Kamus Istilah Geologi

KAMUS istilah GEOLOGI

A

aa lava - Lava whose surface is covered with random masses of angular jagged blocks.

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

abrasion - Erosion of rock material by friction of solid particles moved by gravity, water, ice, or wind.

absolute time - Geologic time measured in years. Compare with relative time.

acid rain - Rain higher in acid than normal. Forms from sulfur dioxide (SO₂) from the burning of fossil fuels; combines with water to form sulfuric acid.

acidic lava - Lava containing 70 percent or more of silica.

actynolite - A metamorphic ferromagnesian mineral; an asbestos.

adiabatic rate - In a body of air moving upward or downward, change in temperature without gain or loss of temperature to air through with it moves.

aftershock - Earthquake that follows a larger earthquake and originates at or near focus of larger earthquake. Major shallow earthquakes are generally followed by many

aftershocks, which decrease in number as time goes on but may continue for days or even months.

agate - Variety of chalcedony with alternating layers of chalcedony and opal.

A horizon - Soil zone immediately below surface, from which soluble material and fine-grained particles have been moved downward by water seeping into soil. Varying amounts of organic matter give A horizon color ranging from gray to black.

Airy hypothesis - Explains isostasy by assuming earth’s crust has same density everywhere and differences in elevation result from differences in thickness of outer layer.

albite - Feldspar in which diagnostic positive ion is Na⁺; sodic feldspar, Na(AlSi₃O₈). One of plagioclase feldspars.

alkali rocks - Igneous rocks in which abundance of alkalies is unusually high, generally indicated by soda pyroxenes, soda amphiboles, and/or feldspathoids.

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.

almandite - A deep-red garnet of iron and aluminum formed during regional metamorphism.

alpha decay - Radioactive decay taking place by loss of an alpha particle from nucleus. Mass of element decreases by 4, and atomic number decreases by 2.

alpha particle - A helium atom lacking electrons and therefore having a double positive charge.

Alpine, or mountain, glacier - See valley glacier.

amorphous - A state of matter in which there is no orderly arrangement of atoms.

amphibole group - Ferromagnesian silicates with a double chain of silicon-oxygen tetrahedra. Common example: hornblende. Contrast with pyroxene group.

amphibolite - A faintly foliated metamorphic rock developed during regional metamorphism of simatic rocks. Composed mainly of hornblende and plagioclase feldspars.

amphibolite facies - An assemblage of minerals formed during regional metamorphism at moderate to high pressures between 450 and 700º C.

andalusite - A silicate of aluminum built around independent tetrahedra, Al₂SiO₅. Characteristic  of middle-grade metamorphism. Compare with kyanite, which has same composition and forms under similar conditions but which has different crystal habit. Contrast with sillimanite, which has same composition but different crystal habit and forms at highest metamorphic temperatures.

andesite - A fine-grained igneous rock with no quartz or orthoclase, composted of about 75 percent plagioclase feldspars, balance ferromagnesian silicates. Important as lavas; possibly derived by fractional crystallization from basaltic magma. Widely characteristic of mountain-making processes around borders of Pacific Ocean. Confined to continental sectors.

andesite line - A map line designating the petrographic boundary of Pacific Ocean. Extrusive rocks on Pacific side of line are basaltic; on other side, andesitic.

angstrom - A unit of length equal to one hundred-millionth of a centimeter (10^-8 cm). Abbreviation, Å.

angular momentum - A vector quantity, product of mass times radius of orbit times velocity. The energy of motion of solar system.

angular unconformity - An unconformity in which older strata dip at different angle from that of younger strata.

anhydrite - Mineral calcium sulfate, CaSO₄, which is gypsum without water.

anion - A negatively charged atom or group of atoms produced by gain of electrons. Compare cation.

anorthite - Feldspar in which diagnostic positive ion is Ca²⁺; calcic feldspar, Ca(Al₂Si₂O₈). One of plagioclase feldspars.

anorthosite - A plutonic igneous rock composed of 90 percent or more of feldspar mineral anorthite. Pyroxene and some olivine usually make up balance of the rock.

Antarctic bottom water - Seawater that sinks to ocean floor off Antarctica and flows equatorward beneath North Atlantic deep water.

Antarctic intermediate water - Seawater that sinks at about 50ºS and flows northward above North Atlantic deep water.

anthracite - Metamorphosed bituminous coal of about 95 to 98 percent carbon.

anticline - A configuration of folded, stratified rocks in which rocks dip in two directions away from a crest, as principal rafters of a common gable roof dip away from ridgepole. Reverse of syncline. The "ridgepole," or crest, is called axis.

aphanitic texture - Individual minerals present but in particles so small that they cannot be identified without a microscope.

aquifer - A permeable material through which groundwater moves.

arête - A narrow, saw-toothed ridge developed between cirques.

arkose - A detrital sedimentary rock formed by cementation of individual grains of sand size and predominantly composed of quartz and feldspar. Derived from disintegration of granite.

arroyo - Flat-floored, vertically walled channel of an intermittent stream typical of semiarid climates; often applied to such features of southwestern United States. Synonymous with wadi and wash.

artesian water - Water under pressure when tapped by a well and able to rise above level at which first encountered. It may or may not flow out at ground level.

asbestos - A general term applied to certain fibrous minerals displaying similar physical characteristics although differing in composition. Some asbestos has fibers long enough to be spun into fabrics with great heat resistance, such as those for automobile brake linings. Types with shorter fibers are compressed into insulating boards, shingles, etc. Most common asbestos mineral (95 percent of United States production) is chrysotile, a variety of serpentine, a metamorphic mineral.

aseismic ridge - An oceanic ridge without extensive seismic activity as, for example, the Hawaiian Island-Emperor Seamount ridge in the Pacific Ocean.

ash - Volcanic fragments of sharply angular glass particles, smaller than cinders. See also volcanic ash.

asphalt - A brown to black, solid or semisolid bituminous substance. Occurs in nature but is also obtained as residue from refining of certain hydrocarbons ("artificial asphalt").

asteroids - Orbiting small bodies believed to be either fragments of a disintegrated planet or of matter that never completed planet forming process.

asthenosphere - A zone within earth's mantle where plastic movements occur to permit isostatic adjustments. Begins 50 to 100 km below surface and extends perhaps to 500 km.

astrogeology - Geology of celestial bodies other than earth. Generally restricted in meaning to geology of extraterrestrial bodies of our planetary system.

asymmetric fold - A fold in which on limb dips more steeply than the other.

Atlantic Ridge - Belt of mountains under Atlantic Ocean from Iceland to Antarctica and nearly midway between continents; also called mid-Atlantic Ridge.

atoll - A ring of low coral islands arranged around a central lagoon.

atom - A building block of matter; combination of protons, neutrons, and electrons, of which 103 kinds are now known.

atomic energy - Energy associated with nucleus of an atom. It is releases when nucleus is split.

atomic mass - The nucleus of an atom contains 99.95 percent of its mass. Total number of protons and neutrons in nucleus is called mass number.

atomic number - Number of positive charges on nucleus of an atom; number of protons in nucleus.

atomic reactor - A huge apparatus in which a radioactive core heats water under pressure and passes it to a heat exchanger.

atomic size - Radius of an atom (average distance from center to outermost electron of neutral atom). Commonly expressed in angstroms.

augite - A rock-forming ferromagnesian silicate mineral built around single chains of silicon-oxygen tetrahedral.

aureole - A zone in which contact metamorphism has taken place.

axial plane - A plane through a rock fold that includes the axis and divides the fold as symmetrically as possible.

axis - Ridge or place of sharpest folding of an anticline or syncline.

B

backset beds - Inclined layers of sand developed on gentler dune slope to windward. These beds may constitute a large part of total volume of a dune, especially if there is sufficient vegetation to trap most sand before it can cross over to slip face.

bank-full stage - Stage of flow at which a stream fills its channel up to level of its bank. Recurrence interval averages 1.5 to 2 years.

barchan - A crescent-shaped dune with wings, or horns, pointing downwing. Has gentle windward slope and steep lee slope inside horns; about 30 m high and 300 m wide from horn to horn. Moves with wind about 15 m/year across flat, hard surface where limited supply of sand is available.

barrier island - A low, sandy island near shore and parallel to it, on a gently sloping offshore bottom.

barrier reef - A reef separated from a landmass by a lagoon of varying width and depth opening to sea through passes in reef.

basal plane - Lower boundary of zone of concentric folding.

basal slip - Movement of an entire glacier over underlying ground surface.

basalt - A fine-grained igneous rock dominated by dark-colored minerals, consisting of plagioclase feldspars (over 50 percent) and ferromagnesian silicates. Basalts and andesites represent about 98 percent of all extrusive rocks.

base flow - That portion of stream flow attributable to groundwater flow.

base level - Level below which a stream cannot erode. There may be temporary base levels along a stream's course, such as those established by lakes or resistant layers of rock. Ultimate base level is sea level.

basement complex - Undifferentiated rocks underlying oldest identifiable rocks in any region. Usually sialic, crystalline, metamorphosed. Often, but not necessarily, Precambrian.

basic lava - Lava containing less than 50 percent silica.

basin - See dome.

batholith - A discordant pluton that increases in size downward, has no determinable floor, and shows an area of surface exposure exceeding 100 km².

bauxite - Chief ore of commercial aluminum. A mineraloid mixture of hydrous aluminum oxides.

bay barrier - A sandy beach, built up across mouth of a bay so that it is no longer connected to main body of water.

bed - (1) A rock mass of large horizontal extent bounded, especially above, by physically different material. (2) A deposit, as of ore, parallel to local stratification.

bedding - (1) A collective term used to signify existence of beds or layers, in sedimentary rocks. (2) Sometimes synonymous with bedding plane.

bedding plane - Surface separating layers of sedimentary rocks. Each bedding plane marks termination of one deposit and beginning of another of different character, such as surface separating a sand bed from a shale layer. Rock tends to separate, or break, readily along bedding planes.

bed load - Material in movement along stream bottom or, if wind is moving agency, along surface. Contrast with material carried in suspension or solution.

belt of soil moisture - Subdivision of zone of aeration. Belt from which water may be used by plants or withdrawn by soil evaporation. some water passes down into intermediate belt, where it may be held by molecular attraction against influence of gravity.

bench mark - A point of known elevation and position - usually indicated on a map by the letters B. M., with the altitude given (on American maps) to the nearest foot. (Some maps now use meters.)

Benioff zone - A seismic zone dipping beneath a continental margin and having a deep-sea trench as surface expression.

bergschrund - Gap, or crevasse, between glacier ice and headwall of a cirque.

berms - In coastline terminology berms are storm-built beach features that resemble small terraces; on seaward edges are low ridges built up by storm waves.

beta decay - Radioactive decay taking place by loss of a beta particle (electron) from a neutron in nucleus. Mass of element remains same, but atomic number increases by 1.

B horizon - Soil zone of accumulation below A horizon. Here is deposited some material moved down from A horizon.

big-bang theory - Theory that presently expanding universe originated as primeval cosmic fireball in very short period of time 10 to 12 billion years ago. Compare steady-state theory.

binding energy - Amount of energy that must be supplied to break an atomic nucleus into its component fundamental particles. It is equivalent to mass that disappers when fundamental particles combine to form a nucleus.

biochemical rock - A sedimentary rock made up of deposits resulting directly or indirectly from life processes of organisms.

biotite - "Black mica," ranging in color from dark brown to green. Rock-forming ferromagnesian silicate mineral with tetrahedra in sheets.

bituminous coal - Soft coal, containing about 80 percent carbon and 10 percent oxygen.

blowout - A basin scooped out of soft, unconsolidated deposits by process of deflation. Ranges from a few meters to several kilometers in diameter.

body wave - Push-pull or shake earthquake wave travelling through a body of a medium; distinguished from waves travelling along free surface.

bornite - A mineral, Cu₅FeS₄; an important ore of copper.

bottomset bed - Layer of fine sediment deposited in a body of standing water beyond advancing edge of a growing delta, which eventually builds up on bottomset beds.

boulder size - Volume greater than that of a sphere with diameter of 256 mm.

boulder train - Series of glacier erratics from the same bedrock source, usually with some property that permits easy identification. Arranged across country in a fan, with apex at source and widening in direction of glacier movement.

Bowen's reaction series - Series of minerals for which any early-formed phase tends to react with melt that remains to yield a new mineral further along in the series. Thus early-formed crystals of olivine react with remaining liquids to form augite crystals; these in turn may further react with liquid then remaining to form hornblende. See also continuous reaction series and discontinuous reaction series.

braided stream - Complex tangle of converging and diverging stream channels separated by sand bars or islands. Characteristic of flood plains where amount of debris is large in relation to discharge.

breccia - Clastic rock made up of angular fragments of such size that an appreciable percentage of rock volume consists of particles of granule size or larger.

brittle - A property of material whereby strength in tension is greatly different from strength in compression. Substance ruptures easily with little or no flow.

brown clay - An extremely fine-grained deposit found on the deep ocean floors, most abundantly in the Pacific Ocean. Derived from the continents and drifted to the open ocean, where the clay-sized particles settle.

burial metamorphism - Changes resulting from pressures and temperatures in rocks buried to depth of several kilometers.

C

calcic feldspar - Anorthite, Ca(Al₂Si₂O₈).

calcite - A mineral composed of calcium carbonate, CaCO₃.

caldera - Roughly circular, steep-sided volcanic basin with diameter at least three or four times depth. Commonly at summit of a volcano. Contrast with crater.

caliche - Whitish accumulation of calcium carbonate in soil profile.

capillary fringe - Belt above zone of saturation, in which underground water is lifted against gravity by surface tension in passages of capillary size.

capillary size - "Hairlike," or very small, such as tubes from 0.0025 to 0.25 cm in diameter.

carbohydrate - compound of carbon, hydrogen, and oxygen. Carbohydrates are chief products of life process in plants.

carbonate mineral - Mineral formed by combination of complex ion (CO₃)^2- with a positive ion. Common example: calcite, CaCO₃.

carbon cycle - Process in sun's deep interior by which radiant energy is generated in formation of helium from hydrogen.

carbon 14 - Radioactive isotope of carbon, which has halflife of 5,730 years. Used to date events up to about 50,000 years ago.

carbon ratio - A number obtained by dividing amount of fixed carbon in coal by sum of fixed carbon and volatile matter and multiplying by 100. This is same as percentage of fixed carbon, assuming no moisture or ash.

cassiterite - A mineral, tin dioxide, SnO₂. Ore of tin with specific gravity 7; nearly 75 percent of world's tin production is from placer deposits, mostly from cassiterite.

cataclastic metamorphism - Textural changes in rocks in which brittle minerals and rocks are broken and flattened as a result of intense folding or faulting; produces fragmentation of rocks as coarse-grained breccias and fine-grained mylonites.

cation - A positively charged atom or group of atoms produced by loss of electrons. Compare anion.

cementation - Process by which a binding agent is precipitated in spaces among individual particles of an unconsolidated deposit. Most common cementing agents are calcite, dolomite, and quartz; others include iron oxide, opal, chalcedony, anhydrite, and pyrite.

cement rock - Clayey limestone used in manufacturing hydraulic cement. Contains lime, silica, and alumina in varying proportions.

chalcedony - General name applied to fibrous cryptocrystalline silica with waxy luster. Deposited from aqueous solutions and frequenly found lining or filling cavities in rocks. Agate is a variety with alternating layers of chalcedony and opal.

chalcocite - A mineral, copper sulfide, Cu₂S; sometimes called copper glance; one of most important ore minerals of copper.

chalcopyrite - A mineral, a sulfide of copper and iron, CuFeS₂; sometimes called copper pyrite or yellow copper ore.

chalk - 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 very fine-grained calcite deposits of biochemical or inorganic-chemical origin.

chemical rock - In terminology of sedimentary rocks chemical rock is composed chiefly of material deposited by chemical precipitation, whether organic or inorganic (compare with detrital sedimentary rock). Chemical sedimentary rocks may have either clastic or nonclastic (usually crystalline) texture.

chemical weathering - Weathering of rock material by chemical processes that transform original material into new chemical combinations. Thus chemical weathering of orthoclase produces clay, some silica, and a soluble salt of potassium.

chert - Granular cryptocrystalline silica, similar to flint but usually light in color. Occurs as compact massive rock or as nodules.

chlorite - Family of tetrahedral sheet silicates of iron, magnesium, and aluminum, characteristic of low-grade metamorphism. Green color, with cleavage like mica except that chlorite small scales are not elastic.

C horizon - Soil zone that contains partially disintegrated and decomposed parent material. Lies directly under B horizon and grades downward into unweathered material.

chromite - Mineral oxide of iron and chromium, FeCr₂O₄, only ore of commercial chromium. One of first minerals to crystallize from magma; is concentrated within the magma.

chrysotile - Metamorphic mineral; an asbestos, fibrous variety of serpentine. Silicate of magnesium, with tetrahedra arranged in sheets.

chute or chute cutoff - Applied to stream flow, chute refers to new route taken by a stream when main flow is diverted to inside of a bend, along trough between low ridges formed by deposition on inside of bend, where water velocity is reduced. Compare with neck cutoff.

cinder cone - Structure built exclusively or predominantly of pyroclastic ejecta dominated by cinders. Parasitic to a major volcano, seldom exceeds 500 m in height. Slopes up 30 to 40º. Eample: Parícutin.

cinders - Volcanic fragments; small, slaglike, solidified pieces of magma 0.5 to 2.5 cm across.

cirque - A steep-walled hollow in a mountainside at high elevation, formed by ice-plucking and frost action and shaped like a half bowl or half amphitheater. Serves as principal gathering ground for ice of a valley glacier.

clastic texture - Texture shown by sedimentary rocks from deposits of minerals and rock fragments.

clay minerals - Finely crystalline, hydrous silicates formed from weathering of such silicate minerals as feldspar, pyroxene, and amphibole. Most common clay minerals belong to kaolinite, montmorillonite, and illite groups.

clay size - Volume less than that of a sphere with diameter of 0.004 mm.

cleavage - (1) Mineral cleavage: property possessed by many minerals of breaking in certain preferred direction along smooth plane surfaces. Planes of cleavage are governed by atomic pattern and represent direction in which atomic bonds are relatively weak. (2) Rock cleavage: property possessed by certain rocks of breaking with relative ease along parallel planes or nearly parallel surfaces. Rock cleavage is designated as slaty, phyllitic, schistose, and gneissic.

coal - Sedimentary rock composed of combustible matter derived from partial decomposition and alteration of plant cellulose and lignin.

cobble size - Volume greater than that of a sphere with diameter of 64 mm and less than that of a sphere with diameter of 256 mm.

coesite - High-pressure form of quartz, with density of 2.92. Associated with impact craters and cryptovolcanic structures.

col - Pass through a mountain ridge. Created by enlargement of two cirques on opposite sides of ridge until headwalls meet an are broken down.

cold glacier - Glacier in which no surface melting occurs during summer months and whose temperature is always below freezing.

colloidal size - between 0.0002 and 0.001 mm.

color - Sensation resulting from stimulation of retina by light waves of certain lengths.

column - Column, or post, of dripstone joining floor and roof of a cave; result of joining of stalactite and stalagmite.

columnar jointing - Pattern of jointing that blocks out columns of rock. Characteristic of tabular basalt flows or sills.

compaction - Reduction in pore space between individual grains from pressure of overyling sediments or pressures of earth movement.

composite volcanic cone - Composed of interbedded lava flows and pyroclastic material and characterized by slopes of close to 30º at summit, progressively reducing to 5º near base. Example: Mayon.

compression - Squeezing stress that tends to decrease volume of a material.

concentric folding - Elastic bending of an originally horizontal sheet with all internal movements parallel to a basal plane (lower boundary of the fold).

conchoidal fracture - A mineral's habit of fracturing to produce curved surfaces like interior of a shell (conch). Typical of glass and quartz.

concordant pluton - An intrusive igneous body with contacts parallel to layering or foliation surfaces of rocks into which it has intruded.

concretion - An accumulation of mineral matter formed around a center, or axis, of deposition after a sedimentary deposit has been laid down. Cementation consolidates the deposit as a whole, but the concretion is a body within hos rock that represents local concentration of cementing material: enclosing rock is less firmly cemented than the concretion. Commonly spheroidal or disk-shaped and composed of such cementing agents as calcite, dolomite, iron oxide, or silica.

cone of depression - A dimple in the water table, which forms as water is pumped from a well.

cone sheet - A dike, part of a concentric set dipping inward, like an inverted cone.

conglomerate - Detrital sedimentary rock made up of more or less rounded fragments of such size that an appreciable percentage of volume of rock consists of particles of granule size or larger.

connate water - Water trapped in a sedimentary deposit when the deposit was laid down.

contact metamorphism - Metamorphism at or very near contact between magma and rock during intrusion.

continent - One of the principal land masses of the earth, usually regarded as including Africa, Antarctica, Asia, Australia, Europe, North America, and South America.

continental drift - Slow, lateral movement of continents; involves rigid plates that may carry both continental and oceanic areas as they move.

continental glacier - An ice sheet that obscures mountains and plains of a large section of a continent. Continental glaciers exist on Greenland and Antarctica.

continental margin - Continental rise, continental slope, and continental shelf together make up the continental margin, particularly on the trailing edge of a continent.

continental rise - In some places base of continental slope is marked by somewhat gentler continental rise, which leads downward to deep ocean floor.

continental shelf - Shallow, gradually sloping zone extending from sea margin to a depth at which there is marked or rather steep descent into ocean depths down continental slope. Seaward boundary of shelf averages about 130 m in depth.

continental slope - Portion of ocean floor extending downward from seaward edge of continental shelves. In some places, such as south of Aleutian Islands, slopes descend directly to ocean deeps. In other places, such as off eastern North America, they grad into somewhat gentler continental rises, which in turn lead to deep ocean floors.

continuous reaction series - Branch of Bowen's reaction series comprising plagioclase feldspars, in which reaction of early-formed crystals with later liquids takes place continuously - that is, without abrupt phase change.

contour interval - The difference in elevation represented by adjacent countour lines.

contour line - A map line connecting points representing places on the earth's surface that have the same elevation. It thus locates the intersection with the earth's surface of a plane at any arbitrary elevation parallel to the datum plane. Contours represent the vertical, or third, dimension on a map, which has only two dimensions. They show the shape and size of physical features such as hills and valleys. A depression is indicated by an ordinary contour line except that hachures, or short dashes, are used on one side and point toward the center of the depression.

convection - Mechanism by which material moves because its density differs from that of surrounding material. Density differences are often brought about by heating.

convection cell - Pair of convection currents adjacent to each other.

convection current - Closed circulation of material sometimes developed during convection. Convection currents normally develop in pairs, each pair called convection cell.

convergent plate boundary - Boundary between two plates moving toward each other. Compare divergent plate boundary.

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

core - Innermost zone of earth, surrounded by mantle.

Coriolis effect - Tendency of any moving body, on or starting from surface of earth, to continue in direction in which earth's rotation propels it. Direction in which the body moves because of this tendency combined with direction in which it is aimed determines ultimate course of the body relative to earth's surface. In the northern hemisphere Coriolis effect causes a moving body to veer or try to veer to right of its direction of forward motion; in the southern hemisphere, to left. Magnitude of effect is proportional to velocity of a body's motion. This effect causes cyclonic storm-wind circulation to be counterclockwise in the northern hemispher and clockwise in the southern hemisphere and determines final course of ocean currents relative to trade winds.

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

covalent bond - Bond in which atoms combine by sharing their electrons.

crater - Roughly circular, steep-sided volcanic basin with diameter less than three times depth. Commonly at summit of a volcano (contrast with caldera). Applied also to depressions caused by meteorites by either direct or explosive impact.

craton - Relatively immobile part of the earth, generally large central portion of a continent.

creep - Applied to soils and surficial material, slow downward plastic movement. As applied to elastic solids, slow permanent yielding to stresses less than yield point if applied for a short time only.

crevasse - (1) Deep crevice, or fissure, in glacier ice. (2) Breach in a natural levee.

cross bedding - See inclined bedding.

crosscutting relationships, law of - A rock is younger than any rock across which it cuts.

crust - Outermost shell of the earth. Continental crust averages 35 km thick, density 2.6 t/m³; oceanic crust, about 5 km thick, density 3 t/m³.

cryptocrystalline - State of matter in which there is actually orderly arrangement of atoms characteristic of crystals but in units so small (material is so fine grained) that crystalline nature cannot be determined with an ordinary microscope.

cryptovolcanic structure - Geologic structure in which rocks appear disrupted by volcanic activity but in which there are no associated volcanic rocks.

crystal - Solid with orderly atomic arrangement, which may or may not develop external faces that give it crystal form.

crystal form - Geometrical form taken by a mineral, giving external expression to orderly internal atomic arrangement.

crystalline structure - Orderly arrangement of atoms in a crystal. Also called crystal structure.

crystallization - Process through which crystals separate from fluid, viscous, or dispersed state.

Curie temperature - Temperature above which ordinarily magnetic material loses magnetism. On cooling below this temperature, it regains magnetism. Example: Iron loses magnetism above 760º C and regains it while cooling below this temperature. Thus 760º C is the Curie temperature of iron.

current bedding - See inclined bedding.

cutoff - See chute cutoff, neck cutoff.

cyclosilicate - Mineral with crystal structure containing silicon-oxygen, (SiO₄)^4-, tetrahedra arranged as rings.

 D

datum plane - The reference surface from which all altitudes on a map are measured. This is usually mean sea level.

debris slide - Small, rapid movement of largely unconsolidated material that slides or rolls downward to produce irregular topography.

decomposition - Synonymous with chemical weathering.

deep focus - Earthquake focus deeper than 300 km. Greatest depth of focus known is 700 km.

deep-sea trenches - See island-arc deeps.

deflation - Erosive process in which wind carries off unconsolidated material.

deformation of rocks - Any change in original shape or volume of rock masses; produced by mountain-building forces. Folding, faulting, and plastic flow are common modes of rock deformation.

delta - Plain underlain by an assemblage of sediments accumulated where a stream flows into a body of standing water, its velocity and transporting power suddenly reduced. Originally named after Greek letter delta () because many are roughly triangular in plan, with apex pointing upstream.

dendritic pattern - An arrangement of stream courses that, on a map or viewed from the air, resemble branching habit of certain trees, such as oaks or maples.

density - Measure of concentrations of matter, expressed as mass per unit volume.

density current - Current due to differences in density of water from place to place caused by changes in temperature and variations in salinity or amount of material held in suspension.

depositional remanent magnetism - Magnetism resulting from tendency of magnetic particles such as hematite to orient themselves in earth's magnetic field as they are deposited. Orientation is maintained as soft sediments are lithified and thus records earth's field when particles are laid down. Abbreviation, DRM.

desert varnish - Thin, shiny layer of iron and manganese oxides that coats some desert-rock surfaces.

detrital sedimentary rock - Rock formed from accumulation of minerals and rocks derived from erosion of previously existing rocks or from weathered products of these rocks.

diabase - Rock of basaltic composition, essentially labradorite and pyroxene, characterized by ophitic texture.

diamond - A mineral composed of elemental carbon; hardest substance known. Used as a gem and, in industry, for cutting tools.

differential erosion - Process by which different rock masses or different parts of same rock erode at different rates.

dike - Tabular discordant pluton.

dike swarm - Group of approximately parallel dikes.

dilatancy - Tendency of rocks to expand along minute fractures immediately prior to failure; stress may be from earth movements or from controlled laboratory experiments.

diorite - Coarse-grained igneous rock with composition of andesite (no quartz or orthoclase), composed of 75 percent plagioclase feldspars and balance ferromagnesian silicates.

dip - (1) Acute angle that a rock surface makes with a horizontal plane. Direction of dip is always perpendicular to strike. (2) See magnetic declination.

dipole - Any object oppositely charged at two points. Most commonly refers to a molecule that has concentrations of positive charge or negative charge at two different points.

dipole magnetic field - Portion of earth's magnetic field best described by a dipole passing through earth's center and inclined to earth's rotation axis. See also nondipole magnetic field and external magnetic field.

dip pole - See magnetic pole.

dip-slip fault - Fault in which displacement is in direction of fault's dip.

discharge - With reference to stream flow, quantity of water that passes a given point in unit time. Measured in cubic meters per second or, often, cubic feet per second (abbreviation, cfs).

disconformity - Unconformity in which beds on opposite sides are parallel.

discontinuity - Within earth's interior, sudden or rapid changes with depth in one or more physical properties of materials constituting the earth, as evidenced by seismic data.

discontinuous reaction series - Branch of Bowen's reaction series including minerals olivine, augite, hornblende, and biotite, for which each series change represents abrupt phase change.

discordant pluton - An intrusive igneous body with boundaries that cut across surfaces of layering or foliation in rocks into which it has intruded.

disintegration - Synonymous with mechanical weathering.

divergent plate boundary - Boundary between two plates moving apart. New oceanic-type lithosphere is created at the opening. Compare convergent plate boundary.

divide - Line separating two drainage basins.

dolomite - Mineral composed of carbonate of calcium and magnesium, CaMg(CO₃)². Also used as rock name for formations composed largely of mineral dolomite.

dome - Anticlinal fold without clearly developed linearity of crest so that beds involved dip in all directions from a central area, like an inverted but usually distorted cup. Reverse of basin.

drainage basin - Area from which a given stream and its tributaries receive water.

drift - Any material laid down directly by ice or deposited in lakes, oceans, or streams as result of glacial activity. Unstratified glacial drift is called till and forms moraines; stratified forms outwash plains, eskers, kames, and varves.

dripstone - Calcium carbonate deposited from solution by underground water entering a cave in zone of aeration. Sometimes called travertine.

DRM - See depositional remanent magnetism.

drumlin - Smooth, streamlined hill composed of till. Long axis oriented in direction of ice movement: Blunt nose points upstream, and gentler slope tails off downstream. In height drumlins range from 8 to 60 m, with average somewhat less than 30 m. Most drumlins are between 0.5 and 1 km in length, the length commonly several times width. Diagnostic characteristics are shape and composition of unstratified glacial drift, in contrast to kames, or stratified glacial drift and random shapes.

dune - Mound or ridge of sand piled by wind.

dust-cloud hypothesis - Hypothesis that solar system was formed from condensation of interstellar dust clouds.

dust size - Volume less than that of sphere with diameter 0.06 mm; used in reference to particles carried in suspension by wind.

E

earthflow - Combination of slump and mudflow.

earth waves - Mechanism for transmitting energy from earthquake focus.

ecliptic - Apparent path of the sun in the heavens; plane of planets' orbits.

eclogite facies - Metamorphic rocks of gabbroic composition, consisting primarily of pyroxene and garnet.

elastic deformation - Nonpermanent deformation, after which body returns to original shape or volume when deforming force removed.

elastic energy - Energy stored within a solid during elastic deformation and released during elastic rebound.

elastic limit - Maximum stress that produces only elastic deformation.

elastic rebound - Recovery of elastic strain, when material breaks or deforming force is removed.

elastic solid - A solid that yields to applied force by changing shape, volume, or both but returns to original conditions when force is removed. Amount of yield is proportional to force.

elasticity - A property of materials that defines extent to which they resist small deformations, from which they recover completely when deforming force is removed. Elasticity equals stress divided by strain.

electric charge - Property of matter resulting from imbalance between number of protons and number of electrons in given piece of matter. The electron has negative charge; the proton, positive charge. Like charges repel each other; unlike charges attract each other.

electric current - Flow of electrons.

electric turbine generator - Apparatus that uses steam from heat exchanger to drive turbine and generate electricity.

electrical energy - Energy of moving electrons.

electron - Fundamental particle of matter, the most elementary unit of negative electrical charge. Mass, 0.00055 u (atomic mass unit or just mass unit).

electron shell - Imaginary spherical surface representing all possible paths of electrons with same average distance from nucleus and with approximately same energy.

electron-capture decay - Radioactive decay that takes place as an orbital electron is captured by a proton in nucleus. Mass of element remains constant, but atomic number decreases by 1.

element - Unique combinations of protons, neutrons, and electrons that cannot be broken down by ordinary chemical methods. Fundamental properties of an element are determined by number of protons, each element assigned a number corresponding to its number of protons. Combinations containing from 1 through 103 protons are now known.

end moraine - Ridge or belt of till marking farthest advances of a glacier. Sometimes called terminal moraine.

energy - Capacity for producing motion. Energy holds matter together and can become mass or be derived from mass. Takes such terms as kinetic, potential, heat, chemical, electrical, and atomic energy; one form of energy can be changed to another.

energy level - Distance from atomic nucleus at which electrons orbit. May be thought of as shell surrounding nucleus.

epicenter - Area on surface directly above focus of earthquake.

epidote - Silicate of aluminum, calcium, and iron characteristic of low-grade metamorphism and associated with chlorite and albite in greenschist facies. Built around independent tetrahedra.

epidote-amphibolite facies - Assemblage of minerals formed between 250 and 450º C during regional metamorphism.

erg - Unit of energy expressing capacity for doing work. Equal to energy expended when force of 1 dyne acts through distance of 1 cm.

erosion - Movement of material from one place to another on earth's surface. Agents of movement include gravity, water, ice, and wind.

erosional flood plain - Flood plain created by lateral erosion and gradual retreat of valley walls.

erratic - In terminology of glaciation a stone or boulder carried by ice to a place where it rests on or near bedrock of different composition.

escape velocity - Minimum velocity an object must have to escape from gravitational field. For moon this is about 2.83 km/s and for earth about 11.2 km/s.

esker - Winding ridge of stratified glacial drift, steep-sided, 3 to 15 m high, and from a fraction of a kilometer to over 160 km long.

eugosyncline - Part of geosyncline in which volcanism is associated with clastic sedimentation, generally located away from craton.

eustatic change - Change in sea level produced entirely by increase or decrease in amount of water in oceans; hence of worldwide proportions.

eutrophic - In lake aging pertains to an old-age lake and indicates high supply of nutrients supporting high biologic productivity.

evaporation - Process by which liquid becomes vapor at temperature below boiling point.

evaporite - rock composed of minerals precipitated from solutions concentrated by evaporation of solvents. Examples: rock salt, gypsum, anhydrite.

exfoliation - Process by which rock plates are stripped from larger rock mass by physical forces.

exfoliation dome - Large, rounded domal feature produced in homogenous coarse-grained igneous rocks and sometimes in conglomerates by process of exfoliation.

external magnetic field - Component of earth's field originating from activity above earth's surface. Small when compared with dipole and nondipole components of field, which originate beneath surface.

extravasate - To cause (molten lava) to pour forth from a volcanic vent.

extrusive rock - Rock solidified from mass of magma poured or blown out upon earth's surface.

facies - Assemblage of mineral, rock, or fossil features reflecting environment in which rock was formed. See sedimentary facies, metamorphic facies.

fault - Surface of rock rupture along which has been differential movement. A fracture in the continuity of a rock formation caused by a shifting or dislodging of the earth's crust, in which adjacent surfaces are differentially displaced parallel to the plane of fracture. Also called shift.

fault-block mountain - Mountain bounded by one or more faults.

feldspars - Silicate minerals composed of silicon-oxygen and aluminum-oxygen tetrahedra linked together in three-dimensional networks with positive ions fitted into interstices of negatively charged framework of tetrahedra. Classed as aluminosilicates. When positive ion is K+, mineral is orthoclase; when Na+, mineral is albite; when Ca+, mineral is anorthite.

felsite - General term for light-colored, fine-grained igneous rocks.

ferromagnesian silicate - Silicate in which positive ions are dominated by iron, magnesium, or both.

fibrous fracture - Mineral habit of breaking into splinters or fibers.

fiery cloud - Avalanche of incandescent pyroclastic debris mixed with steam and other gases, heavier than air, and projected down a volcano's side. Also called nuée ardente.

fiord - Glacially deepened valley now flooded by the sea to form long, narrow, steep-walled inlet.

firn - Granular ice formed by recrystallization of snow. Intermediate between snow and glacier ice. Sometimes called névé.

fissility - Property of splitting along closely spaced planes more or less parallel to bedding. Presence distinguishes shale from mudstone.

fission - Process by which atomic nucleus breaks down to form nuclei of lighter atoms.

fissure eruption - Extrusion of lava from fissure in earth's crust.

flashy stream - Stream with high flood peak of short duration, which may be caused by urbanization.

flint - Granular cryptocrystalline silica, usually dull and dark. Often occurs as lumps or nodules in calcareous rocks, such as Cretaceous chalk beds of southern England.

flood basalt - Basalt poured out from fissures in floods that tend to form great plateaus. Sometimes called plateau basalt.

flood frequency - Time within which a flood of a given size can be expected to occur.

flood plain - Area bordering a stream, over which water spreads in time of flood.

flood plain of aggradation - Flood plain formed by building up of valley floor by sedimentation.

flotation - Process that begins concentration of ore minerals from gangue.

fluid - Material that offers little or no resistance to forces tending to change its shape.

focus - Source of given set of earthquake waves.

fold - Bend, flexure, or wrinkle in rock produced when rock was in a plastic state.

fold mountains - Mountains consisting primarily of elevated, folded sedimentary rocks.

foliation - Layering in some rocks caused by parallel alignment of minerals; textural feature of some metamorphic rocks. Produces rock cleavage.

footwall - One of blocks of rock involved in fault movement. One that would be under feet of person standing in tunnel along or across fault; opposite hanging wall.

fore dune - Dune immediately behind shoreline of ocean or large lake.

foreset beds - Inclined layers of sediment deposited on advancing edge of growing delta or along lee slope of advancing sand dune.

foreshock - Relatively small earthquake that precedes larger earthquake by a few days or weeks and originates at or near focus of larger earthquake.

fossil - Evidence of past life, such as dinosaur bones, ancient clam shell, footprint of long-extinct animal, or impression of leaf in rock.

fossil fuels - Organic remains used to produce heat or power by combustion. Include coal, petroleum, and natural gas.

fractional distillation - Recovery - one or more at a time - of fractions of complex liquid, each of which has different density.

fractionation - Process whereby crystals that formed early from magma have time to settle appreciably before temperature drops much further. They are thus effectively removed from environment in which they formed.

fracture - As mineral characteristic, way in which mineral breaks when it does not have cleavage. May be conchoidal (shell shaped), fibrous, hackly, or uneven.

fracture cleavage - System of joints spaced fraction of centimeter apart.

freeboard of continents - Relative elevation of continents with respect to sea level.

fringing reef - Reef attached directly to landmass.

frost action - Process of mechanical weathering caused by repeated cycles of freezing and thawing. Expansion of water during freezing cycle provides energy for process.

fumarole - Vent for volcanic steam and gases.

fundamental particles - Protons, neutrons, and electrons, which combine to form atoms. Each particle is defined in terms of its mass and its electric charge.

fusion - Process by which nuclei of lighter atoms join to form nuclei of heavier atoms.

gabbro - Coarse-grained igneous rock with composition of basalt.

galaxy - Family of stars grouped in space. The earth belongs to Milky Way galaxy, which contains about 100 billion stars.

galena - A mineral; lead sulfid, PbS. Principal ore of lead.

gangue - Commercially valueless material remaining after ore-mineral extraction from rock.

garnet - family of silicates of iron, magnesium, aluminum, calcium, manganese, and chromium, which are built around independent tetrahedra and appear commonly as distinctive 12-sided, fully developed crystals. Characteristic of metamorphic rocks; generally cannot be distinguished from one another without chemical analysis.

gas - (1) State of matter that has neither independent shape nor volume, can be compressed readily, and tends to expand indefinitely. (2) In geology "gas" is sometimes used to refer to natural gas, gaseous hydrocarbons that occur in rocks, dominated by methane. Compare "oil," referring to petroleum.

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

geographic poles - Points on earth's surface marked by ends of axis in rotation.

geologic column - Chronologic arrangement of rock units in columnar form, with oldest units at bottom and youngest at top.

geologic-time scale - Chronologic sequence of units of earth time.

geologize - To study geology or make geologic investigations.

geology - Organized body of knowledge about the earth, including physical geology and historical geology, among others.

geomagnetic poles - Dipole best approximating earth's observed field is one inclined 11.5º from axis of rotation. Points at which ends of this imaginary magnetic axis intersect earth's surface are known as geomagnetic poles. They should not be confused with magnetic, or dip, poles or virtual geomagnetic poles.

geophysical prospecting - Mapping rock structures by methods of experimental physics. Includes measuring magnetic fields, force of gravity, electrical properties, seismic-wave paths and velocities, radioactivity, and heat flow.

geophysics - Physics of the earth.

geosyncline - Literally, "earth syncline." Term now refers, however, to a basin in which thousands of meters of sediments have accumulated, with accompanying progressive sinking of basin floor explained only in part by load of sediments. Common usage includes both accumulated sediments themselves and geometrical form of basin in which they are deposited. All folded mountain ranges were built from geosynclines, but not all geosynclines have become mountain ranges.

geothermal field - Area where wells drilled to obtain elements contained in solution in hot brines and to tap heat energy.

geothermal gradient - See thermal gradient.

geyser - Special type of thermal spring that intermittently ejects its water with considerable force.

glacier - A mass of ice, formed by recrystallization of snow, that flows forward or has flowed at some past time under influence of gravity. By convention we exclude icebergs even though they are large fragments broken from seaward end of glaciers.

glacier ice - Unique form of ice developed by compression and recrystallization of snow and consisting of interlocking crystals.

glass - Form of matter that exhibits properties of a solid but has atomic arrangements, or lack of order, of a liquid.

Glossopteris flora - A late-Paleozoic assemblage of fossil plants named for seed fern Glossopteris, a plant in the flora. Widespread in South America, South Africa, Australia, India, and Antarctica.

gneiss - Metamorphic rock with gneissic cleavage. Commonly formed by metamorphism of granite.

gneissic cleavage - Rock cleavage where surface may be a few hundredths of a millimeter to a centimeter or more apart.

goethite - Hydrous iron oxide, FeO(OH).

Gondwanaland - Hypothetical continent thought to have broken up in Mesozoic. Resulting fragments are postulated to form presentday South America, Africa, Australia, India, and Antarctica.

graben - Elongated, trenchlike, structural form bounded by parallel normal faults created when block that forms trench floor moves downward relative to blocks that form sides.

grade - Term used to designat extent to which metamorphism has advanced. Found in such combinations as "high-grade" or "low-grade metamorphism." Compare with rank.

graded bedding - Type of bedding shown by sedimentary deposit when particles become progressively finer from bottom to top.

gradient - Slope of stream bed or land surface; expressed in percent, as feet per mile, or meters per kilometer, or in degrees.

granite - Coarse-grained igneous rock dominated by light-colored minerals, consisting of about 50 percent orthoclase, 25 percent quartz, and balance of plagioclase feldspars and ferromagnesian silicates. Granites and granodiorites comprise 95 percent of all intrusive rocks.

granitization - Special type of metamorphism by which solutions of magmatic origin move through solid rocks, change ions with them, and convert them into rocks that achieve granitic character without having passed through magmatic stage.

granodiorite - Coarse-grained igneous rock intermediate in composition between granite and diorite.

granular texture - Composed of mineral grains large enough to be seen by unaided eye; also called phaneritic texture.

granulite facies - Gneissic rocks produced by deep-seated high-grade regional metamorphism.

graphic structure - Intimate intergrowth of potassic feldspar and quartz with long axes of quartz crystals lining up parallel to feldspar axis. Quartz part is dark and feldspar is light in color; so pattern suggests Egyptian heiroglyphs. Commonly found in pegmatites.

graphite - "Black lead." A mineral composed entirely of carbon. Very soft because of crystalline structure; diamond, in contrast, has same composition but its hardest substance known.

gravity anomaly - Difference between observed and computed values of gravity.

gravity fault - Fault in which hanging wall appears to have moved downward relative to footwall. Also called normal fault.

gravity meter - An instrument for measuring force of gravity. Also called gravimeter.

gravity prospecting - Mapping force of gravity at different places to determine differences in specific gravity of rock masses and, through this, distribution of masses of different specific gravity. Done with gravity meter (gravimeter).

graywacke - A variety of sandstone generally characterized by hardness, dark color, and angular grains of quartz, feldspar, and small rock fragments set in matrix of clay-sized particles. Also called lithic sandstone.

greenschist - Schist characterized by green color. Product of regional metamorphism of simatic rocks. (Green color is imparted by mineral chlorite.)

greenschist facies - Assemblage of minerals formed between 150 and 250º C during regional metamorphism.

groundmass - Finely crystalline or glassy portion of porphyry.

ground moraine - Till deposited from a glacier as veneer over landscape and forming gently rolling surface.

groundwater - Underground water within zone of saturation.

groundwater table - Upper surface of zone of saturation for underground water. An irregular surface with slope or shape determined by quantity of groundwater and permeability of earth materials. In general, highest beneath hills and lowest beneath valleys. Also referred to as water table.

guyot - Flat-topped seamount rising from ocean floor like a volcano but planed off on top and covered by appreciable water depth. Synonymous with tablemount.

gypsum - Hydrous calcium sulfate, CaSO₄ · 2H₂O. A soft, common mineral in sedimentary rocks, where it sometimes occurs in thick beds interstratified with limestones and shales. Sometimes occurs as layer under bed of rock salt since it is one of first minerals to crystallize on evaporation of seawater. Alabaster is a fine-grained massive variety of gypsum.

H - Symbol for mineral hardness.

hackly fracture - Mineral's habit of breaking to produce jagged, irregular surfaces with sharp edges.

half-life - Time needed for one-half of nuclei in sample of radioactive element to decay.

halide - Compound made from a halogen, such as chlorine, iodine, bromine, or fluorine.

halite - A mineral; rock salt or common salt, NaCl. Occurs widely disseminated or in extensive beds and irregular masses precipitated from seawater and interstratified with rocks of other types as true sedimentary rock.

hanging valley - A valley that has greater elevation than the valley to which it is tributary, at point of junction. Often (but not always) created by deepening of main valley by a glacier. Hanging valley itself may or may not be glaciated.

hanging wall - One of blocks involved in fault movement. One that would be hanging overhead for person standing in tunnel along or across fault; opposite footwall.

hardness - Mineral's resistance to scratching on a smooth surface. Mohs scale of relative hardness consists of 10 minerals, each scratching all those below it in scale and being scratched by all those above it:

1.     talc

2.     gypsum

3.     calcite

4.     fluorite

5.     apatite

6.     orthoclase

7.     quartz

8.     topaz

9.     corundum

10.   diamond

head - Difference in elevation between intake and discharge points for a liquid. In geology most commonly of interest in connection with movement of underground water.

heat energy - Special manifestation of kinetic energy in atoms. Temperature of a substance depends on average kinetic energy of component particles. When heat is added to a substance, average kinetic energy increases.

heat exchanger - Unit in atomic-power generation that uses water (heated under pressure by atomic reactor) to form steam from water in another system and to drive a turbine for electricity generation.

heat flow - Product of thermal gradient and thermal conductivity of earth materials.
Average over whole earth, 1.2 ± 0.15 µcal/cm²-s.

heave - To displace or move (a vein, lode, or stratum, for example).

height - The vertical difference in elevation between an object and its immediate surroundings.

hematite - Iron oxide, Fe₂O₃. Principal ore mineral for about 90 percent of commercial iron produced in United States. Characteristic red color when powdered.

hinge fault - Fault in which displacement preceptibly dies out along strike and ends at definite point.

historical geology - Branch of geology that deals with history of the earth, including record of life on earth as well as physical changes in earth itself.

horn - Spire of bedrock left where cirques have eaten into a mountain from more than two sides around a central area. Example: Matterhorn of the Swiss Alps.

hornblende - A rock-forming ferromagnesian silicate mineral with double chains of silicon-oxygen tetrahedral. An amphibole.

hornfels - Dense, granular metamorphic rock. Since this term is commonly applied to a metamorphic equivalent of any fine-grained rock, composition is variable.

hornfels facies - Assemblage of minerals formed as temperatures greater than 700º C during contact metamorphism.

horst - Elongated block bounded by parallel normal faults in such a way that it stands above blocks on both sides.

hot spot - Localized melting region in mantle near base of lithosphere, a few hundred kilometers in diameter and persistent over tens of millions of years. Existence of heat is assumed from volcanic activity at surface.

hot spring - Thermal spring that brings hot water to surface. Water temperature usually 6.5º C or more above mean air temperature.

hydration - Process by which water combines chemically with other molecules.

hydraulic gradient - Head of underground water divided by distance of travel between two points: If head is 10 m for two points 100 m apart, hydraulic gradient is 0.1 or 10 percent. When head and distance of flow are same, hydraulic gradient is 100 percent.

hydraulic mining - Use of strong water jet to move deposits of sand and gravel from original site to separating equipment, where sought-for mineral is extracted.

hydrocarbon - Compound of hydrogen and carbon that burns in air to form water and oxides of carbon. There are many hydrocarbons. The simples, methane, is chief component of natural gas. Petroleum is a complex mixture of hydrocarbons.

hydroelectric power - Conversion of energy to electricity by free fall of water. This method supplies about 4 percent of world's electrical energy.

hydrograph - Graph of variation of stream flow over time.

hydrologic cycle - General pattern of water movement by evaporation from sea to atmosphere, by precipitation onto land, and by return to sea under influence of gravity.

hydrothermal solution - Hot, watery solution that usually emanates from magma in late stages of cooling. Frequently contains, and deposits in economically workable concentrations, minor elements that, because of incommensurate ionic radii or electronic charges, have not been able to fit into atomic structures of common minerals of igneous rocks.

hysteresis - Retardation of recovery from elastic deformation after stress is removed.

icecap - Localized ice sheet.

ice sheet - Broad, moundlike mass of glacier ice of considerable extent that has tendency to spread radially under own weight. Localized ice sheets are sometimes called icecaps.

igneous - (1)  Formed by solidification from a molten state. Used of rocks. (2)  Of or relating to rock so formed; pyrogenic.

igneous rock - Aggregate of interlocking silicate minerals formed by cooling and solidification of magma.

illite - Clay mineral family of hydrous aluminous silicates. Structure is similar to that of montmorillonite, but aluminum replaces 10 to 15 percent of silicon, which destroys montmorillonite's property of expanding with addition of water because weak bonds replaced by strong potassium-ion links. Structurally illite intermediate between montmorillonite and muscovite. Montmorillonite converts to illite in sediments; illite, to muscovite under conditions of low-grade metamorphism. Illite is commonest clay mineral in clayey rocks and recent marine sediments and is present in many soils.

ilmenite - Iron titanium oxide. Accounts for much of unique abundance of titanium on moon.

inclined bedding - Bedding laid down at angle to horizontal. Also referred to as cross bedding or current bedding.

index minerals - Chlorite, low-grade metamorphism; almandite, middle-grade metamorphism; sillimanite, high-grade metamorphism.

induced magnetism - In terminology of rock magnetism one of components of rock's natural remanent magnetism. It is parallel to earth's present field and results from it.

inertia member - Central element of a seismograph, consisting of weight suspended by wire or spring so that it acts like pendulum free to move in only one plane.

infiltration - Soaking into ground of water on surface.

inosilicate - Mineral with crystal structure containing silicon-oxygen tetrahedra in single or double chains.

intensity - Measure of effects of earthquake waves on human beings, structures, and earth's surface at particular place. Contrast with magnitude, which is measure of total energy released by an earthquake.

intermediate belt - Subdivision of zone of aeration. Belt that lies between belt of soil moisture and capillary fringe.

intermediate lava - Lava composed of 60 to 65 percent silica.

intrusive rock - Rock solidified from mass of magma that invaded earth's crust but did not reach surface.

ion - Electrically unbalanced form of an atom or group of atoms, produced by gain or loss of electrons.

ionic bond - Bond in which ions are held together by electrical attraction of opposite charges.

ionic radius - Average distance from center to outermost electron of an ion. Commonly expressed in angstroms.

island-arc deeps - Arcuate trenches bordering some continents; some reach depths of 9,000 m or more below sea surface. Also called deep-sea trenches or trenches.

isoclinal folding - Beds on both limbs nearly parallel, whether fold upright, overturned, or recumbent.

isograd - Line connecting those rocks comprising the same metamorphic facies; line connecting similar temperature-pressure values; line marking the boundary between two metamorphic facies.

isoseismic line - Line connecting all points on surface of earth where intensity of shaking from earthquake waves is same.

isostasy - Ideal condition of balance that would be attained by earth materials of differing densities if gravity were the only force governing heights relative to each other.

isotope - Alternative form of an element produced by variations in number of neutrons in nucleus.

jasper - Granular, cryptocrystalline silica usually colored red by hematite inclusions.

joint - Break in rock mass with no relative movement of rock on opposite sides of break

joint system - Combination of intersecting joint sets, often at approximately right angles.

juvenile water - Water brought to surface or added to underground supplies from magma.

kame - Steep-sided hill of stratified glacial drive. Distinguished from drumlin by lack of unique shape and by stratification.

kame terrace - Stratified glacial drift deposited between wasting glacier and adjacent valley wall. When ice melts, this material stands as a terrace along valley wall.

Kansan - Of, or relating to, the second glacial stage of the Pleistocene in North America.

kaolinite - Clay mineral, hydrous aluminous silicate, Al₄Si₄O₁₀(OH)⁸. Structure consists of one sheet of silicon-oxygen tetrahedra each sharing three oxygens to give ratio of (Si₄O₁₀)^4- linked with one sheet of aluminum and hydroxl. Composition of pure kaolinite does not vary as for other clay minerals, montmorillonite and illite, in which ready addition or substitution of ions takes place.

karst topography - Irregular topography characterized by sink holes, streamless valleys, and streams that disappear underground - all developed by action of surface and underground water in soluble rock such as limestone.

Kerguelen-Gaussberg Ridge - Belt of mountains under Indian Ocean between India and Antarctica.

kettle - Depression in ground surface formed by melting of a block of ice buried or partially buried by glacial drift, either outwash or till.

kinetic energy - Energy of movement. Amount possessed by an object or particle depends on mass and speed.

kyanite - A silicate mineral characteristic of temperatures of middle-grade metamorphism. Al₂SiO₅ in bladed blue crystals is softer than a knife along the crystal. Its crystalline structure is based on independent tetrahedra. Compare with andalusite, which has same composition and forms under similar conditions but has different crystal habit. Contrast with sillimanite, which has same composition but different crystal habit and forms at highest metamorphic temperatures.

L - Symbol for earthquake surface waves.

laccolith - Concordant pluton that has domed up strata into which it intruded.

lag time - On stream hydrograph time interval between center of mass of precipitation and center of mass of resulting flood.

laminar flow - Mechanism by which fluid (such as water) moves slowly along a smooth channel or through a tube with smooth walls with fluid particles following straight-line paths parallel to channel or walls. Contrast with turbulent flow.

landslide - General term for relatively rapid mass movement, such as slump, rock slide, debris slide, mudflow, and earthflow.

lapilli - Pyroclastic debris in pieces about walnut size.

large waves - Earthquake surface waves.

latent heat of fusion - Number of calories per unit volume that must be added to a material at melting point to complete process of melting. These calories do not raise temperature.

lateral moraine - Ridge of till along edge of valley glacier. Composed largely of material fallen to glacier from valley walls.

laterite - Tropical soil rich in hydroxides of aluminum and iron and formed under conditions of good drainage.

lava - Magma poured out on surface of earth or rock solidified from such magma.

leading edge - That margin that occurs on the side of a continent which is riding on a plate in motion toward another plate; commonly this margin is very active tectonically.

left-lateral fault - Strike-slip fault where ground opposite you appears to have moved left when you face it.

levee (natural) - Bank of sand and silt built by river during floods, where suspended load deposited in greatest quantity close to river. Process of developing natural levees tends to raise river banks above level of surrounding flood plains. Break in natural levee sometimes called crevasse.

lignite - Low-grade coal, with about 70 percent carbon and 20 percent oxygen. Intermediate between peat and bituminous coal.

limb - One of two parts of anticline or syncline, on either side of axis.

limestone - Sedimentary rock composed largely of mineral calcite, CaCO₃, formed by either organic or inorganic processes. Most limestones have clastic texture, but nonclastic, particularly crystalline, textures are common. Carbonate rocks, limestone and dolomite, constitute estimated 12 to 22 percent of sedimentary rocks exposed above sea level.

limonite - Iron oxide with no fixed composition or atomic structure; a mineraloid. Always of secondary origin, not a true mineral. Is encountered as ordinary rust or coloring material of yellow clays and soils.

liquefaction - Process of changing soil and unconsolidated sediments into water mixture immediately following earthquake; often results in foundation failure, with sliding of ground under building structures.

liquid - State of matter that flows readily so that the mass assumes form of container but retains independent volume.

lithic sandstone - See graywacke.

lithification - Process by which unconsolidated rock-forming materials are converted into consolidated or coherent state.

lithosphere - Rigid outer layer of earth; includes crust and upper part of mantle. Relatively strong layer in contrast to underlying asthenosphere.

loess - Unconsolidated, unstratified aggregation of small, angular mineral fragments, usually buff in color. Generally believed to be wind-deposited; characteristically able to stand on very steep to vertical slopes.

longitudinal dune - Long ridge of sand (dune) oriented in general direction of wind movement. A small one is less than 3 m high and 60 m long. Very large ones are called seif dunes.

longitudinal wave - Push-pull wave.

lopolith - Tabular concordant pluton shaped like spoon bowl, with both roof and floor sagging downward.

magma - Naturally occurring silicate melt, which may contain suspended silicate crystals, dissolved gases, or both. These conditions may be met in general by a mixture containing as much as 65 percent crystals but no more than 11 percent dissolved gases.

magmatic segregation - The process by which heavy, early crstallized minerals settle out from a magma and become concentrated.

magnetic declination - Angle of divergence between geographic meridian and magnetic meridian. Measured in degrees east and west of geographic north.

magnetic inclination - Angle that magnetic needle makes with surface of earth. Also called dip of magnetic needle.

magnetic pole - North magnetic pole is point on earth's surface where north-seeking end of a magnetic needle free to move in space points directly down. At south magnetic pole the same needle points directly up. These poles are also known as dip poles.

magnetic reversal - Shift of 180º in earth's magnetic field such that north-seeking needle of magnetic compass would point south rather than to north magnetic pole.

magnetite - A mineral; iron oxide, Fe₃O₄. Black; strongly magnetic. Important ore of iron.

magnetosphere - Region 1,000 to 64,000 km above earth, where magnetic field traps electrically charged particles from sun and space. First believed to consist of two bands, Van Allen belts.

magnetostratigraphy - Use of magnetized rocks to determine history of events in record of changes in earth's magnetic field in past geologic ages.

magnitude - Measure of total energy released by an earthquake. Contrast with intensity, which is measure of effects of earthquake waves at particular place.

manganes nodule - A nodule found on the deep ocean floors and composed largely of iron and manganese, with smaller amounts of such metals as cobalt, titanium, copper, and nickel.

mantle - Intermediate zone of earth. Surrounded by crust, it rests on core at depth of about 2,900 km.

marble - Metamorphic rock of granular texture, with no rock cleavage, and composed of calcite, dolomite, or both.

maria - Dark-toned "seas" of moon. Mark moons topographically low areas.

marl - Calcareous clay or intimate mixture of clay and particles of calcite or dolomite, usually shell fragments.

marsh gas - Methane, CH₄; simplest paraffin hydrocarbon. Predominant component of natural gas.

mascons - Concentrations of mass located beneath surfaces of lunal maria.

mass - A number that measures quantity of matter. It is obtained on earth's surface by dividing weight of a body by acceleration due to gravity.

mass movement - Surface movement of earth materials induced by gravity.

mass number - Number of protons and neutrons in atomic nucleus.

mass unit - One-twelfth mass of carbon atom. Approximately mass of hydrogen atom.

matter - Anything that occupies space. Usually defined by describing its states and properties: solid, liquid, or gas; possesses mass, inertia, color, density, melting point, hardness, crystal form, mechanical strength, or chemical properties. Composed of atoms.

meander - (1) Turn or sharp bend in stream's course. (2) To turn, or bend, sharply. Applied to stream courses in geological usage.

mechanical weathering - Process by which rock is broken down into smaller and smaller fragments as result of energy developed by physical forces. Also known as disintegration.

medial moraine - Ridge of till formed by junction of two lateral moraines when two valley glaciers join to form single ice stream.

mélange - Heterogenous mixture of rock materials. Mappable body of deformed rocks that may be several kilometers in length and consists of highly sheared clayey matrix, thoroughly mixed with angular native and exotic blocks of diverse origin and geologic age.

Mercalli intensity scale - Scale to evaluate intensity of earthquake shaking on basis of effects at given place.

mesotrophic - In lake aging, stage between oligotrophic and eutrophic.

metal - Substance fusible and opaque, good conductor of electricity, and with characteristic luster. Examples: gold, silver, aluminum. Of the elements 77 are metals.

metallic bonding - Special kind of bonding in atoms of metallic elements whereby outermost electrons are not shared or exchanged but are free to move around and connect to any atoms in solid. Relative freedom of movement of electrons accounts for high level of electrical conductivity in metals.

metalloid - Element of some metallic and some nonmetallic characteristics. There are nine metalloids.

metamorphic facies - Assemblage of minerals that reached equilibrium during metamorphism under specific range of temperature and pressure.

metamorphic rock - "Changed-form rock." any rock changed in texture or composition by heat, pressure, or chemically active fluids after original formation.

metamorphic zone - Area subjected to metamorphism and characterized by certain metamorphic facies formed during process.

metamorphism - A process whereby rocks undergo physical or chemical changes or both to achieve equilibrium with conditions other than those under which they were originally formed (weathering arbitrarily excluded from meaning). Agents of metamorphism are heat, pressure, and chemically active fluids.

metasomatism - Process whereby rocks are altered when volatiles exchange ions with them.

meteor - Transient celestial body that enters earth's atmosphere with great speed, becoming incandescent from heat generated by air resistance.

meteoric water - Groundwater derived primarily from precipitation.

meteorite - Stone or metallic body fallen to earth from outer space.

methane - Simplest paraffin hydrocarbon, CH₄. Principal consituent of natural gas. Sometimes called marsh gas.

micas - Group of silicate minerals characterized by perfect sheet or scale cleavage resulting from atomic pattern, in which silicon-oxygen tetrahedra linked in sheets. Biotite is ferromagnesian black mica. Muscovite is potassic white mica.

microseism - "Small shaking." Specifically limited in technical usage to earth waves generated by sources other than earthquakes and, most frequently, to waves with periods of from 1 to about 9 s, from sources associated with atmospheric storms.

mid-Atlantic Ridge - See Atlantic Ridge.

midocean ridge - Continuous, seismically active, median mountain range extending through North and South Atlantic, Indian, and South Pacific Oceans.

migmatite - Mixed rock produced by intimate interfingering of magma and invaded rock.

mineral - Naturally occurring solid element or compound, exclusive of biologically formed carbon components. Has definite composition or range of composition and orderly internal atomic arrangement (crystalline structure), which gives unique physical and chemical properties, including tendency to assume certain geometrical forms known as crystals.

mineral deposit - Occurrence of one or more minerals in such concentration and form as to make possible removal and processing for use at profit.

mineraloid - Substance that does not yield definite chemical formula and shows no sign of crystallinity. Examples: bauxite, limonite, and opal.

miogesyncline - That part of a geosyncline in which volcanism is absent, generally located near craton.

modulus of elasticity - Slope of graph line relating stress to strain in elastic deformation.

Mohorovicic discontinuity (Moho) - Base of crust marked by abrupt increases in velocities of earth waves.

molecule - Smallest unit of compound that displays properties of that compound.

Monel metal - Steel containing 68 percent nickel.

monocline - Double flexure connecting strata at one level with same strata at another level.

montmorillonite - Clay mineral family, hydrous aluminous silicate with structural sandwich of one ionic sheet of aluminum and hydroxl between two (Si₄O₁₀)^4- sheets. Sandwiches piled on each other with water between and with nothing but weak bonds to hold them together. As result, additional water can enter lattice readily, causing mineral to swell appreciably and further weakening attraction between structural sandwiches. Consequntly a lump of montmorillonite in a bucket of water slumps rapidly into a loose, incoherent mass. Compare with other clay minerals, kaolinite and illite.

moon - A natural satellite.

moraine - General term applied to certain landforms composed of till.

mountain - Any part of landmass projecting conspicuously above its surroundings.

mountain chain - Series of more or less parallel ridges, all of which formed within a single geosyncline or on its borders.

mountain structure - Structure produced by deformation of rocks.

mudcracks - Cracks caused by shrinkage of drying deposit of silt or clay under surface conditions.

mudflow - Flow of well-mixed mass of rock, earth, and water that behaves like a fluid and moves down slopes with consistency similar to that of newly mixed concrete.

mudstone - Fine-grained, detrital sedimentary rock made up of silt- and clay-sized particles. Distinguished from shale by lack of fissility.

muscovite - "White mica." Nonferromagnesian rock-forming silicate mineral with tetrahedra arranged in sheets. Sometimes called potassic mica.

mylonite - Fine-grained rock formed by grinding during intense folding or faulting associated with cataclastic metamorphism.

native state - State in which an element occurs uncombined in nature. Usually applied to metals, as in "native copper," "native gold," etc.

natural gas - Gaseous hydrocarbons that occur in rocks. Dominated by methane.

natural remanent magnetism - Magnetism of rock. May or may not coincide with present magnetic field of earth. Abbreviation, NRM.

natural resources - Energy and materials made available by geological processes.

neck cutoff - Breakthrough of a river across narrow neck separating two meanders, where downstream migration of one has been slowed and next meander upstream has overtaken it. Compare with chute cutoff.

negative charge - Condition resulting from surplus of electrons.

nesosilicate - Mineral with crystal structure containing silicon-oxygen tetrahedra arranged as isolated units.

neutron - Proton and electron combined and behaving like fundamental particle of matter. Electrically neutral with mass of 1.00896 u. If isolated, decays to form proton and electron.

névé - Granular ice formed by recrystallization of snow. Intermediate between snow and glacier ice. Sometimes called firn.

Nichrome - Steel alloy with 35 to 38 percent nickel.

nickel steel - Steel containing 2.5 to 3.5 percent nickel.

nodule - Irregular, knobby-surfaced mineral body that differs in composition from rock in which formed. Silica in form of chert or flint is common component of nodules. They are commonly found in limestone and dolomite.

nonclastic texture - Applied to sedimentary rocks in which rock-forming grains are interlocked. Most sedimentary rocks with nonclastic texture are crystalline.

nonconformity - Unconformity separating younger rocks from distinctly different older high metamorphosed or igneous rocks.

nondipole magnetic field - Portion of earth's magnetic field remaining after dipole field and external field are removed.

nonferromagnesians - Silicate minerals that do not contain iron or magnesium.

nonmetal - Element that does not exhibit metallic luster, conductivity, or other features of metal. Of the elements 17 are non-metals.

normal fault - Fault in which hanging wall appears to have moved downward relative to footwall; opposite of thrust fault. Also called gravity fault.

North Atlantic deep water - Seawater in Arctic that sinks in North Atlantic and drifts southward as far as 60º S.

northeast trades - Winds blowing from the northeast toward the equator in the northern hemisphere.

NRM - See natural remanent magnetism.

nucleus - Protons and neutrons constituting central part of an atom.

nuée ardente - "Hot cloud." French term applied to highly heated mass of gas-charged lava ejected from vent or pocket at volcano summit more or less horizontally onto an outer slope, down which it moves swiftly, however slight the incline, because of its extreme mobility.

obduction - Process whereby part of the subducted plate and/or associated igneous rocks and deep-sea sediments are broken off and pushed up onto the overriding plate.

oblique slip fault - Fault with components of relative displacement along both strike and dip.

obsidian - Glassy equivalent of granite.

oil - In geology refers to petroleum.

oil shale - Shale containing such proportion of hydrocarbons as to be capable of yielding petroleum on slow distillation.

oligotrophic - In lake aging pertains to a youthful lake and indicates water low in accumulated nutrients and high in dissolved oxygen.

olivine - Rock-forming ferromagnesian silicate mineral that crystallizes early from magma and weathers readily at earth's surface. Crystal structure based on isolated (SiO₄)^4- ions and positive ions of iron, magnesium, or both. General formula: (Mg, Fe)₂SiO₄.

oölites - Spheroidal grains of sand size, usually composed of calcium carbonate, CaCO₃, and thought to have originated by inorganic percipitation. Some limestones largely made up of oölites.

ooze - Deep-sea deposit consisting of 30 percent or more by volume of hard parts of very small, sometimes microscopic, organisms. If particular organism dominant, its name is used as modifier, as in globigerina ooze, or radiolarian ooze.

opal - Amorphous silica, with varying amounts of water; a mineral gel.

open-pit mining - Surface mining represented by sand and gravel pits, stone quarries, and copper mines of some western states.

ophitic - Rock texture in which lath-shaped plagioclase crystals are enclosed wholly or in part in later-formed augite, as commonly occurs in diabase.

order of crystallization - Chronological sequences in which crystallization of various minerals of an assemblage takes place.

ore deposit - Metallic minerals in concentrations that can be worked at profit.

orogeny - Process by which mountain structures develop.

orthoclase - Feldspar in which K+ is diagnostic positive ion; K(AlSi₃O₈).

orthoquartzite - Sandstone composed completely - or almost completely - of quartz grains. Quartzose sandstone is synonym.

outwash - Material carried from a glacier by meltwater and laid down in stratified deposits.

outwash plain - Flat or gently sloping surface underlain by outwash.

overbank deposits - Sediments (usually clay, silt, and fine sand) deposited on flood plain by river overflowing banks.

overturned fold - Fold with at least one limb rotated through more than 90º.

oxbow - Abandoned meander isolated from main stream channel by deposition and filled with water.

oxide mineral - Mineral formed by direct union of an element with oxygen. Examples: ice, corundum, hematite, magnetite, cassiterite.

P - Symbol for earthquake primary waves.

pahoehoe lava - Lava whose surface is smooth and billowy, frequently molded into forms resembling huge rope coils. Characteristic of basic lavas.

paired terraces - Terraces that face each other across stream at same elevation.

paleomagnetism - Study of earth's magnetic field as has existed during geologic time.

paleosol - Soil formed in past environment; often buried.

Pangaea - Hypothetical continent from which all others are postulated to have originated through process of fragmentation and drifting.

parabolic dune - Dune with long, scoop-shaped form that, when perfectly developed, exhibits prabolic shape in plan, with horns pointing upwind. Contrast barchan, in which horns point downwind. Characteristically covered with sparse vegetation; often found in coastal belts.

paternoster lakes - Chain of lakes resembling string of beads along glaciated valley where ice plucking and gouging have scooped out series of basins.

peat - Partially reduced plant or wood material, containing approximately 60 percent carbon and 30 percent oxygen. An intermediate material in process of coal formation.

pebble size - Volume greater than that of sphere with diameter of 4 mm and less than that of a spere of 64 mm.

pedalfer - Soil characterized by accumulation of iron salts or iron and aluminum salts in B horizon. Varieties of pedalfers include red and yellow soils of southeastern United States and podsols of northeastern quarter of United States.

pedocal - Soil characterized by accumulation of calcium carbonate in its profile. Characteristic of low rainfall. Varieties include black and chestnut soils of northern Plains states and red and gray desert soils of drier western states.

pedology - Science that treats of soils - origin, character, and utilization.

pegmatite - Small pluton of exceptionally coarse texture, with crystals up to 12 m in length,  commonly formed at margin of batholith. Nearly 90 percent of all petmatites are simple pegmatites of quartz, orthoclase, and unimportant percentages of micas; others are extremely rare ferromagnesian pegmatites and complex pegmatites. Complex pegmatites have as major components sialic minerals of simple pegmatites but also contain a variety of rare minerals.

pelagic deposit - Material formed in deep ocean and deposited there. Example: ooze.

pendulum - Inertia member so suspended that, after displacement, restoring force will return it to starting position. If displaced and then released, oscillates, completing one to-and-fro swing in time called period.

Pennsylvanian - Of, belonging to, or being the geologic time, system of rocks, and sedimentary deposits of the sixth period of the Paleozoic Era, characterized by the development of coal-bearing rock formations.

perched water table - Top of zone of saturation that bottoms on impermeable horizon above level of general water table in area. Is generally near surface and frequently supplies a hillside spring.

peridotite - Coarse-grained igneous rock dominated by dark-colored minerals, consisting of about 75 percent ferromagnesian silicates and balance plagioclase feldspars.

period - For oscillating systems, length of time required to complete on oscillation.

permeability - For rock or earth material, ability to transmit fluids. Permeability equal to velocity of flow divided by hydraulic gradient.

petroleum - An oily mixture of hydrocarbons extracted from subsurface earth structures. Thought to result from physical and chemical conversion of remains of animals and plants. A fuel in natural or refined state, yielding on distillation such products as gasoline, kerosene, naphtha.

phaneritic texture - Individual grains large enough to be identified without the aid of a microscope; also called granular texture.

phase - (1) Homogenous, physically distinct portion of matter in physical-chemical system not homogenous, as in three phases of ice, water, and aqueous vapor. (2) Group of seismic waves of one type.

phenocryst - A crystal significantly larger than crystals of surrounding minerals.

phosphate rock - Sedimentary rock containing calcium phosphate.

photosynthesis - Process by which carbohydrates are compounded from carbon dioxide and water in presence of sunlight and chlorophyll.

phyllite - Clayey metamorphic rock with rock cleavage intermediate between slate and schist. Commonly formed by the regional metamorphism of shale or tuff. Micas characteristically impart a pronounced sheen to rock cleavage surfaces. Has phyllitic cleavage.

phyllitic cleavage - Rock cleavage in which flakes are produced barely visible to unaided eye. Coarser than slaty and finer than schistose cleavage.

phyllosilicate - Mineral with crystal structure containing silicon-oxygen tetrahedra arranged as sheets.

physical geology - Branch of geology that deals with nature and properties of material composing the earth, distribution of materials throughout globe, processes by which they are formed, altered, transported, and distorted, and nature and development of landscape.

piedmont glacier - Glacier formed by coalescence of valley glaciers and spreading over plains at foot of mountains from which valley glaciers come.

pipe - (1) A vertical, cylindrical vein of ore. (2) An eruptive passageway opening into the crater of a volcano.

placer - A concentration of relatively heavy and resistant minerals in stream or beach deposits; two examples are some deposits of gold and of diamonds.

planet - Heavenly body that changes position from night to night with respect to background of stars.

planetology - Organized body of knowledge about planetary system.

plastic deformation - Permanent change in shape or volume not involving failure by rupture and, once started, continuing without increase in deforming force.

plastic solid - Solid that undergoes deformation continuously and indefinitely after stress applied to it passes a critical point.

plate - Earth's lithosphere, varying in thickness from several tens of kilometers to as much as 100 km and including crust and part of upper mantle above asthenosphere.

plate tectonics - Theory of worldwide dynamics involving movement and interactions of the many rigid plates of earth's lithosphere.

plateau basalt - Basalt poured out from fissures in floods that tend to form great plateaus. Sometimes called flood basalt.

playa - Flat-floored center of undrained desert basin.

playa lake - Temporary lake formed in a playa.

pleochroic halo - Minute, concentric-spherical zones of darkening or coloring that form around inclusions of radioactive minerals in biotite, chlorite, and a few other minerals. About 0.075 mm in diameter.

plume - Pipelike convection cells thought to carry heat and mantle material from lower mantle up to crust, producing hot spots at surface.

plunge - Acute angle that axis of folded rock mass makes with horizontal plane.

pluton - A body of igneous rock formed beneath earth surface by consolidation from magma. Sometimes extended to include bodies formed beneath surface by metasomatic replacement of older rock.

plutonic igneous rock - Rock formed by slow crystallization, which yields coarse texture. Once believed to be typical of crystallization at great depth, but not a necessary condition.

pluvial lake - Lake formed during a pluvial period.

pluvial period - Period of increased rainfall and decreased evaporation; prevailed in nonglaciated areas during time of ice and advance elsewhere.

podsol - Ashy-gray or gray-brown soil of pedalfer group. Highly bleached soil, low in iron and lime, formed under moist and cool conditions.

point bars - Accumulations of sand and gravel deposited in slack waters on inside of bends of winding, or meandering, river.

Poisson's ratio - Ratio of change of diameter per unit diameter to change of length per unit length in elastic stretching or compression of cylindrical specimen.

polar compound - Compund,such as water, with a molecule that behaves like small bar magnet with positive charge on one end negative charge on other.

polarity epoch - Interval of time during which earth's magnetic field has been oriented dominantly in either normal or reverse direction. May be marked by shorter intervals of opposite sign, called polarity events.

polarity event - See polarity epoch.

polar wandering, or migration - Movement of position of magnetic pole during past time in relation to present position.

polymorphism - Existence of several different morphologic kinds occurring species or mineral.

porosity - Percentage of open space or interstices in rock or other earth material. Compare with permeability.

porphyritic - Textural term for igneous rocks in which large crystals, called phencrysts, are set in finer groundmass, which may be crystalline or glass or both.

porphyry - Igneous rock containing conspicuous pheocrysts in fine-grained or glassy groundmass.

Portland cement - Hydraulic cement consisting of compounds of silica, lime, and alumina.

positive charge - Condition resulting from deficiency of electrons.

potassic feldspar - Orthoclase, K(AlSi₃O₈).

potential energy - Stored energy waiting to be used. Energy that a piece of matter possesses because of position or because of arrangement of parts.

prairie soils - Transitional soils between pedalfers and pedocals.

Pratt hypothesis - Explains isostasy by assuming all portions of the crust have same total mass above certain elevation, called level of compensation. Higher sections would have proportionately lower density.

precipitation - Discharge of water, in rain, snow, hail, sleet, fog, or dew, on land or water surface. Also, process of separating mineral constituents from solution by evaporation (halite, anhydrite) or from magma to form igneous rocks.

precursor - Relating to earthquakes, refers to events immediately preceding actual shaking of ground. Includes changes in seismic velocities, groundwater levels, and tilt of ground surface.

pressure - Force per unit area.

prevailing westerlies - Winds blowing from the west and characteristic of the middle latitudes both north and south of the equator.

primary wave - Earthquake body wave that travels fastest and advances by push-pull mechanism. Also known as longitudinal, compressional, or P wave.

proton - Fundamental particle of matter with positive electrical charge of 1 unit (equal in amount, but opposite in effect, to the charge of electron) and mass of 1.00758 u.

proton-proton fusion - Rapidly moving protons in hot interior of stars collide and fuse to form atoms of helium from atoms of hydrogen in continuous buildup of higher elements.

pumice - Pieces of magma up to several centimeters across that have trapped bubbles of steam or other gases as they were thrown out in eruption. Sometimes they have sufficient buoyancy to float on water.

push-pull wave - Wave that advances by alternate compression and rarefaction of medium, causing particles in path to move forward and backward along direction of wave's advance. In connection with earth waves, also known as primary wave, compressional wave, longitudinal wave, or P wave.

pyrite - A sulfide mineral, iron sulfide, FeS₂.

pyroclastic debris - Fragments blown out by explosive volcanic eruptions and subsequently deposited on ground. Include ash, cinders, lapilli, blocks, bombs, and pumice.

pyroxene group - Ferromagnesian silicates with a single chain of silicon-oxygen tetrahedra. Common example: augite. Compare with amphibole group (example: hornblende), which has a double chain of tetrahedra.

pyrrhotite - A mineral, iron sulfide. So commonly associated with nickel minerals that has been called "world's greatest nickel ore."

quartz - A silicate mineral, SiO₂, composed exclusively of silicon-oxygen tetrahedra, with all oxygens joined in a three-dimensional network. Crystal form is six-sided prism tapering at end, with prism faces striated transversely. An important rock-forming mineral.

quartzite - Metamorphic rock commonly formed by metamorphism of sandstone and composed of quartz. No rock cleavage. Breaks through sand grains in contrast to sandstone, which breaks around grains.

Quaternary - Of, belonging to, or designating the geologic time, system of rocks, and sedimentary deposits of the second period of the Cenozoic Era, from the end of the Tertiary Period through the present, characterized by the appearance and development of human beings and including the Pleistocene Epoch and the Holocene Epoch.

radar - Ultrahigh-frequency electromagnetic radiation.

radial drainage - Arrangement of stream courses in which streams radiate outward in all directions from central zone.

radiant energy - Electromagnetic waves travelling as wave motion.

radioactivity - Spontaneous breakdown of atomic nucleus, with emission of radiant energy.

rain wash - Water from rain after it has fallen on ground and before concentrated in definite stream channels.

range - Elongated series of mountain peaks considered to be a part of one connected unit, such as Appalachian Range or Sierra Nevada Range.

rank - Term used to designate extent to which metamorphism has advanced. Compare with grade. Rank is more commonly employed in designating stage of metamorphism of coal.

ray craters - Lunar craters marked by rays. Young on lunar time scale.

rays - Light-toned streaks that spread outward from such lunar craters as Tycho, Kepler, and Copernicus.

reaction series - See Bowen's reaction series.

recessional moraine - Ridge or belt of till marking period of moraine formation, probably in period of temporary stability or slight readvance, during general wastage of a glacier and recession of its front.

recorder - Part of a seismograph that makes record of ground motion.

rectangular pattern - Arrangement of stream courses in which tributaries flow into larger streams at angles approaching 90º.

recumbent fold - Fold with axial plane more or less horizontal.

reflection seismic prospecting - Uses reflected waves and places seismographs at distances only a fraction of depths investigated.

refraction seismic prospecting - Uses travel times of refracted waves and spreads seismographs over lines roughly four times depth being investigated.

refractory - Mineral or compound that resists action of heat and chemical agents.

regional metamorphism - Metamorphism occurring over tens or scores of kilometers.

rejuvenation - Change in conditions of erosion that causes a stream to begin more active erosion and a new cycle.

relative time - Dating of events by place in chronologic order of occurrence rather than in years. Compare with absolute time.

relief - The difference in elevation of an area between tops of hills and bottoms of valleys.

reverse fault - Fault in which hanging wall appears to have moved upward relative to footwall; contrast with normal, or gravity fault. Also called thrust fault.

rhyolite - Fine-grained igneous rock with composition of granite.

rift zone - System of fractures in earth's crust. Often associated with lava extrusion.

right-lateral fault - Strike-slip fault in which ground opposite you appears to have moved right when you face it.

rigidity - Resistance to elastic shear.

rill - Miniature stream channel which forms along axis of broad, shallow trough carrying sheet wash, or sheet flow.

rilles - Trenchlike depressions on moon's surface. Some are straight walled, others sinuous.

ring dike - Arcuate (rarely circular) dike with steep dip.

ripple marks - Small waves produced in unconsolidated material by wind or water. See ripple marks of oscillation.

ripple marks of oscillation - Ripple marks formed by oscillating movement of water such as may be found along sea coast outside surf zone. Symmetrical, with sharp or slightly rounded ridges separated by more gently rounded troughs.

rock - Aggregate of minerals of one or more kinds in varying proportions.

rock cycle - Concept of sequences through which earth materials may pass when subjected to geological processes.

rock flour - Finely divided rock material pulverized by glacier and carried by streams fed by melting ice.

rock-forming silicate minerals - Minerals built around framework of silicon-oxygen tetrahedra: olivine, augite, hornblende, biotite, muscovite, orthoclase, albite, anorthite, quartz.

rock glacier - Tongue of rock waste found in valleys of certain mountainous regions. Characteristically lobate and marked by series of arcuate, rounded ridges that give it aspect of having flowed as viscous mass.

rock melt - Liquid solution of rock-forming mineral ions.

rock salt - Halite, or common salt, NaCl.

rock slide - Sudden and rapid slide of bedrock along planes of weakness.

Rossi-Forel scale - Scale for rating earthquake intensities, devised in 1878.

runoff - Water that flows off land.

rupture - Breaking apart or state of being broken apart.

S - Symbol for secondary wave.

salt - In geology this term usually refers to halite, or rock salt, NaCl, particularly in such combinations as salt water and salt dome.

saltation - Mechanism by which a particle moves by jumping from one point to another.

salt dome - Mass of NaCl generally of roughly cylindrical shape and with diameter of about 2 km near top. Such mass has been pushed through surrounding sediments into present position. Reservoir rocks above and alongside salt domes trap oil and gas.

saltwater wedge - Body of water, found in some estuaries, which thins toward head of estuary and is overridden by fresh water from land.

sand - Clastic particles of sand size, commonly but not always composed of mineral quartz.

sand size - Volume greater than that of a sphere with diameter of 0.0625 mm and less than that of a sphere with diameter of 2 mm.

sandstone - Detrital sedimentary rock formed by cementation of individual grains of sand size and commonly composed of mineral quartz. Sandstones constitute estimated 12 to 28 percent of sedimentary rocks.

sapropel - Aquatic ooze or sludge rich in organic matter. Believed to be source material for petroleum and natural gas.

satellite crater - Crater formed by impact of a fragment ejected during creation of a primary crater. Also called secondary crater.

scale - The ratio on a map of the distance between two points on the ground and the same two points on the map. It may be expressed in three ways:

Fractional scale - If two points are 1 km apart in the field, they may be represented on the map as separated by some fraction of that distance, say, 1 cm. In this instance, the scale is 1 cm to the kilometer. There are 100,000 cm in 1 km; so this scale can be expressed as the fraction, or ratio, 1:100,000. Many topographic maps of the United States Geological Survey have a scale of 1:62,500; and many recent maps have a scale of 1:31,250, and others of 1:24,000.

Graphic scale - This scale is a line printed on the map and divided into units that are equivalent to some distance, such as 1 km or 1 mi.

Verbal scale - This is an expression in common speech, such as "four centimeters to the kilometer," "an inch to a mile," or "two miles to the inch."

schist - Metamorphic rock dominated by fibrous or platy minerals. Has schistose cleavage and is product of regional metamorphism.

schistose cleavage - Rock cleavage with grains and flakes clearly visible and cleavage surfaces rougher than in slaty or phyllitic cleavage.

sea-floor spreading - Process by which ocean floors spread laterally from crests of main ocean ridges. As material moves laterally from ridge, new material is thought to replace it along ridge crest by welling upward from mantle.

seamount - Isolated, steep-sloped peak rising from deep ocean floor but submerged beneath surface. Most have sharp peaks, but some have flat tops and are called guyots, or tablemounts. Seamounts are probably volcanic in origin.

secondary crater - See satellite crater.

secondary wave - Earthquake body wave slower than primary wave. Shear, shake, or S wave.

secular variation of magnetic field - Change in inclination, declination, or intensity of earth's magnetic field. Detectable only from long historical records.

sedimentary - Of or relating to rocks formed by the deposition of sediment.

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

sedimentary rock - Rock formed from accumulations of sediment, which may consist of rock fragments of various sizes, remains or products of animals or plants, products of chemical action or of evaporation, or mixtures of these. Stratification is single most characteristic feature of sedimentary rocks, which cover about 75 percent of land area.

sedimentation - Process by which mineral and organic matters are laid down.

seif dune - Very large longitudinal dune. As high as 100 m and as long as 100 km.

seismic prospecting - Method of determining nature and structure of buried rock formations by generating waves in ground (commonly by small explosive charges) and measuring length of time these waves require to travel different paths.

seismic sea wave - Large wave in ocean generated at time of earthquake. Popularly but incorrectly known as tidal wave. Sometimes called tsunami.

seismogram - Record obtained on a seismograph.

seismograph - Instrument for recording vibrations, most commonly employed for recording earth vibrations during earthquakes.

seismology - Scientific study of earthquakes and other earth vibrations.

senile - Worn away nearly to the base level, as at the end of an erosion cycle.

series - A group of rock formations closely related in time of origin and distinct as a group from other formations.

serpentine - Silicate of magnesium common among metamorphic minerals. Occurs in two crystal habits: platy, known as antigorite; fibrous, known as chrysotile, an asbestos. "Serpentine" comes from mottled shades of green on massive varieties, suggestive of snake markings.

S.G. - Symbol for specific gravity.

shake wave - Wave that advances by causing particles in path to move from side to side or up and down at right angles to direction of wave's advance, a shake motion. Also called shear wave, or secondary wave.

shale - Fine-grained, detrital sedimentary rock made up of silt- and clay-sized particles. Contains clay minerals as well as particles of quartz, feldspar, calcite, dolomite, and other minerals. Distinguished from mudstone by presence of fissility.

shear - Change of shape without change of volume.

shear wave - Wave that advances by shearing displacements (which change shape without changing volume) of medium. This causes particles in path to move from side to side or up and down at right angles to direction of wave's advance. Also called shake wave, or secondary wave.

sheet - A broad, relatively thin deposit or layer of igneous or sedimentary rock.

sheet flow - See sheet wash.

sheeting - Joints essentially parallel to ground surface. More closely spaced near surface and become pregressively farther apart with depth. Particularly well developed in granitic rocks, but sometimes in other massive rocks as well.

sheet wash - Water accumulating on a slope in thin sheet of water. May begin to concentrate in rills. Also called sheet flow.

shield - Nucleus of Precambrian rocks around which a continent has grown.

shift - Fault.

shield volcano - Volcano built up almost entirely of lava, with slopes seldom as great as 10º at summit and 2º at base. Examples: five volcanoes on island of Hawaii.

sial - A term coined from chemical symbols for silicon and aluminum. Designates composite of rocks dominated by granites, granodiorites, and their allies and derivatives, which underlie continental areas of globe. Specific gravity considered to be about 2.7.

sialic rock - Igneous rock composed predominantly of silicon and aluminum, from whose chemical symbols term is constructed. Average specific gravity, about 2.7.

siderite - A mineral; iron carbonate, FeCO₃. An ore of iron.

silicate minerals - Minerals with crystal structure containing silicon-oxygen tetrahedra arranged as isolated units (nesosilicates), single or double chains (inosilicates), sheets (phyllosilicates), or three-dimensional frameworks (tectosilicates).

silicon-oxygen tetrahedron - Complex ion composed of silicon ion surrounded by four oxygen ions. Negative charge of 4 units, and represented by symbol (SiO₄)^4-. Diagnostic unit of silicate minerals, and makes up central building unit of nearly 90 percent of materials of earth's crust.

sill - An approximately horizontal sheet of igneous rock intruded between older rock beds.

sillimanite - A silicate mineral, Al₂SiO₅, characteristic of highest metamorphic temperatures and pressures. Occurs in long slender crystals, brown, green, white. Crystalline structure based on independent tetrahedra. Contrast with kyanite and andalusite, which have same composition but different crystal habits and form at lower temperatures.

silt size - Volume greater than that of a sphere with diameter of 0.0039 mm and less than that of a sphere with diameter of 0.0625 mm.

Silurian - Of, belonging to, or being the geologic time, system of rocks, or sedimentary deposits of the third period of the Paleozoic Era, characterized by the development of early invertebrate land animals and land plants.

sima - Term coined from silicon and magnesium. Designates worldwide shell of dark, heavy rocks. Sima believed to be outermost rock layer under deep, permanent ocean basins, such as Pacific. Originally sima considered basaltic in composition, with specific gravity of about 3.0. It has been suggested also, however, that it may be peridotitic in composition, with specific gravity of about 3.3.

simatic rock - Igneous rock composed perdominantly of ferromagnesian minerals. Average specific gravity, 3.0 to 3.3.

sink - See sinkhole.

sinkhole - Depression in surface of ground caused by collapse of roof over solution cavern.

sinter - A chemical sediment or crust, as of porous silica, deposited by a mineral spring.

slate - Fine-grained metamorphic rock with well-developed slaty cleavage. formed by low-grade regional metamorphism of shale.

slaty cleavage - Rock cleavage in which ease of breaking occurs along planes separated by microscopic distances.

slip - (1)  A smooth crack at which rock strata have moved on each other. (2)  A small fault. (3)  The relative displacement of formerly adjacent points on opposite sides of a fault.

slip face - Steep face on lee side of a dune.

slope failure - See slump.

slump - Downward and outward movement of rock or unconsolidated material as unit or as series of units. Also called slope failure.

snowfield - Stretch of perennial snow existing in area where winter snowfall exceeds amount of snow that melts away during summer.

snow line - Lower limit of perennial snow.

soapstone - See talc.

sodic feldspar - Albite, Na(AlSi₃O₈).

soil - Superficial material that forms at earth's surface as result of organic and inorganic processes. Soil varies with climate, plant and animal life, time, slope of land, and parent material.

soil horizon - Layer of soil approximately parallel to land surface with observable characteristics produced through operation of soil-building processes.

solar constant - Average rate at which radiant energy received by earth from sun. Equal to little less than 2 cal/cm²-min on plane perpendicular to sun's rays at outer edge of atmosphere, when earth is at mean distance from sun.

solar system - Sun with group of celestial bodies held by its gravitational attraction and revolving around it.

sole mark - Cast of sedimentary structures such as cracks, tracks, or grooves formed on lower surface or underside of sandstone bed, commonly revealed after original underlying sedimentary layer has weathered away.

solid - Matter with definite shape and volume and some fundamental strength. May be crystalline, glassy, or amorphous.

solid solution - Single crystalline phase that may vary in composition within specific limits.

solifluction - Mass movement of soil affected by alternate freezing and thawing. Characteristic of saturated soils in high latitudes.

sorosilicates - Mineral with crystal structure containing silicon-oxygen tetrahedra arranged as double units.

southeast trades - Winds blowing from the southeast toward the equator in the southern hemisphere.

space lattice - In crystalline structure of mineral, three-dimensional array of points representing pattern of locations of identical atoms or groups of atoms constituting a mineral's unit cell. There are 230 pattern types.

specific gravity - Ratio between weight of given volume of material and weight of equal volume of water at 4º C.

specific heat - Amount of heat necessary to raise temperature of 1 g of any material 1º C.

sphalerite - A mineral; zinc sulfide, ZnS. Nearly always contains iron, (Zn, Fe)S. Principal ore of zinc. (Also known as zinc blende or blackjack.)

spheroidal weathering - Spalling off of concentric shells from rock masses of various sizes as result of pressures built up during chemical weathering.

spit - Sandy bar built by currents into a bay from a promontory.

spring - Place where water table crops out at ground surface and water flows out more or less continuously.

stack - Small island that stands as isolated, steep-sided rock mass just off end of promontory. Has been isolated from land by erosion and weathering concentrated behind end of a headland.

stage - A subdivision in the classification of stratified rocks, ranking just below a series and representing rock formed during a chronological age.

stalactite - Icicle-shaped accumulation of dripstone hanging from cave roof.

stalagmite - Post of dripstone growing upward from cave floor.

star - A heavenly body that seems to stay in same position relative to other heavenly bodies.

staurolite - Silicate mineral characteristic of middle-grade metamorphism. Crystalline structure based on independent tetrahedra with iron and aluminum. Has unique crystal habit that makes it striking and easy to recognize; six-sided prisms intersecting at 90º to form cross or at 60º to form an X shape.

steady-state theory - Theory that universe is developing by continuous creation of matter as newly formed galaxies replace those expanding out of sight, thus keeping mass density of universe constant. Compare with big-ban theory.

stock - Discordant pluton that increases in size downward, has no determinable floor, and shows area of surface exposure less than 100 km². Compare with batholith.

stoping - Mechanism by which batholiths have moved into crust by breaking off and foundering of blocks of rock surrounding magma chamber.

strain - Change of dimensions of matter in response to stress: Commonly, unit strain, such as change in length per unit length (total lengthening divided by original length), change in width per unit width, change in volume per unit volume.

straticulate - Having thin layers.

stratification - Structure produced by deposition of sediments in layers or beds.

stratigraphic trap - Structure that traps petroleum or natural gas because of variation in permeability of reservoir rock or termination of inclined reservoir formation on up-dip side.

stratum - A bed or layer of sedimentary rock having approximately the same composition throughout.

streak - Color of fine powder of mineral; may be different from color of hand specimen. Usually determined by rubbing mineral on piece of unglazed porcelain (hardness about 7) known as a "streak plate," which is, of course, useless for minerals of greater hardness.

stream order - Hierarchy in which segments of a stream system are arranged.

stream terrace - Surface representing remnants of stream's channel or flood plain when stream was flowing at higher level. Subsequent downward cutting by stream leaves remnants of old channel or flood plain standing as terrace above present stream level.

strength - Stress at which rupture occurs or plastic deformation begins.

stress - Force applied to material that tends to change dimensions: commonly, unit stress or total force divided by the area over which applied. Contrast with strain.

striations - (1) Scratches, or small channels, gouged by glacial action. Bedrock, pebbles, and boulders may show striations produced when rocks trapped by ice were ground against bedrock or other rocks. Striations along bedrock surface are oriented in direction of ice flow across that surface. (2) In minerals, parallel, threadlike lines, or narrow bands, on face of mineral. Reflect internal atomic arrangement.

strike - Direction of line formed by intersection of a rock surface with a horizontal plane. Strike is always perpendicular to direction of dip.

strike-slip fault - Fault in which movement is almost in direction of fault's strike.

strip mining - Surface mining in which soil and rock covering sought-for commodity are moved to one side. Some coal mining is pursued in this manner.

structural - Of or relating to the structure of rocks and other aspects of the earth's crust.

structural relief - Difference in elevation of parts of deformed stratigraphic horizon.

structure - Attitudes of deformed masses of rock.

subduction - Acto of one tectonic unit's descending under another (commonly slab of lithosphere).

subduction zone - Elongate region along which lithospheric block descends relative to another lithospheric block.

sublimation - Process by which solid material passes into gaseous state without first becoming liquid.

subsequent stream - Tributary stream flowing along beds of less erosional resistance and parallel to beds of greater resistance. Course determined subsequent to uplift that brought more resistant beds within sphere of erosion.

subsurface water - Water below ground surface. Also referred to as underground water and subterranean water.

subterranean water - Water below ground surface. Also referred to as underground water and subsurface water.

subtropical high pressure zones - Zones of atmospheric high pressure located about 30º north and 30º south of the equator.

sulfate mineral - Mineral formed by combination of complex ion (SO₄)²- with positive ion. Common example: gypsum, CaSO₄ · 2H₂O.

sulfide mineral - Mineral formed by direct union of element with sulfur. Examples: argentite, chalcocite, galena, sphalerite, pyrite, and cinnabar.

superposition - Law by which, if series of sedimentary rocks has not been overturned, topmost layer is always youngest and lowermost always oldest.

surface wave - Wave that travels along free surface of medium. Earthquake surface waves sometimes represented by symbol L.

surge - Applied to glaciers; rapid and sometimes catastrophic advance of ice.

suspended water - Underground water held in zone of aeration by molecular attraction exerted on water by rock and earth materials and by attraction exerted by water particles on one another.

suspension - Process by which material is buoyed up in air or water and moved about without making contact with surface while in transit. Contrasts with traction.

symmetrical fold - Fold in which axial plane is essentially vertical. Limbs dip at similar angles.

syncline - A configuration of folded, stratified rocks in which rocks dip downward from opposite directions to come together in a trough. Reverse of anticline.

synclinal - Relating to, formed by, or forming a syncline

tablemount - See guyot.

tabular - (1) Shape with large area relative to thickness. (2) Tending to split into thin flat pieces.

taconite - Unleached iron formation of Lake Superior District. Consists of chert with hematite, magnetite, siderite, and hydrous iron silicates. Ore of iron, averaging 25 percent iron, but natural leaching turns it into ore with 50 to 60 percent iron.

talc - Silicate of magnesium common among metamorphic minerals. Crystalline structure based on tetrahedra arranged in sheets; greasy and extremely soft. Sometimes known as soapstone.

talus - Slope established by accumulation of rock fragments at foot of cliff or ridge. Rock fragments that form talus may be rock waste, slide rock, or pieces broken by frost action. Actually, term "talus" widely used to mean rock debris itself.

tarn - Lake formed in bottom of cirque after glacier ice has disappeared.

tectonic - Relating to, causing, or resulting from structural deformation of the earth's crust.

tectonic change of sea level - Change in sea level produced by land movement.

tectonism - (1) The structural behavior of an element of the earth's crust. (2)  Crustal instability.

tectosilicate - Mineral with crystal structure containing silicon-oxygen tetrahedra arranged in three-dimensional frameworks.

temporary base level - Nonpermanent base level, such as that formed by lake.

tension - Stretching stress that tends to increase volume of a material.

terminal moraine - Ridge or belt of till marking farthest advance of a glacier. Sometimes called end moraine.

terminal velocity - Constant rate of fall eventually attained by grain or body when acceleration caused by influence of gravity is balanced by resistance of fluid through which grain falls or air through which body falls.

terrace - Nearly level surface, relatively narrow, bordering a stream or body of water and termination in a steep bank. Commonly term is modified to indicate origin, as in stream terrace and wave-cut terrace.

terrae - Light-toned highlands of moon.

terrigenous - Derived from the land, especially by erosive action. Used primarily of sediments.

terrigenous deposit - Material derived from above sea level and deposited in deep ocean. Example: volcanic ash.

Tertiary - Of, belonging to, or being the geologic time, system of rocks, and sedimentary deposits of the first period of the Cenozoic Era, extending from the Cretaceous Period of the Mesozoic Era to the Quaternary Period of the Cenozoic Era, characterized by the appearance of modern flora and of apes and other large mammals.

tetrahedron - A four-sided solid. Used commonly in describing silicate minerals as shortened reference to silicon-oxygen tetrahedron.

texture - General physical appearance of rock, as shown by size, shape, and arrangement of particles that make it up.

thermal gradient - In earch, rate at which temperature increases with depth below surface.

thermal pollution - Increase in normal temperatures of natural waters caused by intervention of human activities.

thermal spring - Spring that brings warm or hot water to surface. Temperature usually 6.5º C or more above mean air temperature. Sometimes called warm spring, or hot spring.

thermoremanent magnetism - Magnetism acquired by igneous rock as it cools below Curie temperature of magnetic minerals in it. Abbreviation, TRM.

thin section - Slice of rock ground so thin as to be translucent.

tholeiite - Group of basalts primarily composed of plagioclase (approximately An₅₀), pyroxene, and iron oxides as phenocrysts in glassy groundmass of quartz and alkali feldspar; little or no olivine present.

throw - The amount of vertical displacement of a fault.

thrust fault - Fault in which hangin wall appears to have moved upward relative to footwall; opposite of gravity, or normal, fault. Also called reverse fault.

tidal current - Water current generated by tide-producing forces of sun and moon.

tidal inlet - Waterway from open water into a lagoon.

tidal wave - Popular but incorrect designation for tsunami.

tide - Alternate rising and falling of surface of coean, other bodies of water, or earth itself in response to forces resulting from motion of earth, moon, and sun relative to each other.

till - Unstratified, unsorted glacial drift deposited directly by glacier ice.

tillite - Rock formed by lithification of till.

time-distance graph - Graph of travel time against distance.

tombolo - Sand bar connecting an island to mainland or joining two islands.

topographic deserts - Deserts deficient in rainfall because they are either located far from oceans toward center of continents or cut off from rain-bearing winds by high mountains.

topography - Shape and physical features of land.

topset bed - Layer of sediment constituting surface of delta. Usually nearly horizontal and covers edges of inclined foreset beds.

tourmaline - Silicate mineral of boron and aluminum with sodium, calcium, fluorine, iron, lithium, or magnesium. Formed at high temperatures and pressures through agency of fluids carrying boron and fluorine. Particularly associated with pegmatites.

township and range - Most of the area of the United States has been subdivided by a system of land survey in which a square 6 mi on a side is the basic unit, called a township when measured north and south of a given base line, and called a range when measured east and west of a given principal meridian.

traction - Process of carrying material along bottom of a stream. Traction includes movement by saltation, rolling, or sliding.

trailing edge - The margin of a continent that occurs on the side opposite the margin moving toward another continental or oceanic plate (leading edge); commonly is less active tectonically, behaving in a passive manner. The margin of a continent moving away from the locus of spreading along a midoceanic ridge as, for example, the eastern coasts of North and South America.

transcurrent fault - See strike-slip fault.

transducer - Device that picks up relative motion between mass of seismograph and ground and converts this into form that can be recorded.

transform fault - Point at which strike-slip displacements stop and another structural feature, such as a ridge, develops.

transition element - Element in series in which inner shell is being filled with electrons after outer shell has been started. All transition elements metallic in free state.

transpiration - Process by which water vapor escapes from a living plant and enters atmosphere.

transverse dune - Dune formed in areas of scanty vegetation and in which sand has moved in ridge at right angles to wind. Exhibits gentle windward slope and steep leeward slope characteristic of other dunes.

transverse wave - See shear, or shake, wave.

trap rock - Popular synonym for basalt.

travel time - Total elapsed time for wave to travel from source to designated point.

travertine - Form of calcium carbonate, CaCO₃, which forms stalactites, stalagmites, and other deposits in limestone caves or incrustations around mouths of hot and cold calcareous springs. Sometimes known as tufa, or dripstone.

trellis pattern - Roughly rectilinear arrangement of stream courses in pattern reminiscent of garden trellis, developed in region where rocks of differing resistance to erosion have been folded, beveled, and uplifted.

trenches - See island-arc deeps.

TRM - See thermoremanent magnetism.

tropical deserts - Deserts lying between 5º to 30º north and south of equator.

truncated spur - Beveled end of divide between two tributary valleys where they join a main valley that has been glaciated. Glacier of main valley has worn off end of divide.

tsunami - Large wave in ocean generated at time of earthquake. Popularly but incorrectly known as tidal wave. Sometimes called seismic sea wave.

tufa - Calcium carbonate, CaCO₃, formed in stalactites, stalagmites, and other deposits in limestone caves, as incrustations around mouths of hot and cold calcareous springs, or along streams carrying large amounts of calcium carbonate in solution. Sometimes known as travertine or dripstone.

tuff - Rock consolidated from volcanic ash.

tundra - Stretch of Arctic swampland developed on top of permanently frozen ground. Extensive tundra regions have developed in parts of North America, Europe, and Asia.

turbidites - Sedimentary deposits settled out of turbid water carrying particles of widely varying grade size. Characteristically display graded bedding.

turbidity current - Current in which limited volume of turbid or muddy water moves relative to surrounding water because of current's greater density.

turbulent flow - Mechanism by which fluid (such as water) moves over or past a rough surface. Fluid not in contact with irregular boundary outruns that slowed by friction or deflected by uneven surface. Fluid particles move in series of eddies or whirls. Most stream flow is turbulent; turbulent flow is important in both erosion and transportation. Contrast with laminar flow.

type locality - The place or region in which a rock, series of rock, or formation is typically exposed.

ultimate base level - Sea level, lowest possible base level for a stream.

ultrabasic - Containing magnesium and iron and only a very small amount of silica. Used of igneous rock.

ultramatic - Ultrabasic.

unconformity - Buried erosion surface separating two rock masses, older exposed to erosion for long interval of time before deposition of younger. If older rocks were deformed and not horizontal at time of subsequent deposition, surface of separation is angular unconformity. If older rocks remained essentially horizontal during erosion, surface separating them from younger rocks is called disconformity. Unconformity that develops between massive igneous or metamorphic rocks exposed to erosion and then covered by sedimentary rocks is called nonconformity.

unconformable - Showing unconformity.

underground water - Water below ground surface. Also referred to as subsurface water and subterranean water.

uneven fracture - Mineral habit of breaking along rough, irregular surfaces.

uniformitarianism - Concept that present is key to past. This means that processes now operating to modify earth's surface have also operated in geologic past, that there is uniformity of processes past and present.

unit cell - In crystalline structure of mineral, three-dimensional grouping of atoms arbitrarily selected so that mineral's strucutre represented by periodic repetition of this unit in space lattice.

unpaired terrace - A terrace formed when an eroding stream, swinging back and forth across a valley, encounters resistant rock beneath unconsolidated alluvium and is deflected, leaving behind single terrace with no corresponding terrace on other side of stream.

upheaval - A raising of a part of the earth's crust.

uplift - An upheaval.

upthrow - An upward displacement of rock on one side of a fault.

upper - Of, relating to, or being a later division of the period named.

Upper Carboniferous - Pennsylvanian.

valley glacier - Glacier confined to stream valley. Usually fed from a cirque. Sometimes called Alpine glacier or mountain glacier.

valley train - Gently sloping plain underlain by glacial outwash and confined by valley walls.

varve - Pair of thin sedimentary beds, one coarse, one fine. This couplet has been interpreted as representing a cycle of 1 year or interval of thaw followed by interval of freezing in lakes fringing a glacier.

vein - A regularly shaped and lengthy occurrence of an ore; a lode.

ventifact - Pebble, cobble, or boulder that has had its shape or surface modified by wind-driven sand.

vesicle - Small cavity in aphanitic or glassy igneous rock, formed by expansion of bubble of gas or steam during solidification of rock.

virtual geomagnetic pole - Pole consistent with magnetic field as measured at any one locality. Refers to magnetic-field direction of single point, in contrast to geomagnetic pole, which refers to best fit of geocentric dipole for entire earth's field. Most paleomagnetic readings expressed as virtual geomagnetic poles.

viscocity - An internal property of rock that offers resistance to flow. Ratio of deforming force to rate at which changes in shape are produced.

volatile components - Materials in magma, such as water, carbon dioxide, and certain acids, whose vapor pressures are high enough to cause them to become concentrated in any gaseous phase that forms.

volcanic ash - Dust-sized pyroclastic particle: volume equal to, or less than, that of sphere with diameter of 0.06 mm.

volcanic block - Angular mass of newly congealed magma blown out in eruption. Contrast with volcanic bomb.

volcanic bomb - Rounded mass of newly congealed magma blown out in eruption. Contrast with volcanic block.

volcanic breccia - Rock formed from relatively large blocks of congealed lava embedded in mass of ash.

volcanic dust - Pyroclastic detritus consisting of particles of dust size.

volcanic earthquakes - Earthquakes caused by movements of magma or explosions of gases during volcanic activity.

volcanic eruption - Explosive or quiet emission of lava, pyroclastics, or volcanic gases at earth's surface, usually from volcano but rarely from fissures.

volcanic mountains - Mountains built up from extrusion of lava and pyroclastic debris.

volcanic neck - Solidified material filling vent, or pipe, of dead volcano.

volcanic tremor - Continuous shaking of ground associated with certain phase of volcanic eruption.

volcano - Landform developed by accumulation of magmatic products near central vent.

vug - Small unfilled cavity in rock, usually lined with crystalline layer of different composition from surrounding rock.

wadi - See arroyo.

warm glacier - Glacier which reaches melting temperature throughout thickness during summer season.

warm spring - Thermal spring that brings warm water to surface. Temperature usually 6.5º C or more above mean air temperature.

warp - Large section of continent composed of horizontal strata gently bent upward or downward.

wash - See arroyo.

water gap - Gap cut through resistant ridge by superimposed or antecedent stream.

water table - Upper surface of zone of saturation for underground water. An irregular surface with slope or shape determined by quantity of groundwater and permeability of earth materials. In general, highest beneath hills and lowest beneath valleys.

wave - Configuration of matter that transmits energy from one point to another.

weathering - Response of materials once in equilibrium within earth's crust to new conditions at or near contact with water, air, or living matter. See also chemical weathering and mechanical weathering.

wrinkle ridges - Ridges found on surfaces of lunar maria and flooded craters. May be caused by uplift due to volcanism or to compression.

xenolith - Rock fragment foreign to igneous rock in which it occurs. Commonly inclusion of country rock intruded by igneous rock

yazoo-type river - Tributary unable to enter main stream because of natural levees along main stream. Flows along backswamp zone parallel to main stream.

yield point - Maximum stress that solid can withstand without undergoing permanent deformation, either by plastic flow or by rupture.

zone of aeration - Zone immediately below ground surface, in which openings partially filled with air and partially with water trapped by molecular attraction. Subdivided into (1) belt of soil moisture. (2) intermediate belt, and (3) capillary fringe.

zone of saturation - Underground region within which all openings filled with water. Top of zone of saturation is called water table. Water contained within zone of saturation is called groundwater.

zones of regional metamorphism – High-grade, above 700º C; middle-grade, 400 to 700º C; low-grade, 150 to 400º C.