Glossary of Geologic Terms

Introduction

This glossary contains brief definitions of technical geologic terms prepared by the NPS Geologic Resources Inventory (GRI) program and used in GRI publications and NPS geology and minerals websites.

GRI Glossary

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GRI Glossary of Geologic Terms

TERMS	DEFINITIONS

TERMS	DEFINITIONS
aa	A Hawaiian term for lava flows typified by a rough, jagged, or clinkery surface.
abandoned mineral lands (AML)	Lands, waters, and surrounding watersheds that contain facilities, structures, improvements, and disturbances associated with past mineral exploration, extraction, processing, and transportation, including oil and gas features and operations, for which the National Park Service takes action under various authorities to mitigate, reclaim, or restore in order to reduce hazards and impacts to resources.
abandoned mineral lands (AML) feature	An individual element of an AML site, such as vertical shaft, adit, open stope, open pit, highwall, and prospect. Features include structures such as headframes, mills, wellheads, and storage facilities; landform modifications such as access roads, drainage diversions, and drill pads; and piles of ore, protore (marginal-grade ore), waste rock, soil stockpiles, and hardrock or placer tailings.
abandoned mineral lands (AML) site	An area composed of AML features grouped by past ownership, geographical, or other logical grouping containing facilities, structures, improvements, and disturbances associated with past mineral exploration, extraction, processing, and transportation operations.
ablation	All processes by which snow and ice are lost from a glacier, including melting, evaporation (sublimation), wind erosion, and calving.
ablation till	Loosely consolidated rock debris, formerly in or on a glacier, that accumulated in place as ablation removed the surface ice.
absolute age	The geologic age (in years) of a fossil, rock, feature, or event. The term is now in disfavor as it implies a certainty or exactness that may not be possible by present dating methods. See “isotopic age” and “radiometric age.”
abyssal plain	A flat region of the deep ocean floor.
accessory mineral	A mineral whose presence in a rock is not essential to the proper classification of the rock.
accretion (sedimentary)	The gradual addition of new land to an existing landmass by the deposition of sediment, for example, on a beach by the washing up of sand from the sea.
accretion (streams)	The filling-up of a stream channel as a result of such factors such as silting or wave action.
accretion (structural geology)	The addition of island-arc or continental material to a continent via collision, welding, or suturing at a convergent plate boundary.
accretionary prism	A wedge-shaped body of deformed rock consisting of material scraped off oceanic crust as it descends at a subduction zone.
acmite	Synonymous with aegirine. A brown or green monoclinic mineral of the clinopyroxene group: NaFe3+[Si2O6]. It occurs in certain alkali-rich igneous rocks.
actinolite	A bright green or grayish green monoclinic mineral of the amphibole group: Ca(Mg,Fe)5(OH)2[Si8O22]. It may contain manganese. It sometimes occurs in the form of asbestos, and also in fibrous, radiated, or columnar forms in metamorphic rocks (such as schists) and in altered igneous rocks.
active margin	A tectonically active plate boundary where lithospheric plates are converging, diverging, or sliding past one another.
adit	A horizontal passage into a mine from the surface.
aeolian	Describes materials formed, eroded, or deposited by or related to the action of wind.
agglomerate	A consolidated pyroclastic rock made primarily of bombs. Roughly synonymous with spatter.
agglutinate	A welded pyroclastic deposit characterized by vitric material binding the individual clasts that commonly became fused while hot and viscous.
aggradation	The building up of Earth’s surface by depositional processes.
alaskite	An intrusive igneous rock with a high percentage of sodium- or potassium-rich feldspar minerals and a low percentage of mafic minerals.
albite	A silicate (silicon + oxygen) mineral of the plagioclase group composed of sodium and aluminum, NaAlSi3O8.
algal ridge	A low ridge or elevated margin at the seaward (outer) edge of a reef flat, composed of the calcium-carbonate secretions of actively growing calcareous algae.
alkali	Describes silicate (silicon + oxygen) minerals that contain alkali metals such as sodium and potassium.
alkalic	Describes a rock that is enriched in sodium and potassium.
alkali feldspar	A group of potassium-rich feldspar minerals containing alkali metals.
alkali metals	Any of the soft, white, low-density, low-melting, highly reactive metallic elements, including lithium, sodium, potassium, rubidium, cesium, and francium, belonging to Group 1A of the periodic table.
allanite	A brownish black silicate (silicon + oxygen) mineral of the epidote group.
allochthon	A mass of rock or a fault block that tectonic processes moved from its place of origin; commonly underlain by décollements.
allochthonous	Describes rocks or minerals formed elsewhere than their present place.
alloformation	A mappable body of sedimentary rock with roughly parallel layers that is defined and identified on the basis of bounding discontinuities (i.e., erosional truncations or breaks in sedimentation) rather than on the basis of lithologic change.
alluvial fan	A low, relatively flat to gently sloping, fan-shaped mass of loose rock material deposited by a stream, especially in a semiarid region, where a stream issues from a canyon onto a plain or broad valley floor.
alluvial terrace	A stream terrace composed of unconsolidated alluvium produced by a rejuvenated stream via renewed downcutting of the floodplain or valley floor, or by the covering of a terrace with alluvium.
alluvium	Stream-deposited sediment.
alpine glacier	A small glacier in a mountain range (i.e., not an ice cap or ice sheet) that usually originates in a cirque and may flow down into a valley previously carved by a stream. Synonymous with “mountain glacier” and “valley glacier.”
altiplanation	Solifluction and related slope movements that tend to produce flat or terracelike surfaces, especially at high elevations and latitudes.
Altithermal	A dry postglacial interval (from about 8,000 to 4,000 calendar years ago) during which temperatures were higher than at present.
aluminosilicate	A silicate (silicon + oxygen) mineral in which aluminum substitutes for some of the silicon in the SiO4 tetrahedra.
ammonite	Any ammonoid belonging to the suborder Ammonitina, characterized by a thick, ornamental shell with sutures having finely divided lobes and saddles. Range: Jurassic to Cretaceous.
amphibole	A group of silicate (silicon + oxygen) minerals composed of hydrous calcium and magnesium with the general formula (Ca2Mg5)Si8O22(OH)2.
amphibolite	A metamorphic rock consisting mostly of the minerals amphibole and plagioclase, with little or no quartz.
amygdaloidal	Describes rocks with amygdules.
amygdule	A gas cavity or vesicle in an igneous rock that has become filled with secondary minerals.
andesite	A volcanic rock characteristically medium dark in color and containing approximately 57%–63% silica and moderate amounts of iron and magnesium.
angular unconformity	An unconformity between two groups of rocks whose bedding planes are not parallel or in which the older, underlying rocks dip at a different angle (usually steeper) than the younger, overlying strata.
anhedral	A grain lacking well-developed crystal faces.
anhydrite	A sulfate (sulfur + oxygen) mineral composed of anhydrous calcium sulfate, CaSO4; readily alters to gypsum (hydrous calcium sulfate).
anorthite	A silicate (silicon + oxygen) mineral composed of calcium and aluminum, CaAl2Si2O8.
anthodite	A pencil-like speleothem usually composed of aragonite; commonly occurs in clusters radiating out from a common base.
anthracite	An organic sedimentary rock referred to as “hard coal,” formed from bituminous coal by pressures and temperatures existing in the metamorphic environment of deepest burial accompanied by folding deformation of enclosing rocks. The fixed carbon content reaches a maximum of 85% to 90%. Under still more intense folding deformation and metamorphism, graphite or pure carbon is formed from anthracite.
anticline	A fold, generally convex upward (“A”-shaped) whose core contains the stratigraphically older rocks.
anticlinorium	A large anticlinal structure of regional extent composed of lesser folds.
apatite	A group of phosphate (phosphorus + oxygen) minerals composed of calcium together with fluorine, chlorine, hydroxyl, or carbonate in varying amounts and having the general formula Ca5(F,OH,Cl)(PO4,CO3)3.
aphanitic	Describes the texture of fine-grained igneous rock in which different components are not distinguishable by the unaided eye.
aphyric	Describes the texture of a fine-grained igneous rock that lacks coarse crystals.
aplite	A light-colored, fine-grained intrusive igneous rock emplaced at relatively shallow depths beneath Earth’s surface.
aquiclude	A saturated geologic unit that is incapable of transmitting significant quantities of water under ordinary hydraulic gradients. Replaced by the term “confining bed.”
aquifer	A rock or sedimentary unit that is sufficiently porous to hold water, sufficiently permeable to allow water to move through it, and saturated to some level.
aragonite	A carbonate (carbon + oxygen) mineral of calcium, CaCO?; the second most abundant cave mineral after calcite, differing from calcite in its crystal structure.
arenaceous	Describes a sediment or sedimentary rock consisting of sand-sized fragments, or having a sandy texture or the appearance of sand.
arenite	A general term for sedimentary rocks composed of sand-sized fragments.
arête	A sharp-edged rocky ridge, commonly present above snowline in rugged, glacially sculpted mountains; results from the continued backward growth of the walls of adjoining cirques.
argillaceous	Pertaining to, largely composed of, or containing clay-size particles or clay minerals.
argillite	A weakly metamorphosed rock, derived from mudstone or shale, but more highly indurated; lacks the fissility of shale and the cleavage of slate.
arkose	A commonly coarse-grained, pink or reddish sandstone consisting of abundant feldspar minerals.
arroyo	A small, deep, flat-floored channel or gully of an ephemeral stream in the arid and semiarid regions of the southwestern United States.
artesian	Describes groundwater confined under hydrostatic pressure.
artesian pressure	Hydrostatic pressure of artesian water, often expressed in terms of pounds per square inch at the land surface, or in terms of the height (in feet above the land surface) of a column of water that would be supported by the pressure.
artesian spring	A spring from which water flows under artesian pressure, usually through a fissure or other opening in the confining bed above the aquifer.
artesian system	Any system incorporating the following: a water source, a body of permeable rock bounded by bodies of distinctly less permeable rock, and a structure enabling water to percolate into and become confined in the permeable rock under pressure distinctly greater than atmospheric.
artesian well	A well that taps confined groundwater. Water in the well rises above the level of the top of the aquifer under artesian pressure.
artiodactyla	Any of the order Artiodactyla of ungulates (e.g., camel or pig) with an even number of functional toes on each foot.
ash	Fine-grained material, less than 2 mm (0.08 in) across, ejected from a volcano.
ash fall	Airborne ash that falls from an eruption cloud, and the resulting deposit.
ash flow	A density current, generally a hot mixture of volcanic gases and tephra that travels across the ground surface; produced by the explosive disintegration of viscous lava in a volcanic center, or from a fissure or group of fissures. The solid materials contained in a typical ash flow are generally unsorted and ordinarily include volcanic dust, pumice, scoria, and blocks in addition to ash.
ash-flow tuff	A tuff deposited by an ash flow.
asthenosphere	Earth’s relatively weak layer below the rigid lithosphere where isostatic adjustments take place, magmas may be generated, and seismic waves are strongly attenuated; part of the upper mantle.
astronomical tide	The periodic rise and fall of a body of water resulting from gravitational interactions between the Sun, Moon, and Earth. Synonymous with “tide,” but used to emphasize the absence of atmospheric influences.
atoll	A ring or horseshoe-shaped coral reef rising from deep water; commonly 1 km (0.6 mi) to more than 100 km (60 mi) across.
augen	Describes large lenticular minerals that are eye-shaped in cross section.
augite	A dark-green to black silicate (silicon + oxygen) mineral of the pyroxene group that contains abundant aluminum, iron, and magnesium.
aulacogen	Meaning “furrow” in Greek. A sediment-filled continental rift that trends at a high angle to the adjacent continental margin or orogeny. Also, a narrow, elongate basin that extends into the craton either from a passive-margin basin or from a mountain belt that formed from a passive-margin basin.
aureole	A zone surrounding an igneous intrusion in which the country rock shows the effects of contact metamorphism.
authigenic	Formed or generated in place, specifically said of rock constituents and minerals that have not been transported or that crystallized locally at the spot where they are now found, and of minerals that came into existence at the same time as, or subsequently to, the formation of the rock of which they constitute a part.
autochthon	A body of rocks in the footwall of a fault that has not moved substantially from its site of origin; the rocks may be slightly to considerably deformed, however.
autochthonous	A widely applied term, meaning formed or produced in the place where now found. It is similar in meaning to authigenic, which refers to constituents rather than whole formations.
avalanche	A large mass of snow, ice, soil, or rock, or a mixture of these materials, falling, sliding, or flowing very rapidly under the force of gravity; velocities may exceed 500 kph (300 mph). Commonly, in geologic writing, an avalanche other than a “snow avalanche” includes a descriptive term such as “debris avalanche.”
avulsion	The sudden cutting off or separation of land by a flood or an abrupt change in a stream course.
axial stream	The main stream of an intermontane valley, flowing in the deepest part of the valley parallel to it longest dimension. Also, a stream that follows a syncline or anticline.
axis	A straight-line approximation of the trend of a fold along the boundary between its two limbs. “Hinge line” is a preferred term.
azimuth	Direction of a horizontal line as measured on an imaginary horizontal circle.
badlands	Eroded topography characterized by steep slopes and surfaces with little or no vegetative cover; composed of unconsolidated or poorly cemented clays or silts.
back barrier	The landward side of a barrier island.
back reef	The landward side of a reef.
backshore	The upper or inner, usually dry, zone of a shore or beach. Also, the area lying immediately at the base of a sea cliff. Synonymous with “backbeach.”
backwasting	Wasting (gradual erosion) that causes a slope to retreat without changing its gradient.
backwater	A body of water that is parallel to a river but is stagnant or little affected by the river’s currents.
bajada	A broad, continuous alluvial slope or gently inclined detrital surface formed from the coalescence of alluvial fans along a basin margin.
bank	A submerged ridge of sand in the sea, a lake, or a river, usually exposed during low tide or low water.
bank atoll	A ring-shaped coral reef or reef complex rising from an oceanic plateau or submarine bank.
barchan dune	A crescent-shaped dune in which the arms or horns of the crescent point downwind; characteristic of inland desert regions.
barrier island	A long, low, narrow island consisting of a ridge of sand that parallels the coast.
basalt	A volcanic rock that is characteristically dark in color (gray to black), contains approximately 53% silica or less, and is rich in iron and magnesium.
basaltic andesite	A volcanic rock that is commonly dark gray to black and contains approximately 53%–57% silica.
base flow	Streamflow supported by groundwater and not attributed to direct runoff from precipitation or snow melt.
base level	The lowest level to which a stream channel can erode. The ultimate base level is sea level, but temporary, local base levels exist.
basement	The undifferentiated rocks, commonly igneous and metamorphic, that underlie rocks exposed at the surface, commonly sedimentary. In many regions the basement is of Precambrian age, but it may be much younger. Also, Earth’s crust below sedimentary deposits that extends down to the Mohorovicic discontinuity.
basic	Describes an igneous rock that is relatively low in silica and relatively rich in iron, magnesium, and/or calcium.
basin (structural)	A doubly plunging syncline in which rocks dip inward from all sides.
basin (sedimentary)	Any depression, from continental to local scale, into which sediments are deposited.
batholith	A large, generally discordant plutonic body having an aerial extent of 40 mi2 (100 km2) or more and no known floor.
bathymetry	The measurement of ocean or lake depths and the charting of the topography of the ocean or lake floor.
bayhead bar	An elongate, offshore ridge or mass, usually of sand, formed a short distance from the shore and across a bay near its head; similar to a bayhead barrier, but smaller and generally submerged.
bayhead barrier	An elongate, offshore ridge or mass, usually of sand, rising above the high-tide level, formed a short distance from the shore and across a bay near its head. It commonly has a narrow inlet.
baymouth bar	A long, narrow accumulation of sand or gravel, deposited entirely or partly by waves across the mouth or entrance of a bay.
beach	The unconsolidated material at the shoreline that covers a gently sloping zone, typically with a concave profile, extending landward from the low-water line to the place where there is a definite change in material or physiographic form (e.g., a cliff), or to the line of permanent vegetation (usually the effective limit of the highest storm waves).
beach face	The section of the beach normally exposed to the action of wave uprush.
beachrock	A poorly to well-cemented sedimentary rock formed in the intertidal zone, consisting of sand and gravel cemented with calcium carbonate.
bed	The smallest sedimentary stratigraphic unit, commonly ranging in thickness from about 1 cm (0.4 in) to 1 to 2 m (40 to 80 in) and distinguishable from beds above and below.
bedding	Depositional layering or stratification of sediments.
bed load	The part of the total stream load that is moved on or immediately above the stream bed.
bedrock	Solid rock that underlies unconsolidated sedimentary deposits and soil.
bedset	A succession of beds bounded by surfaces of erosion or non-deposition.
benthic	Pertaining to the ocean bottom or organisms living on or in substrate; also, referring to that environment.
bentonite	Soft clay or greasy claystone composed mostly of the clay mineral smectite, formed by the chemical alteration of glassy volcanic ash in contact with water.
bergschrund	The crevasse at the head of an alpine glacier that separates the moving snow and ice from the relatively immobile snow and ice adhering to the headwall of a cirque.
berm	A low, impermanent, nearly horizontal or landward-sloping bench, shelf, or ledge above the high-water line on a beach.
bioherm	A moundlike, domelike, lenslike, or reeflike mass of rock built up by sedentary organisms, composed almost exclusively of their calcareous remains, and enclosed or surrounded by rock of different lithology.
biotite	A dark-colored, shiny silicate mineral (silicon + oxygen) of the mica group composed of magnesium and/or iron, K(Mg,Fe)Si3O10(OH)2; characterized by perfect cleavage, readily splitting into thin sheets.
bioturbation	The reworking of sediments by organisms.
bituminous	Describes rocks that contain (and sometimes smell of) asphalt, tar, or petroleum.
bituminous coal	An organic sedimentary rock referred to as “soft coal,” resulting from deeper burial with rising pressures and temperatures, driving hydrogen and other volatiles and leaving a fixed carbon content of 50% to 60%.
bituminous limestone	A dark, dense limestone containing abundant organic matter that probably accumulated under stagnant conditions; commonly emits a fetid odor when freshly broken or vigorously rubbed.
bivalve	Having a shell composed of two distinct, but equal or nearly equal, movable valves, which open and shut.
block	A pyroclast ejected in a solid state; it has a diameter greater than 64 mm (2.5 in).
block (fault)	A segment of Earth’s crust bounded completely or partially by faults.
blowout	A general term for a small saucer-, cup-, or trough-shaped hollow or depression formed by wind erosion on a preexisting dune or other sand deposit, especially in an area of shifting sand or loose soil, or where protective vegetation is disturbed or destroyed.
body fossil	Evidence of past organisms such as bones, teeth, shells, or leaf imprints.
bog iron ore	A soft, spongy, and porous deposit of impure hydrous iron oxide, formed in bogs, marshes, swamps, peat mosses, and shallow lakes by precipitation from iron-bearing waters and by the oxidizing action of algae, iron bacteria, or the atmosphere.
bolson	An extensive, flat-floored basin covered with alluvium into which drainage from the surrounding mountains flows toward a playa or central depression. The term is used in the desert regions of the southwestern United States.
bomb	A viscous pyroclast more than 64 mm (2.5 in) in diameter ejected then shaped while in flight; it commonly has a hollow or vesicular interior.
boudinage	A structure in highly deformed sedimentary or metamorphic rocks, in which an originally continuous layer or bed has been stretched, thinned, and broken at regular intervals into bodies resembling “boudins” (sausages).
boulder	A detached rock mass larger than a cobble (see below), having a diameter greater than 256 mm (10 in); the largest rock fragment recognized by sedimentologists.
Bouma cycle	A fixed, characteristic succession of five intervals that makes up a complete sequence of a turbidite, from bottom top (oldest to youngest): (1) graded, (2) lower parallel laminations, (3) current ripple laminations, (4) upper parallel laminations, and (5) politic; one or more intervals may be missing. Named after Arnold Bouma (1932–2011), a renowned researcher, educator, and sedimentologist.
bounding surface	An erosion truncation surface that separates groups of cross-beds on various scales.
boxwork	A network of resistant, intersecting fins, usually of calcite, that project outward from the intervening, more deeply weathered bedrock of a cave wall.
brachiopod	Any marine invertebrate belonging to the phylum Brachiopoda, characterized by two bilaterally symmetrical valves that are commonly attached to a substratum but may also be free. Range: Lower Cambrian to Holocene.
braided stream	A sediment-clogged stream that forms multiple channels that divide and rejoin.
breakdown	Collapse of a cave ceiling or walls of a cave. Also, the accumulation of debris thus formed.
breakwater	An offshore, generally shore-parallel structure that breaks the force of the waves.
breccia	A coarse-grained, generally unsorted sedimentary rock consisting of cemented angular clasts more than 2 mm (0.08 in) across.
breccia (volcanic)	A coarse-grained, generally unsorted volcanic rock consisting of partially welded angular fragments of ejected material.
brecciated	Converted into, characterized by, or resembling a breccia.
brecciation	Formation of a breccia, as by crushing or breaking a rock into angular fragments.
brittle	Describes a rock that fractures before sustaining deformation.
bryozoan	Any invertebrate belonging to the phylum Bryozoa; characterized by colonial growth and a calcareous skeleton. Range: Ordovician (and possibly Upper Cambrian) to Holocene.
burrow	A tubular or cylindrical hole or opening, made in originally soft or loose sediment by a mud-eating worm, mollusk, or other invertebrate; may be later filled with clay or sand and preserved.
calcareous	Describes a substance that contains calcium carbonate. When applied to a rock name it implies that as much as 50% of the rock is calcium carbonate.
calcarenite	Limestone consisting of more than 50% sand-size carbonate grains.
calcium carbonate	CaCO3. A solid occurring in nature as primarily calcite and aragonite.
calc-silicate rock	A metamorphic rock consisting mainly of calcium-bearing silicates and formed by metamorphism of impure limestone or dolomite.
calcic	Describes a mineral or igneous rock containing a significant amount of calcium.
calcite	A carbonate (carbon + oxygen) mineral of calcium, CaCO?; calcium carbonate. It is the most abundant cave mineral.
calcrete	A deposit of calcium carbonate that forms a cemented layers within a soil profile. Synonymous with “caliche,” especially used in the southwestern United States.
caldera	A large, more-or-less circular, basin-shaped volcanic depression formed by collapse during an eruption.
caliche	A hard layer of cemented calcium carbonate, commonly on or near the surface in arid and semiarid regions.
calving	The breaking away of a mass of ice from a near-vertical ice face into a body of water.
cape	An extensive, somewhat rounded irregularity of land jutting out from the coast into a large body of water, either as a peninsula (e.g., Cape Cod, Massachusetts) or as a projecting point (e.g., Cape Hatteras, North Carolina). Also, the part of the projection extending farthest into the water.
capillary action	The action by which a fluid, such as water, is drawn up in small interstices or tubes as a result of surface tension.
capillary fringe	The lowest part of the unsaturated zone immediately above the water table in which the interstices (intervening spaces) are filled with water as a result of capillary action.
carbonaceous	Describes a rock or sediment with considerable carbon, especially organic material, hydrocarbon, or coal.
carbonate	A mineral group composed of carbon and oxygen plus an element or elements; for example calcite, CaCO3; and dolomite, CaMg(CO3)2.
carbonate rock	A rock, for example, limestone, calcite, and dolomite, that consist primarily of carbonate minerals.
cataclastic	Describes a structure in a rock, such as bending, breaking, or crushing of minerals, that results from extreme stress during metamorphism.
cataclasite	A fine-grained rock formed by pervasive fracturing, milling, crushing, and grinding by brittle deformation, typically under high pressure.
cauldron	An inclusive term for all volcanic subsidence structures regardless of shape or size, depth of erosion, or connection with the surface; the term includes caldron subsidences and collapse calderas.
cay	A small, low, coastal island or emergent reef of sand or coral. Also, a flat mound of sand and coral fragments built up on a reef flat at or just above high-tide level. The term is used especially in the Caribbean where it is pronounced “key” and sometimes spelled “kay.”
cement (sedimentary)	Mineral material, usually chemically precipitated, that occurs in the spaces among the individual grains of a sedimentary rock, thus binding the grains together.
cementation	The process by which clastic sediments become lithified or consolidated into hard, compact rocks, usually through deposition or precipitation of minerals in the spaces among the individual grains of the sediment; may occur simultaneously with sedimentation or at a later time.
cephalopod	A marine mollusk of the class Cephalopoda, characterized by a head surrounded by tentacles and, in most fossil forms, a straight, curved, or coiled calcareous shell divided into chambers. Range: Cambrian to Holocene.
chalcedony	A cryptocrystalline variety of quartz.
channel	The bed of a stream or river. Also, a natural passageway or depression of perceptible extent containing continuously or periodically flowing water, or forming a connecting link between two bodies of water.
channel (coastal)	A relatively narrow sea or stretch of water between two close landmasses and connecting two larger bodies of water.
channel bar	An elongated deposit of sand and gravel in the course of a stream; commonly in braided streams.
chemical weathering	Chemical breakdown of minerals at Earth’s surface via reaction with water, air, or dissolved substances; commonly results in a change in chemical composition providing more stablity in the current environment.
chert	An extremely hard sedimentary rock with conchoidal (resembling a conch shell) fracturing, consisting mostly of interlocking crystals of quartz.
chertification	A type of silicification in which fine-grained quartz or chalcedony is introduced into limestone or dolomite.
chlorite	A group of silicate (silicon + oxygen) minerals composed of iron, magnesium, and aluminum with the general formula (Mg,Fe)6(AlSiO3)O10(OH)8.
chronology	The arrangement of events in their proper sequence in time.
chronostratigraphy	The branch of stratigraphy that deals with the ages of strata and their time relations.
chute cutoff	A narrow “short cut” across a meander bend.
cinder	A glassy, vesicular, pyroclastic fragment that falls to the ground in an essentially solid condition.
cinder cone	A conical hill, commonly steep, ranging from tens to hundreds of meters tall, formed by the accumulation of solidified fragments of lava that fell around the vent during a basaltic or andesitic eruption.
cirque	A semicircular, topographic hollow eroded by alpine glaciers; commonly contains a small round lake, and may or may not be occupied by ice or snow.
clast	An individual constituent, grain, or fragment of a rock or unconsolidated deposit, produced by the mechanical or chemical disintegration of a larger rock mass.
clastic	Describes rocks or sediments made of fragments of preexisting rocks.
clastic dike	A tabular mass of sedimentary material that cuts across the structure or bedding of preexisting rock in the manner of an igneous dike; formed by filling cracks or fissures from below, above, or laterally.
clay	Minerals and sedimentary fragments that are less than 1/256 mm (0.00015 in) across.
clay mineral	Any mineral occurring in the clay-sized fraction with the understanding that size imposes physical and chemical characteristics.
claystone (igneous)	A dull, altered, feldspathic igneous rock in which the groundmass or the whole rock has been reduced to a compact mass of earthy or clayey alteration products.
claystone (sedimentary)	An indurated rock with more than 67% clay-sized minerals.
cleavage	The tendency of a mineral to break along planes of weak bonding.
climbing dune	A dune formed by aeolian piling-up of sand by wind against a cliff or mountain slope.
clinopyroxene	A group name for pyroxene minerals that crystallize in the monoclinic system and sometimes contain considerable calcium with or without aluminum and the alkali metals.
coal	An organic sedimentary rock consisting of carbon, hydrogen, and oxygen, with some sulfur and nitrogen. Formed by the destructive distillation of plant remains under anaerobic and progressively rising pressures and temperatures.
coalification	The alteration or metamorphism of plant material into coal.
coarse-grained	Describes a crystalline rock and texture in which the individual minerals are relatively large, specifically an igneous rock whose particles have an average diameter greater than 5 mm (0.2 in). Also, describes sediment or sedimentary rock and texture in which the individual constituents are easily seen with the unaided eye, specifically sediment or rock whose particles have an average diameter greater than 2 mm (0.08 in).
coastal plain	Any lowland area bordering a sea or ocean, extending inland to the nearest elevated land, and sloping very gently seaward; may result from the accumulation of material along a coast.
cobble	A rock fragment larger than a pebble and smaller than a boulder, having a diameter in the range of 64–256 mm (2.5–10 in), being somewhat rounded or otherwise modified by abrasion in the course of transport.
cohesion	The intermolecular attraction by which the elements of a body are held together.
coignimbrite	Fallout tephra deposited from a pyroclastic flow, composed of near-vent breccias with large lithic clasts, which dropped from pyroclastic flows, and fine-grained ash elutriated (separated based on size) from the top of a pyroclastic flow by the turbulent rise of hot gases.
collapse structure	Any rock structure resulting from the removal of support and consequent collapse by the force of gravity, for example, gravitational sliding on fold limbs, salt solution causing collapse of overlying rocks in salt basins, sinkhole collapse, or collapse into mine workings.
colluvium	A general term applied to loose and incoherent deposits, usually at the foot of a slope or cliff and brought there chiefly by gravity.
columnar joints	Parallel, prismatic columns, polygonal in cross section, in basaltic flows and sometimes in other extrusive and intrusive rocks; form as a result of contraction during cooling.
compaction	The process whereby fine-grained sediment is converted to consolidated rock.
compression	A decrease in volume of material (including Earth’s crust) as it is pressed or squeezed together.
composite volcano	Steep, conical volcanoes built by the eruption of viscous lava flows, tephra, and pyroclastic flows; usually constructed over tens to hundreds of thousands of years and may erupt a variety of magma types (basalt to rhyolite); typically consist of many separate vents. Synonymous with “stratovolcano.”
conchoidal	Resembling the curve of a conch shell and used to describe a smoothly curved surface on a rock or mineral; characteristic of quartz and obsidian.
concordant	Describes a stratum with contacts parallel to the orientation of adjacent strata.
concretion	A hard, compact aggregate of mineral matter, rounded to irregularly shaped; composition generally differs from that of the rock in which it occurs.
confined aquifer	An aquifer bounded above and below by confining beds. An aquifer containing confined groundwater.
confined groundwater	Groundwater under pressure significantly greater than that of the atmosphere. Its upper surface is the bottom of a confining bed.
confining bed	A body of relatively impermeable or distinctly less permeable material stratigraphically adjacent to one or more aquifers. Replaced the term “aquiclude.”
conglomerate	A coarse-grained, generally unsorted, sedimentary rock consisting of cemented, rounded clasts larger than 2 mm (0.08 in) in diameter.
conjugate set	A pair of intersecting joints, usually formed during compression; commonly described on the basis of the angle of intersection.
conodont	One of a number of small fossil elements assigned to the order Conodontophorida; commonly toothlike in form but not necessarily in function.
contact	The surface between two types or ages of rocks.
contact metamorphism	One of the principal local processes of thermal metamorphism, genetically related to the intrusion and extrusion of magmas and taking place in rocks at or near their contact with a body of igneous rock. Metamorphic changes are effected by the heat and fluids emanating from the magma and by some deformation connected with the emplacement of the igneous mass.
continental	Formed on land rather than in the sea. Continental deposits may be of lake, swamp, wind, stream, or volcanic origin.
continental drift	A term for the process by which continents move relative to one another; it is a consequence of plate tectonics.
continental crust	Earth’s crust that is rich in silica and aluminum and underlies the continents and the continental shelves; ranges in thickness from about 25 km (15 mi) to more than 70 km (40 mi) under mountain ranges, averaging about 40 km (25 km) thick.
continental rise	Gently sloping region from the foot of the continental slope to the deep ocean abyssal plain; generally consists of smooth topography but may have submarine canyons.
continental shelf	The shallowly submerged—covered by water depths of less than 200 m (660 ft)—part of a continental margin that extends from the shoreline to the continental slope.
continental shield	A large area of exposed basement at the interior of a continent, commonly with a very gently convex surface, surrounded by sediment-covered platforms. The rocks of virtually all shield areas are Precambrian.
continental slope	The relatively steep slope from the outer edge of the continental shelf down to the more gently sloping ocean depths of the continental rise or abyssal plain.
convergent plate boundary	A boundary between two plates that are moving toward each other. Essentially synonymous with “subduction zone” but used in different contexts.
coprolite	Fossilized feces.
coquina	Limestone composed of cemented shell fragments.
coral	Any of a large group of bottom-dwelling, sessile, marine invertebrate organisms (polyps) that belong to the class Anthozoa (phylum Cnidaria), characterized by production of an external skeleton of calcium carbonate; may exist as solitary individuals or grow in colonies. Range: Abundant in the fossil record in all periods later than the Cambrian.
coralline algae	Calcareous algae that form encrustations resembling coral.
cordillera	An extensive assemblage of more or less parallel chains of mountains (together with their associated valleys, basins, plains, plateaus, rivers, and lakes), especially the main mountain axis of a continent.
core	Earth’s center, consisting of an inner core and outer core: The inner core extends from a depth of about 5,200 km (3,200 mi) to the center of the Earth at 6,371 km (3,959 mi). The outer core extends from a depth of 2,900 km (1,800 mi) to the inner core and is presumed to be liquid.
core (drill)	A cylindrical section of rock or sediment. usually 5–10 cm (2–4 in) across and up to several meters long, taken as a sample of the interval penetrated by a core bit, and brought to the surface for geologic examination and/or laboratory analysis.
country rock	The rock surrounding an igneous intrusion or pluton; also, the rock enclosing or traversed by a mineral deposit.
craton	The relatively old and geologically stable interior of a continent.
creep	The slow, imperceptible downslope movement of mineral, rock, and soil particles under gravity.
crevasse	A deep fissure or crack in a glacier, resulting from differential movement over an uneven surface; may be as much as 100 m (300 ft) deep.
crinoid	A marine invertebrate (echinoderm) that uses a stalk to attach itself to a substrate; “arms” are used to capture food. Range: Paleozoic to Holocene, through very common in the Paleozoic and rare today.
cross-bed	A single bed, inclined at an angle to the main planes of stratification; the term is commonly restricted to a bed that is more than 1 cm (0.4 in) thick.
cross-bedding	Uniform to highly varied sets of inclined beds deposited by wind or water that indicate flow conditions such as direction and depth.
cross section	A graphic interpretation of geology, structure, or stratigraphy based on mapped and measured geologic extents and attitudes, depicted in a vertical plane (i.e., a cut or profile view).
cross-stratification	Arrangement of strata inclined at an angle to the main stratification. This is a general term that is commonly divided into cross-bed, which is cross-strata thicker than 1 cm (0.4 in); and cross-lamination, which is cross-strata thinner than 1 cm (0.4 in).
crust	Earth’s outermost layer or shell.
cryptocrystalline	Describes a rock texture in which individual crystals are too small to be recognized or distinguished with an ordinary microscope.
crystalline	Describes a regular, orderly, repeating geometric structural arrangement of atoms.
crystal structure	The orderly and repeated arrangement of atoms in a crystal.
cuesta	A hill or ridge with a gentle slope on one side and a steep slope on the other.
cumulate	An igneous rock formed by accumulation of crystals that settle out from a magma by the action of gravity.
cutbank	A steep, bare, slope formed by lateral erosion of a stream.
cycad	A member of a group of seed-bearing plants (Cycadales) with pinnately compound leaves, poorly developed wood, and ovules borne on loosely or compactly arranged spore-bearing leaves in terminal cones. Range: late Carboniferous (?) to Holocene.
dacite	A volcanic rock that is characteristically light in color and contains approximately 63%–68% silica and moderate amounts of sodium and potassium.
debris	Any surficial accumulation of loose material detached from rock masses by chemical or mechanical means; consists of rock fragments, soil material, and sometimes organic matter.
debris cone	A cone or mound of ice or snow on a glacier, covered with a veneer of debris thick enough to protect the underlying material from ablation.
debris flow	A moving mass of rock fragments, soil, and mud, with more than half of the particles larger than sand size. Slow debris flows may move less that 1 m (3 ft) per year; rapid ones reach 160 kph (100 mph).
décollement	A large (kilometers to tens of kilometers) fault of shear zone that is shallowly dipping to nearly horizontal; typically, faults and folds in rocks above the décollement do not extend across it. Synonymous with “detachment fault.”
dedolomitization	A process resulting from metamorphism, wherein part or all of the magnesium in a dolomite or dolomitic limestone is used for the formation of magnesium oxides, hydroxides, and silicates and resulting in an enrichment in calcite.
deflation	The removal of material from a beach, desert, or other land surface by wind action.
deformation	The process of folding, faulting, shearing, or fabric development in rocks as a result of Earth stresses.
delta	The low, nearly flat, alluvial tract of land at or near the mouth of a river, commonly forming a triangular or fan-shaped plain of considerable area; resulting from the accumulation of sediment supplied by the river in such quantities that it is not removed by tides, waves, and currents.
delta fan	A deposit formed by the merging of an alluvial fan with a delta.
delta plain	The level or nearly level surface composing the landward part of a large or compound delta; strictly, an alluvial plain characterized by repeated channel bifurcation and divergence, multiple distributary channels, and interdistributary flood basins.
dendritic	Describes a branching pattern.
dendrochronology	The study of annual growth rings of trees for dating of the recent past.
depocenter	An area or site of maximum deposition.
depth zone	One of five oceanic environments, or ranges of oceanic depths: (1) littoral zone, between high and low tides; (2) neritic zone, between low-tide level and 200 m (660 ft) above the continental shelf; (3) bathyal zone, between 200 and 3,500 m (660 and 11,500 ft); (4) abyssal zone, between 3,500 and 6,000 m (11,500 and 20,000 ft); and (5) hadal zone 6,000 m (20,000 ft) and deeper.
desert pavement	A natural residual concentration of wind-polished, closely packed pebbles, boulders, and other rock fragments, mantling a desert surface where wind action or sheetwash have removed all smaller particles.
desiccation	A complete or nearly complete drying-out or drying-up.
detachment fault	A regionally extensive, gently dipping normal fault that is commonly associated with extension in a metamorphic core complex. Synonymous with “décollement.”
detrital	Pertaining to or formed from detritus.
detrital mineral	Any mineral grain resulting from mechanical disintegration of parent rock, especially a heavy mineral found in a sediment, or weathered and transported from a vein or lode and found in a placer or alluvial deposit.
detritus	Loose rock and mineral material that is worn off or removed by mechanical processes.
devitrification	Conversion of glass to crystalline material.
diabase	An intrusive igneous rock consisting primarily of the minerals labradorite and pyroxene.
diamictite	Nonsorted or poorly sorted, noncalcareous, terrigenous sedimentary rock that contains a wide range of particle sizes, such as a rock with sand and/or larger particles in a muddy matrix. The term implies no origin.
diamicton	The nonlithified equivalent of diamictite.
diatom	A microscopic, single-celled alga that secretes walls of silica, called frustules; lives in freshwater or marine environments.
diatomite	A light-colored, soft, silica-rich sedimentary rock consisting mostly of diatoms.
diatreme	A breccia-filled volcanic pipe formed by gaseous explosion.
differential erosion	Erosion that occurs at irregular or varying rates due to differences in the resistance and hardness of surface material: softer and weaker rocks are rapidly worn away; harder and more resistant rocks remain to form ridges, hills, or mountains.
dike	A narrow igneous intrusion that cuts across bedding planes or other geologic structures.
diopside	A silicate (silicon + oxygen) mineral of the pyroxene group; occurs in mafic and ultramafic igneous rocks and some areas of contact metamorphism.
diorite	A coarse-grained, intrusive igneous rock characteristically containing plagioclase, as well as dark-colored amphibole (especially hornblende), pyroxene, and sometimes a small amount of quartz; diorite grades into monzodiorite with the addition of alkali feldspar.
dip	The angle between a bed or other geologic surface and the horizontal plane.
dip-slip fault	A fault with measurable offset where the relative movement is parallel to the dip of the fault.
discharge	The rate of flow of surface water or groundwater at a given moment, expressed as volume per unit of time.
disconformity	An unconformity in which the bedding of strata above and below is parallel.
discordant	Describes an igneous intrusion having contacts that are not parallel to foliation or bedding in the country rock.
displacement	The relative movement of the two sides of a fault; also, the specific amount of such movement.
divergent plate boundary	A boundary between two plates that are moving apart, characterized by mid-ocean ridges at which sea-floor spreading occurs.
doline	A type of sinkhole or a karst collapse feature.
dolomite (mineral)	A carbonate (carbon + oxygen) mineral of calcium and magnesium, CaMg(CO3)2.
dolomite (rock)	A carbonate sedimentary rock containing more than 50% of the mineral dolomite (calcium-magnesium carbonate). The rock is sometimes referred to as “dolostone” to distinguish it from the mineral.
dolomitic	Describes a rock containing dolomite, especially one that contains 5%–50% of the mineral dolomite in the form of cement and/or grains or crystals.
dolomitization	The process by which limestone is wholly or partly converted to dolomite or dolomitic limestone through the replacement of the original calcium carbonate (calcite) by magnesium carbonate (mineral dolomite), usually through the action of magnesium-bearing water (seawater or percolating meteoric water).
dome	Any smoothly rounded landform or rock mass; more specifically, an elliptical uplift in which rocks dip gently away in all directions.
downcutting	Stream erosion in which cutting is directed primarily downward, as opposed to laterally.
downwarping	Subsidence of Earth’s crust on a regional scale as a result of crustal loading by ice, water, sediments, or lava flows.
drainage	The manner in which the waters of an area flow off in surface streams or subsurface conduits; also, the processes of surface drainage of water from an area by streamflow and sheet flow, and the removal of excess water from soil by downward flow.
drainage basin	A region or area bounded by a drainage divide and occupied by a drainage system, specifically the tract of country that gathers water originating as precipitation and contributes it to a particular stream channel or system of channels, or to a lake, reservoir, or other body of water.
dredge (engineering)	A floating machine for excavating sedimentary material from the bottom of a body of water.
dredge (oceanography)	An ocean-bottom sampler that scrapes material from the sea floor as the device is dragged behind a slowly moving ship.
driblet	Pyroclastic material ejected from a vent. Synonymous with “spatter.”
drift	All rock material (clay, silt, sand, gravel, and boulders) transported and deposited by a glacier, or by running water emanating from a glacier.
dripstone	A mineral deposit formed in a cave by dripping water.
dropstone	An oversized stone in laminated sediment that depresses the underlying laminae and may be covered by other laminae that drape over the stone; commonly deposited from ice rafts.
drumlin	A low, smoothly rounded, elongated oval hill, mound, or ridge of till that formed under the ice margin and was shaped by glacial flow; the long axis is parallel to the direction of ice movement.
ductile	Describes a rock that is able to sustain deformation such as folding, bending, or shearing before fracturing.
dune	A low mound or ridge of sediment, usually sand, deposited by the wind.
dune field	Extensive deposits of sand in an area where the supply is abundant; individual dunes resemble barchans dunes but are highly irregular in shape and crowded.
dunite	Peridotite in which the mafic mineral is almost entirely olivine, with accessory chromite almost always present.
ebb-tidal delta	A tidal delta formed on the seaward side of a tidal inlet.
edifice	The constructional mass of a volcano.
efflorescence	A whitish fluffy or crystalline powder produced as a surface encrustation on a rock or soil in an arid region by evaporation of water brought to the surface by capillary action; it may consist of one or several minerals, commonly soluble salts such as gypsum and halite. Also, the process by which an efflorescent salt or crust is formed.
effusive eruption	An eruption that produces mainly lava flows and domes.
electrical resistivity survey	A measure of the difficulty with which an electric current flows through unconsolidated sediment and rock.
electromagnetic survey	An electrical exploration method based on the measurement of alternating magnetic fields associated with currents artificially or naturally maintained in the subsurface.
emergence	A change in the levels of water and land such that the land is relatively higher and areas formerly under water are exposed; results from either an uplift of land or fall of water level.
enclave	A compositionally or texturally distinct part of an igneous rock, commonly elongated or ellipsoidal in shape and typically smaller than 1 m (3 ft) across.
en echelon	Describes geologic features, particularly faults, that overlap in a steplike pattern.
entrainment	The process of picking up and transporting sediment, commonly by wind or water.
entrenched meander	An incised meander carved downward into the surface of the valley in which the meander originally formed; preserves its original pattern with little modification, suggesting rejuvenation of a meandering stream as a result of rapid vertical uplift or a lowering of base level; exhibits a symmetric cross profile in a gorge or canyon setting.
entrenched stream	A stream, often meandering, that flows in a narrow canyon or valley (i.e., “trench”) cut into a plain or relatively level upland; specifically a stream that has inherited its course from a previous cycle of erosion and that cuts into bedrock with little modification of the original course.
ephemeral lake	A short-lived lake.
ephemeral stream	A stream that flows briefly, only in direct response to precipitation, and whose channel is always above the water table.
epicenter	The point on Earth’s surface directly above the initial rupture point of an earthquake.
epiclastic rock	A rock formed at Earth’s surface by consolidation of fragments of preexisting rocks.
epicontinental	Describes a geologic feature on the continental shelf or interior.
epidote	A characteristically green silicate (silicon + oxygen) mineral, commonly occurring as slender, grooved crystals in hand specimens.
epidosite	A metamorphic rock consisting of epidote and quartz, and generally containing other secondary minerals such as uralite and chlorite.
epigenesist (metamorphism)	The change in the mineral character of a rock as a result of external influences operating near the Earth’s surface.
epigenesis (sedimentology)	The changes, transformations, or processes that occur at low temperatures and pressures, which affect sedimentary rocks subsequent to their compaction, exclusive of surficial alteration (weathering) and metamorphism.
epigenetic (economic geology)	Describes a mineral deposit of origin later than that of the enclosing rocks.
epigenetic (sedimentology)	Describes a sedimentary mineral, texture, or structure formed after the deposition of the sediment.
equigranular	Said of the texture of a rock having crystals of the same or nearly the same size.
erg	A regionally extensive tract of sandy desert; a “sand sea.”
erosion	The general process or group of processes that loosen, dissolve, wear away, and simultaneously move from one place to another, the materials of Earth’s crust; includes weathering, solution, abrasive actions, and transportation, but usually excludes slope movements.
erratic	A rock fragment carried by glacial ice deposited at some distance from the outcrop from which it was derived, and generally, though not necessarily, resting on bedrock of different lithology.
escarpment	A steep cliff or topographic step resulting from vertical displacement on a fault or as a result of slope movement or erosion. Synonymous with “scarp.”
estuary	The seaward end or tidal mouth of a river where freshwater and seawater mix.
euhedral	A grain bounded by perfect crystal faces; well-formed.
eustatic	Describes a worldwide rise or fall in sea level.
eutaxitic	Describes a stratified mineral deposit.
evaporite	A sedimentary rock composed primarily of minerals produced from a saline solution as a result of extensive or total evaporation of the solvent (usually water).
evapotranspiration	Loss of water from a land area through transpiration (passage of water vapor from a living body through a membrane) of plants and evaporation from the soil and surface-water bodies. Also, the volume of water lost through evapotranspiration.
exfoliation	The spalling, peeling, or flaking of layers or concentric sheets from an exposed rock mass caused by a change in heat or a reduction in pressure when overlying rocks erode away.
explosive eruption	An energetic eruption that produces mainly ash, pumice, and fragmental ballistic debris.
exsolution	The process whereby an initially homogeneous solid solution separates into two (or possibly more) distinct crystalline phases without addition or removal of material.
extension	Deformation of Earth’s crust whereby rocks are pulled apart.
extrusion	The emission of lava onto Earth’s surface; also, the rock so formed.
extrusive	Describes an igneous rock that has been erupted onto the surface of the Earth. Extrusive rocks include lava flows and pyroclastic material such as volcanic ash.
fabric	The complete spatial and geometrical configuration of all components that make up a deformed rock, including texture, structure, and preferred orientation.
facies (metamorphic)	The pressure and temperature conditions that result in a particular suite of metamorphic minerals.
facies (sedimentary)	The depositional or environmental conditions reflected in the sedimentary structures, textures, mineralogy, fossils, and other components of a sedimentary rock.
fan delta	A gently sloping alluvial deposit produced where a mountain stream flows out onto a lowland.
fanglomerate	A sedimentary rock consisting of waterworn fragments of various sizes deposited in an alluvial fan and later cemented into rock.
fault	A break in rock characterized by displacement of one side relative to the other.
fault gouge	Soft, uncemented, pulverized, claylike material found along some faults; formed by friction during fault movement.
feldspar	A group of abundant silicate (silicon + oxygen) minerals, comprising more than 60% of Earth’s crust and occurring in all types of rocks.
feldspathic	Describes a rock containing feldspar.
feldspathoid	A group of comparatively rare rock-forming minerals consisting of aluminosilicates of sodium, potassium, or calcium and having too little silica to form feldspar. Feldspathoids are chemically related to the feldspars, but differ from them in crystal form and physical properties; they take the places of feldspars in igneous rocks that are undersaturated with respect to silica or that contain more alkalies and aluminum than can be accommodated in the feldspars.
felsenmeer	From the German word meaning “sea of rocks”; a block field consisting of usually angular blocks with no fine sizes in the upper part, over solid or weathered bedrock, colluvium, or alluvium, without a cliff or ledge above as an apparent source.
felsic	Derived from feldspar + silica to describe an igneous rock having abundant light-colored minerals such as quartz, feldspars, or muscovite; also, describes those minerals.
felsite	Any light-colored, fine-grained extrusive igneous or hypabyssal rock composed mostly of quartz and feldspar.
felty	Said of the texture of the groundmass of a holocrystalline (composed entirely of crystals) igneous rock in which lath-shaped microlites (microscopic crystals that polarize light; typically plagioclase) are interwoven in an irregular unoriented fashion.
fenestral	Having openings or transparent areas in a rock.
ferric	Of, relating to, or containing iron.
ferromagnesian	Containing iron and magnesium.
fetch	The area of the ocean surface over which a constant and uniform wind generates waves.
fiamme	From “fiamma,” meaning “flame” in Italian. Dark, vitric lenses in welded tuffs, averaging a few centimeters in length, perhaps formed by the collapse of fragments of pumice.
fine-grained	Describes sediment or sedimentary rock and texture in which the individual constituents are too small to distinguish with the unaided eye, specifically sediment or rock whose particles have an average diameter less than 1/16 mm (0.002 in), that is, silt-size particles and smaller. Also, describes a crystalline or glassy rock and texture in which the individual minerals are relatively small, specifically an igneous rock whose particles have an average diameter less than 1 mm (0.04 in).
fissile	Capable of being easily split along closely spaced planes.
fissure	A fracture or crack in rock along which there is a distinct separation; commonly filled with mineral-bearing materials.
fissure (volcanic)	An elongated fracture or crack at the surface from which lava erupts.
fissure vent	A volcanic conduit having the form of a crack or fissure at Earth’s surface.
fissure volcano	One of a series of volcanic vents in a pattern of eruption along a fissure.
fjard	A small, narrow, irregular inlet or bay, shorter, shallower, and broader in profile than a fjord.
fjord	A long, narrow, winding, glacially eroded inlet or arm of the sea, U-shaped and steep-walled, generally several hundred meters deep, between high rocky cliffs or slopes along a mountainous coast; usually represents the seaward end of a deeply excavated glacial-trough valley that is partially submerged by drowning after the melting of glacial ice.
flame structure	A wave- or flame-shaped plume of mud that squeezed irregularly upward into an overlying sedimentary layer.
flat slab subduction	Refers to the subduction of one tectonic plate beneath another at a relatively shallow angle.
flint	The homogeneous, dark-gray or black variety of chert.
float	A general term for isolated, displaced fragments of a rock, especially on a hillside below an outcropping ledge or vein.
floodplain	The surface or strip of relatively smooth land composed of alluvium and adjacent to a river channel, constructed by the present river in its existing regimen and covered with water when the river overflows its banks. A river has one floodplain and may have one or more terraces representing abandoned floodplains.
floodplain splay	A small alluvial fan or other outspread deposit formed where an overloaded stream breaks through a levee (artificial or natural) and deposits its material (often coarse-grained) on the floodplain. Synonymous with “overbank splay,” “sand splay,” “channel splay,” or simply “splay.”
flood-tidal delta	A tidal delta formed on the landward side of a tidal inlet.
flowstone	Any deposit of calcium carbonate or other mineral formed by flowing water on the walls or floor of a cave.
fluvial	Of or pertaining to a river or rivers.
fluvial channel	A natural passageway or depression produced by the action of a stream or river.
fluviatile	Belonging to a river; produced by river action; growing or living in freshwater rivers. It is an approximate synonym of “fluvial,” but particularly used for the physical products of river action such as “fluviatile dam.”
flysch	A marine sedimentary facies characterized by a thick sequence of poorly fossiliferous, thinly bedded, graded marls and sandy and calcareous shales and muds, rhythmically interbedded with conglomerates (rare), coarse sandstones, and graywackes; typically deposited in deep ocean basins near convergent plate boundaries and rising mountains.
fold	A curve or bend in an originally flat structure, such as a rock stratum, bedding plane, or foliation; usually a product of deformation.
folia	Speleothems, commonly of calcite, that are downward-sloping, shelflike, interleaved tiers.
foliation	A preferred arrangement of crystal planes in minerals. Primary foliation develops during the formation of a rock and includes bedding in sedimentary rocks and flow layering in igneous rocks. In metamorphic rocks, the term commonly refers to a parallel orientation of planar minerals such as micas. Secondary foliation develops during deformation and/or metamorphism and includes cleavage, schistosity, and gneissic banding.
footwall	The lower wall of a fault.
foraminifer	Any protozoan belonging to the subclass Sarcodina, order Foraminiferida, characterized by the presence of a test of one to many chambers composed of secreted calcite (rarely silica or aragonite) or of agglutinated particles; most foraminifers are marine but freshwater forms are known. Range: Cambrian to Holocene.
fore reef	The seaward side of a reef.
foreshore	The part of the beach face that lies between the berm crest and the low-water line; it is regularly covered and uncovered by the rise and fall of the tide and is shaped by tides and waves.
formation	Fundamental rock-stratigraphic unit that is mappable, lithologically distinct from adjoining strata, and has definable upper and lower contacts.
fossil	A remain, trace, or imprint of a plant or animal that has been preserved in the Earth’s crust since some past geologic time; loosely, any evidence of past life.
fracture	The breaking of a mineral other than along planes of cleavage. Also, any break in a rock such as a crack, joint, or fault.
freeze-thaw	The mechanical weathering process caused by alternate or repeated cycles of freezing and thawing water in pores, cracks, and other openings of rock and unconsolidated deposits, usually at the surface.
friable	Describes a rock or mineral that is easily crumbled.
frost action	The mechanical weathering process caused by alternate or repeated cycles of freezing and thawing of water in pores, cracks, and other openings, usually at the surface.
frost heaving	The uneven lifting or upward movement, and general distortion, of surface soils, rocks, vegetation, and structures such as pavements, due to subsurface freezing of water and growth of ice masses.
frost wedging	A type of frost action by which jointed rock is pried and dislodged by ice acting as a wedge.
fulgurite	An irregular glassy tube or crust produced by the fusion of loose sand by lightning, occurring primarily on exposed mountain tops or in dune areas.
fumarole	A vent, usually volcanic, from which gases and vapors are emitted.
gabbro	A group of dark-colored, coarse-grained intrusive igneous rocks composed of plagioclase, pyroxene, amphibole, and olivine.
garnet	A hard silicate (silicon + oxygen) mineral with a glassy luster, and commonly well-defined crystal faces; characteristically dark red but occurs in a variety of colors.
gastropod	Any mollusk belonging to the class Gastropoda, characterized by a distinct head with eyes and tentacles and, in most, by a single calcareous shell that is closed at the apex, sometimes spiraled, not chambered, and generally asymmetrical. Range: Upper Cambrian to Holocene.
geoarchaeology	The application of concepts and methods of the earth sciences to archaeological problems and vice versa. It provides evidence for the development, preservation, and destruction of archaeological sites, and for regional-scale environmental change and the evolution of the physical landscape, including the impact of human groups.
geodetic surveying	Surveying that takes into account the figure and size of Earth, with corrections made for curvature; used where the areas or distances involved are so great that the desired accuracy and precision cannot be obtained by plane (ordinary field and topographic) surveying.
geodesy	The science concerned with the determination of the size and shape of the Earth and the precise location of points on its surface.
geology	The study of Earth, including its origin, history, physical processes, components, and morphology.
geomorphology	The study of the general configuration of surface landforms and their relationships to underlying structures, and of the history of geologic changes as recorded by these surface features.
geothermal	Pertaining to the heat of the interior of the Earth.
geothermal gradient	The rate of change of temperature with depth in the Earth.
glaciolacustrine	Pertaining to, derived from, or deposited in glacial lakes, especially referring to deposits and landforms composed of suspended material transported by meltwater streams flowing into lakes bordering a glacier.
glaciomarine	Describes the accumulation of glacially eroded, terrestrially derived sediment in a marine environment.
glassy	Describes the texture of certain extrusive igneous rocks that is similar to glass and developed as a result of rapid cooling of the lava, without distinctive crystallization. Synonymous with “vitreous.”
glauconite	A greenish silicate (silicon + oxygen) mineral, (K,Na)(Fe,Al,Mg)2(Si,Al)4O10(?H)2, characterized by a micaceous structure, commonly interstratified with smectite; may serve as an indicator of very slow sedimentation.
glaze	The surface of a lava flow that resembles a fired glass surface or ceramics.
gneiss	A foliated metamorphic rock with alternating bands of dark and light minerals. Varieties are distinguished by texture (e.g., augen gneiss), characteristic minerals (e.g., hornblende gneiss), or general composition (e.g., granite gneiss).
Gondwana	The late Paleozoic continent of the Southern Hemisphere and counterpart of Laurasia of the Northern Hemisphere; both were derived from the supercontinent Pangaea.
gossan	An iron-bearing weathered product overlying a sulfide deposit.
gradient	A degree of inclination (steepness of slope), or a rate of ascent or descent, of an inclined part of Earth’s surface with respect to the horizontal; expressed as a ratio (vertical to horizontal), a fraction (such as m/km or ft/mi), a percentage (of horizontal distance), or an angle (in degrees).
granite	A coarse-grained, intrusive igneous rock in which quartz constitutes 10%–50% of the felsic (“light-colored”) components and the alkali feldspar/total feldspar ratio is generally restricted to the range of 65% to 90%; perhaps the best known of all igneous rocks.
granitoid	A granitic rock.
graben	An elongated, downdropped trough or basin, bounded on both sides by high-angle normal faults that dip toward one another.
granoblastic	Describes the texture of a metamorphic rock in which recrystallization formed crystals of nearly the same size in all directions.
granodiorite	A coarse-grained intrusive igneous rock intermediate in composition between quartz diorite and quartz monzonite, containing quartz, plagioclase, and potassium feldspar as the felsic (“light-colored”) components, with biotite, hornblende, or, more rarely, pyroxene, as the mafic (“dark-colored”) components.
gravel	An unconsolidated, natural accumulation of typically rounded rock fragments resulting from erosion; consists predominantly of particles larger than sand; that is, greater than 2 mm (1/12 in) across.
graywacke	A dark gray, firmly indurated, coarse-grained sandstone that consists of poorly sorted angular to subangular grains of quartz and feldspar, with a variety of dark rock and mineral fragments embedded in a compact clayey matrix.
greenschist	A schistose metamorphic rock whose green color is due to the presence of the mineral chlorite, epidote, or actinolite.
greenstone	A general term for any compact, dark-green, altered or metamorphosed basic igneous rock with a green color due to chlorite, actinolite, or epidote mineral content.
groundmass	The finer grained and/or glassy material between the large crystals of an igneous rock. Also, sometimes used for the matrix of a sedimentary rock.
groundwater	That part of subsurface water that is in the zone of saturation, including underground streams.
groundwater basin	An area of bedrock in a karst spring that collects drainage from all the sinkholes and sinking streams in its drainage area.
group	The formal lithostratigraphic unit next in rank above formation. A group includes two or more associated formations with significant features in common.
grus	A silica-rich sand derived from the weathering of a parent rock, usually granite.
gully	A small channel produced by running water in unconsolidated material.
gypcrete	A gypsum deposit that forms a cemented layer within a soil profile.
gypsiferous	Gypsum-bearing.
gypsite	An earthy variety of gypsum containing dirt and sand, occurring in arid regions as an efflorescent deposit over an outcrop of gypsum or in gypsum-bearing strata.
gypsum	A sulfate (sulfur + oxygen) mineral of calcium and water, CaSO4 • 2H2O.
halide	A mineral group composed of chlorine or fluorine plus an element or elements, for example, calcium in fluorite, CaF2; and sodium in halite, NaCl.
halite	A halide (chlorine or fluorine) mineral composed of sodium and chloride, NaCl. Synonymous with “native salt,” “rock salt,” and “common salt.”
hanging valley	A tributary glacial valley whose mouth is high above the floor of the main valley, which was eroded by the main body of the glacier.
hanging wall	The upper wall of a fault.
hardground	A planar surface of rock, generally composed of limestone and colonized by corals and other attached benthos.
hardpan	A hard, impervious layer of soil lying at or just below the surface; physical characteristics limit root penetration and restrict water movement.
harzburgite	A peridotite composed chiefly of olivine and orthopyroxene.
hawaiite	An olivine-rich basalt. Also, a pale-green, iron-poor gem variety of olivine from the lavas of Hawaii.
heavy mineral	A detrital mineral from a sedimentary rock, having a specific gravity higher than a standard (usually 2.85). It is commonly a minor constituent or accessory mineral of the rock and less than 1% in most sands.
helictite	A delicate speleothem that grows in all directions.
hematite	An oxide mineral composed of oxygen and iron, Fe2O3.
hermatypic	Describes a type of reef-building coral that is incapable of adjusting to conditions lacking sunlight.
heterogeneous	Consisting of dissimilar or diverse ingredients or constituents.
highstand	The interval of time during one or more cycles of relative sea-level change when sea level is above the edge of the continental shelf in a given area.
hinge line	The axis along which the curvature of a fold is greatest. Also, a line or boundary between a stable region and one undergoing upward or downward movement.
hogback	A ridge resembling in outline the back of a hog with a sharp summit and steep slopes of nearly equal inclination on both flanks.
holocrystalline	Describes an igneous rock texture composed entirely of crystals.
homogeneous	Of uniform structure or composition throughout.
hoodoo	A bizarrely shaped column, pinnacle, or pillar of rock, commonly produced in a region of sporadic heavy rainfall by differential weathering or erosion of horizontal strata, facilitated by layers of varying hardness and joints.
hornblende	A silicate (silicon + oxygen) mineral of sodium, potassium, calcium, magnesium, iron, and aluminum; commonly black and occurring in distinct crystals or in columnar, fibrous, or granular forms in hand specimens. The most common mineral of the amphibole group.
horizon (archeology)	A period during which the influence of a specific culture spread rapidly over a defined area.
horn	A high pyramidal peak with steep sides formed by the intersection of the walls of three or more cirques.
hornfels	A fine-grained metamorphic rock composed of a mosaic of grains without preferred orientation that have the same or nearly the same diameter in all directions; typically formed by contact metamorphism.
hornito	A small mound of spatter built on the back of a lava flow, formed by the gradual accumulation of lava clots ejected through an opening in the roof of an underlying lava tube.
horst	An elongated, uplifted block that is bounded on both sides by normal faults that dip away from one another.
hot spot	A volcanic center, 100–200 km (60–120 mi) across, persistent for at least a few tens of millions of year, with a surface expression, commonly at the center of a plate, that indicates a rising plume of hot mantle material.
hot-spot track	A ridge of volcanic rock formed when a lithospheric plate moves over a hot spot. The active hot-spot volcano lies at the end of the track. Extinct volcanoes lie along the track, with the oldest extinct volcano farthest from the active hot spot.
hot spring	A thermal spring whose temperature is above that of the human body.
hurricane	The term applied in the Northern Hemisphere for an atmospheric low-pressure system with a closed, roughly circular, wind motion that is counterclockwise, and sustained near-surface wind speed equal to or exceeding 64 knots (73 mph).
hyaloclastite	A deposit formed by the flow or intrusion of lava or magma into water, ice, or water-saturated sediment, which shatters the lava or magma into small angular fragments.
hydraulic conductivity	The ease with which water moves through spaces or pores in soil or rock.
hydrostatic pressure	The pressure exerted by the water at any given point in a body of water at rest. The hydrostatic pressure of groundwater is generally due to the weight of water at higher levels in the saturated zone.
hydrogeology	The science that deals with subsurface waters and related geologic aspects of surface waters, including the movement of groundwater; the mechanical, chemical, and thermal interaction of groundwater with the porous medium; and the transport of energy and chemical constituents by the flow of groundwater. Synonymous with “geohydrology.”
hydrology	The study of liquid and solid water properties, circulation, and distribution, on and under the Earth’s surface and in the atmosphere.
hydrolysis	A decomposition reaction involving water, frequently involving silicate (silicon + oxygen) minerals.
hydrothermal	Of or pertaining to hot water, to the action of hot water, or to the products of this action.
hydrothermal water	Subsurface water whose temperature is high enough to make it geologically or hydrologically significant, whether or not it is hotter than the rock containing it.
hydrovolcanic	A term encompassing all volcanic activity that results from the interaction among lava, magmatic heat, or gases and water at or near Earth’s surface.
hypabyssal	Describes an igneous rock or intrusive body formed at shallow depth.
hypersthene	A silicate (silicon + oxygen) mineral of the pyroxene group consisting of magnesium and iron.
iddingsite	A reddish-brown mixture of silicate (silicon + oxygen) minerals, including iron, calcium, and magnesium, formed by the alteration of olivine; forms rust-colored patches in basic igneous rocks.
igneous	Describes a rock or mineral that solidified from molten or partly molten material; also, describes processes leading to, related to, or resulting from the formation of such rocks. One of the three main classes or rocks—igneous, metamorphic, and sedimentary.
ignimbrite	A pyroclastic flow deposit.
ilmenite	Iron titanium oxide. An oxide mineral composed of oxygen, titanium, and iron FeTiO3; it commonly contains considerable magnesium and manganese. A common accessory mineral in basic igneous rocks (e.g., gabbro), and commonly concentrated in mineral sands.
imbrication	Consistent orientation of rocks or their clasts; commonly displayed by pebbles on a streambed that have been tilted by flowing water so that their flat surfaces dip upstream.
incised meander	An old stream meander that has become deepened by rejuvenation and that is more or less closely bordered or enclosed by valley walls. The term is used collectively and includes “entrenched meanders” and “ingrown meanders.”
incision	Downward erosion by a stream, resulting in a deepened channel and commonly a narrow, steep-walled valley.
indurated	Describes a rock or soil hardened or consolidated by pressure, cementation, or heat.
induration	Hardening by heat, pressure, or the introduction of cementing material, especially the process by which relatively consolidated rock is made harder or more compact.
inequant	Describes a mineral grain whose length and width are not the same.
inflation	Process by which a local area of pahoehoe lava swells as a result of injection of lava beneath its surface crust.
ingrown meander	A continually growing or expanding incised meander formed during a single cycle of erosion by the enlargement of an initial minor meander while the stream was actively downcutting; exhibits a pronounced asymmetric cross profile (a well-developed, steep undercut slope on the outside of the meander, a gentle slip-off slope on the inside) and is produced when the rate of downcutting is slow enough to afford time for lateral erosion.
inlet	A small, narrow opening, recess, indentation, or other entrance into a shoreline through which water penetrates into the land; or a waterway entering a sea, lake, or river. Also, a short, narrow waterway between islands, or connecting a bay, lagoon, or similar body of water with a larger body of water.
intercalated	Describes the presence or insertion of a layer of material between layers of a different type.
interdune	The relatively flat surface between dunes, commonly a long, troughlike, wind-swept passage between parallel dunes; may be covered with sand or sand free.
interfluve	The area between rivers, especially the relatively undissected upland or ridge between two adjacent valleys containing streams flowing in the same general direction.
intermediate	Said of an igneous rock that is transitional between felsic and mafic, generally having a silica content of 54%–65%.
intermediate magma	Describes magma that contains between 62% and 63% silica and is moderately viscous, gas-rich, and sometimes erupts explosively, though it may also produce lava flows.
interstitial	Said of a mineral deposit in which the minerals fill the pores of the host rock.
intertidal	Pertaining to the benthic ocean environment or depth zone between high water and low water; also, pertaining to the organisms of that environment. Synonymous with “littoral.”
intertonguing	The overlapping of markedly different rocks through vertical succession of wedge-shaped layers; results in the disappearance of sedimentary bodies in laterally adjacent masses.
intrusion	The process of emplacement of magma into preexisting rock. Also, the igneous rock mass formed.
intrusive	Pertaining to intrusion, both the process and the rock body.
island arc	An offshore, generally curved belt of volcanoes above a subduction zone.
isoclinal	Describes a fold with parallel limbs.
isograd	A line on a map joining points where metamorphism proceeded under similar pressures and temperatures, as indicated by rocks containing a diagnostic mineral or mineral assemblage.
isostacy	The condition of equilibrium, comparable to floating, of the units of the lithosphere above the asthenosphere.
isotopic age	An age (in years) calculated from the quantitative determination of radioactive elements and their decay products.
isotopic dating	Calculating an age in years for geologic materials by measuring the presence of a short-lived radioactive element (e.g., carbon-14) or by measuring the presence of a long-lived radioactive element plus its decay product (e.g., potassium-40/argon-40). The term applies to all methods of age determination based on nuclear decay of naturally occurring radioactive isotopes.
isotropy	The condition of having uniform properties in all directions.
isthmus	A narrow strip or neck of land, bordered on both sides by water, connecting two larger land areas.
jameo	A large sink formed by the collapse of the roof of more than one level of a multi-level lava-tube cave.
jasper	A variety of chert associated with iron ores and containing iron-oxide impurities that give it various colors, especially red.
joint	A break in rock without relative movement of rocks on either side of the fracture surface.
jökulhlaup	An Icelandic term for a glacial outburst flood.
juvenile (geomorphology)	Youthful.
juvenile (volcanology)	Formed directly from magma reaching the surface.
kame	A mound, knob, or short irregular ridge, composed of stratified sand and gravel deposited by a subglacial stream as a fan or delta at the margin of a melting glacier.
kame delta	A flat-topped, steep-sided hill of well-sorted sand and gravel deposited by a meltwater stream flowing into a proglacial or other ice-marginal lake; the proximal margin of the delta was built in contact with a glacier.
kaolinite	A common white clay mineral composed of mostly aluminum oxide.
karren	Channels or furrows caused by solution on massive bare limestone surfaces.
karstification	The action of water, mainly solutional but also mechanical, that produces features of karst topography.
karst	A type of topography that is formed on limestone, gypsum, and other soluble rocks, primarily by dissolution. It is characterized by sinkholes, caves, and underground drainage.
karst valley	A closed depression formed by the coalescence of several sinkholes.
karst window	A collapse sinkhole opening into a cave.
keratophyre	A salic extrusive igneous rock formed at shallow depth characterized by the presence of albite or albite-oligoclase and chlorite, epidote, and calcite, all of metamorphic origin.
key	One of the coral islets or barrier islands off the southern coast of Florida. Synonymous with “cay.”
kipuka	An area surrounded by a lava flow.
klippe	An erosional remnant of a thrust sheet that is completely surrounded by exposure of the footwall. Commonly used in the plural, “klippen.”
knickpoint	Any interruption or break in slope.
labradorite	A silicate (silicon + oxygen) mineral of the plagioclase group with the general formula (Ca,Na)(Al,Si)4O8.
laccolith	A mushroom-shaped pluton that has intruded sedimentary strata and domed up the overlying sedimentary layers.
lacustrine	Describes a process, feature, or organism pertaining to, produced by, or inhabiting a lake.
lag gravel	An accumulation of coarse material remaining on a surface after finer material has been blown or washed away.
lagomorph	Any member of the order Lagomorpha of which there are two living families: Leporidae (hares and rabbits) and the Ochotonidae (pikas).
lagoon	A narrow body of water that is parallel to the shore and between the mainland and a barrier island; characterized by minimal or no freshwater influx and limited tidal flux, which cause elevated salinities. Also, a shallow body of water enclosed or nearly enclosed within an atoll.
lahar	A mixture of water and volcanic debris that moves rapidly down the slope of a volcano, characterized by a substantial component (>50%) of fine-grained material that acts as a matrix to give the deposit the strength it needs to carry the bigger clasts.
lamina	The thinnest recognizable unit layer of original deposition in a sediment or sedimentary rock, differing from other layers in color, composition, or particle size; specifically a sedimentary layer less than 1 cm (0.4 in) thick and commonly 0.05–1.00 mm (0.002–0.04 in) thick. Synonymous with “lamination”; lamination is also the formation of such layers.
laminated	Consisting of very thin compositional layers.
landslide	A collective term covering a wide variety of slope-movement landforms and processes that involve the downslope transport of soil and rock material en masse under the influence of gravity.
lapilli	Pyroclastic materials ranging between 2 and 64 mm (0.08 and 2.5 in) across with no characteristic shape; may be either solidified or still viscous upon landing. An individual fragment is called a lapillus.
latite	A porphyritic volcanic rock with phenocrysts of plagioclase and potassium feldspar minerals in nearly equal amounts, little or no quartz, and a finely crystalline to glassy groundmass.
Laurasia	The late Paleozoic continent of the Northern Hemisphere and counterpart of Gonwana of the Southern Hemisphere; both were derived from the supercontinent Pangaea.
lava	Molten or solidified magma that has been extruded though a vent onto Earth’s surface.
lavacicle	A protrusion of lava that resembles an icicle in form, resulting from the dripping of lava into a lava tube.
lava dome	A steep-sided mass of viscous, commonly blocky, lava extruded from a vent; typically has a rounded top and covers a roughly circular area; may be isolated or associated with lobes or flows of lava from the same vent; typically silicic (rhyolite or dacite) in composition.
lava lake	A lake of molten lava, usually basaltic, in a volcanic crater or depression. Also refers to solidified and partly solidified stages.
lava tube	Conduits through which lava travels beneath the surface of a lava flow; also, a cavernous segment of the conduit remaining after the flow of lava ceases.
law of faunal succession	A general law of geology: Fossil organisms (faunas and floras) succeed one another in a definite and recognizable order, each geologic formation having a different total aspect of life from that in the formations above it and below it; or, the relative age of rocks can be determined from their fossil content.
law of original horizontality	A general law of geology: Water-laid sediments are deposited in strata that are horizontal or nearly horizontal, and parallel or nearly parallel to the Earth’s surface.
law of superposition	A general law upon which all geologic chronology is based: In any sequence of layered rocks, sedimentary or volcanic, that has not been overturned, the youngest stratum is at the top and the oldest at the base; that is, each bed is younger that the bed beneath, but older than the bed above it.
lawsonite	A colorless to grayish-blue orthorhombic mineral: CaAl2Si2O7(OH)2 • H2O.
leaching	The separation, selective removal, or dissolving-out of soluble constituents from a rock or orebody by the natural action of percolating water.
left-lateral fault	A strike-slip fault on which the side opposite the observer has been displaced to the left. Synonymous with “sinistral fault.”
lens	A sedimentary deposit that resembles a convex lens and is characterized by converging surfaces, thick in the middle and thinning out toward the edges.
lenticular	Resembling in shape the cross section of a lens.
lentil	A minor rock-stratigraphic unit of limited geographic extent that thins out in all directions.
leuco-	A prefix meaning “light-colored” and applied to light-colored igneous rocks that are relatively poor in mafic (“dark-colored”) minerals.
leucogranite	A light-colored intrusive igneous rock rich in potassium feldspars and aluminum.
levee	A long broad low embankment of sand and coarse silt built by floodwater overflow along both banks of a stream channel.
levee (speleology)	A retaining wall of hardened lava alongside a lava channel; also, the freestanding cooled edge of a lava tongue or flow left after evacuation of the molten lava.
lherzolite	Peridotite composed chiefly of olivine, orthopyroxene, and clinopyroxene, in which olivine is generally most abundant.
Liesgang rings	Concentric rings or bands caused by precipitation of minerals within a fluid-saturated rock. Synonymous with “Liesgang banding.”
lidar (light detection and ranging)	A method and instrument that measures distance to a reflecting object by emitting timed pulses of light and measuring the time between emission and reception of reflected pulses; the measured interval is converted to distance.
lignite	An organic sedimentary rock, referred to as “brown coal,” with a low fixed carbon content of 25% caused by compaction by overlying sediments.
limb	One side of a structural fold.
lime	Calcium oxide, CaO.
limestone	A carbonate sedimentary rock consisting of more than 95% calcite and less than 5% dolomite.
lineament	A linear topographic feature of regional extent that probably reflects an underlying crustal structure. Also, any extensive linear surface feature (e.g., fault lines, aligned volcanoes, and straight stream courses).
limonite	A general field term for any brown, amorphous, to cryptocrystalline naturally occurring coatings (such as ordinary rust), loose or dense earthy masses, and pseudomorphs after other iron minerals formed by oxidation of iron or iron-bearing minerals; may also be formed as an inorganic or biogenic precipitate in bogs, lakes, springs, or marine deposits. Synonymous with “bog iron ore,” “brown iron ore,” and “brown hematite.”
liquefaction	The transformation of loosely packed sediment into a more tightly packed fluid mass.
litharenite	A sedimentary rock, specifically sandstone, that consists of more than 25% fine-grained rock fragments, less than 10% feldspar, and less than 75% quartz, quartzite, or chert. Short for “lithic arenite.”
lithic	Described a medium-grained sedimentary rock or pyroclastic deposit that contains abundant fragments of previously formed rocks.
lithic tuff	A dense deposit of volcanic ash that includes fragments of previously formed rocks that solidified in a vent and were then ejected.
lithification	The conversion of sediment into solid rock.
lithify	To change to stone, or to petrify; especially to consolidate from a loose sediment to solid rock.
lithofacies	A lateral, mappable subdivision of a designated stratigraphic unit distinguished from adjacent subdivisions on the basis of lithology.
lithology	The physical description or classification of a rock or rock unit based on characteristics such as color, mineral composition, and grain size.
lithophysa	A hollow, bubblelike structure composed of concentric shells of finely crystalline potassium-rich feldspar, quartz, and other minerals; occurs in silica-rich volcanic rocks as rhyolite and obsidian.
lithosphere	Earth’s relatively rigid outer shell that consists of the entire crust plus the uppermost mantle. It is broken into about 20 plates, and according to the theory of plate tectonics, movement and interaction of these plates is responsible for most geologic activity.
lithostratigraphy	The branch of stratigraphy that deals with the description and systematic organization of the rocks of Earth’s crust into distinctive named units based on the lithologic character of the rocks and their stratigraphic relations.
littoral	Pertaining to the benthic ocean environment or depth zone between high water and low water; also, pertaining to the organisms of that environment. Synonymous with “intertidal.”
littoral cone	A mound of hyaloclastic debris constructed by steam explosions at the point where lava enters the sea or other body of water. Littoral cones lack feeding vents connected to subsurface magma supplies.
loam	A rich permeable soil composed of a mixture of clay, silt, sand, and organic matter.
lodgment till	Successive layers of till plastered beneath a glacier, commonly characterized by a compact fissile structure and stones whose long axes are oriented generally parallel to the direction of ice flow.
loess	Windblown silt-sized sediment.
longitudinal dune	A long, narrow sand dune, usually symmetrical in cross profile, oriented parallel with the direction of the prevailing wind; characteristically wider and steeper on the windward side and tapering to a point on the leeward side; may be a few meters high and as much as 100 km (60 mi) long.
longshore current	A current parallel to a coastline caused by waves approaching the shore at an oblique angle.
lower flow regime	A condition of stream flow that is characterized by a unidirectional current and relatively low sediment transport rates.
lowstand	The interval of time during one or more cycles of relative sea-level change when sea level is below the edge of the continental shelf in a given area.
lunar tide	The part of the tide caused solely by the tide-producing force of the Moon.
luster	The reflection of light from the surface of a mineral, described by its quality and intensity.
maar	A low-relief, broad volcanic crater formed by multiple shallow explosive eruptions. It is surrounded by a low-relief rim of fragmental material, and may be filled by water.
mafic	Derived from magnesium + ferric (Fe is the chemical symbol for iron) to describe an igneous rock having abundant dark-colored, magnesium- or iron-rich minerals such as biotite, pyroxene, or olivine; also, describes those minerals.
magma	Molten rock beneath Earth’s surface capable of intrusion and extrusion.
magma reservoir	A chamber in the shallow part of the lithosphere from which volcanic materials are derived; the magma in the reservoir ascended from a deeper source.
magmatic arc	An arcuate line of plutons, volcanic rocks, or active volcanoes formed at a convergent plate boundary.
magmatism	The development and movement of magma, and its solidification as igneous rock.
magnetite	Iron oxide. An oxide mineral composed of oxygen and iron, Fe3O4; commonly contains manganese, nickel, chromium, and titanium. A very common and widely distributed accessory mineral in rocks of all kinds and as a “heavy mineral” in sand.
mantle	The zone of the Earth below the crust and above the core.
mantle plume	A vertical cylindrical part of Earth’s mantle, hotter than its surroundings, within which larger-than-normal amounts of heat are conducted upward to form a hot spot at the Earth’s surface.
mappability	The quality or state of being mappable, determined by such considerations as the scale of base maps, purpose of the project and time assigned for completing the mapping, kind and number of exposures of the strata, the experience and skill of the mapper(s), and extent of the previous geologic study and mapping of surrounding areas.
marine terrace	A relatively flat-topped, horizontal or gently inclined, surface of marine origin along a coast, commonly veneered by a marine deposit (typically silt, sand, or fine gravel).
marker bed	A well-defined, easily identifiable stratum or body of strata that has sufficiently distinctive characteristics (such as lithology or fossil content) to facilitate correlation in field mapping or subsurface work. Also, a geologic formation that serves as a marker.
marl	An unconsolidated sedimentary rock or soil consisting of clay and lime (CaO).
marl	A term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting primarily of a mixture of clay and calcium carbonate; specifically an earthy substance containing 35%–65% clay and 65%–35% carbonate.
marlstone	A sedimentary rock composed of marl.
massif	A topographic and structural feature, especially in an orogenic belt, commonly formed of rocks more rigid than those of its surroundings; may consist of protruding bodies of basement rocks that consolidated during earlier orogenies, or younger plutonic bodies.
mass wasting	Dislodgement and downslope transport of a mass of rock and/or unconsolidated material under the direct influence of gravity. In contrast to “erosion,” the debris removed is not carried within, on, or under another medium. Synonymous with “slope movement.”
matrix	The fine-grained material between coarse grains in an igneous or sedimentary rock. Also refers to rock or sediment in which a fossil is embedded.
maximum credible earthquake	The largest hypothetical earthquake that may be reasonably expected to occur along a given fault.
meander	One of a series of sinuous curves, bends, or turns in the course of a stream, produced by a mature stream swinging from side to side as it flows across its floodplain or shifts its course laterally toward the convex side of an original curve.
mechanical weathering	The physical breakup of rocks without change in composition.
medium-grained	Describes an igneous rock and texture in which the individual crystals have an average diameter in the range of 1 to 5 mm (0.04 to 0.2 in.). Also, describes sediment or sedimentary rock and texture in which the individual particles have an average diameter in the range of 1/16 to 2 mm (0.002 to 0.08 in), that is, sand size.
megabreccia	A rock produced by brecciation on a very large scale; individual blocks are as much as 400 m (1,300 ft) long.
mélange	A body of jumbled rock that is mappable at a scale of 1:24,000 or smaller and includes fragments and blocks of all sizes embedded in a fragmented and generally sheared matrix.
mélange	A mappable chaotic body of mixed rocks with a block-in-matric fabric whose internal structure and evolution are intimately linked to the structural, sedimentary, magmatic, and metamorphic processes in their tectonic setting of formation (Festa et al. 2010).
member	A lithostratigraphic unit with definable contacts; a subdivision of a formation.
mesa	A broad, flat-topped erosional hill or mountain with by steeply sloping sides or cliffs.
meta–	A prefix used with the name of a sedimentary or igneous rock, indicating that the rock has been metamorphosed.
metamorphic	Pertaining to the process of metamorphism or to its results.
metamorphic core complex	A domed or arched uplift of deformed metamorphic and plutonic rocks overlain by relatively unmetamorphosed, cover rocks.
metamorphic rock	Any rock derived from preexisting rocks that was altered in response to marked changes in temperature, pressure, shearing stress, and chemical environment. One of the three main classes of rock—igneous, metamorphic, and sedimentary.
metamorphism	The mineralogical, chemical, and structural changes of solid rocks, generally imposed at depth below the surface zones of weathering and cementation.
meteoric water	Water of recent atmospheric origin.
metavolcanic	Describes a volcanic rock that shows evidence of metamorphism.
miarolitic	Describes a small irregular cavity in igneous rock into which small crystals of the rock-forming minerals protrude.
mica	A group of abundant silicate (silicon + oxygen) minerals characterized by perfect cleavage, readily splitting into thin sheets. Examples include “biotite” and “muscovite.”
micaceous	Consisting of, containing, or pertaining to mica; also, resembling mica, for example, a “micaceous mineral” capable of being easily split into thin sheets.
micrite	The semiopaque crystalline matrix in limestones, consisting of chemically precipitated carbonate mud with crystals less than 4 microns in diameter.
microcrystalline	Describes a rock texture consisting of crystals visible only with a microscope.
micrographic	Describes the graphic texture of an igneous rock, distinguishable only with a microscope.
microlite	A microscopic crystal that polarizes light and has some determinable optical properties.
microplate	A small lithospheric plate.
mid-ocean ridge	The continuous, generally submarine and volcanically active mountain range that marks the divergent tectonic margins in Earth’s oceans.
migmatite	Literally “mixed rock,” exhibiting both igneous and metamorphic characteristics due to partial melting during metamorphism.
mineral	A naturally occurring inorganic crystalline solid with a definite chemical composition or compositional range.
miogeocline	A wedge of shallow-water sediment building outward from the edge of the continent.
Mohorovicic discontinuity	The seismic-velocity discontinuity or boundary surface that separates the Earth’s crust from the subjacent mantle. Named in honor of its discoverer, Andrija Mohorovicic (1857–1936), a Croatian seismologist.
mold	An impression made in the surrounding earth by the exterior or interior of a fossil shell or other organic structure and then preserved. Also, a cast of the inner surface of a fossil shell.
mole track	A small, geologically short-lived ridge formed by the humping up and cracking of ground along a strike-slip fault.
monadnock	A conspicuous hill or mountain consisting of resistant rock that remains after the surrounding, less resistant rock has worn away. Type locality: Mount Monadnock in New Hampshire.
monocline	A one-limbed fold in strata that are otherwise flat-lying.
monoclinic system	One of the six crystal systems of minerals, characterized by either a single twofold axis of symmetry or a single plane of symmetry, or a combination of the two. Of the three nonequivalent axes, one is perpendicular to the plane formed by the other two.
monogenetic	Resulting from one process, derived from one source, or originating or developing at one place and time.
monument	An isolated pinnacle, column, or pillar of rock resulting from erosion that resembles a human-made monument or obelisk.
montmorillonite	A clay mineral of the smectite group, which is common in soils, sedimentary rocks, and some mineral deposits. They are derived from the alteration of volcanic glass and the weathering of primary silicates.
monzo–	Derived from Monzoni in northern Italy for monzonite, a prefix that indicates that various intrusive igneous rocks contain a significant amount of both alkali feldspar and plagioclase.
monzodiorite	An intrusive igneous rock intermediate in composition between monzonite and diorite and containing nearly equal amounts of plagioclase and alkali feldspar. The presence of alkali feldspar distinguishes monzodiorite from diorite.
monzonite	An intrusive igneous rock, intermediate in composition between syenite and diorite, containing approximately equal amounts of alkali feldspar and plagioclase and very little quartz. Monzonite contains less quartz and more plagioclase than granite.
moonmilk	A soft, white, initially deformable deposit that occurs on the walls of caves.
moraine	A mound, ridge, or other distinct accumulation of unsorted, unstratified glacial drift, predominantly till, deposited mostly by direct action of a glacier.
moulin	A roughly cylindrical, nearly vertical hole or shaft in a glacier, scoured out by swirling meltwater as it pours down from the surface.
morphometry	The quantitative measurement the shape of a landform.
mud	A mixture of water with silt- or clay-sized earth material.
mud crack	A crack formed in clay, silt, or mud by shrinkage during dehydration at Earth’s surface.
mud flat	A relatively level area of fine silt along a shore or around an island, alternately covered and uncovered by the tide, or covered by shallow water; a muddy tidal flat, barren of vegetation.
mudflow	A general term for a landform and process characterized by a flowing mass of predominantly fine-grained earth material possessing a high degree of fluidity during movement.
mud pot	A type of hot spring that contains boiling mud, which is usually sulfurous and commonly multicolored.
mudstone	Hardened mud having the texture and composition of shale but lacking its fissility.
mugearite	An extrusive or hypabyssal igneous rock of the alkali basalt suite containing oligoclase, alkali feldspar, and mafic minerals.
mollusk	A solitary invertebrate such as gastropods, bivalves, and cephalopods belonging to the phylum Mollusca. Range: Lower Cambrian to Holocene.
mush	Partially crystallized magma.
muscovite	A light-colored silicate (silicon + oxygen) mineral of the mica group, KAl3Si3O10(OH)2, characterized by perfect cleavage in one direction and the ability to split into thin, clear sheets.
mylonite	A fine-grained, foliated metamorphic rock typically in localized zones of ductile deformation, commonly formed at great depths under high temperature and pressure.
mylonite structure	A flowlike appearance, characteristic of mylonite, produced by intense small-scale crushing, breaking, and shearing of rock.
nappe	A sheetlike, allochthonous rock unit, which has moved on a predominantly horizontal surface.
native elements	A mineral group that is composed of a single element, for example, gold, Au; or copper, Cu.
neck	An eroded, vertical, pipelike intrusion that represents the vent of a volcano.
neoglacial	Describes a period of glacial readvance during the late Holocene Epoch. The Little Ice Age (1500s to mid-1800s) was the most recent.
nepheline	A hexagonal feldspathoid mineral: (Na,K)AlSiO4. It occurs in glassy crystals or colorless grains, or as coarse crystals or green to brown masses or greasy luster without cleavage, in alkalic rocks; it is an essential constituent of sodium-rich rocks.
neritic	Describes the ocean environment or depth zone between low tide and 200 m (660 ft), or between low tide and approximately the edge of the continental shelf; also, describes the organisms living in that environment.
nonconformity	A type of unconformity developed between sedimentary rocks and older rocks (plutonic igneous or massive metamorphic rocks) that had been exposed to erosion before the overlying sediments covered them.
normal fault	A fault in which the hanging wall appears to have moved downward relative to the footwall; the angle of dip is usually 45°–90°.
nuée ardente	A swiftly flowing, turbulent, sometimes incandescent gaseous cloud erupted from a volcano, containing ash and other pyroclastic materials in its lower part. Synonymous with “pyroclastic flow,” which is the preferred term.
numerical age	The geologic age of a fossil organism, rock, or geologic feature or event given in units of time, usually years. Commonly used as a synonym of “isotopic age,” but may also refer to ages obtained from tree rings, varves, and other dating methods.
obduction	The overriding of oceanic crust onto the leading edge of a continental lithospheric plate.
oblique fault	A fault in which motion includes both dip-slip and strike-slip components.
obsidian	A black or dark-colored volcanic glass, usually of rhyolite composition, characterized by conchoidal fracture.
oceanic crust	Earth’s crust that underlies the ocean basins and is rich in iron and magnesium; ranges in thickness from about 5 to 10 km (3 to 6 mi).
oceanic trench	A narrow, elongated depression, which may be thousands of kilometers long, of the deep-sea floor associated with a subduction zone, oriented parallel to a volcanic arc and usually to the edge of the adjacent continent; commonly 2 km (1 mi) or more deeper than the surrounding ocean floor.
oil field	A geographic region rich in petroleum resources and containing one or more wells that produce, or have produced, oil and/or gas.
olivine	A silicate (silicon + oxygen) mineral of magnesium and iron, (Mg,Fe)2SiO4; commonly olive-green and an essential mineral in basalt, gabbro, and peridotite.
oligoclase	A silicate (silicon + oxygen) mineral of the plagioclase group, intermediate in chemical composition and crystallographic and physical characteristics between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8).
olistolith	A large block or other rock mass, usually more than 10 m (30 ft) long, transported by submarine, gravity-driven sliding or slumping and included within a debris-flow deposit called an olistostrome.
olistostrome	A debris-flow deposit consisting of a chaotic mass of intimately mixed heterogeneous materials (such as blocks and muds) that accumulated by submarine gravity sliding or slumping of unconsolidated sediments. It is a mappable, lens-like stratigraphic unit lacking true bedding but intercalated among normally bedded sequences.
ooid	One of the small round or ovate accretionary bodies in a sedimentary rock, resembling the roe of fish, formed by accretion around a nucleus such as a shell fragment, algal pellet, or sand grain; laminated grains may reach 2 mm (0.08 in) across, but are commonly 0.5 to 1 mm (0.02 to 0.04 in) across. Synonymous and preferred to “oolith” (to avoid confusion with “oolite”).
oolite	A sedimentary rock, usually limestone, composed of ooids.
opal	A hydrous silicate (silcon + oxygen) mineral or mineral gel, SiO2 • nH2O, consisting of packed spheres of silica and varying amounts of water (as much as 20% but usually 3%–9%).
ophiolite	An assemblage of ultramafic and mafic intrusive and extrusive igneous rock, probably representing oceanic crust.
orogeny	A mountain-building event.
ortho–	A prefix used with the names of metamorphic rocks, indicating that the rock was derived from an igneous rock; it may also indicate the primary origin of a crystalline sedimentary rock, e.g., “orthoquartzite” as distinguished from “metaquartzite.”
orthoclase	A colorless, white, cream-yellow, flesh-pink, or gray silicate (silicon + oxygen) mineral of the alkali feldspar group, KAlSi3O8, characterized by potassium ions in its crystal structure.
orthopyroxene	A mineral of the pyroxene group that crystallized in the orthorhombic system and usually contains no calcium and little or no aluminum.
orthopyroxenite	An ultramafic igneous rock composed primarily of orthopyroxene.
orthoquartzite	A clastic sedimentary rock that is composed of almost exclusively of quartz sand.
orthorhombic system	One of the six crystal systems of minerals, characterized by three axes of symmetry that are mutually perpendicular and of unconstrained relative lengths.
ostracode	Any aquatic crustacean belonging to the subclass Ostracoda, characterized by a two-valved (shelled), generally calcified carapace with a hinge along the dorsal margin. Range: Lower Cambrian to Holocene.
outcrop	Any part of a rock mass or formation that is exposed or “crops out” at Earth’s surface.
outer trench swell	A subtle ridge on the seafloor near an oceanic trench formed where a subducting plate begins to flex and fault into the trench.
outwash	Glacial sediment transported and deposited by meltwater streams.
outwash plain	A broad, gently sloping sheet of outwash deposited by meltwater streams flowing from the front of or beyond a glacier, and formed by coalescing outwash fans.
overbank deposit	Fine-grained sediment (silt and sand) deposited on a floodplain by floodwaters.
overburden (economic geology)	Rock material, either loose or consolidated, that overlies a mineral deposit and must be removed prior to mining.
overburden (sedimentary geology)	Loose soil, silt, sand, gravel, or other unconsolidated material overlying bedrock.
overdeepening	Process by which an eroding glacier excessively deepens and broadens a preexisting valley.
overthrust	A low-angle thrust fault of large scale, with displacement generally measured in kilometers.
oxbow	A closely looping meander in a stream that resembles the U-shaped collar of an ox yoke. Synonymous with “horseshoe bend.”
oxidation	The process of combining with oxygen.
oxide	A mineral group composed of oxygen plus an element or elements, for example, iron in hematite, Fe2O3; or aluminum in corundum, Al2O3.
pahoehoe	A Hawaiian term for a lava flow typified by a smooth, billowy, or ropy surface.
palagonite	An altered volcanic, specifically basaltic, glass that becomes pillow lava or occurs in amygdules.
palagonite tuff	A pyroclastic rock consisting of angular fragments of hydrothermally altered or weathered palagonite, produced by explosive interaction of mafic magma and water.
paleochannel	A remnant of a stream channel cut in older rock or sediment and filled by the younger overlying rock or sediment; a buried stream channel.
paleofill	Sediment that filled caves and sinkholes that existed before the present cave passage formed.
paleofluvial channel	A natural passageway or depression produced by the action of a stream or river at a particular time in the geologic past.
paleogeography	The study, description, and reconstruction of the physical landscape in past geologic periods.
paleontology	The study of the life and chronology of Earth’s geologic past based on the fossil record.
paleosol	An soil layer preserved in the geologic record.
paleotopography	The topographic relief of an area at a particular time in the geologic past.
parabolic dune	Crescent-shaped dune with horns or arms that point upwind.
Pangaea	A supercontinent that existed from about 300 million to about 200 million years ago and included most of the continental crust of the Earth, from which the present continents were derived by fragmentation and continental drift. During an intermediate stage of the fragmentation—between the existence of Pangaea and that of the present continents—Pangaea split into two large fragments, Laurasia in the Northern Hemisphere and Gondwana in the Southern Hemisphere.
parent material	The unconsolidated organic and mineral material from which soil forms.
parent rock	Rock from which soil, sediment, or other rock is derived.
parting	A plane or surface along which a rock readily separates.
passive margin	A continental plate boundary where no plate-scale tectonism is taking place; plates are not converging, diverging, or sliding past one another.
patterned ground	Well-defined, more or less symmetrical forms such as circles, polygons, nets, steps, and stripes in surficial material that develop as a result of intense frost action.
peat	An accumulation of partly decomposed plant remains in swampy lowlands. It is an early stage or rank in the development of coal.
pebble	A small rounded rock, especially a waterworn stone, between 4 and 64 mm (0.16 and 2.5 in) across.
pediment	A gently sloping, erosional bedrock surface at the foot of a mountain or plateau escarpment.
pegmatite	An intrusive igneous rock consisting of exceptionally coarse-grained, interlocking crystals, generally granitic in composition and commonly in irregular dikes, lenses, and veins, especially at the margins of batholiths.
pelecypod	Any benthic aquatic mollusk belonging to the class Pelecypoda, characterized by a bilaterally symmetrical bivalve shell, a hatchet-shaped foot, and sheetlike gills. Range: Ordovician to Holocene.
pelite	A sediment or sedimentary rock composed of the finest detritus (clay- or mud-sized particles), or a calcareous sediment composed of clay and minute particles of quartz.
pelitic	Describes a sedimentary rock composed of pelite or a metamorphic rock derived by metamorphism of pelite.
pendant	A solutional remnant hanging from the ceiling or wall of a cave.
peneplain	A broad area of low topographic relief resulting from long-term, extensive erosion.
peperite	A breccialike material in sedimentary rocks, interpreted as a mixture of lava with sediment or as shallow intrusions of magma into wet sediment.
perched aquifer	An aquifer that is separated from (“perched” above) the water table by an unsaturated zone.
peridotite	A coarse-grained intrusive igneous rock composed primarily of olivine and other mafic minerals; commonly alters to serpentinite.
period	The fundamental unit of the worldwide geologic time scale. It is lower in rank than era and higher than epoch. The geochronologic unit during which the rocks of the corresponding system were formed.
periphyton	Microorganisms, primarily algae and heterotrophic microbes, that coat rocks, plants, and other surfaces at the bottom of a body of water.
perlite	A volcanic glass having the composition of rhyolite, a perlitic texture, and a generally higher water content than obsidian.
perlitic	Describes the texture of glassy volcanic rocks characterized by numerous curving cracks roughly concentric around closely spaced centers.
permafrost	Any soil, subsoil, or other surficial deposit, or even bedrock, occurring in arctic, subarctic, and alpine regions at a variable depth beneath Earth’s surface in which a temperature below freezing has existed continuously for a long time (from two years to tens of thousands of years).
permeability	A measure of the relative ease with which a fluid moves through the pore spaces of a rock or unconsolidated deposit.
perovskite	A yellow, brown, or grayish-black cubic mineral: CaTiO3.
perthite	An intergrowth of two feldspars—a potassium-rich phase and a sodium-rich phase; typically form strings, lamellae, blebs, films, or irregular small veins that are visible to the naked eye.
phenocryst	A coarse-grained crystal in a porphyritic igneous rock.
phonolite	A group of fine-grained volcanic rocks primarily composed of alkali feldspar.
phosphate	A mineral group composed of phosphorus and oxygen plus an element or elements, for example, calcium in apatite, Ca5(F,Cl,OH)PO4; and copper and aluminum in turquoise, CuAl6(PO4)4(OH)8.
phlogopite	A magnesium-rich mineral of the mica group: KMg3Si3AlO10(F,OH)2. It is a yellowish brown to brownish red or copper-colored, and usually occurs in crystalline limestones as a result of dedolomitization.
phosphatic	Pertaining to or containing phosphate minerals.
phreatic	Of or relating to groundwater.
phreatic explosion	A volcanic explosion of steam, mud, or other material that is not incandescent (hot enough to glow); caused by the heating and consequent expansion of groundwater due to an underlying igneous heat source.
phreatic zone	The zone of saturation.
phreatomagmatic	See “hydrovolcanic.”
phreatophyte	A deeply rooted plant that obtains water from the water table or through the overlying capillary fringe.
phyllite	A metamorphic rock, intermediate between slate and mica schist, with minute crystals of graphite, sericite, or chlorite that impart “schistosity” (a silky sheen).
picrite	A dark-colored, generally hypabyssal, extrusive igneous rock composed of <52% silica and containing abundant olivine along with pyroxene, biotite, possibly amphibole, and <10% plagioclase.
pictograph	A picture painted on a rock by primitive peoples.
piedmont	A gently sloping area at the base of a mountain front. Synonymous with “bajada.” Also, describes a feature (e.g., plain, slope, or glacier) that lies or formed at the base of a mountain or mountain range.
piedmont glacier	A thick, continuous sheet of ice on land at the base of a mountain range, formed by spreading out and coalescing of valley glaciers from higher elevations.
piedmont lake	A lake occupying a partly overdeepened basin that was excavated in rock or dammed by a moraine of a piedmont glacier.
pillow lava	A general term for lavas displaying pillow structures and considered to have formed in a subaqueous environment; such lava is usually basaltic or andesitic.
pillow structure	A structure observed in certain extrusive igneous rocks that is characterized by discontinuous bun-shaped masses ranging in size from a few centimeters to a meter or more in greatest dimension. They are considered to be the result of subaqueous extrusion.
piping	Erosion or solution by percolating water in a layer of subsoil, resulting in the formation of narrow conduits, tunnels, or “pipes” through which soluble or granular soil material is removed.
pipy concretion	A hard, compact mass of mineral matter formed in an elongated or pipelike shape.
pisolite	A sedimentary rock, usually limestone, made up of pisoids (round or ellipsoidal accretionary bodies commonly composed of calcium carbonate) cemented together.
placer	A concentrated deposit of minerals, usually heavy, such as gold, cassiterite, or rutile, in a beach or stream deposit.
placer mining	The extraction of metals or minerals from placers, usually involves running water.
plagioclase	A silicate (silicon + oxygen) mineral of the feldspar group that contains both sodium and calcium ions that freely substitute for one another; characterized by striations (parallel lines) in hand specimens.
planar	Lying or arranged as a plane or in planes, usually implying more or less parallelism, as in bedding or cleavage.
plastic	Describes a material capable of permanently deforming without rupturing.
plate	Thin, flat fragment of rock, such as a slab of flagstone.
plate boundary	A zone of seismic and tectonic activity along the edges of lithospheric plates, resulting from the relative motion among plates.
plate tectonics	A theory of global tectonics in which the lithosphere is divided into about 20 rigid plates that interact with one another at their boundaries, causing seismic and tectonic activity along these boundaries.
plateau	A broad, flat-topped topographic high (terrestrial or marine) of great extent and elevation above the surrounding plains, canyons, or valleys.
platform	Any level or nearly level surface.
platy	Describes a sedimentary particle whose length is more than three times its thickness; also describes a sandstone or limestone that splits into thin layers, 2 to 10 mm (0.08 to 0.4 in) thick.
playa	A dry, vegetation-free, flat area at the lowest part of an undrained desert basin.
playa lake	A shallow, intermittent lake in an arid region, covering up or occupying a playa in the wet season but subsequently drying up.
Plinian eruption	An explosive eruption characterized by large amounts of tephra and a tall eruption column from which a steady, turbulent stream of fragmented magma and magmatic gas is released at a high velocity.
plug	A vertical, pipelike body of magma that represents the conduit to a former vent. Also, a lava-filled crater whose surrounding material has been removed by erosion.
plume	A persistent, pipelike body of hot material moving upward from Earth’s mantle into the crust.
pluton	A deep-seated igneous intrusion.
plutonic	Describes an igneous rock or intrusive body formed at great depth beneath Earth’s surface.
pluvial	Describes a geologic process or feature resulting from rain.
poikilitic	Describes the texture of an igneous rock in which small grains of one mineral are irregularly scattered within a typically anhedral larger crystal of another mineral.
point bar	A low ridge of sand and gravel deposited in a stream channel on the inside of a meander, where flow velocity slows.
polymorph	A crystal form of a substance that displays polymorphism.
polymorphism	The characteristic of a chemical substance to crystallize in more than one form.
polyp	An individual coelenterate (e.g., coral) with a hollow tubular or columnar body terminating at the top in a central mouth surrounded by upward directed tentacles.
porosity	The percentage of total void space in a volume of rock or unconsolidated deposit.
porphyritic	Describes an igneous rock of any composition that contains conspicuous phenocrysts (larger crystals) in a fine-grained groundmass.
porphyry	An igneous rock consisting of abundant coarse-grained crystals in a fine-grained groundmass.
porphyroclast	A partly crushed, unmetamorphosed rock fragment within a finer-grained matrix in a metamorphic rock.
potassium feldspar	A feldspar mineral rich in potassium such as orthoclase, microcline, and sanidine.
potentiometric surface	A surface representing the total head of groundwater and defined by the level to which water will rise in tightly cased wells.
Precambrian	A commonly used term to designate all rocks older than the Cambrian Period of the Standard Global Chronostratigraphic Scale. It includes the Archean and Proterozoic eons and represents 90% of geologic time.
pressure ridge (sea ice)	A rugged, irregular wall of floating broken ice buckled upward by the lateral pressure of wind or current forcing or squeezing one floe against another.
pressure ridge (volcanic)	An elongate uplift of the congealing crust of a lava flow, buckled upward by laterally directed pressure.
principle of uniformity	The assumption of uniformity of causes or processes throughout time and space; the uniformity of natural laws. Not synonymous with “uniformitarianism.”
prodelta	The part of a delta below the level of wave erosion.
progradation	The seaward building of land as a result of sedimentary deposition.
propylitization	Describes low-pressure and low-temperature alteration around many ore bodies.
protalus rampart	An arcuate ridge of coarse, angular blocks of rock that mark the downslope edge of an existing or melted snowbank.
protolith	The unmetamorphosed rock from which a given metamorphic rock was formed. Synonymous with “parent rock.” Also, the parent rock from which regolith is formed.
provenance	A place of origin, specifically the area from which the constituent materials of a sedimentary rock were derived.
pseudomorph	A mineral whose outward crystal form resembles that of another mineral; described as being “after” the mineral whose outward form it has (e.g., quartz after fluorite).
pull-apart basin	A topographic depression created by an extensional bend or extensional overstep along a strike-slip fault.
pumice	A highly vesicular pyroclast with very low bulk density and thin vesicle walls.
pumiceous	Describes a texture of volcanic rock consisting of tiny gas holes such as in pumice; finer than scoriaceous.
pyroclast	An individual particle ejected during a volcanic eruption; usually classified according to size.
pyroclastic	Describes clastic rock material formed by volcanic explosion or aerial expulsion from a vent; also, describes a rock texture of explosive origin. It is not synonymous with “volcanic.”
pyroclastic flow	A hot, typically >800°C (1,500°F), chaotic mixture of rock fragments, gas, and ash that travels rapidly (tens of meters per second) away from a volcanic vent or collapsing flow front.
pyroclastic surge	Low-density, dilute, turbulent pyroclastic flow. The deposits may be thinly bedded, laminated, and cross-bedded.
pyroxene	A group of silicate (silicon + oxygen) minerals composed of magnesium and iron with the general formula (Mg,Fe)SiO3; characterized by short, stout crystals in hand specimens.
quartz	Silicon dioxide, SiO2. The only silicate (silicon + oxygen) mineral consisting entirely of silicon and oxygen. Synonymous with “crystalline silica.”
quartzite	Metamorphosed quartz sandstone. A medium-grained, nonfoliated metamorphic rock composed mostly of quartz.
quartz monzonite	An intrusive igneous rock of granitic composition but with about as much plagioclase as alkali feldspar.
quartzose	Containing quartz as a principal constituent.
radioactivity	The spontaneous decay or breakdown of unstable atomic nuclei.
radiocarbon age	An isotopic age expressed in years and calculated from the quantitative determination of the amount of carbon-14 remaining in an organic material. Synonymous with “carbon-14 age.”
radiolarian	Any actinopod (protozoan) belonging to the subclass Radiolaria, characterized by a siliceous skeleton and a marine pelagic environment. Range: Cambrian to Holocene.
radiometric age	An age (in years) calculated from the quantitative determination of radioactive elements and their decay products. The preferred term is “isotopic age.”
ravinement	From the French meaning “hollowing out (by waters),” an irregular junction that marks a break in sedimentation, such as an erosion line occurring where shallow–water marine deposits have “scooped down into” (or “ravined”) slightly eroded underlying beds; a small-scale disconformity caused by periodic invasions of the sea over a deltaic area.
rebound	Upward flexing of Earth’s crust. Synonymous with “upwarping.”
recharge	The addition of water to the saturated zone below the water table.
red bed	Sedimentary strata that is predominantly red due to the presence of ferric iron oxide (hematite) coating individual grains; usually sandstone, siltstone, or shale.
reef	A ridgelike or moundlike structure, layered or massive, built by sedentary calcareous organisms (e.g., corals) and consisting mostly of their remains.
reef crest	The sharp break in slope at the seaward margin of a reef flat, located at the top of the reef front; marked by dominance of a particular coral species (such as Acropora palmata throughout the Caribbean) or by an algal ridge and/or surge channels.
reef flat	A stony platform of reef rock, landward of the reef crest at or above the low tide level, commonly strewn with coral fragments and coral sand and less commonly covered by patches of living coral and associated organisms.
reef front	The upper part of the outer or seaward slope of a reef.
reef rock	A resistant massive unstratified rock composed of the calcareous remains of reef-building organisms and commonly carbonate sand, entirely cemented by calcium carbonate.
reflection shooting	A type of seismic survey based on measurement of the travel times of seismic waves that originate from an artificially produced disturbance and are reflected back at near-vertical incidence from subsurface boundaries, resulting in a separation of media of different densities and/or elastic-wave velocities.
refraction	The deflection of a ray of light or of an energy wave (such as a seismic wave) due to its passage from one medium to another of differing density, which changes its velocity.
refraction shooting	A type of seismic survey based on the measurement of the travel times of seismic waves that have traveled nearly parallel to the bedding in high-velocity layers, in order to map such layers.
regional metamorphism	A type of metamorphism that affects an extensive region, as opposed to local metamorphism that affects a relatively restricted area.
regolith	From the Greek “rhegos” (blanket) + “lithos” (stone), the layer of unconsolidated rock material that forms the surface of the land and overlies or covers bedrock; includes rock debris of all kinds, volcanic ash, glacial drift, alluvium, loess, and aeolian deposits, vegetal accumulations, and soil.
regression	Long-term seaward retreat of the shoreline or relative fall of sea level.
rejuvenation	The renewal of any geologic process.
relative dating	The chronological placement and ordering of rocks, events, or fossils with respect to the geologic time scale and without reference to numerical ages.
relict	Describes a topographic feature that remains after other parts of the feature have been removed or have disappeared, for example, a “relict beach ridge” or a “relict hill.” Also, a landform made by processes no longer operative such as glaciated forms in the northern United States or sand dunes in rain forests.
reverse fault	A contractional high-angle (greater than 45°) dip-slip fault in which the hanging wall moves up relative to the footwall.
reservoir	An artificial or natural storage place for water, such as a lake, pond, or aquifer, from which the water may be withdrawn for such purposes as irrigation, municipal water supply, or flood control.
reservoir rock	A permeable rock layer where hydrocarbons have migrated and are held underground.
resurgent caldera	A caldera in which the downdropped block is uplifted by magmatic intrusion following crater formation.
resurgent cauldron	A cauldron in which the cauldron block, following subsidence, has been uplifted, usually in the form of a structural dome.
rhizome	A somewhat elongated, usually horizontal underground plant stem that is often thickened by storage of reserve food material, produces shoots above and roots below, and is distinguished from the true root in possessing buds, nodes, and usually scalelike leaves.
rhyodacite	A volcanic rock that contains approximately 68%–72% silica and is intermediate in composition between rhyolite and dacite.
rhyolite	A volcanic rock that is characteristically light in color, contains approximately 72% or more of silica, and is rich in potassium and sodium.
rift	A region of Earth’s crust where extension results in formation of many related normal faults, commonly associated with volcanic activity.
rift valley	A depression formed by grabens along the crest of a mid-ocean ridge or in a continental rift zone.
right-lateral fault	A strike-slip fault on which the side opposite the observer has been displaced to the right.
rill	A very small brook or trickling stream of water usually without any tributaries. Also, the channel formed by such a stream.
rille	A trenchlike or cracklike valley, commonly occurring on a planetary surface subjected to plains volcanism; may be irregular with a meandering course (sinuous rille) or relatively straight (normal rille).
rincon	Meaning “inside corner or nook” in Spanish and used in the southwestern United States for a square-cut recess or hollow in a cliff or a reentrant in the borders of a mesa or plateau. Also used for a small, secluded valley, and for a bend in a stream.
rip current	A strong, narrow, surface or near-surface current of short duration (a few minutes to an hour or two) and high velocity (up to 2 knots), flowing seaward from the shore through the breaker zone at nearly right angles to the shoreline, appearing as a visible band of agitated water returning to the sea after being piled up on the shore by incoming waves and wind.
ripple marks	The undulating, approximately parallel and usually small-scale pattern of ridges formed in sediment by the flow of wind or water.
riprap	A layer of large, durable rock fragments placed in an attempt to prevent erosion by water and thus preserve the shape of a surface, slope, or underlying structure.
rip-up clast	A mud clast (usually flat) that has been “ripped up” by currents from a semiconsolidated mud deposit, transported, and deposited elsewhere; commonly associated with a storm or other high-energy event.
roche moutonnée	A glacially sculpted, elongated bedrock knob or hillock.
rock	An aggregate of one or more minerals (e.g., granite), a body of undifferentiated mineral matter (e.g., obsidian), or a body of solid organic material (e.g., coal).
rockfall	The most rapid type of slope movement in which a newly detached fragment of bedrock of any size falls from a cliff or other very steep slope, traveling straight down or in a series of leaps and bounds down a slope.
rock glacier	A mass of poorly sorted angular boulders and fine material, with interstitial ice a meter or so below the surface (ice-cemented) or containing a buried ice glacier (ice-cored).
rodent	Any of the order Rodentia of relatively small, gnawing animals such as a mouse, squirrel, or beaver that have in both jaws a single pair of incisors with a chisel-shaped edge.
roundness	The relative amount of curvature of the “corners” of a sediment grain.
roundstone	Any naturally rounded rock fragment larger than a sand grain.
runup	The advance of water up the foreshore (between high- and low-water marks) of a beach, following the breaking of a wave.
rutile	A reddish-brown oxide mineral of titamium, TiO2.
sabkha	Any flat area, either coastal or interior, where saline minerals crystallize near or at the surface as a result of deflation or evaporation.
salic	Derived from silicon + aluminum + ic to describe an igneous rock having abundant light-colored, silicon- or aluminum-rich minerals such as quartz and feldspar; also, describes those minerals.
saltation	A mode of sediment movement, driven by wind or water, whereby materials move through a series of intermittent leaps or jumps.
sand	A clastic particle smaller than a granule and larger than a silt grain, with a diameter ranging from 1/16 to 2 mm (0.0025 to 0.08 in).
sand sheet	A sheetlike body of surficial sediment, commonly sand, that veneers the underlying stratigraphic units (unconsolidated deposits or bedrock) and can range in thickness from a few centimeters to tens of meters, with a lateral persistence of a few meters to tens of kilometers.
sandstone	Clastic sedimentary rock composed of predominantly sand-sized grains.
sanidine	A silicate (silicon + oxygen) mineral of the alkali feldspar group.
sapping	The natural process of erosion along the base of a cliff by the wearing-away of softer layers, commonly involving the weakening or rock by groundwater conducted along the contact between rock strata, and thus removing the support for the upper mass which breaks off into large blocks falling from a cliff face.
saprolite	A soft, earthy, typically clay-rich, thoroughly decomposed rock, formed in place by chemical weathering of igneous, sedimentary, and metamorphic rocks.
saturated zone	A subsurface zone in which all the interstices are filled with water under pressure greater than that of the atmosphere; separated from the unsaturated zone (above) by the water table.
scarp	A steep cliff or topographic step resulting from displacement on a fault or as a result of slope movement or erosion. Synonymous with “escarpment.”
schist	A medium- to coarse-grained, strongly foliated, metamorphic rock with eminently visible mineral grains, particularly mica, which are arranged parallel, imparting a distinctive sheen, or “schistosity,” to the rock.
schistose	Describes a rock displaying schistosity, or foliation, which imparts a silky sheen.
schistosity	The foliation in schist or other coarse-grained, crystalline rock resulting from the parallel alignment of platy mineral grains of mica or inequant crystals of other minerals.
schorlomite	A black or brown-black cubic mineral of the garnet group: Ca3(Ti4+, Fe3+)2(Si,Fe3+)3O12.
scoria	A bomb-size pyroclast that is irregular in form and generally very vesicular.
scoria cone	A conical volcanic feature formed by the accumulation of scoria and other pyroclasts, usually of basaltic or andesitic composition.
scoriaceous	Describes a texture of volcanic rock consisting of relatively large gas holes such as in vesicular basalt; coarser than pumiceous.
scour	The powerful and concentrated clearing and digging action of flowing water, air, or ice.
seafloor spreading	A process whereby new oceanic crust is formed by upwelling of magma at the center of mid-ocean ridges and by a moving-away of the new material from the site of upwelling at rates of 1 to 10 cm (2 to 25 in) per year. This movement provides the source of seafloor within the theory of plate tectonics, which also contains a provision for destruction of seafloor by subduction.
seagrass	Flowering plants from four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae, and Cymodoceaceae) all in the order Alismatales, which grow in marine environments.
seamount	An elevated portion of the sea floor, 1,000 m (3,300 ft) or higher, either flat-topped or peaked.
sediment	An eroded and deposited, unconsolidated accumulation of rock and mineral fragments.
sedimentary	Pertaining to or containing sediment.
sedimentary rock	A rock resulting from the consolidation of loose sediment that has accumulated in layers; it may be “clastic,” consisting of mechanically formed fragments of older rock; “chemical,” formed by precipitation from solution; or “organic,” consisting of the remains of plants and animals. One of the three main classes of rock—igneous, metamorphic, and sedimentary.
sedimentation	The process of forming or accumulating sediment into layers, including the separation of rock particles from parent rock, the transportation of these particles to the site of deposition, the actual deposition or settling of the particles, the chemical and other changes occurring in the sediment, and the ultimate consolidation of the sediment into solid rock.
seiche	An oscillation of a body of water in an enclosed or semi-enclosed basin that varies in period—depending on the physical dimensions of the basin—from a few minutes to several hours, and in height from several centimeters to a few meters; primarily caused by local changes in atmospheric pressure, aided by winds, tidal currents, and earthquakes.
seismic	Pertaining to an earthquake or Earth vibration, including those that are artificially induced.
seismicity	The phenomenon of movements in the Earth’s crust. Synonymous with “seismic activity.”
septaria	A large, 8–90 cm (3–35 in) across, roughly spheroidal concretion characterized by a series of radiating cracks that are invariably filled or partly filled by crystalline minerals, most commonly calcite.
septarian	Describes the irregular polygonal pattern of internal cracks developed in septaria; closely resembles the desiccation structure of mud cracks.
sequence	A succession of geologic events, processes, or rocks, arranged in chronologic order to show their relative position and age with respect to geologic history as a whole. Also, a rock-stratigraphic unit that is traceable over large areas and defined by sediment associated with a major sea level transgression–regression.
seriate	A variety of igneous porphyritic texture in which the sizes of grains range gradually down to the size of groundmass grains.
sericite	An alteration product of various aluminosilicate minerals—specifically a white, fine-grained potassium mica, usually muscovite or very close to muscovite in composition—occurring as small scales and flakes in metamorphic rocks.
series	A chronostratigraphic unit next in rank below system and above stage; the rocks formed during an “epoch” of geologic time.
serpentine	A group of silicate (silicon + oxygen) minerals with the general formula (Mg,Al,Fe,Mn,Ni,Zn)2–3(Si,Al,Fe)2O5(OH)4, characterized by a greasy or silky luster, a slightly soapy feel, and conchoidal fracture.
serpentinite	A nonfoliated, metamorphic rock characterized by mottled shades of green and a resemblance to the skin of a serpent; consists almost entirely of serpentine minerals.
serpulid	Any annelid (ocean worm) that belongs to the family Serpulidae and that characteristically builds a contorted calcareous or leathery tube on a submerged surface. Range: Middle Triassic to Holocene.
shale	A clastic sedimentary rock made of clay-sized particles and characterized by fissility.
shear	Deformation resulting from stresses that cause contiguous parts of a body to slide relatively to each other in a direction parallel to their plane of contact.
sheet	A general term for a tabular igneous intrusion, especially those that are concordant (parallel to preexisting rock bodies) or only slightly discordant (cutting across preexisting rock bodies).
shear zone	A zone of rock that has been crushed and brecciated by many parallel fractures as a result of shearing.
sheet erosion	The removal of thin layers of surface material more or less evenly from an extensive area of gently sloping land by broad continuous sheets of running water, rather than by streams flowing in well-defined channels.
sheetflood	A broad expanse of moving, storm-borne water that spreads as a thin, continuous, relatively uniform film over a large area in an arid region and that is not concentrated into well-defined channels; its distance of flow is short and its duration is measured in minutes or hours, commonly occurring after a period of sudden and heavy rainfall.
sheet flow	The downslope movement or overland flow of water, in the form of a thin, continuous film, over relatively smooth soil or rock surfaces and not concentrated into channels larger than rills.
sheetwash	A sheetflood occurring in a humid region. Also, the material transported and deposited by such a sheetflood. Used as a synonym of “sheet flow” and “sheet erosion.”
shield volcano	A broad shield-shaped volcano that is built up by successive, mostly effusive, eruptions of low-silica lava.
shoal	A relatively shallow place in a stream, lake, sea, or other body of water.
shoaling	To become shallow gradually, to fill up or block off with a shoal, or to proceed from a greater to a lesser depth of water.
shoreface	The zone between the seaward limit of the shore and the more nearly horizontal surface of the offshore zone; typically extends seaward to storm wave depth or about 10 m (30 ft).
shoreface ravinement surface	An abrupt, scoured surface that is interpreted to have been formed by the erosive action of shoreface currents and waves during landward movement of the shoreline. Marine strata, which in general display a deepening-upward trend, must directly overlie the surface.
sierra	A high range of hills or mountains, especially one having jagged or irregular peaks that resemble the teeth of a saw.
silica	Silicon dioxide, SiO2, an essential constituent of many minerals, occurring as crystalline quartz, cryptocrystalline chalcedony, and amorphous opal.
silicate	A mineral group composed of silicon (Si) and oxygen (O) plus an element or elements, for example, quartz, SiO2; olivine, (Mg, Fe)2SiO4; and pyroxene, (Mg,Fe)SiO3; as well as the amphiboles, micas, and feldspars.
silicate tetrahedron	A complex ion formed by four oxygen ions surrounding a silicon ion in a tetrahedral configuration, SiO4; the basic unit of the silicates.
silicic	Describes a silica-rich igneous rock or magma.
silicic magma	Describes magma that contains more than 65% silica; generally viscous, gas-rich, and tends to erupt explosively.
siliciclastic	Describes noncarbonate clastic rocks.
siliceous	Describes a rock or other substance containing abundant silica.
sill	An igneous intrusion that parallels the bedding of preexisting sedimentary rock or the foliation of preexisting metamorphic rock.
silt	Clastic sedimentary material intermediate in size between fine-grained sand and coarse clay, 0.0039 to 0.063 mm (0.00015 to 0.0025 in) across.
silting	The accumulation of silt suspended throughout a body of standing water or in some considerable portion of it. In particular, the choking, filling, or covering with stream-deposited silt behind a dam or other place of retarded flow, or in a reservoir. Synonymous with “siltation.”
siltstone	A clastic sedimentary rock composed of silt-sized grains.
sinkhole	A circular, commonly funnel-shaped depression in a karst area with subterranean drainage.
sinter	The lightweight, porous, opaline variety of silica that is white or nearly white and deposited as an incrustation by precipitation from the waters of geysers and hot springs.
skarn	Rocks composed mostly of lime-bearing silicates derived from nearly pure limestones and dolomites into which large amounts of silica, aluminum, iron, and magnesium have been introduced.
slabbing	Rock splitting into slabs along closely spaced parallel fissures, commonly falling under the force of gravity from a cliff face.
slate	A fine-grained, foliated metamorphic rock that easily splits into slabs and thin plates.
slickenside	A smoothly polished and commonly striated surface representing a fault plane.
slip face	The steeply sloping surface on the lee side of a dune, standing at or near the angle of repose of loose sand, and advancing downwind by a succession of slides wherever that angle is exceeded.
slope	The inclined surface of any part of Earth’s surface, such as a hillslope. Also, a broad part of a continent descending into an ocean.
slope movement	The gradual or rapid downslope movement of soil or rock under gravitational stress. Synonymous with “mass wasting.”
slope wash	Soil and rock material that is or has been transported down a slope under the force of gravity and assisted by running water not confined to channels; also, the process by which slope-wash material is moved.
slump	A generally large, coherent slope movement with a concave failure surface and subsequent backward rotation relative to the slope.
smectite	A group of expanding-lattice clay minerals derived from the alteration of volcanic glass and from the weathering of primary silicates.
soapstone	A soft metamorphic rock with a fibrous or flakey texture and a soapy feel, composed mostly of talc with lesser amounts of other minerals.
soil	The unconsolidated portion of the Earth’s crust modified through physical, chemical, and biotic processes into a medium capable of supporting plant growth.
solar tide	The part of the tide caused solely by the tide-producing force of the Sun.
solifluction	The slow downslope movement of waterlogged soil, normally at rates of 0.5 to 5.0 cm (0.2 to 2 in) per year; especially, the flow occurring at high elevations in regions underlain by frozen ground, which acts as a downward barrier to water percolation.
solifluction lobe	An isolated, tongue-shaped feature, as much as 25 m (80 ft) wide and 150 m (490 ft) long, formed by more rapid solifluction on certain sections of a slope; commonly has a steep front (15°–25°) and a relatively smooth upper surface.
solute	A dissolved substance.
sorted	Describes an unconsolidated sediment consisting of particles of essentially uniform size.
sorting	The dynamic process by which sedimentary particles having some particular characteristic (such as similarity of size, shape, or specific gravity) are naturally selected and separated from associated but dissimilar particles by the agents of transportation.
source rock	In petroleum geology: a rock layer from which hydrocarbons have been generated or are capable of being generated. Source rocks contain sufficient organic matter to be a future source of hydrocarbons.
spalling	The process by which scales, plates, or flakes of rock, from less than a centimeter to several meters thick, successively fall from the bare surface of a large rock mass; a form of exfoliation.
spar	A term loosely applied to any transparent or translucent light-colored crystalline material, usually readily cleavable and somewhat lustrous.
spatter	An accumulation of initially very fluid pyroclasts, usually stuck together, coating the surface around a vent.
spatter cone	A low, steep-sided cone of spatter built up on a fissure or vent, usually composed of basaltic material.
specific conductance	The measure of discharge of a water well per unit of drawdown.
speleothem	Any secondary mineral deposit that forms in a cave.
sphene	A usually yellow or brown accessory mineral in granitic rocks and in calcium-rich metamorphic rocks; also called titanite.
spreading center	A divergent plate boundary where two lithospheric plates are spreading apart. It is a source of new crustal material.
spring	A place where groundwater flows naturally from a rock or the soil onto the land surface or into a body of surface water.
squeeze-up	A small accumulation of viscous lava extruded under pressure from a fracture or opening onto the solidified surface of a lava flow.
stade	An interval of a glacial stage marked by glacial readvance.
stage (glacial)	A major subdivision of a glacial epoch, particularly one of the cycles of growth and disappearance of the Pleistocene ice sheets.
stage (stratigraphy)	A chronostratigraphic unit next in rank below series and above substage, based on biostratigraphic zones considered to approximate time-equivalent deposits; rocks formed during an “age” of geologic time.
stalactite	A conical or cylindrical speleothem that hangs from the ceiling or wall of a cave, deposited from drops of water and usually composed of calcite but may be formed of other minerals.
stalagmite	A conical or cylindrical speleothem that is developed upward from the floor of a cave by the action of dripping water, usually formed of calcite but may be formed of other minerals.
staurolite	A silicate (silicon + oxygen) mineral of iron and aluminum, commonly with some magnesium and zinc, (Fe,Mg)4Al17(Si,Al)8O45(OH)3, characterized by twinned crystals that resemble a cross.
stock	A relatively small plutonic body having an aerial extent less than 100 km2 (40 mi2) and no known floor.
stone stream	An accumulation of boulders or angular blocks, with no fine sizes in the upper part, over solid or weathered bedrock, colluvium, or alluvium; usually occur at the heads of ravines but may extend into forests or fill a valley floor; may exist on any slope angle, but ordinarily not steeper than 40°.
stope	An underground excavation formed by the extraction of ore.
storm surge	An abnormal, sudden rise of sea level along an open coast during a storm, caused primarily by strong winds offshore, or less frequently, a drop in atmospheric pressure, resulting in water piled up against the coast. It is most severe during high tide.
storm tide	The total observed seawater level during a storm, resulting from the combination of storm surge and the astronomical tide. An erroneous synonym of “storm surge.”
strand plain	A shore built seaward by waves and currents, extending continuously for some distance along the coast.
strata	Tabular or sheetlike layers of sedimentary rock that are visually distinctive from other layers above and below. The singular form of the term is stratum, but is less commonly used.
strath	An extensive terracelike remnant of a broad valley floor that has undergone dissection, commonly covered by a veneer of alluvium.
stratification	The accumulation or layering of sedimentary rocks as strata. Tabular, or planar, stratification refers to essentially parallel surfaces. Cross-stratification refers to strata inclined at an angle to the main stratification.
stratigraphic	Of or pertaining to strata.
stratigraphy	The geologic study of the origin, occurrence, distribution, classification, correlation, and age of rock layers, especially sedimentary rocks.
stratotype	The original or subsequently designated standard of reference of a named layered stratigraphic unit or of a stratigraphic boundary. Also, a specific interval or point in a sequence of rock strata that constitutes the standard of the definition and characterization of the stratigraphic unit or boundary being defined. Synonymous with “type section.”
stratovolcano	A volcano that is constructed of alternating layers of lava and pyroclastic deposits, along with abundant dikes and sills. Viscous, high-silica lava may flow from fissures radiating from a central vent, from which pyroclastic material is ejected. Synonymous with “composite volcano.”
stream	Any body of water moving under gravity flow in a clearly confined channel.
stream capture	The natural diversion of the headwaters of one stream into the channel of another stream having greater erosional activity. Synonymous with “stream piracy.”
stream channel	A long, narrow depression shaped by the concentrated flow of stream water.
stream terrace	A planar surface alongside a stream valley representing the remnants of an abandoned floodplain, stream bed, or valley floor produced during a former stage of erosion or deposition.
striations	Parallel scratches or lines.
striations (glacial)	One of a series of long, delicate, finely cut, commonly straight and parallel furrows or lines inscribed on a bedrock surface by the rasping and rubbing of rock fragments embedded at the base of a moving glacier, usually oriented in the direction of ice movement; also form on the rock fragments transported by a glacier.
strike	The compass direction of the line of intersection of an inclined surface with a horizontal plane.
strike-slip fault	A fault with measurable offset where the relative movement is parallel to the strike of the fault. Described as left-lateral (sinistral) when relative motion of the block opposite the observer is to the left, and right-lateral (dextral) when relative motion is to the right.
stromatolite	An sedimentary structure produced by sediment trapping, binding, and/or precipitation as a result of the growth and metabolic activity of microorganisms, principally cyanophytes (blue-green algae); occurs in a variety of forms, from nearly horizontal to markedly columnar, domal, or subspherical.
structural geology	The branch of geology that deals with the description, representation, and analysis of structures, primarily on a moderate to small scale. The subject is similar to tectonics, but the latter term is generally used for the analysis of broader regional or historical phases.
structure	The attitudes and relative positions of the rock masses of an area resulting from such processes as faulting, folding, and igneous intrusion.
subaerial	Describes a condition or process that exists or operates in the open air on or immediately adjacent to the land surface.
subangular	Said of a sedimentary particle showing effects of slight abrasion, retaining its original form, and have faces that are virtually untouched and edges and corners that are rounded off to some extent.
subaqueous	Describes conditions and processes, or features and deposits, that exist or are situated in or under water.
subbituminous	Describes black coal, intermediate in rank between lignite and bituminous coal; distinguished from lignite by higher carbon and lower moisture content.
subduction	The process of one lithospheric plate descending beneath another.
subduction zone	A long, narrow belt in which subduction takes place.
subhedral	A grain partly bounded by crystal faces; intermediate between euhedral and anhedral.
sublittoral	Pertaining to the ocean environment or depth zone between low-tide level and 200 m (660 ft), or between low-tide level and approximately the edge of the continental shelf. Synonmous with “neritic.”
submarine	Something situated or living under the surface of the sea.
submergence	A rise of water level in relation to land, so that areas of formerly dry land become inundated; results from either a sinking of the land or rise of water level.
subparallel	Almost parallel.
subrounded	Said of a sedimentary particle showing considerable abrasion and an original general form that is still discernable; many of its edges and corners are considerable rounded off to smooth curves.
subsidence	The sudden sinking or gradual downward settling of part of Earth’s surface.
superposition	The order in which rocks are placed or accumulated in beds one above the other; the highest bed is the younges. Also, the process by which successively younger sedimentary layers are deposited on lower and older layers.
supralittoral	Describes a feature or process at an elevation higher than normal tidal range on a given shoreface. Synonymous with “supratidal.”
sulfate	A mineral group composed of sulfur (S) and oxygen (O) plus an element or elements, for example, calcium in anhydrite, CaSO4, and gypsum, CaSO4 • 2H2O; and barium in barite, BaSO4.
sulfide	A mineral group composed of sulfur plus an element or elements, for example, lead in galena, PbS, and iron in pyrite, FeS2.
sump	An excavated area in which drainage water is collected for subsequent use in irrigation or wild-fowl conservation. Also, a dialect term for a swamp or morass, and for a stagnant pool or puddle of dirty water.
surge channel	A channel extending across the crest of a coral reef, in which water level fluctuates according to the height and arrival time of breakers. The in- and out-flow is normally vigorous and commonly extremely dangerous.
suture	The linear zone where two continental landmasses become joined via obduction.
swallow holes	Points along streams and in sinkholes where surface-water flow is lost to underground conduits.
syenite	An intrusive igneous rock containing alkali feldspar, a small amount of plagioclase (less than in monzonite), one or more mafic minerals (especially amphibole), and little or no quartz. Unlike granite, syenite is an uncommon rock, and occurs as small intrusive bodies or dikes, and as quartz-poor borders of large granitic bodies.
syncline	A generally concave upward fold of which the core contains the stratigraphically younger rocks.
synclinorium	A composite synclinal structure of regional extent composed of lesser folds.
synorogenic	Describes a geologic process or event during a period of orogenic activity; also describes a rock or feature formed by such a process or event.
system (geology)	A group of related natural features, objects, or forces.
system (stratigraphy)	The fundamental unit of chronostratigraphic classification of Phanerozoic rocks; each unit represents a time span and an episode of Earth history sufficiently great to serve as a worldwide reference unit. It is the temporal equivalent of a period.
tabular	Said of a feature having two dimensions that are much larger or longer than the third.
taiga	A swampy area of coniferous forest, commonly between tundra and steppe regions.
talus	Rock fragments, usually coarse and angular, lying at the base of a cliff or steep slope from which they have fallen.
tectonic	Describes a feature or process related to large-scale movement and deformation of Earth’s crust.
tectonics	The geologic study of the broad structural architecture and deformational processes of the lithosphere and asthenosphere.
tectonite	A rock whose fabric has been modified substantially by deformation processes.
tephra	A collective term used for all pyroclastic material, regardless of size, shape, or origin, ejected into the air during a volcanic eruption.
terrace	Any long, narrow, relatively level or gently inclined surface (i.e., a bench or steplike ledge) that is bounded along one edge by a steeper descending slope and along the other edge by a steeper ascending slope, thus breaking the continuity of the slope; commonly occurs along the margin and above the level of a body of water, marking a former water level.
terrane	A fault-bounded body of rock of regional extent, characterized by a geologic history different from that of contiguous terranes or bounding continents.
terrestrial	Describes a feature, process, or organism related to land, Earth, or its inhabitants.
terrigenous	Describes material or a feature derived from the land or a continent.
test	The shell or internal skeleton of many invertebrates.
thalweg	The line connecting the lowest/deepest points along a stream bed; the line of maximum depth.
theory	A hypothesis that has been rigorously tested against further observations or experiments; a generally accepted tenet of science.
thermal	Pertaining to or caused by heat.
thermal spring	A spring whose water temperature is appreciably higher than the local mean annual atmospheric temperature. A thermal spring may be a “hot spring” or a “warm spring.”
thermokarst topography	An irregular land surface containing lakes, bogs, caverns, pits, and other small depressions formed in permafrost by the melting of ground ice.
tholeiite	A volcanic rock, specifically a type of basalt characterized by the presence of orthopyroxene.
thrust fault	A dip-slip fault with a shallowly dipping (less than 45°) fault surface where the hanging wall moves up and over relative to the footwall.
tidal delta	A delta formed at the mouth of a tidal inlet on either the seaward or the lagoon side of a barrier island or baymouth bar by changing tidal currents that sweep sand in and out of the inlet.
tidal inlet	Any inlet through which water alternately floods landward with the rising tide and ebbs seaward with the falling tide; specifically a natural inlet maintained by tidal currents.
till	Unstratified drift deposited directly by a glacier without reworking by meltwater and consisting of a mixture of clay, silt, sand, gravel, and boulders ranging widely in size and shape.
titanomagnetite	A titaniferous variety of magnetite: (Fe3+, Fe2+, Ti4+)2[Fe2+O4].
tonalite	An intrusive igneous rock composed primarily of quartz and plagioclase with 10% or less alkali feldspar.
tongue	An extension, projection, or offshoot of a larger body of rock, commonly occurring as wedges that disappear away from the main body.
topography	The general morphology of Earth’s surface, including relief and locations of natural and human-made features.
tourmaline	A silicate (silicon + oxygen) mineral of sodium and aluminum whose composition varies widely because of substitutions of elements; occurs as a semi-precious stone in a wide variety of colors.
trace (structural geology)	The intersection of a geological surface with another surface, for example, the trace of bedding on a fault surface, or the trace of a fault or outcrop on the ground.
trace fossil	A fossilized feature such as a track, trail, burrow, or coprolite (dung), that preserves evidence of an organism’s life activities, rather than the organism itself. Compare to “body fossil.”
trachyte	A group of fine-grained, generally porphyritic, volcanic rocks containing alkali feldspar and minor mafic minerals.
transcurrent fault	A continental strike-slip fault that does not terminate at a lithospheric plate boundary.
transform fault	A strike-slip fault that links two other faults or plate boundaries such as two segments of a mid-ocean ridge.
transform plate boundary	A type of plate boundary at which lithosphere is neither created nor destroyed, and plates slide past each other on a strike-slip fault.
transgression	Landward migration of the sea as a result of a relative rise in sea level.
transpression	A combination of crustal shortening and strike-slip movement.
transpressional fault	A strike-slip fault across which shortening occurs.
transtension	A combination of extension and strike-slip faulting.
transverse	Said of an entity that is extended in a crosswise direction, especially of a topographic feature that is oriented at right angles to the grain or general strike of a region.
transverse dune	A dune that is elongated perpendicular to the prevailing wind direction; the leeward slope stands at or near the angle of repose of sand, whereas the windward slope is comparatively gentle.
travertine	A chemical sedimentary rock composed of precipitated calcium carbonate (predominantly calcite and aragonite) from spring-fed, heated and/or ambient-temperature waters. The spongy or less compact variety is tufa.
tree mold	A cylindrical hollow in a lava flow formed by the envelopment of a tree by the flow, solidification of the lava in contact with the tree, and disappearance of the tree by burning and subsequent removal of the charcoal and ash. The inside of the mold preserves the surficial features of the tree.
tremolite	A white to dark-gray silicate (silicon + oxygen) mineral of the amphibole group, Ca2(Mg,Fe)5Si8O22(OH)2, with varying amounts of iron and may contain manganese and chromium; characterized by prismatic crystals.
trend	The direction or bearing of an outcrop of a geologic feature such as an ore body, fold, or orogenic belt.
trilobite	Any marine arthropod belonging to the class Trilobita, characterized by a three-lobed ovoid outer skeleton, divided lengthwise into axial and side regions and transversely into cephalon (“head”), thorax (middle), and pygidium (“tail”). Range: Lower Cambrian to Permian.
trondhjemite	A light-colored, intrusive igneous rock composed mostly of plagioclase feldpar, quartz, sparse biotite, and little or no alkali feldspar.
tufa	A chemical sedimentary rock composed of calcium carbonate, formed by evaporation commonly as an incrustation around the mouth of a spring, along a stream, or less commonly as a thick, concretionary deposit in a lake or along its shore. It may also be precipitated by algae or bacteria. The hard, dense variety of travertine.
tuff	Consolidated or cemented volcanic ash and lapilli.
tuffaceous	Describes non-volcanic, clastic sediments that contain ash-size pyroclasts.
tumulus	A dome or small mound on the crust of a lava flow, caused by pressure due to the difference in the rate of flow between the cooler lava crust and the more fluid underlying lava.
turbidite	Sediment or rock deposited from a turbidity current (underwater flow of sediment) and characterized by graded bedding, moderate sorting, and well-developed primary structures in the sequence noted by the Bouma cycle.
twilight zone	The part of a cave where daylight penetrates and gradually diminishes to zero light.
type area	The geographic area or region that encompasses the stratotype or type locality of a stratigraphic unit or stratigraphic boundary.
type locality	The place where a geologic feature such as an ore occurrence, a particular kind of igneous rock, or the type specimen of a fossil species was first recognized and described.
type section	The originally described sequence of strata that constitute a stratigraphic unit. It serves as an objective standard with which spatially separated parts of the unit may be compared. It is preferably in an area where the unit shows maximum thickness and is completely exposed (or at least shows top and bottom).
ultramafic	Describes an intrusive igneous rock primarily composed of mafic minerals.
unconfined groundwater	Groundwater that has a water table; water not confined under pressure beneath a confining bed.
unconformable	Describes strata that do not succeed the underlying rocks in immediate order of age or in parallel position, especially younger strata that do not have the same dip and strike as the underlying rocks. Also, describes the contact between unconformable rocks.
unconformability	The quality, state, or condition of being unconformable, such as the relationship of unconformable strata.
unconformity	A substantial break or gap in the geologic record where a rock unit is overlain by another that is not next in stratigraphic succession, resulting from either a change that caused deposition to cease for a considerable span of time or erosion with loss of the previously formed record.
undercutting	The removal of material at the base of a steep slope by the erosive action of water (such as a meandering stream), sand-laden wind in a desert, or waves along a coast.
underfit stream	A stream that appears to be too small to have eroded the valley in which it flows. Also, a stream whose volume is greatly reduced or whose meanders show a pronounced shrinkage in radius.
uniformitarianism	The assumption that “the geological forces of the past differ neither in kind nor in energy from those now in operation,” which was the basis advocated by Charles Lyell for interpreting past phenomena by analogy with modern ones. Thus the cliché, “the present is the key to the past.”
unsaturated zone	A subsurface zone between the land surface and the water table that includes air, gases, and water held by capillary action. Synonymous with “vadose zone” and “zone of aeration.”
uplift	A structurally high area in Earth’s crust produced by movement that raises the rocks.
upper flow regime	A condition of stream flow that is characterized by a unidirectional current and relatively high sediment transport rates.
upwarping	Upward flexing of Earth’s crust on a regional scale as a result of the removal of ice, water, sediments, or lava flows.
vadose	Refers to the unsaturated zone between the land surface and the water table that includes air, gases, and water held by capillary action.
vadose water	Water of the unsaturated zone.
vapor-phase crystallization	The crystallization of minerals from hot gases escaping through a volcanic body. Cooling of the escaping gases, which carry elements in solution, promotes the crystallization of mineral in rock cavities.
varve	Any cyclic, genetically related paired sedimentary layer, generally occurring in a repeating series, such as seasonally deposited glacial varves, but also laminated nonglacial shales, evaporites, and other sediments.
varve (glacial)	A sedimentary layer deposited in a body of still water within one year’s time, specifically a thin pair of graded glaciolacustrine layers seasonally deposited, usually by meltwater streams into a glacial lake or other body of still water in front of a glacier.
vent	Any opening at Earth’s surface through which magma erupts or volcanic gases are emitted.
vesicle	A cavity of variable shape formed by the entrapment of a gas bubble during solidification of lava.
vesicular	Describes the texture of a rock, especially lava, characterized by abundant vesicles formed as a result of the expansion of gases during the fluid stage of the lava.
viscosity	The property of a substance to offer internal resistance to flow.
vitreous	Having the luster and appearance of glass.
vitreous luster	Luster that resembles glass. Synonymous to “glassy.”
vitric	Describes pyroclastic material that is characteristically glassy.
vitrophyre	Any porphyritic igneous rock with a glassy groundmass.
volatile	Readily vaporizable.
volatile component	Material in magma, such as water or carbon dioxide, whose vapor pressures is sufficiently high to be concentrated as a gas.
volcanic	Pertaining to the activities, structures, or rock types of a volcano. A synonym of extrusive.
volcanic arc	A large-scale (hundreds of kilometers) generally curved belt of volcanoes above a subduction zone.
volcanic exhalation	An emission of gas or ash from a vent in a relatively short burst.
volcaniclastic	Pertaining to all clastic volcanic materials formed by any process of fragmentation, dispersed by any kind of transporting agent, deposited in any environment, or mixed in any significant portion with nonvolcanic fragments.
volcanogenic	Formed by processes directly connected with volcanism.
volcanism	The processes by which magma and its associated gases rise into Earth’s crust and are extruded onto the surface and into the atmosphere.
vug	A small cavity in rock, commonly lined with crystals of a different mineral composition from the enclosing rock.
warm spring	A thermal spring whose temperature is appreciably above the local mean annual atmospheric temperature, but below that of the human body.
wash	A broad, gravelly, dry stream bed, generally in the bottom of a canyon that is periodically swept by a torrent of water. The term is used especially in the southwestern United States.
water table	The surface between the saturated zone and the unsaturated zone. Synonymous with “groundwater table” and “water level.”
wave base	The depth at which wave activity no longer stirs up sediments, usually at about 10 to 20 m (30 to 70 ft) below water level.
wave-cut bench	A level or gently sloping narrow surface or platform produced by wave erosion, extending outward from above the base of the wave-cut cliff and occupying all of the shore zone and part or all of the surface.
weathering	The physical, chemical, and biological processes by which rock is broken down, particularly at Earth’s surface.
welded tuff	A glass-rich pyroclastic rock that has been indurated by the welding together of glass shards under the combined action of the heat retained by the particles, the weight of overlying material, and hot gases. Cinders and bombs may also be welded together to form solid outcrops of rock.
welding	Consolidation of sediments under pressure. Also, the diagenetic process whereby discrete crystals and/or grains become attached to each other during compaction.
wildcat well	An exploratory well drilled for oil or gas on a geologic feature not yet proven to be productive.
wind tide	The part of the tide produced by wind, rather than astronomical forces; specifically the change in water level as a result of wind moving across the surface.
Wisconsinan	Pertaining to the classical fourth glacial stage of the Pleistocene Epoch in North America, following the Sangamonian interglacial stage and preceding the Holocene Epoch.
xenocryst	A crystal that resembles a phenocryst in igneous rock but is foreign to the body of rock in which it occurs.
xenolith	A rock particle, formed elsewhere, entrained in magma as an inclusion.
yazoo	A tributary that is parallel to the main stream channel for a considerable distance before joining it, especially a stream forced to flow along the outer base of a natural levee formed by the main stream. The type example is the Yazoo River in western Mississippi, joining the Mississippi River at Vicksburg.
zeolite	A group of silicate (silicon + oxygen) minerals that commonly occur as well-formed crystals in the cavities of mafic igneous rocks, particularly basalt.
zircon	A very durable silicate mineral (silicon + oxygen), ZrSiO4. When cut and polished, the colorless variety provides exceptionally brilliant gemstones.

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Sources

Terms in the GRI glossary were prepared by GRI report authors or adapted from primary sources, including:

• Bates, R. L., and J. A. Jackson, editors. 1984. Dictionary of geological terms. Third edition. American Geological Institute, Alexandria, Virginia.
• Neuendorf, K. K. E., J. P. Mehl Jr., and J. A. Jackson. 2005. Glossary of geology. Fifth edition. American Geological Institute, Alexandria, Virginia.

Related Links

• NPS—Glossary of Volcanic Terms

• The American Geosciences Institute

Geologic Resources Inventory