the Earth's surface or overlain by unconsolidated material.

beheaded stream The headwaters of a stream that have been captured by another stream. compare stream piracy .

berm A small terrace in the backshore area of the coast with its terrace facing seaward.

beta decay The process of radioactive decay in which a neutron loses a beta particle, which is physically identical to an electron. This increases the atomic number of the atom by one by turning the neutron into a proton. The atom's atomic mass number stays the same because the total number of protons and neutrons remain the same. The most common form of radioactive decay.

BIF see banded iron formation

binding energy The energy that holds the particles in the nucleus of an atom together. It is this energy, when released, that is used to generate nuclear power.

biogenic sediment Sediments produced directly by the life processes of plants or animals.

biogenic sedimentary rock A sedimentary rock composed primarily of biogenic sediments.

bioturbation The turning and mixing of sediments by organisms.

black smoker A vent on the seafloor from which hydrothermal fluids are emitted. Upon mixing with seawater and cooling, the fluids precipitate a cloud of fine-grained sulfide minerals that resembles a cloud of black smoke.

blind valley A valley in karst that ends abruptly downstream at the point where its stream disappears underground as a sinking stream.

blowout A an irregular depression excavated by wind, usually in previously deposited blown sand.

body wave Any seismic wave that travels through the body of the Earth, rather than along its surface. compare surface wave.

bond (ionic, covalent, Van der Waals, metallic) see chemical bond

bottomset bed Layer of fine sediment deposited in a body of standing water beyond the edge of a growing delta and which is eventually built over by the advancing delta. Similarly bottomset beds may accumulate in the wind shadow of a sand dune and be preserved beneath it as the dune advances.

boudinage A structure in which brittle beds bounded by more ductile ones have been divided into segments during metamorphism.

boulder train Clusters of erratics from same source, with some distinctive characteristic that makes their common source easily recognizable.

boundary The tectonic region in which two plates meet. compare margin.

Bowen's Reaction Series A series of minerals formed during crystallization of a magma, in which the formation of minerals alters the composition of the remaining magma. Mafic minerals comprise a discontinuous series, in which successive minerals form at the expense of early-formed ones. The plagioclase feldspars form in a continuous series, in which the composition of plagioclase becomes progressively sodium rich, but the crystal structure of the mineral does not change.

braided stream A stream with a complex tangle of converging and diverging channels separated by sand bars or islands.

branch work cave Cave with passage ways formed along bedding planes and with an areal pattern similar to that of surface streams.

breakwater A protective wall built offshore and usually parallel to the shore.

breccia A clastic rock in which the gravel-sized particles are angular in shape and make up an appreciable volume of the rock.

breeder reactor A nuclear reactor in which 238U or 232Th, which are not easily fissionable, absorb neutrons to become atoms of 239Pu or 236U, which can later be used as fuels in fission reactors. Breeder technology is not yet feasible.

brittle Structural behavior in which a material deforms permanently by fracturing.

brittle limit The stress limit beyond which a material fractures, rather than behaving in a ductile or elastic fashion.

burial metamorphism Takes place in an environment where pressure and temperature are barely more intense than during diagenesis , typically in a deepening sequence of sediments.

calcarenite A sandstone in which the sand-sized grains are calcite.

caldera A large, basin-shaped volcanic depression, more or less circular in form. Typically steep-sided, found at the summit of a shield volcano .

caliche Gravel, sand, or desert debris cemented by calcium carbonate, an accumulated product of chemical weathering in a dry climate. compare claypan, fragipan, hardpan.

calving The breaking away of ice from the front of the glacier when it ends in a lake or an ocean. Produces icebergs.

cap rock A comparatively impervious stratum immediately overlying an oil- or gas-bearing rock.

capacity The total amount of material a stream is able to carry under given conditions.

capillary water Water in the zone of aeration held to soil particles by surface tension of the water molecules for each other and for the soil particles.

carbonate conservation depth The water depth below which the calcium carbonate produced in the ocean is completely dissolved. There is no calcium carbonate deposition below this level.

carbonate rock A rock consisting primarily of a carbonate mineral such as calcite or dolomite, the chief minerals in limestone and dolostone, respectively.

cataclastic metamorphism Takes place in an environment where intense pressure due to shearing is common, as in a major fault zone.

cation An ion that has a positive electrical charge. That is, an atom that has lost one or more electrons.

cave  A natural open space underground, large enough for a person to enter. Most commonly occur by the dissolution of soluble rocks, generally limestone.

cementation Process by which a binding, or cementing, agent is precipitated in spaces among individual particles of a deposit. Common cementing agents are calcite, quartz, and dolomite.

Cenozoic The current geologic era, which began 66.4 million years ago and continues to the present.

chain reaction A self-sustaining nuclear reaction, made possible when neutrons released by fission of some atoms in a nuclear reactor strike other atoms, causing them to fission as well.

chalcedony A cryptocrystalline form of quartz, microscopically fibrous with waxy luster. May be transparent or translucent, and with a uniform tint of white, gray, pale blue, and, less often, black.

chalk A variety of limestone made up in part of biochemically derived calcite, in form of skeletons or skeletal fragments of microscopic oceanic plants and animals mixed with fine-grained calcite deposits of biochemical or inorganic-chemical origin.

chemical bond The interactions among the electrons of atoms that hold atoms together to form chemical compounds. If electrons cluster primarily around one atom of a pair, the bond is ionic . If they are shared more or less equally, it is covalent . If electrons move freely between atoms over an extended region, the bond is metallic. A weak electrostatic bond due to uneven distribution of electrons around atoms or groups of atoms is a Van der Waals bond.

chemical element A fundamental substance that cannot be further refined or subdivided by chemical means. All atoms of a chemical element have the same number of protons.

chemical remanent magnetism Acquired as magnetic minerals form and align themselves to the global magnetic field during diagenesis of a sedimentary deposit.

chemical sediment Sediment formed by chemical precipitation from water. Example: halite precipitated as the result of the evaporation of sea water.

chemical sedimentary rock A sedimentary rock made up of chemical sediments. Example: rock salt.

chemical weathering see decomposition

chert A cryptocrystalline form of quartz, microscopically granular. Occurs as nodules and as thin, continuous layers. Duller, less waxy luster than chalcedony. Occurs in limestone, dolostone, and mudstones.

chlorofluorocarbons (CFC) Gases that can be dissociated by solar radiation, which releases chlorine, which in turn destroys ozone.

chute cutoff A narrow "short cut" across a meander bend, formed in flood as the main stream flow is diverted into a trough between point bars. Sometimes called simply a '"chute".

cinder cone  A conical volcano formed by the accumulation of pyroclastic debris around a vent.

cirque A steep-walled hollow in a mountain side, shaped like an amphitheater, or bowl, with one side partially cut away. Place of origin of a mountain glacier.

clastic Refers to rock or sediments made up primarily of broken fragments of pre-existing rocks or minerals.

clay 1. The name for a family of finely-crystalline sheet silicate minerals. 2. Fine-grained soil consisting of mineral particles, not necessarily clay minerals, that are less than 0.074 mm in their maximum dimension.

claypan A layer of stiff, compact, relatively impervious clay which is not cemented. compare caliche , fragipan, hardpan.

cleavage 1.  of a mineral: The tendency of a mineral to split along planes determined by the crystal structure. 2.  of a rock: see slaty cleavage

coal Sedimentary rock composed of combustible matter derived from the partial decomposition of plant material.

coast A narrow strip of land along the margin of the ocean extending inland for a variable distance from low water mark.

col Mountain pass formed by enlargement of two opposing cirques until their head walls meet and are broken down.

column Pillar formed as a stalactite and stalagmite meet.

columnar jointing The type of jointing that breaks rock, typically basalt, into columnar prisms. Usually the joints form a more or less distinct hexagonal pattern.

compaction Reduction of pore space between individual particles as the result of overlying sediments or of tectonic movements.

competence The maximum size of particle that a stream can carry.

composite volcano see stratovolcano

Comprehensive Soil Classification System (CSCS) The classification system in most common use by North American soil scientists. Categories are based on the chemical and physical characteristics of a soil. compare USDA Soil Classification System .

compression Squeezing a material from opposite directions.

concordant Lying parallel to, rather than cutting across surrounding strata.

concretion  A compact mass of mineral matter, usually spherical or disk-like in shape and embedded in a host rock of different composition. They form by precipitation of mineral matter about a nucleus such as a leaf, or a piece of shell of bone.

conduction Heat transport by direct transfer of energy from one particle to another, without moving the particle to a new location. compare convection , radiation .

cone of depression A downward distortion or dimple in the water table that forms as a well pumps water faster than it can flow through the aquifer.

conglomerate A clastic sedimentary rock composed of lithified beds of rounded gravel mixed with sand.

Constancy of Interfacial Angles The statement that the angles between congruent crystal faces on samples of a single mineral are always identical. A consequence of, and therefore evidence for the existence of crystalline structure in minerals.

contact metamorphism Metamorphism genetically related to the intrusion (or extrusion) of magmas and taking place in rocks at or near their contact with a body of igneous rock.

continental arc A belt of volcanic mountains on the continental mainland that lie above a subduction zone. compare island arc .

continental crust  The part of the crust that directly underlies the continents and continental shelves. Averages about 35 km in thickness, but may be over 70 km thick under largest mountain ranges.

continental deserts Located in continental interior far from moisture-bearing winds.

continental divide A major drainage divide separating the drainage to one ocean from another.

continental drift The theory that explained the relative positions and shapes of continents, the formation of mountains, and other large-scale geologic phenomena as results of the lateral movement of continents. The crust of ocean basins was assumed to be relatively immobile. compare plate tectonics , sea floor spreading .

continental ice glacier An ice sheet that obscures all but the highest peaks of a large part of a continent.

continental rise The portion of the continental margin that lies between the abyssal plain and the continental slope. The continental rise is underlain by crustal rocks of the ocean basin.

continental shelf The portion of the continental margin that extends as a gently sloping surface from the shoreline seaward to a marked change in slope at the top of the continental slope . Seaward depth averages about 130 m.

continental slope That part of the continental margin that lies between the continental shelf and the continental rise. Slope relatively steep, 3o - 6o. The continental slope is underlain by crustal rocks of the continent.

convection Heat transport by moving particles, and the thermal energy that they carry, to a new location. compare conduction , radiation .

convection cell A cyclical pattern of movement in a fluid body such as the ocean, the atmosphere, or the Earth's mantle, driven by density variations which in turn are the result of differences in temperature from one part of the fluid to another.

convergent boundary A boundary between two plates of the Earth's crust that are pushing together.

co-product A mineral commodity that is recovered from a mining operation for some other mineral product. Example: Platinum is commonly a co-product of nickel mining.

coquina A coarse-grained, porous variety of clastic limestone made up chiefly of shells and shell fragments.

core Innermost zone of Earth. Consists of two parts, an outer liquid section and an inner solid section, both chiefly of iron and nickel with about 10 percent lighter elements. It is surrounded by the mantle.

correlation Process of establishing contemporaneity of rocks or events in one area with rocks or events in another area.

crater 1. A steep-walled, usually conical depression at the summit or on the flanks of a volcano, resulting from the explosive ejection of material from a vent. 2.  A bowl-shaped depression with a raised, overturned rim produced by the impact of a meteorite or other energetic projectile.

craton The stable portions of the continents that have escaped orogenic activity for the last 2 billion years. Made predominantly of granite and metamorphic rocks. compare orogen .

creep 1. The very slow, generally continuous downslope movement of soil and debris under the influence of gravity. 2. The movement of sand grains along the land surface.

crevasse 1. Breach in a natural levee . 2. Deep crevice or open fracture in glacier ice.

cross-bedding see inclined bedding .

cross-cutting relationships Geologic discontinuities that suggest relative ages: A geologic feature is younger than the feature it cuts. Thus, a fault cutting across a rock is younger than the rock.

crust  The upper part of the lithosphere , divided into oceanic crust and continental crust .

crystal The multi-sided form of a mineral, bounded by planar growth surfaces, that is the outward expression of the ordered arrangement of atoms within it.

crystal settling Gravitational sinking of crystals from the liquid in which they formed, by virtue of their greater density. A type of igneous differentiation.

crystal structure The regular and repeated three-dimensional arrangement of atoms or ions in a crystal.

crystalline 1. Having a crystal structure. 2. When referring to sedimentary rocks, crystalline designates a texture in which mineral crystals have formed in an interlocking pattern. see nonclastic. 3. As a generic term, geologists use the term "crystalline rocks" as a rough synonym for "igneous or metamorphic rocks".

cumulate An igneous rock that forms by crystal settling .

Curie point The temperature above which a mineral loses its magnetism.

current ripple mark  An asymmetric ripple mark formed by wind or water moving generally in one direction. Steep face of ripple faces in direction of current. compare oscillation ripple mark .

cyclothem A series of beds, of interest because they include coal, which were associated with unstable shelf or interior basin conditions in which alternating marine transgressions and regressions occurred.

Darcy's law A formula describing the flow of water through an aquifer.

daughter An atom that results from the radioactive decay of a parent atom.

debris flow Fast-moving, turbulent mass movement with a high content of both water and rock debris. The more rapid debris flows rival the speed of rock slides.

decay rate The rate at which a population of radioactive atoms decays into stable daughter atoms. Rate often expressed in terms of half life of the parent isotope .

decomposition (chemical weathering) Weathering processes that are the result of chemical reactions. Example: the transformation of orthoclase to kaolinite.

deflation A process of erosion in which wind carries off particles of dust and sand.

dehydration Any process by which water bound within a solid material is released. Example: Gypsum (CaSO4·2H2O) becomes anhydrite (CaSO4) by dehydration.

delta An assemblage of sediments accumulated where a stream flows into a body of standing water and its velocity and transporting power are suddenly reduced. . A "delta plain" is the upper surface of a delta.

dendritic drainage  A stream pattern that, when viewed on a map or from the air, resembles the branching pattern of a deciduous tree such as a maple or oak.

denudation The sum of the processes that result in the wearing away or the progressive lowering of the Earth's surface by weathering, erosion, mass wasting, and transportation.

depositional environment The nature of the environment in which sediments are laid down. They are immensely varied and may range from the deep ocean to the coral reef and the glacial lake of the high mountains. The nature of the depositional environment may be deduced from the nature of the sediments and rock deposited there.

depositional remanent magnetism Develops as magnetic minerals settle through water and align themselves in the Earth's magnetic field.

desert pavement A lag accumulation of pebbles or boulders that cuts off further deflation .

desertification A process of land degradation initiated by human activity, particularly in the zones along the margins of deserts.

detrital sedimentary rock A sedimentary rock made up of detrital sediments.

detrital sediments Sediments made of fragments or mineral grains weathered from pre-existing rocks.

diagenesis All the physical, chemical, and biologic changes undergone by sediments from the time of their initial deposition, through their conversion to solid rock, and subsequently to the brink of metamorphism.

differential weathering Weathering that occurs at different rates, as the result of variations in composition and mechanical resistance of rocks, or differences in the intensity of weathering processes.

differentiation The process of developing more than one rock type, in situ, from a common magma.

dike A tabular igneous intrusion that cuts across the surrounding rock.

dilatancy An increase in the bulk volume of rock during deformation. Possibly related to the migration of water into microfractures or pores.

dip The angle that a structural surface such as a bedding plane or fault surface makes with the horizontal, measured perpendicular to the strike and in the vertical plane.

dip pole see magnetic pole

dip slip fault A fault on which the movement is parallel to the dip of the fault plane.

directed pressure Pressure applied predominately in one direction, rather than uniformly.

discharge In a stream, the volume of water passing through a channel in a given time.

disconformity An unconformity in which the beds above the unconformity are parallel to the beds below the unconformity.

discordant Cutting across surrounding strata.

disintegration (mechanical weathering) The processes of weathering by which physical actions such as frost wedging break down a rock into fragments, involving no chemical change.

dissolution A chemical reaction in which a solid material is dispersed as ions in a liquid. Example: Halite (NaCl) undergoes dissolution when placed in water.

dissolved load Amount of material water carries in solution.

distributary channels Stream channels that fan out from the upstream point of the delta and carry the sediments that build the delta.

divergent boundary Boundary between two crustal plates that are pulling apart.

dolostone A carbonate rock made up predominately of the mineral dolomite, CaMg(C03)2.

dome An uplift or anticlinal structure, roughly circular in its outcrop exposure, in which beds dip gently away from the center in all directions.

drag fold A minor fold produced within a weak bed or adjacent to a fault by the movement of surrounding rocks in opposite directions.

drainage basin  The area from which a stream and its tributaries receives its water.

drainage divide  The line that separates one drainage basin from another.

drift Glacial deposits laid down directly by glaciers or laid down in lakes, ocean, or streams as result of glacial activity.

dripstone  Calcium carbonate deposited from solution as water enters a cave through the zone of aeration. Forms stalactites, stalagmites and other cave deposits.

drumlin Streamlined hill, largely of till, with blunt end pointing into direction from which ice moved. Occur in clusters called drumlin fields.

dry farming Farming without irrigation in drylands.

drylands A general term for semiarid and desert lands.

ductile Structural behavior in which a material deforms permanently without fracturing.

dust bowl An area subject to dust storms, especially south central United States.

dust devil A small, dust-bearing whirlwind.

dust storm Large volume of dust-sized particles lifted high into the atmosphere.

Earth system System involving continuous interaction of the solid Earth, the atmosphere, the oceans and living things.

earthflow A form of slow, but perceptible, mass movement, with high content of water and rock debris. Lateral boundaries are well-defined and the terminus is lobed. With increasing moisture content grades into a mudflow.

eccentricity of the Earth's orbit A measure of the circularity of the Earth's orbit. It varies in cycles of about 100,000 and 400,000 years.

elastic Non-permanent structural deformation during which the amount of deformation (strain) is proportional to the stress.

elastic rebound The statement that movement along a fault is the result of an abrupt release of a progressively increasing elastic strain between the rocks on either side of the fault.

elasticity The tendency for a body to return to its original shape and size when a stress is removed.

electron A fundamental unit of matter, negatively charged and disposed in a cloud surrounding the nucleus of an atom.

electron capture Nuclear decay in which a proton in the nucleus acquires an electron from the outer cloud of the atom's electrons. This converts the proton to a neutron, reduces the number of protons in the nucleus by one and atomic number of the original element by one. Atomic mass number remains constant because the total number of protons and neutrons is unchanged.

electron shell A characteristic energy level with which an electron is associated. Electrons occupy discrete shells within the cloud surrounding an atom's nucleus. These may be thought of, loosely, as if they represented orbits at distinct heights above the nucleus.

element see chemical element

end moraine see terminal moraine.

eon The primary division of geologic time which are, from oldest to youngest, the Hadean, Archean, Proterozoic, and Phanerozoic eons.

epicenter The point on the Earth's surface that is directly above the focus of an earthquake.

epoch A division of geologic time next shorter than a period. Example: the Pleistocene epoch is in the Quaternary period.

equilibrium line On a glacier the line separating the zone of accumulation from the zone of ablation .

era A division of geologic time next smaller than the eon and larger than a period. Example: The Paleozoic era is in the Phanerozoic eon and includes, among others, the Devonian period.

erratic  A stone or boulder, glacially transported from place of origin and left in an area of different bedrock composition.

esker A winding ridge of stratified drift . Forms in a glacial tunnel and, when ice melts, stands as ridge up to 15 m high and kilometers in length.

ETP curve see Milankovitch curve .

eustatic change in sea level A worldwide change in sea level, such as caused by melting glaciers.

eutrophication The process of aging of lakes by the addition of nutrients.

evaporite A mineral or rock deposited directly from a solution (commonly seawater) during evaporation. For example, gypsum and halite are evaporite minerals.

exfoliation The process by which concentric scales, plates, or shells of rock are stripped or spall from the bare surface of a large rock mass.

exfoliation dome A large dome-shaped form that develops in homogeneous crystalline rocks as the result of exfoliation.

exotic river A river that is able to maintain its flow through a desert because of water received from outside the desert.

extrusive  Pertaining to igneous rocks or features formed from lava released on the Earth's surface.

facies see metamorphic facies , sedimentary facies

failed rift A rift emanating from a plate triple junction along which minimal divergence has taken place.

fall When applied to mass movement of material refers to free fall of material moving without contact with the surface.

fault The surface of rock rupture along which there has been differential movement of the rock on either side.

fault gouge Soft, uncemented, pulverized clay-like material found along some faults.

ferromagnesian Containing iron and magnesium, applied to the mafic minerals. Example: olivine.

fetch Distance over which wave-forming winds blow.

field capacity see specific retention.

fiery cloud see nuée ardente .

fjord Glaciated valleys now flooded by the sea.

firn (névé) Granular ice formed by the recrystallization of snow. Intermediate between snow and glacier ice.

fission The spontaneous or induced splitting, by particle collision, of a heavy atomic nucleus into a pair of fragments plus some neutrons. Controlled induced fission can be used as a source of nuclear power.

fission track dating Dating of minerals by fission tracks, damage tracks left in a mineral by spontaneous alpha emissions.

fissure eruption An eruption of lava that takes place from a fracture, usually without producing a cone.

flash flood A flood that rises and falls very rapidly.

flashy stream A stream with a high, short flood peak and short lag time.

flint A variety of chert , often black because of included organic matter.

flood Peak flow that tops the banks of a stream channel.

flood recurrence interval The number of years of record plus 1 divided by the rank of each maximum annual flood.

floodplain Area bordering a stream over which water spreads when the stream tops its channel banks.

flow When applied to mass movement, refers to a chaotic movement of material in continuous contact with the ground surface, commonly involving a moderate to high amount of water.

flow folding A fold formed in relatively fluid rocks that have flowed toward a synclinal trough.

flowstone  General term for deposits formed by dripping and flowing water on walls and floors of caves.

fluid inclusion  A tiny cavity in a crystal, commonly 1 to 100 microns in diameter, containing liquid and/or gas. Formed by the entrapment of fluid during the growth or subsequent deformation of the crystal.

focus The point within the Earth which is the center of an earthquake, at which strain energy is first released and converted to elastic wave energy.

foliation  A planar structure that develops in metamorphic rocks as a result of directed pressure .

fold and thrust mountains Mountains, characterized by extensive folding and thrust faulting, that form at convergent plate boundaries on continents.

foot wall block The body of rock that lies below an inclined fault plane. compare hanging wall block

formation water The water, held in pore volume in sedimentary rocks, that has persisted with little change in composition since it was buried with the sediment.

foreset bed Inclined layers of sediment deposited on the advancing margin of a growing delta or along the slip face of a sand dune.

foreshock A minor tremor that precedes an earthquake. An increase in seismicity may signal that a major release of strain energy is about to occur.

foreshore Lies between low and high tide marks.

fossil Evidence in rock of the presence of past life, such as a dinosaur bone, an ancient clam shell, or the footprint of a long-extinct animal.

fossil fuel A hydrocarbon (coal or petroleum) that can be extracted from the Earth for use as a fuel. Fossil fuels are non-renewable energy sources.

fractional crystallization A sequence of crystallization from magma in which the early-formed crystals are prevented from reacting with the remaining magma, resulting in a magma with an evolving chemical composition.

fragipan A dense layer of soil, containing silt and sand but no organic matter and little clay, whose extreme hardness and impermeability are due primarily to compaction. compare caliche , claypan, hardpan.

free oscillation A vibration of a body such as a bell or the Earth that continues without further influence after an initial event.

fringing reef  A coral reef attached directly to the mainland.

frost wedging A type of disintegration in which jointed rock is forced apart by the expansion of water as it freezes in fractures.

fusion The combination of two light nuclei to form a heavier nucleus, with the accompanying release of energy. This is the source of energy in a hydrogen bomb. If it could be controlled, it could serve as an alternative to fission in nuclear power generation.

gabbro A coarse-grained igneous rock, chemically equivalent to a basalt .

gardening The constant and slow churning of the lunar regolith as the result of meteorite impacts.

geanticline An anticlinal structure presumed to form in the context of geosynclinal evolution. Not in current use since the development of plate tectonic theory.

geode Roughly spherical, hollow or partially hollow accumulation of mineral matter. A few centimeters to nearly 0.5 m in diameter. Outer layer of chalcedony lined with crystals that project toward the hollow center. Crystals, often perfectly formed, usually quartz although calcite and dolomite and - more rarely - other minerals. Most commonly occur in limestone, and less often in shale.

geologic column The arrangement of rock units in the proper chronological order from youngest to oldest.

geologic time scale The chronological sequence of units of Earth time.


As the name implies, geophysics involves the application of physical theories and
measurements to discover the properties of the earth. The disciplines dates to antiquity, mainly
as a scientific approach to earthquake prediction (A problem still unsolved), but major progress
began in the late 1500's with initial work in such areas as magnetism and gravity. Tremendous
improvements in instrumentation in the early years of the20th century generated rapid progress
in geophysics and ultimately led, in the 1960's, to the theory of plate tectonics.

     Plate tectonics, the study of the interior structure of the earth, and such related areas as global
and regional processes are known collectively as solid earth geophysics. The subdiscipline
known as exploration geophysics involves the use of geophysical theory and instrumentation to
locate petroleum and other mineral sources. Unlike solid earth geophysics, exploration
geophysics generally concentrates on finding lateral heterogeneities in a relatively small part of
the earth's crust.

     Geophysics has increased dramatically man's ability to exploit natural resources. Human
senses cannot quantify, or even detect (e.g., magnetism) many physical phenomena. Humans
cannot detect variations in he earth's gravitation field of one part per million, but modern gravity
meters can (in fact, to 0.02 parts per million or better). Seismology, the primary method of
petroleum exploration, requires exact timing and recording of very low-amplitude vibrators,
vibrations (or shaking) that is far below that which a human would sense.

     The following definitions are from Robert E. Sheriff's "Encyclopedic Dictionary of Exploration


      1.The study of the earth by quantitative physical method, especially by seismic reflection
        and refraction, gravity, magnetic, electrical, electromagnetic, and radioactivity methods. 
      2.The application of physical principles to studies of the earth. Includes the branches of (a)
        seismology (earthquakes and elastic waves); (b) geothermometry (heating of the earth,
        heat flow, volcanology, and hot springs); (c) hydrology (ground and surface water,
        sometimes including glaciology); (d) physical oceanography; (e) meteorology; (f) gravity
        and geodesy (the earth's gravitational field and the size and form of the earth); (g)
        atmospheric electricity and terrestrial magnetism (including ionosphere, Van Allen belts,
        telluric currents, etc.); (h) tectonophysics (geological processes in the earth); and (i)
        exploration and engineering geophysics. Geochronology (the dating of earth history) and
        geocosmogony (the origin of the earth) are sometimes added to the foregoing list. 
      3.Often refers to solid-earth geophysics only, thus excluding (c), (d), (e), and portions of
        other subjects from the above list. 
      4.Exploration geophysics is the use of seismic, gravity, magnetic, electrical,
        electromagnetic, etc., methods in the search for oil, gas, minerals, water, etc., with the
        objective of economic exploitation. 


      1.One who studies the physical properties of the earth or applies physical measurements to
        geologic problems; a specialist in geophysics.

geosyncline A downwarping of the Earth's crust, either elongate or basin-like, measured in scores of kilometers, in which sedimentary and volcanic rocks accumulate to thicknesses of thousands of meters. Not in current use since the development of plate tectonic theory.

geothermal energy Heat extracted from the Earth for use as an power source.

geothermal gradient The rate at which temperature increases with depth below the surface.

geyser A type of thermal spring which ejects water intermittently with considerable force.

glaciation The formation, advance and retreat of glaciers and the results of these activities.

glacier A mass of ice, formed by the recrystallization of snow, that flows forward, or has flowed at some time in the past.

glacier ice Ice with interlocking crystals that makes up the bulk of a glacier.

glass An inorganic solid in which there is no crystalline structure .

glassy A texture of extrusive igneous rocks that develops as the result of rapid cooling, so that crystallization is inhibited.

global warming The prediction that climate will warm as a result of the addition to the atmosphere of humanly produced greenhouse gases.

gneiss A coarse, foliated metamorphic rock in which bands of granular minerals (commonly quartz and feldspars) alternate with bands of flaky or elongate minerals (e.g., micas, pyroxenes). Generally less than 50% of the minerals are aligned in a parallel orientation.

gneissosity The style of foliation typical of gneiss.

Gondwana The southern portion of the late Paleozoic supercontinent known as Pangea. It means, literally "Land of the Gonds" (a people of the Indian subcontinent). The variant Gondwanaland found in some books, therefore, is a tautology.

gouge see fault gouge

graben see rift

graded bedding Type of bedding sedimentary deposits in which individual beds become finer from bottom to top.

gradient Slope of a stream bed or hillside. The vertical distance of descent over horizontal distance of slope.

granite Light colored, coarse grained, intrusive igneous rock characterized by the minerals orthoclase and quartz with lesser amounts of plagioclase feldspar and iron-magnesium minerals. Underlies large sections of the continents.

granitic belt A region of granitic rock, one of two characteristic regions within cratons .

granitization A metamorphic process by which solid rock is converted into granite by the addition or removal of material, without passing through a magmatic stage. compare metasomatism .

gravitational heating Planetary heating caused by the conversion of potential energy into heat. Associated with the iron catastrophe .

gravitational moisture Water in the zone of aeration that is moving down toward the zone of saturation.

graywacke (lithic sandstone) A variety of sandstone characterized by angular-shaped grains of quartz and feldspar, and small fragments of dark rock all set in a matrix of finer particles.

greenhouse gases Gases (primarily water and carbon dioxide, but also a variety of sulfur and nitrogen compounds and gaseous hydrocarbons) that trap the Sun's heat in the atmosphere.

greenstone An altered or metamorphosed mafic igneous rock that owes its dark color to the presence of chlorite, epidote, or amphiboles.

greenstone belt A region of greenstones, one of two characteristic regions within cratons .

groin A wall built out from the shore, usually at perpendicular to it to trap sand carried by longshore currents .

groove A broad, deep, generally straight furrow carved in bed rock by the abrasive action of debris embedded in a moving glacier. Larger and deeper than a glacial striation.

ground moraine Till deposited from main body of glacier during ablation.

ground water table see water table .

ground water Water beneath the Earth's surface.

guyot see seamount

habit A general term for the outward appearance of a mineral, defined by the relative sizes and arrangement of characteristic crystal faces.

Hadean The oldest eon in Earth history, extending from the origin of the Earth to about 3.9 billion years ago.

half-life The amount of time that it takes for one half of an original population of atoms of a radioactive isotope to decay.

hanging valley  A valley whose mouth is high above the floor of the main valley to which it is tributary. Usually, but not always, the result of mountain glaciation.

hanging wall block The body of rock that lies above an inclined fault plane. compare foot wall block

hardness Resistance of a mineral to scratching, determined on a comparative basis by the Mohs scale .

hardpan A general term for a relatively hard layer of soil at or just below the ground surface, cemented by silica, iron oxide, calcium carbonate, or organic matter. compare caliche , claypan, fragipan.

head (hydraulic head) The level to which ground water in the zone of saturation will rise.

heat flow The amount of thermal energy leaving the Earth per cm2/sec.

heave In mass movement the upward motion of material by expansion as, for example, the heaving caused by freezing water.

hiatus A gap or interruption in the continuity of the geologic record either because the record was never formed or because it was destroyed by erosion. It represents the time interval spanned by an unconformity .

high level nuclear waste Radioactive waste from defense activities of the U. S. government and from spent fuel rods from nuclear reactors.

hinge fault A fault along which there is increasing offset or separation along the strike of the fault plane, from an initial point of no separation.

hoodoo A column or pillar of rock produced by differential weathering in a region of sporadic heavy rainfall, commonly facilitated by joints and by rock layers of varying hardness.

Hooke's Law A statement of elastic deformation, that strain is directly proportional to stress.

horn The sharp spire of rock formed as glaciers in several cirques erode into a central mountain peak.

horst compare rift

hot spot A region of high heat flow on the Earth's surface, thought to lie above a mantle plume .

humus The generally dark, more or less stable part of the organic matter in a soil, so well decomposed that the original sources cannot be identified.

hydraulic conductivity Measure of permeability in Earth materials.

hydraulic gradient The slope of the water table. Measured by the difference in elevation between two points on the slope of the water table and the distance of flow between them.

hydraulic head see head.

hydrograph Graph of variation of stream flow over time.

hydrologic system (or hydrologic cycle) The pattern of water circulation from the ocean to the atmosphere to the land and back to the ocean.

hydrolysis A decomposition reaction involving water, in which hydrogen ions (H+) or hydroxyl ions (OH-) replace other ions. The result is a new residual mineral. Example: the addition of water to orthoclase produces kaolinite and releases K+ and silica into solution.

ice sheet A broad, mound-like mass of glacier ice that usually spreads radially outward from a central zone.

ice shelf A floating ice sheet extending across water from a land-based glacier.

icecap A small ice sheet.

igneous rock A rock that has crystallized from a molten state.

inclined bedding (cross-bedding) Bedding laid down at an angle to the horizontal, as in many sand dunes.

inclined fold A fold whose axial plane is inclined from the vertical, but in which the steeper of the two limbs is not overturned. compare overturned fold.

inclusion (xenolith) A fragment of older rock caught up in an igneous rock.

index fossil A fossil that identifies and dates the strata in which it is typically found. To be most useful, an index fossil must have broad, even worldwide distribution and must be restricted to a narrow stratigraphic range.

index mineral  A mineral formed under a particular set of temperature and pressure conditions, thus characterizing a particular degree of metamorphism.

inertia The tendency of a body to resist acceleration . A moving body tends to keep moving at a constant speed in the same direction, and a stationary body tends to remain in one place, unless acted upon by an outside force.

inner core  The solid innermost part of the core with a diameter of a little over 1,200 km.

intensity  A measure of the size of an earthquake in terms of the damage it causes.

interlobate moraine Ridge formed along junction of adjacent glacier lobes.

intrusive Pertaining to igneous rocks or features formed by the emplacement of magma in pre-existing rock.

ion An atom that has an electrical charge, by virtue of having gained or lost electrons. see cation, anion

ionic radius The effective distance from the center of an ion to the edge of its electron cloud.

ionic substitution The replacement of one or more ions in a crystal structure by others of similar size and electrical charge. Example: Fe2+ is interchangeable with Mg2+ in most ferromagnesian minerals.

iron catastrophe The period in the Hadean eon during which much of the iron in outer portions of the Earth migrated toward the center of the planet, producing the core and releasing large amounts of gravitational heat.

ironpan A hardpan in which iron oxides are the primary cementing agents.

island arc A curved belt of volcanic islands lying above a subduction zone. compare continental arc.

isochemical reaction A reaction in which chemical constituents of a rock are rearranged to form a new mineral assemblage, but no material is added to or lost from the rock as a whole. Applied generally to diagenetic or metamorphic environments.

isoclinal fold A fold in which the limbs are parallel.

isograd  A line on a map joining points at which metamorphism took place under similar temperature and pressure conditions, as indicated by rocks belonging to the same metamorphic facies . Generally, the line separates two adjacent metamorphic zones, as indicated by specific .

isotope Atoms that differ in atomic mass number , but not in atomic number , are called isotopes. For example, oxygen (atomic number 8) may have an atomic mass number of 16, 17, or 18, depending on whether it has 8, 9, or 10 neutrons. It therefore has three isotopes.

isoseismal line A line on a map joining points of equal earthquake intensity.

isostasy The condition of equilibrium, comparable to floating, of units of the lithosphere above the asthenosphere .

isostatic change in sea level A sea level change due to change in load on Earth's crust.

jasper A red variety of chert , its color coming from minute particles of included hematite.

jet flow Flow in which fluid moves at high speed in jet-like surges as does water in free fall over a falls.

jetty Similar to a groin but built to keep sand out of a harbor entrance.

joint A surface of fracture in a rock, without displacement parallel to the fracture.

juvenile hydrothermal fluid A hot fluid, largely water, presumed to have been released from a magma.

kame Stratified drift deposited in depressions and cavities in stagnant ice and left as irregular, steep sided hills when the ice is melts.

kame terrace Stratified drift deposited between wasting glacier and adjacent valley wall. Stands as a terrace when glacier melts.

karst  A landscape that develops from the action of ground water in areas of easily soluble rocks. Characterized by caves, underground drainage and sinkholes.

kettle Depression in ground surface formed by the melting of a block of glacier ice buried or partially buried by drift .

komatiite An ultramafic rock with a non-cumulate texture, presumed to be extrusive.

laccolith A concordant igneous intrusion with a flat floor and a convex upper surface, usually less than 8 km across and from a few meters to a few hundred meters thick at its thickest point.

lag time The delay in the response of stream flow between precipitation and flood peak.

lahar A mudflow composed chiefly of pyroclastic material on the flanks of a volcano.

laminar flow Fluid flow in which flow lines are distinct, and parallel and do not mix. compare turbulent flow .

lateral continuity The extent of a rock unit over a considerable but definite area.

lateral moraine Moraine formed by valley glaciers along valley sides.

laterite A highly weathered red soil rich in iron and aluminum oxides. Typically formed in a tropical to temperate climate where intense chemical weathering is common.

Laurasia The northern portion of the late Paleozoic supercontinent called Pangea.

lava   Molten rock that flows at the Earth's surface.

lava dome A steep-sided rounded extrusion of highly viscous lava squeezed out from a volcano and forming a dome-shaped or bulbous mass above and around the volcanic vent. The structure generally develops inside a volcanic crater.

lava flood (plateau basalt) A term applied to large areas of basaltic lava presumably extruded from fissures.

lava lake A lake of lava, usually basaltic, in a volcanic caldera.

layered complex An intrusive igneous body in which there are layers of varying mineral content.

levees Banks of sand and silt along stream bank built by deposition in small increments during successive floods.

limb The portions of a fold that are away from the hinge; the "sides" of the fold.

limestone A sedimentary rock composed mostly of the mineral calcite, CaCO3.

linear dune Long, straight dune with slip faces on each side.

lineation A general term applying to any linear feature in a metamorphic rock.

liquefaction The transformation of a soil from a solid to a liquid state as the result of increased pore pressure.

lithic sandstone see graywacke.

lithification The process by which an unconsolidated deposit of sediments is converted in to solid rock. Compaction, cementation and recrystallization are involved.

lithophile Said of an element that has a greater chemical affinity for silicate rocks than for sulfides or for a metallic state. Example: Aluminum.

lithosphere  The rigid outer shell of the Earth. It includes the crust and uppermost mantle and is on the order of 100 km in thickness.

lithostatic stress The confining (non-directed) pressure imposed by the weight of overlying rock.

littoral current see longshore current .

load Of a stream, the amount that it carries at any one time.

loess Deposits of wind-borne dust.

longshore current (littoral current) A current that flows parallel to the shore just inside the surf zone. Also called the littoral current.

longshore drift The general movement of sediment parallel to the shoreline. Waves generally carry sediment up the shore face at an angle to the shoreline, but carry it straight out again, resulting in a net longshore displacement.

Love wave A seismic surface wave that has a horizontal (side-to-side) component but no vertical component.

low level nuclear waste (TRU) Comes largely from national defense utilities and includes contaminated lab coasts, gloves, and laboratory equipment.

low velocity zone The seismic region within the upper mantle that corresponds to the asthenosphere .

luster The manner in which light reflects from the surface of a mineral, described by its quality and intensity.

mafic Referring to a generally dark-colored igneous rock with significant amounts of one or more ferromagnesian minerals, or to a magma with significant amounts of iron and magnesium.

magma Molten rock, containing dissolved gases and suspended solid particles. At the Earth's surface, magma is known as lava .

magma ocean A global-scale ocean of magma, according to some calculations several hundred kilometers deep, thought to have existed during the final stages of accretion as the Earth was forming.

magnetic anomaly The amount by which a measurement of the local magnetic field intensity exceeds or falls below the intensity of the global magnetic field.

magnetic chron Time during which magnetic polarity is dominantly normal or dominantly reversed.

magnetic declination Angle of divergence between true north and magnetic north. Measured in degrees east or west of true, or geographic north.

magnetic equator Lies half way between the north and south magnetic poles.

magnetic inclination The angle of dip of the compass needle as it varies from horizontal at the magnetic equator to vertical at the magnetic poles.

magnetic polarity The direction, north (normal) or south (reversed), that a magnetic compass needle points.

magnetic polarity time scale A chronology based on the shifting polarity of the Earth's magnetic field.

magnetic pole The point on the Earth's surface where a magnetic needle points vertically downward (north magnetic pole) or vertically upward (south magnetic pole).

magnetic stratigraphy A stratigraphic sequence based on the magnetic polarity of the rocks.

magnetic subchron A period during a magnetic chron when the magnetic polarity is the opposite from that of the magnetic chron.

magnetic polarity The direction, north (normal) or south (reversed) that a magnetic needle points.

magnitude A measure of the strength of an earthquake based on the amount of movement recorded by a seismograph . compare Richter scale.

mantle  That portion of the Earth below the crust and reaching to about 2,780 km, where a transition zone of about 100 km thickness separates it from the core.

mantle plume A hypothetical column of hot, partially molten material that rises from an indeterminate depth in the mantle and is thought by some geologists to provide a driving force for plate movement. compare hot spot .

marble A metamorphic rock composed largely of calcite. The metamorphic equivalent of limestone.

margin The tectonic region that lies at the edge of a continent, whether it coincides with a plate boundary or not.

mass movement The downslope movement of material under the influence of gravity.

maze cave Caves in which passageways have interconnecting loops that form a maze-like pattern.

meander A sharp bend, loop or turn in a stream's course. When abandoned, called a meander scar or an oxbow .

medial moraine Formed by the merging of lateral moraines as two valley glaciers join.

mélange (clastic wedge) A mappable body of rock characterized by blocks and fragments of all sizes, embedded in a sheared matrix. A tectonic mélange commonly forms in the upper portions of a subduction zone, where crustal rock is crushed and sheared.

mesosphere A zone in the Earth between 400 and 670 km below the surface separating the upper mantle from the lower mantle.

Mesozoic An era of time during the Phanerozoic eon lasting from 245 million years ago to 66.4 million ago.

metal porphyry deposit  A mineral deposit genetically related to a pluton of porphyritic rock, commonly granodiorite. Scarce metals are typically enriched by the passage of hydrothermal fluids through rocks surrounding the intrusion, with the result that a metal-rich halo forms there.

metamorphic facies  A set of metamorphic mineral assemblages, repeatedly associated in space and time, such that there is a constant and therefore predictable relationship between mineral composition and chemical composition. That relationship is a consequence of conditions of temperature and pressure under which the assemblages are stable.

metamorphic rock A rock changed from its original form and/or composition by heat, pressure, or chemically active fluids, or some combination of them.

metamorphic zone  A mappable region in which rocks have been metamorphosed to the same degree, as evidenced by the similarity of mineral assemblages in them.

metamorphism The processes of recrystallization, textural and mineralogical change that take place in the solid state under conditions beyond those normally encountered during diagenesis.

metasomatism  The metamorphic processes that occur as a result of the passage of chemically active fluids through a rock, adding to or removing constituents during metamorphism.

microplate see terrane

migmatite A composite rock composed of igneous and metamorphic materials, the result of partial melting at the upper limit of metamorphism.

Milankovitch curve (ETP curve ) A graph representing the amount of solar radiation received at the Earth's surface at a particular latitude and time and based on the variations in the Earth's orbital motions.

mineral  A naturally occurring inorganic solid that has a well-defined chemical composition and in which atoms are arranged in an ordered fashion.

mineral deposit Any natural concentration of a valuable material in the Earth's crust, whether that material can be extracted profitably or not.

Modified Mercalli Scale A commonly used scale of earthquake intensity.

Mohorovicic discontinuity (Moho) The sharp seismic velocity discontinuity that separates the crust and the mantle.

Mohs scale The ten-point scale of mineral hardness , keyed arbitrarily to the minerals talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum, and diamond.

molecule The smallest unit of matter that has the chemical and physical properties of a particular chemical compound.

momentum transfer In a rock slide the forward transfer of energy by the collision of one block of rock with the next block forward. The process makes possible progressively more rapid movement of material in downslope positions.

monocline A simple fold, described as a local steepening in strata with an otherwise uniform dip.

moraine Landform made largely of till.

mountain glacier see valley glacier .

mud cracks Cracks, generally polygonal, caused by the shrinking of a deposit of clay or silt under surface conditions.

mudflow Form of mass movement similar to a debris flow but containing less rock material.

mudstone A fine-grained detrital sedimentary rock made up of clay- and silt-sized particles.

mylonite A chert-like rock without cleavage but with a banded or streaky structure produced by extreme shearing of rocks that have been pulverized and rolled during intense dynamic metamorphism.

nappe A sheet of rock that has moved over a large horizontal distance by thrust faulting, recumbent folding, or both, so that it lies on rocks of markedly different age or lithologic character.

neck cutoff Occurs as a river cuts through the narrow neck of a meander. Sometimes called simply a "cutoff."

neutron A particle in the nucleus of an atom, which is without electrical charge and with approximately the same mass as a proton.

névé see firn.

nivation Erosion beneath and around edges of a snow bank. Results can foreshadow a cirque.

nodule A small, irregular, knobby-surfaced rock body that differs in composition from the rock that encloses it. Formed by the replacement of the original mineral matter. Quartz in the form of flint or chert is the most common component. Most common in limestone and dolostone.

nonclastic A term applied to sedimentary rocks that are not composed of fragments of pre-existing rocks or minerals. The term "crystalline" is more commonly used.

nonconformity An unconformity that separates profoundly different rock types, such as sedimentary rocks from metamorphic rocks.

normal fault A dip-slip fault on which the hanging wall block is offset downward relative to the foot wall block . compare reverse fault .

normal polarity Time when the compass needle points to the magnetic north pole.

north magnetic pole The point on the Earth where the north-seeking end of a magnetic needle, free to swing in space, points directly down.

nuclear power Power generated by controlled fission or (potentially) fusion reactions, the heat from which is used to produce steam and drive turbines.

nucleus (atomic) The center of an atom, containing both protons and (except for 1H) neutrons.

nuée ardente (fiery cloud) A dense, hot (sometimes incandescent) cloud of volcanic ash and gas produced in a Pelean eruption.

oblique-slip fault A fault with both dip-slip and strike-slip components of movement.

obliquity of the Earth's ecliptic Tilt of the Earth's rotational axis in relation to the plane in which the Earth circles to Sun. Cycles from about 21.5o through 24.5o and back to 21.5o every 41,000 years.

oceanic crust  That part of the crust underlying the ocean basins. Composed of basalt and having a thickness of about 5 km.

oil shale A mudrock that will yield liquid or gaseous hydrocarbons upon distillation.

oolite Spheroidal grains of sand size, usually composed of calcite and thought to have formed by inorganic precipitation.

open pit mining Surficial mining, in which the valuable rock is exposed by removal of overlying rock or soil.

ophiolite A suite of mafic and ultramafic rocks and associated marine cherts and their metamorphic equivalents.

ore The naturally occurring material from which a mineral or minerals of economic value can be extracted at a profit.

ore deposit A continuous well-defined mass of material of sufficient ore content to make extraction economically feasible. compare mineral deposit .

original horizontality Refers to the condition of beds or strata as being horizontal or nearly horizontal when first formed.

orogen Linear to arcuate in plan, intensely deformed crustal belt associated with mountain building. compare craton .

orogeny The process of mountain building.

oscillation ripple mark  A symmetric ripple mark formed by waves, which move water back and forth. compare current ripple mark .

outer core  The outermost part of the core. It is liquid, about 1,700 km thick, and separated from the inner, solid core by a transition zone about 565 km thick.

outwash Beds of sand and gravel laid down by glacial melt water

outwash plain A plain underlain by outwash.

overbank deposits Sediments deposited from flood water on the flood plain.

overturned fold An inclined fold in which one limb has been tilted beyond the vertical, so that the stratigraphic sequence within it is reversed. compare inclined fold.

oxbow An abandoned meander .

oxbow lake A lake in an abandoned meander.

oxidation The decomposition process by which iron or other metallic elements in a rock combine with oxygen to form residual oxide minerals.

ozone hole Decrease of ozone in the stratosphere.

P- wave (primary wave, compressional wave) A seismic body wave that involves particle motion, alternating compression and expansion, in the direction of wave propagation. It is the fastest seismic wave. compare S-wave .

pahoehoe  A Hawaiian term for a basaltic lava flow with a smooth, or ropy surface. compare aa .

paleomagnetism Study of the Earth's past magnetism as it is recorded in the rocks.

paleosol A buried soil horizon of the geologic past.

Paleozoic An era of geologic time lasting from 570 to 245 million years ago.

Pangea A supercontinent that existed from about 300 to 200 million years ago, and included most of the continental crust of the Earth.

parabolic dune A sand dune that is parabolic in plan with slip face convex downwind.

parent A radioactive element whose decay produces stable daughter elements.

partial melting The igneous process in which a rock begins to melt at the lower end of its melting interval, yielding a magma with a chemical composition different from the bulk composition of the parent rock.

pascal A unit of pressure, equal to 1/100,000 of a bar .

pedalfer A generic term used to describe the soils typically formed in a humid region. Characteristically have an accumulation of iron and aluminum oxides and hydroxides.

pedocal A generic term used to describe the soils typically found in an arid or semiarid region. Characteristically have an accumulation of carbonates, particularly calcite.

pegmatite An extremely coarse-grained igneous rock with interlocking crystals, usually with a bulk chemical composition similar to granite but commonly containing rare minerals enriched in lithium, boron, fluorine, niobium, and other scarce metals. Pegmatites are also the source for many gem-quality precious and semiprecious stones.

pegmatitic Having the texture of a pegmatite.

pelagic ooze A deep ocean sediment consisting of at least 30% skeletal remains of calcareous or siliceous microorganisms, the rest being clay minerals.

Peléan eruption A type of volcanic eruption characterized by nuées ardentes and the development of lava domes .

peneplain Low gently, rolling landscapes produced by long-continued erosion.

perched water table A water table that develops at a higher elevation than the main water table. <peridotite An ultramafic igneous rock, the major constituent of the mantle.

periglacial Refers to conditions in a near glacial climate.

period In the geologic time scale a unit of time less than an era and greater than an epoch. Example: The Tertiary period was the earliest period in the Cenozoic era and included, among others, the Eocene epoch.

permafrost table The depth in a permafrost region at which the maximum temperature reaches 0o C.

permafrost Soil conditions prevailing in area whose mean annual temperature is 0o C.

permeability The capacity of material to transmit water or other fluids.

petroleum A general term including both oil and natural gas.

phaneritic A textural term meaning "coarse-grained" that applies to igneous rocks.

Phanerozoic the most recent eon of geologic time beginning 570 million years ago and continuing to the present.

phenocryst Any relatively large, conspicuous crystal in a porphyritic igneous rock. compare porphyroblast.

phyllite A metamorphosed mudstone with a silky sheen, more coarse-grained than a slate and less coarse-grained than a schist.

piedmont glacier A glacier that spreads out at the foot of mountains, formed by the coalescence of two or more valley glaciers.

pillow A structure observed in certain igneous rocks extruded into water, characterized by discontinuous, close-fitting, pillow-shaped masses, commonly 30 to 60 cm across.

pipe A vertical conduit through the Earth's crust below a volcano, through which magma has passed.

pirate stream A stream that captures the headwaters of another stream.

placer A surficial mineral deposit formed by mechanical concentration of valuable minerals from weathered debris, usually through the action of stream currents or of waves.

plate A rigid segment of the Earth's lithosphere that moves horizontally and adjoins other plates along zones of seismic activity. Plates may include portions of both continents and ocean basins.

plate boundaries The zones of seismic activity long which plates are in contact. These may coincide with continental margins , but usually do not. Movement between plates is predominately horizontal, and may be divergent, or convergent, or side-by-side.

plate tectonics  A theory of global tectonics according to which the lithosphere is divided into mobile plates. The entire lithosphere is in motion, not simply those segments composed of continental material. compare continental drift

plate triple junction A point from which three rifts emanate at roughly 120 degree angles. Example: the Afar triangle in East Africa.

playa A broad flat desert basin, often containing an ephemeral playa lake.

plucking (quarrying) A process of erosion in which the glacier pulls loose pieces of bed rock.

plume The movement of water along flow lines from a point source of ground water pollution toward its eventual emergence at the surface.

plunging fold A fold in which the axis is inclined at an angle from the horizontal.

pluton An igneous intrusion.

pluvial lake A lake formed during a pluvial period.

pluvial period Time when a dryland area had greater effective moisture than at present.

pocket beach Small, narrow beach, usually crescentic, at head of a bay or small inlet.

point bar Accumulations of sand and gravel deposited in slack water on inside of a winding or meandering river.

polar deserts Deserts in which most moisture is locked up in ground ice and unavailable as liquid water.

polar glacier A glacier whose temperature throughout is always below freezing.

polish A smooth, polished surface imparted to some rock types by glacier abrasion.

polymetamorphism A series of events in which two or more metamorphic episodes have left their imprint on the same rocks.

polymorphism The circumstance in which two minerals with different crystalline structures have identical chemical compositions. Example: Diamond and graphite.

porosity The percentage of material occupied by pore space.

porphyritic A texture of an igneous rock in which large crystals (phenocrysts) are set in a matrix of relatively finer-grained crystals or of glass.

porphyroblast  A large crystal of a mineral such as garnet or staurolite set in a matrix of much finer-grained minerals in a metamorphic rock. compare phenocryst.

potentiometric surface The level to which water will rise in an artesian system when its confining aquitard is pierced.

pothole A hole or basin cut into bedrock of a stream by the abrasive action of pebbles and sand swirled by turbulent stream flow.

Precambrian An informal term to include all geologic time from the beginning of the Earth to the beginning of the Cambrian period 570 million years ago.

precession of the equinox The wobble of the Earth as it spins changes the direction in which its axis of rotation points. One wobble takes about 23,000 years.

pressure melting The phenomenon causing increased melting of ice by increase of pressure.

Principle of faunal and floral succession Groups of animals and plants have succeeded one another in a definite and discernible order.

prograde A succession of metamorphic conditions, each of which is at a higher temperature and/or pressure than the preceding one.

Proterozoic The geologic eon lying between the Archean and Phanerozoic eons, beginning about 2.5 billion years ago and ending about 0.57 billion years ago.

proton A fundamental particle of matter. Provides a positive charge in the nucleus of an atom.

pyroclastic  Pertaining to clastic material formed by volcanic explosion or aerial expulsion from a volcanic vent.

quarrying 1. The process by which building stone, usually in blocks or sheets, is extracted from the Earth.. 2. see plucking

quartz arenite A sandstone in which the sand grains are predominantly quartz.

quartzite A metamorphic rock consisting largely of interlocking quartz grains; the metamorphic equivalent of a sandstone or chert.

radial drainage A pattern in which streams radiate outward from a high central zone.

radiation Heat transport without the intervention of matter, as in the transport of heat from the Sun to the Earth. compare conduction , convection .

radioactivity The spontaneous decay of the nucleus of an element. It involves the change in the number of protons in the nucleus and therefore creates an atom of a new element.

radiocarbon 14C derived from 14N as cosmic ray bombardment adds a neutron to its nucleus and the nucleus emits a proton. Radiocarbon decays back to 14N by beta decay . Half life is 5730 ± 30 years.

rain shadow deserts Deserts formed by blocking moisture-bearing winds with mountain barriers.

ramp The planar surface sloping seaward from the foot of the shore face.

rapids Turbulent stream water flow down a steep gradient, but not as steep as in a waterfall.

Rayleigh wave A type of seismic surface wave in which particles follow a backward elliptical orbit in a vertical plane.

reaction rim A peripheral zone around a mineral grain, composed of another mineral.

recessional moraine Ridges of glacial till marking halt and slight readvance of glacier during its general retreat.

rectangular drainage A pattern in which a stream and its tributaries follow courses marked by nearly right angle bends.

recumbent fold A fold in which the axial plane is horizontal.

refraction 1. Bending of waves or rays of energy, e.g. seismic waves. 2. As applies to the near shore environment, the bending of wave crests as they approach the shore.

regional metamorphism  Metamorphism affecting an extensive region, associated with orogeny .

regolith A layer of unconsolidated fragmental rock material.

rejuvenation Renewed stream erosion, generally as the result of uplift. Generates features of youthful topography on a landscape that was previously worn down to a base level.

relative time Dating of rocks and geologic events by their positions in chronological order without reference to number of years before the present.

remanent magnetism Magnetism acquired by a rock as some time in the past.

reserves That portion of the resources for a valuable mineral commodity that can be extracted from the Earth at a profit today.

reservoir rock Any porous and permeable rock that yields oil or natural gas.

residual (resistant) mineral A mineral that persists in soil after weathering, either because it was resistant to weathering or because it was formed during the weathering process.

residual soil A soil presumed to have developed in place as the product of decomposition and disintegration of bedrock .

resources The reserves of a valuable mineral commodity plus all other mineral deposits that may eventually become available, even those that are presumed to exist but have not yet been discovered and those that are not economically or technologically exploitable at the moment. The total mineral endowment ultimately available for extraction.

retrograde A succession of metamorphic conditions, each one of which is at a lower temperature and/or pressure than the preceding one.

reverse fault A dip-slip fault on which the hanging wall block is offset upward relative to the foot wall block . compare normal fault .

reversed polarity Time when a magnetic needle points to the south pole.

rhyolite A fine-grained silica-rich igneous rock, the extrusive equivalent of granite.

Richter scale A commonly used measure of earthquake magnitude , based on a logarithmic scale. Each integral step on the scale represents a tenfold increase in the extent of ground shaking, as recorded on a seismograph.

rift (graben) A valley caused by extension of the Earth's crust. Its floor forms as a portion of the crust moves downward along normal faults .

rip current Carries excess water in the longshore current out through the surf zone where it dissipates.

ripple marks of oscillation  Symmetrical ripple marks formed by oscillating movement of water such as may be found along the coast just outside the surf zone.

ripple marks  Small waves produced by wind or water moving across deposits of sand or silt.

rock An aggregate of one or more minerals in varying proportions.

rock avalanche see rockslide.

rock cleavage see cleavage

rock cycle The concept of a sequence of events involving the formation, alteration, destruction and reformation of rocks as a result of geologic processes and which is recurrent, returning to a starting point. It represents a closed system. compare rock system.

rock flour Finely divided rock material ground by glacial action and fed by streams fed by melting glaciers.

rock glacier A mass of ice-cemented rock rubble found on slopes of some high mountains. Movement is slow, averaging 30 to 40 cm/yr.

rock record The history recorded in rocks.

rockslide (rock avalanche) A slide involving a downward and usually sudden movement of newly detached segments of bedrock sliding or slipping over an inclined surface of weakness such as a bedding plane, fault plane, or joint surface.

rock system The concept of a sequence of events involving the formation, alteration, destruction and reformation of rocks as a result of geologic processes. Unlike the rock cycle it is an open system and does not return to a starting point. compare rock cycle

rock varnish A thin, shiny veneer of clay minerals and iron and manganese oxides deposited on some rocks in a desert environment.

rock waste Angular fragments of rock. Forms a talus if abundant enough.

rockfall The sudden fall of one or more large pieces of a rock from a cliff.

roundness The degree to which a sedimentary particle's corners and edges are rounded.

runoff The precipitation that runs directly off the surface to stream or body of standing water.

S wave (secondary wave , shear wave) A seismic body wave that involves particle motion from side to side, perpendicular to the direction of wave propagation. S-waves are slower than P-waves and cannot travel through a liquid. compare P-wave .

salinization. A process by which salts accumulate in soil

salt-water invasion Displacement of fresh surface or ground water by the advance of salt water.

saltation A process of sediment transport in which a particle jumps from one point to another.

sand dune An accumulation of wind driven sand into a distinctive shape.

sand sea A large area completely, or nearly completely, covered with sand dunes.

sandstone A clastic sedimentary rock in which the particles are dominantly of sand size, from 0.062 mm to 2 mm in diameter.

sandstorm A blanket of wind-driven sand with an upper surface about a meter above ground level.

sanitary land fill An artificial hill formed by the refuse of present-day civilization.

schist A strongly foliated, coarsely crystalline metamorphic rock, produced during regional metamorphism, that can readily be split into slabs or flakes because more than 50% of its mineral grains are parallel to each other.

schistosity The foliation in a schist, due largely to the parallel orientation of micas.

seafloor spreading  Process by which ocean floors spread laterally from crests of main ocean ridges. As material moves laterally from the ridge, new material replaces it along the ridge crest by welling upward from the mantle. compare continental drift , plate tectonics

seamount (guyot)  A volcanic mountain on the seafloor. If flat-topped, it is a guyot.

seawall A wall at the shore and parallel to it for protection against wave erosion

sedimentary facies An accumulation of deposits that exhibits specific characteristics and grades laterally into other sedimentary accumulations that were formed at the same time but exhibit different characteristics.

sedimentary rock Rock formed from the accumulation of sediment, which may consist of fragments and mineral grains of varying sizes from pre-existing rocks, remains or products of animals and plants, the products of chemical action, or mixtures of these.

seismic gap A segment of an active fault zone that has not experienced a major earthquake during a time period when most other segments of the zone have. Generally regarded as having a higher potential for future earthquakes.

seismic sea wave (tsunami) A sea wave produced by any large-scale, short duration disturbance on the seafloor, commonly a shallow submarine earthquake but possibly also a submarine slide or volcanic eruption.

seismic tomography A technique for three-dimensional imaging of the Earth's interior by using a computer to compare the seismic records from a large number of stations. Similar in concept to a CAT scan used for medical purposes.

seismograph  An instrument that detects, magnifies, and records vibrations of the Earth, especially earthquakes.

seismology The study of earthquakes, and of the structure of the Earth by both natural and artificially generated seismic waves.

seismoscope An instrument that merely indicates the occurrence of an earthquake.

self-exciting dynamo In reference to the Earth, the suggestion that movements in the fluid core may help initiate the Earth's magnetic field.

shadow zone A region 100º to 140º from the epicenter of an earthquake in which, due to refraction from below the core-mantle boundary, no direct seismic waves can be detected.

shale A mudstone that splits or fractures readily.

shatter cone  A distinctively striated conical structure in rock, ranging from a few centimeters to a few meters in length, believed to have been formed by the passage of a shock wave following meteorite impact.

shear Rock deformation involving movement past each other of adjacent parts of the rock and parallel to the plane separating them.

shear strength The resistance of a body to shear stress.

shear stress The stress on an object operating parallel to the slope on which it lies.

sheeting A type of jointing produced by pressure release (unloading) or exfoliation .

shield volcano A volcano in the shape of a flattened cone, broad and low, built by very fluid flows of basaltic lava.

shock lamellae Closely spaced microscopic planes, distinct from cleavage planes, that occur in shock-metamorphosed minerals and are regarded as important indicators of shock metamorphism.

shock metamorphism  Metamorphism induced in rock by the passage of a high-pressure shock wave acting over a period of time from a few microseconds to a fraction of a minute. The only known natural cause of shock metamorphism is the hypervelocity impact of a meteorite.

shore Seaward edge of coast between low tide and effective wave action.

shore face The concave section of the beach from high tide mark down to the ramp between 5 and 20 m off shore.

shore platform A surface of erosion that slopes gently seaward from a cliff base to the low tide mark.

shoreline The line separating land and water. Fluctuates as water rises and falls.

sial The upper layer of the continental crust, so called because it is rich in silica and aluminum oxide. compare sima.

sialic Enriched in sial.

silica Silicon dioxide (SiO2) as a pure crystalline substance makes up quartz and related forms such as flint and chalcedony . More generally, silica is the basic chemical constituent common to all silicate minerals and magmas.

silica tetrahedron The basic structural unit of which all silicates are composed, consisting of a silicon atom surrounded symmetrically by four oxygen atoms. The structure, therefore, has the form of a tetrahedron with an oxygen atom at each corner.

sill A tabular igneous intrusion that parallels the planar structure of the surrounding rock.

sima The oceanic crust, also the lower layer of the continental crust, so called because it is enriched in silica and magnesium oxide. compare sial.

sinkhole Depression in ground surface caused by collapse into a cave below.

sinking stream A stream that empties into the underground into a cave, usually through a sinkhole.

slate A compact, fine-grained metamorphic rock that has slaty cleavage.

slaty cleavage  A style of foliation common in metamorphosed mudstones, characterized by nearly flat, sheet-like planes of breakage, similar in appearance to a deck of playing cards. compare cleavage

slickenside A polished and smoothly striated surface that results from friction along a fault plane.

slide A mass movement in which material maintains continuous contact with the surface on which it moves.

slip face Steep face on lee side of sand dune.

slump Downward and outward rotational movement of Earth materials traveling as a unit or series of units.

smelting The process of removing metal from ore.

snow line The elevation at which snow persists throughout the year.

snowfields Expanses of snow that lie above the snow line.

soil All unconsolidated materials above bedrock. Natural earthy materials on the Earth's surface, in places modified or even made by human activity, containing living matter, and supporting or capable of supporting plants out of doors.

soil horizon A layer of soil that is distinguishable from adjacent layers by characteristic physical properties such as texture, structure, or color, or by chemical composition.

soil moisture Ground water in the zone of aeration

soil structure The combination of soil particles into aggregates or clusters which are separated from adjacent aggregates by surfaces of weakness.

soil texture The physical nature of the soil, according to its relative proportions of sand, clay, and silt.

sole mark Develops as an irregularity on the bottom of a stratum. It is a cast of a depression on the top surface of the immediately underlying bed.

solifluction Turbulent movement of saturated soil or surficial debris.

sorting The range of particle sizes in a sedimentary deposit. A deposit with a narrow range of particle sizes is termed "well-sorted."

south magnetic pole The point on the Earth where a north seeking magnetic needle free to swing in space points directly up.

specific gravity The ratio of the density of a material to the density of water.

specific retention (field capacity) The amount of capillary water retained in a soil after the drainage of gravitational moisture.

sphericity A descriptive term to describe how close a particle's shape is to a sphere.

spit A sandy bar built out from the land into a body of water.

spoil Overburden or non-ore removed in mining or quarrying.

spreading axis (spreading center) A region of divergence on the Earth's surface, as at a rift .

spreading pole A rotational pole around which a plate appears to rotate on the Earth's surface.

spring Occurs at the intersection of the water table with the ground surface.

stack An isolated, steep-sided, rocky mass or island just offshore from a rocky headland, usually on a shore platform.

stalactite  An icicle-shaped accumulation of dripstone hanging from cave roof.

stalagmite  A post of dripstone growing up from a cave floor.

star dune A sand dune built by winds alternating through several directions. Builds vertically rather than migrating and growing laterally.

stick-slip A jerky, sliding motion associated with fault movement.

stock A small batholith.

stoping A process of magmatic intrusion that involves detaching and engulfing pieces of the surrounding rock, so that the magma moves slowly upward.

storm surge A ridge of high water associated with a hurricane and which floods over the shore .

strain Change in the shape or volume of a body as a result of stress.

strain rate The rate at which a body changes shape or volume as a result of stress.

strain seismograph A seismograph that is designed to detect deformation of the ground by measuring relative displacement of two points.

stratification The accumulation of material in layers or beds.

stratified drift Debris washed from a glacier and laid down in well-defined layers.

stratigraphy The succession and age relation of layered rocks.

stratovolcano (composite volcano) A volcano that is composed of alternating layers of lava and pyroclastic material, along with abundant dikes and sills. Viscous, intermediate lava may flow from a central vent. Example: Mt. Fuji in Japan.

streak The color of a mineral in its powdered form, usually obtained by rubbing the mineral against an unglazed porcelain tile to see the mark it makes. A mineral harder than the tile must be pulverized by crushing.

stream capture see stream piracy.

stream order A classification of the relative hierarchy of stream segments in a drainage network.

stream piracy (stream capture) The natural diversion of the headwaters of one stream into the channel of another stream that has greater erosional activity and flows at a lower level.

stream terrace A relatively flat surface along a valley, with a steep bank separating it either from the floodplain, or from a lower terrace.

strength The ability to withstand a stress without permanent deformation.

stress The force per unit area acting on any surface within a solid; also, by extension, the external pressure which generates the internal force.

striations Scratches, or small channels, gouged by glacier action. Occur on boulders, pebbles, and bedrock. Striations along bedrock indicate direction of ice movement.

strike The compass direction of the intersection between a structural surface (e.g., a bedding plane or a fault surface) and the horizontal.

strike-slip fault (transcurrent fault) A fault on which the movement is parallel to the fault's strike.

strip mining Open pit mining, typically for coal.

subduction zone A narrow, elongate region in which one lithospheric plate descends relative to another.

sublimation The process by which matter in the solid state passes directly to the gaseous state without first becoming liquid.

subtropical deserts Deserts in zones of descending air between 25 degrees and 30 degrees north and south latitude.

superimposed stream A stream that was established on a new surface and then, as it cut downward, maintained its course despite encountering different lithologies in the process.

superposition A statement of relative age in layered rocks: In a series of sedimentary rocks that has not been overturned, the topmost layer is always the youngest and the bottommost layer is always the oldest.

surf Produced as a wave steepens and falls forward as the wave nears the shore.

surface of discontinuity In sand dune formation the surface between quiet air of the wind shadow and the rapidly moving air above.

surface wave compare body wave

surging glacier A glacier that moves rapidly (tens of meters per day) as it breaks away from the ground surface on which it rests.

suspended load The amount of material a stream carries in suspension.

suspension A method of sediment transport in which the turbulence of a fluid is able to keep particles supported in the fluid.

suture The line of juncture where continental rocks on two converging plates meet. Example: The region in the Himalayas where the Eurasian and Indian-Australian plates meet.

swash and back wash Uprush of a wave onto the beach followed by the return flow of the water down the beach slope in the intervals between waves.

swells Persistence of wind-formed waves after wind ceases.

syncline A fold that is convex downward, or that had such an attitude at some stage in its development. compare anticline.

taconite A bedded ferruginous chert containing at least 25% iron. A potential iron ore.

tailings Washed or milled ore that is too poor to be further treated.

talus A slope built up by the accumulation of rock waste at the foot of a cliff or ridge.

tar A thick brown to black viscous organic liquid, too thick to migrate easily through most porous sediment.

tar sand A sand containing tar or asphalt, from which the hydrocarbons may potentially be extracted by distillation.

tarn A lake in the bedrock basin of a cirque.

tell An artificial hill formed by the debris of successive human settlements.

temperate glacier A glacier whose temperature throughout is at, or close to, the pressure point of ice, except in winter when it is frozen for a few meters below the surface.

tensile fracture A fracture caused by tensional stress in a rock.

tension A stress that tends to pull a body apart.

tephra A general term for all pyroclastic material.

terminal moraine (end moraine) Ridge of till marking farthest extent of glacier.

terrane (microplate) A fragment of the lithosphere, smaller than a plate, that forms a portion of an accreted terrane margin .

texture The general appearance of a rock as shown by the size, shape, and arrangement of the materials composing it.

"The present is the key to the past" A shorthand reference to the principle of uniformitarianism .

thermal conductivity A measure of the ability of a material to conduct heat.

thermal gradient see geothermal gradient.

thermal spring A spring whose temperature is 6.5o C or more above mean annual air temperature.

thermoremanent magnetism The magnetism of a mineral that it is acquired as it cools below its Curie point.

threshold of movement The point at which a slope or slope material crosses from a condition of stability to one of instability and movement begins.

thrust fault A reverse fault on which the dip angle of the fault plane is 15 degrees or less.

thrust sheet A body of rock above a large-scale thrust fault.

tidal delta A delta formed at both sides of a tidal inlet.

tidal inlet Waterway from open ocean into a lagoon.

tidal power Power generated by harnessing the energy of tidal motion in the ocean.

till (unstratified drift) Glacial drift composed of rock fragments that range from clay to boulder size and randomly arranged without bedding.

topset bed Layer of sediments deposited over surface of a delta, nearly horizontal and covering the tops of the inclined foreset beds.

triangulation The method of locating an epicenter by determining how far it lies from three widely separated seismographs.

transcurrent fault see strike-slip fault

transform boundary A plate boundary in which plates on opposite sides of the boundary move past each other in opposite directions.

transform fault A plate boundary that ideally shows pure strike-slip movement. Associated with the offset segments of midocean ridges.

transported soil A soil that has been moved from the site of its parent rock.

transverse dune A long, straight dune, perpendicular to direction of wind.

trap 1. Any barrier to the upward migration of petroleum, allowing it to accumulate. 2. Any dark colored extrusive igneous rock. A reference to the tendency of basalt and similar rocks to form columnar joints.

travel - time diagram  A plot of seismic wave travel time against distance on the Earth's surface from the epicenter of an earthquake.

travertine (tufa) Variety of limestone which forms stalactites and stalagmites and other deposits in limestone caves (dripstone) and the mouths of hot and cold calcareous springs.

trellis drainage A drainage pattern in which a stream and its tributaries resemble the pattern of a vine on a trellis.

trench Along, narrow, steep-walled, often arcuate depression in the ocean floor, much deeper than the adjacent ocean and associated with a subduction zone .

troughs and bars Linear features in unconsolidated sediments at the foot of the shoreface, the result of breaking waves.

TRU see Low level nuclear waste.

truncated spur  The beveled end of a ridge separating two valleys where they join a larger glaciated valley. Glacier of main valley has eroded back the end of the ridge.

tufa see travertine .

tuff A general term for all consolidated pyroclastic rock. Not to be confused with tufa.

turbidite Sedimentary deposit settled out of turbid water carrying particles of widely varying grade size. Characteristically displays graded bedding.

turbulent flow Fluid flow in which the flow lines are confused and mixed. Fluid moves in eddies and swirls. compare laminar flow.

U-shaped valley  A valley carved by glacier erosion and whose cross-valley profile has steep sides and a nearly flat floor, suggestive of a large letter "U".

unconformity A buried erosion surface separating two rock masses.

uniformitarianism The principle that applies to geology our assumption that the laws of nature are constant As originally used it meant that the processes operating to change the Earth in the present also operated in the past and at the same rate and intensity and produced changes similar to those we see today. The meaning has evolved and today the principle of uniformitarianism acknowledges that past processes, even if the same as today, may have operated at different rates and with different intensities than those of the present. The term "actualism" is sometimes used to designate this later meaning.

unloading The release of confining pressure associated with the removal of overlying material. May result in expansion of rock, accompanied by the development of joints or sheeting .

unstratified drift see till

USDA Soil Classification System A classification of soils on the basis of the processes and conditions by which they form. compare Comprehensive Soil Classification System.

valley glacier( alpine glacier, mountain glacier )  Streams of ice that flow down valleys in mountainous areas.

valley train Outwash plain contained within valley walls.

varve A pair sedimentary units, one coarse-grained, the other fine-grained, interpreted as representing one year of sedimentation.

velocity Distance of travel in unit of time

velocity profile A plot of seismic velocity against depth in the Earth.

ventifact A pebble, cobble, or boulder faceted by wind driven sand.

vesicle A cavity in a lava, formed by the entrapment of a gas bubble during solidification of the lava.

vesicular A textural term applied to an igneous rock containing abundant vesicles, formed by the expansion of gases initially dissolved in the lava.

viscosity The internal resistance to flow in a liquid.

volcanic ash  The dust-sized, sharp-edged, glassy particles resulting from an explosive volcanic eruption.

volcanic cinder A pyroclastic fragment, 0.5 to 2.5 cm in diameter, formed as magma spatters into the air during a volcanic eruption and cools as it falls to Earth.

volcano A vent in the surface of the Earth, from which lava, ash, and gases erupt, forming a structure that is roughly conical.

volcanogenic massive sulfide deposit A mineral deposit of metallic sulfides formed directly through processes associated with volcanism, commonly in a submarine setting.

vulnerable mineral A mineral that does not easily resist decomposition .

Wadati-Benioff zone An inclined plane, roughly coincident with a subduction zone, along which the foci of earthquakes cluster.

Waste Isolation Pilot Plant A pilot plant near Carlsbad, New Mexico, for the storage of low level nuclear waste.

water gap A gap in a ridge or mountain through which a stream flows.

water power Power generated through the agency of moving water.

water table The surface between the zone of saturation and the zone of aeration.

waterfall The perpendicular or very steep descent of a stream.

wave base A depth equal to one half the wave length of waves in deep water, below which stirring due to wind is negligible.

wave crest The top of a wave.

wave height The vertical distance between the crest and adjacent trough of a wave.

wave length The distance between two successive wave crests or troughs.

wave trough The low spot between two successive waves.

weathering The process by which Earth materials change when exposed to conditions at or near the Earth's surface and different from the ones under which they formed. compare decomposition , disintegration .

welded tuff A pyroclastic rock in which glassy clasts have been fused by the combination of the heat retained by the clasts, the weight of overlying material, and hot gases.

well An artificial intersection of the surface and the water table.

Wilson Cycle The opening and closing of ocean basins through plate tectonics.

wilting point The stage at which all water available to plants has been used.

wind farm An area in which a large number of windmills have been erected to generate electrical power.

wind gap An abandoned water gap.

wind power Power generated by using the force of the wind.

wind shadow An area of quiet air in lee of an obstacle. Zone of sand accumulation in lee of sand dune.

xenolith see inclusion

X-ray diffraction The diffraction of a beam of X-rays by the three dimensional periodic array of atoms in a crystal structure . The identity and arrangement of atomic in the structure can be determined by interpreting the angles at which X-rays are scattered by the structure and the intensities of scattered beams.

yardang Sharp, irregularly-crested ridges carved by wind and oriented parallel to wind.

yazoo-type river A tributary stream unable to enter a main stream because of natural levees along the main stream. It flows in a backswamp area, parallel to the main stream until it finds an entry to the main stream.

yield point The stress limit at which permanent deformation takes place in a non-brittle material.

Yucca Mountain Site Site in Nevada proposed for the storage of high level nuclear waste.

zone of ablation The area of wastage in a glacier.

zone of accumulation 1. The B horizon in a residual soil. 2. The area in which ice accumulates in a glacier.

zone of aeration Zone immediately below the ground surface within which pore spaces are partially filled with water and partially filled with air.

zone of flow The zone in a glacier that flows by deforming along planes of weakness in the ice crystals.

zone of fracture The near surface zone in a glacier that behaves like a brittle substance.

zone of leaching The upper horizons in a soil, through which gravitational moisture travels, removing soluble decomposition products.

zone of saturation The zone below the zone of aeration in which all pore spaces are filled with water. 


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