DICTIONARY OF GEOLOGICAL TERMS

A | B | C | D | E | F | G | H | I | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Z

Alkali metal: A strongly basic metal like potassium or sodium.

Alluvial fan: A low, cone shaped deposit of terrestrial sediment formed where a stream undergoes an abrupt reduction of slope.

Alluvium: An unconsolidated terrestrial sediment composed of sorted or unsorted sand, gravel, and clay that had been deposited by water.

Angle of repose: The steepest slope angle in which a particular sediment will lie without cascading down.

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

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

Arkose: A variety of sandstone containing abundant feldspar and quartz, frequently in angular, poorly sorted grains.

Artesian well: A well that penetrates an aquiclude to reach an aquifer containing water under pressure. Thus water in the well rises above the surrounding water table.

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Banded iron ore: A sediment consisting of layers of chert alternating with bands of ferric iron oxides (hematite and limonite) in valuable concentrations.

Basalt: A fine-grained, dark, mafic igneous rock composed largely of plagioclase feldspar and pyroxene.

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

Basic rock: Any igneous rock containing mafic minerals rich in iron and magnesium, but containing no quartz and little sodium rich plagioclase feldspar.

Basin: In tectonics, a circular, syncline-like depression of strata. In sedimentology, the site of accumulation of a large thickness of sediments.

Batholith: A great irregular mass of coarse-grained igneous rock with an exposed surface of more than 100 square kilometers, which has either intruded the country rock or been derived from it through metamorphism.

Bedding: A characteristic of sedimentary rocks in which parallel planar surfaces separating different grain sizes or compositions indicate successive depositional surfaces that existed at the time of sedimentation.

Biochemical precipitate: A sediment, especially of limestone or iron, formed from elements extracted from sea water by living organisms.

Block fault: A structure formed when the crust is divided into blocks of different elevation by a set of normal faults.

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Caldera: A large, circular depression in a volcanic terrane, typically originating in collapse, explosion, or erosion.

Carbonate rock: A rock composed of carbonate minerals, especially limestone and dolomite.

Chemical sediment: One that is formed at or near its place of deposition by chemical precipitation, usually from sea water.

Chemical weathering: The total set of all chemical reactions that act on rock exposed to water and atmosphere and so change it minerals to stable forms.

Chert: A sedimetary form of amorphous or extremely fine-grained silic, partially hydrous, found in concretions and beds.

Cinder cone: A steep, conical hill built up about a volcanic vent and composed of coarse pyroclasts expelled from the vent by escaping gases.

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

Clastic rock: A sedimentary rock formed from mineral particles (clasts) that were mechanically transported.

Clay: Any of a number of hydrous aluminosilicate minerals formed by weathering and hydration of other silicates; also, any mineral fragment smaller than 1/255 mm.

Contact metamorphism: Mineralogical and textural changes and deformation of rock resulting from the head and pressure of an igneous intrusion in the near vicinity.

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Declination: At any place on Earth, the angle between the magnetic and rotational poles.

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

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

Deposition: A general term for the accumualtion of sediments by either physical or chemical sedimentation.

Detrital sediment: A sediment deposited by a physical process.

Diatomite: A siliceous chert-like sediment formed from the hard parts of diatoms.

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

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

Dome: In structural geology, a round or elliptical upwarp of strata resembling a short anticline.

Drift (glacial): A collective term for all te rock, sand, and clay that is transported and deposited by a glacier either as till or as outwash.

Drumlin: A smooth, streamlined hill composed of till.

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Eclogite: An extremely high pressure metamorphic rock containing garnet and pyroxene.

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

Eon: The largest division of geologic time, embracing several Eras, for example, the Phanerozoic, 600 m.y. ago to present); also any span of one billion years.

Epoch: One subdivision of a geologic period, often chosen to correspond to a stratigraphic series. Also used fo a division of time corresponding to a paleomagnetic interval.

Era: A time period including several periods, but smaller than an eon. Commonly recognized eras are Precambrian, Paleozoic, Mesozoic, and Cenozoic.

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

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

Evaporite: A chemical sedimentary rock consisting of minerals precipitated by evaporating waters, especially sal and gypsum.

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

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Facies: The set of all characteristics of a sedimetary rock that indicates its particular environment of deposition and which distinguish it from other facies in the same rock.

Fault: A planar or gently curved fracture in the Earth's crust across which there has been relative displacement.

Fault plane: The plane that best approximates the fracture surface of a fault.

Felsic: An adjective used to describe a light-colored igneous rock poor in iron and magnesium content, abundant in feldspars and quartz.

Fluid inclusion: A small body of fluid that is entrapped in a crystal and has the same composition as the fluid from which the crystal formed.

Flume: A laboratory model of stream flow and sedimentation consisting of a rectangular channel filled with sediment and running water.

Fold: A planar feature, such as a bedding plane, that has been strongly warped, presumably by deformation.

Foliation: Any planar set of minerals or banding of mineral concentrations including cleavage, found in a metamorphic rock.

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

Fossil: An impression, cast, outline, or track of any animal or plant that is preserved in rock after the original organic material is transformed or removed.

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Gabbro: A black, coarse-grained, intrusive igneous rock, composed of calcic feldspars and pryoxene. The intrusive equivalent of basalt.

Geosyncline: A major downwarp in the Earth's crust, usually more than 1000 kilometers in length, in which sediments accumulate to thicknesses of many kilometers. The sediments may eventually be deformed and metamorphosed during a mountain-building episode.

Glacial striations: Scratches left on bedrock and boulders by overriding ice, and showing the direction of motion.

Glacier: A mass of ice and surficial snow that persists throughout the year and flows downhill under its own weight. The size range is from 100 meters to 10,000 kilometers.

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

Graben: A downthrown block between two normal faults of parallel strike but converging dips; hence a tensional feature. See also horst.

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

Granite: A coarse-grained, intrusive igneous rock composed of quartz, orthoclase feldspar, sodic plagioclase feldspar, and micas. Also sometimes a metamorphic product.

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

Gravel: The coarsest of alluvial sediments, containing mostly particles larger than 2 mm in size and including cobbles and boulders.

Gravity anomaly: The value of gravity left after subtracting from a gravity measurement the reference value based on latitude, and possibly the free-air and Bouguer corrections.

Gravity survey: The measurement of gravity at regularly-spaced grid points with repetitions to control instrument drift.

Greenschist: A metamorphic schist containing chlorite and epidote (which are green) and formed by low-temperature, low-pressure metamorphism.

Groundwater: The mass of water in the ground below the phreatic zone, occupying the total pore space in the rock and moving slowly downhill where permeability allows.

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Hard water: Water that contains sufiqcient dissolved calcium and magnesium to cause a carbonate scale to form when the water is boiled or to prevent the sudsing of soap.

Hill: A natural land elevation, usually less than 1000 feet above its surroundings, with a rounded outline. The distinction between hill and mountain depends on the locality.

Hornfels: A high-temperature, low-pressure metamorphic rock of uniform grain size showing no folia-tion. Usually formed by contact metamorphism.

Horst: An elongate, elevated block of crust forming a ridge or plateau, typically bounded by parallel, outward-dipping normal faults.

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

Hydration: A chemical reaction, usually in weathering, which adds water or OH to a mineral structure.

Hydrocarbon: An organic chemical compound made up of carbon and hydrogen atoms arranged in chains or rings.

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

Hydrothermal activity: Any process involving high-temperature groundwaters, especially the alteration and emplacement of minerals and the formation of hot springs and geysers.

Hydrothermal vein: A cluster of minerals precipitated by hydrothermal activity in a rock cavity.

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Igneous rock: A rock formed by congealing rapidly or slowly from a molten state.

Inclination: The angle between a line in the Earth's magnetic field and the horizontal plane; also a synonym for dip.

Intrusion: An igneous rock body that has forced its way in a molten state into surrounding country rock.

Intrusive rock: Igneous rock that is interpreted as a former intrusion from its cross-cutting contacts, chilled margins, or other field relations.

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

Joint: A large and relatively planar fracture in a rock across which there is no relative displacement of the two sides.

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Kimberlite: A peridotite containing garnet and olivine and found in volcanic pipes, through which it may come from the upper Mantle.

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Laccolith: A sill-like igneous intrusion that forces apart two strata and forms a round, lens-shaped body many times wider than it is thick.

Laminar flow: A flow regime in which particle paths are straight or gently curved and parallel.

Lapilli: A fragment of volcanic rock formed when magma is ejected into the air by expanding gases. The size of the fragments ranges from sand- to cobble-size.

Lateral moraine: A moraine formed along the side of a valley glacier and composed of rock scraped off or fallen from the valley sides.

Lava: Magma or molten rock that has reached the surface.

Leaching: The removal of elements from a soil by dissolution in water moving downward in the ground.

Limb (fold): The relatively planar part of a fold or of two adjacent folds (for example, the steeply dipping part of a stratum between an anticline and syncline).

Limestone: A sedimentary rock composed principally of calcium carbonate (CaCO2), usually as the mineral calcite.

Lineation: Any linear arrangement of features found in a rock.

Lithology: The systematic description of rocks, in terms of mineral composition and texture.

Lode: An unusually large vein or set of veins containing ore minerals.

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

Lopolith: A large laccolith that is bowl-shaped and depressed in the center, possibly by subsidence of an emptied magma chamber beneath the intrusion.

Luster: The general textural impression of a mineral surface, given by the light reflected from it. Terms such as metallic, submetallic are standardized but subjective.

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Mafic mineral: A dark-colored mineral rich in iron and magnesium, especially a pyroxene, amphibole, or olivine.

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

Magmatic water: Water that is dissolved in a magma or that is derived from such water.

Magnetic anomaly: The value of the local magnetic field remaining after the subtraction of the dipole portion of the Earth's field.

Magnetometer: An instrument for measuring either one orthogonal component or the entire intensity of the Earth's magnetic field at various points.

Mantle: The main bulk of the Earth, between the crust and core, ranging from depths of about 40 to 3480 kilometers. It is composed of dense mafic silicates and divided into concentric layers by phase changes that are caused by the increase in pressure with depth.

Massive rock: A rock that is little or not at all broken by joints, cracks, foliation, or bedding, tending to present a homogeneous appearance.

Mass spectrometer: An instrument for separating ions of different mass but equal charge (mainly isotopes in geology) and measuring their relative quantities.

Mechanical weathering: The set of all physical processes by which an outcrop is broken up into small particles.

Medial moraine: A long stripe of rock debris carried on or within a glacier resulting from the convergence of lateral moraines where two glaciers join.

Metamorphism: The changes of mineralogy and texture imposed on a rock by pressure and temperature in the Earth's interior. Meteoric water: Rainwater, snow, hail, and sleet.

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

Mineral: A naturally occurring element or compound with a precise chemical formula and a regular internal lattice structure. Organic products are usually not included.

Mineralogy: The study of mineral composition, structure, appearance, stability, occurrence, and associations.

Mohs scale of hardness: An empirical, ascending scale of mineral hardness with talc as 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, and diamond 10.

Monocline: The S-shaped fold connecting two horizontal parts of the same stratum at different elevations. Its central limb is usually not overturned.

Moraine: A glacial deposit of till left at the margin of an ice sheet. See specifically by name, ground moraine, longitudinal moraine, medial moraine, and terminal moraine.

M.y.: Abbreviation for "million years."

Mylonite: A very fine lithified fault breccia commonly found in major thrust faults and produced by shearing and rolling during fault movement.

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Native metal: A natural deposit of a metallic element in pure metallic form, neither oxidized nor combined with sulfur or other elements.

Normal fault: A dip-slip fault in which the block above the fault has moved downward relative to the block below.

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Obsidian: Dark volcanic glass of felsic composition.

Oil shale: A dark-colored shale containing organic material that can be crushed and heated to liberate gaseous hydrocarbons.

Opaque mineral: A mineral which transmits no light through a thin section under a microscope. Usually a native metal, sulfide, or metallic oxide mineral.

Ore: A natural deposit in which a valuable metallic element occurs in high enough concentration to make mining economically feasible.

Ore mineral: The mineral of an ore that contains the useful element.

Orogenic belt: A linear region, often a former geo-syncline, that has been subjected to folding, and other deformation in a mountain-building episode.

Outcrop: A segment of bedrock exposed to the atmosphere.

Oxidation: A chemical reaction in which electrons are lost from an atom and its charge becomes more positive.

Oxidized element: An element occurring in the more positively charged of two common ionic forms.

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Paleontology: The science of fossils, of ancient life-forms, and their evolution.

Pegmatite: An igneous rock with extremely large grains, more than a centimeter in diameter. It may be of any composition but most frequently is granitic.

Peridotite: A coarse-grained mafic igneous rock composed of olivine with accessory amounts of pyroxene and amphibole but little or no feldspar.

Potable water: Water that is agreeable to the taste and not dangerous to the health.

Ppm: Abbreviation for "parts per million."

Proven reserves: Deposits of fossil fuels whose location and extent are known, as opposed to potential but unproved ('*discovered") deposits.

Pumice: A form of volcanic glass, usually of silicic composition, so filled with vesicles that it resembles a sponge and is very light.

Pyroclastic rock: A rock formed by the accumulation of fragments of volcanic rock scattered by volcanic explosions.

Pyroclastic texture: The unsorted, angular, and un-rounded texture of the fragments in a pyroclastic rock.

Pyroxene granulite: A coarse-grained contact metamorphic rock containing pyroxene, formed at high temperatures and low pressures.

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Quartz arenite: A sandstone containing very little except pure quartz grains and cement.

Quartzite: (1) A very hard, clean, white metamorphic rock formed from a quartz arenite sandstone. (2) A quartz arenite containing so much cement that it resembles ( 1 ).

Quartzose sandstone: (1) A quartz arenite. (2) A clean quartz sandstone, less pure than a quartz arehire, that may contain a moderate amount of other detrital minerals and/or calcite cement.

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Recrystallization: The growth of new mineral grains in a rock at the expense of old grains, which supply the material.

Recumbent fold: An overturned fold with both limbs nearly horizontal.

Regional metamorphism: Metamorphism occurring over a wide area and caused by deep burial and high internal temperatures of the Earth.

Regolith: Any solid material lying on top of bedrock. Includes soil, alluvium, and rock fragments weathered from the bedrock.

Regression: A drop in sea level that causes an area of the Earth to be uncovered by sea water, ending marine deposition.

Relief: The maximum regional difference in elevation.

Remote sensing: The study of Earth surface conditions and materials from airplanes and satellites by means of photography, spectroscopy, or radar.

Replacement deposit: A deposit of ore minerals by hydrothermal solutions that have first dissolved the original mineral to form a small cavity.

Rhyolite: The fine-grained volcanic or extrusive equivalent of granite, light brown to gray and compact. Richter magnitude scale: See Magnitude.

Rift valley: A fault trough formed in a divergence zone or other area of tension.

Right-lateral fault: A strike-slip fault on which the displacement of the far block is to the right when viewed from either side.

Ring dike: A dike in the form of a segment of a cone or cylinder, having an arcuare outcrop.

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

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Sandstone: A detrital sedimentary rock composed of grains from 1/16 to 2 millimeters in diameter, dominated in most sandstones by quartz, feldspar, and rock fragments, bound together by a cement of silica, carbonate, or other minerals or a matrix of clay minerals.

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

Schistosity: The parallel arrangement of sheely or prismatic minerals like micas and amphiboles resulting from nonhydrostatic stress in meramorphism.

Scoria: Congealed lava, usually of mafic composition, with a large number of vesicles formed by gases coming out of solution.

Sedimentary rock: A rock formed by the accumulation and cementation of mineral grains transported by wind, water, or ice to the site of deposition or chemically precipitated at the depositional site.

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

Sedimentation: The process of deposition of mineral grains or precipitates in beds or other accumulations. Seif dune: A longitudinal dune that shows the sculpturing effect of cross-winds not parallel to its axis.

Seismic reflection: A mode of seismic prospecting in which the seismic profile is examined for waves that have reflected from near-horizontal strata below the surface.

Seismic refraction: A mode of seismic prospecting in which the seismic profile is examined for waves that have been refracted upward from seismic discontinuities below the profile. Greater depths may be reached than through seismic reflection.

Stratification: A structure of sedimentary rocks, which have recognizable parallel beds of considerable lateral extent.

Stratigraphic sequence: A set of beds deposited that reflects the geologic history of a region.

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

Stratovolcano: A volcanic cone consisting of both lava and pyroclastic rocks, often conical.

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

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

Strike-slip fault: A fault whose relative displacement is purely horizontal.

Subduction zone: A dipping planar zone descending away from a trench and defined by high seismicity, interpreted as the shear zone between a sinking oceanic plate and an overriding plate.

Sublimation: A phase change from the solid to the gaseous state, without passing through the liquid state.

Subsidence: A gentle epeirogenic movement where a broad area of the crust sinks without appreciable deformation.

Syncline: A large fold whose limbs are higher than its center; a fold with the youngest strata in the center.

System (stratigraphy): A stratigraphic unit larger than a series, consisting of all the rocks deposited in one period of an era.

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

Tectonics: The study of the movements and deformation of the crust on a large scale, including epeirogeny, metamorphism, folding, faulting, and plate teetonics.

Terminal moraine: A sinuous ridge of unsorted glacial till deposited by a glacier at the line of its farthest advance.

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

Thrust fault: A dip-slip fault in which the upper block above the fault plane moves up and over the lower block, so that older strata are placed over younger.

Tidal flat: A broad, flat region of muddy or sandy sediment, covered and uncovered in each tidal cycle.

Till: An unconsolidated sediment containing all sizes of fragments from clay to boulders deposited by glacial action, usually unbedded.

Time scale: The division of geologic history into eras, periods, and epochs accomplished through stratig-raphy and paleontology.

Topographic map: See Contour map; also a schematic drawing of prominent landforms indicated by conventionalized symbols, such as hachures or contours.

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

Trace element: An element that appears in minerals in a concentration of less than l percent (often less than 0.001 percent).

Trench: A long and narrow deep trough in the sea floor; interpreted as marking the line along which a plate bends down into a subduction zone.

Tuff: A consolidated rock composed of pyroclastic fragments and fine ash. If particles are melted slightly together from their own heat, it is a "welded tuff."

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

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

Unconsolidated material: Nonlithified sediment that has no mineral cement or matrix binding its grains.

Uplift: A broad and gentle epeirogenic increase in the elevation of a region without a eustatic change of sea level.

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

Vesicle: A cavity in an igneous rock that was formerly occupied by a bubble of escaping gas.

Viscosity: A measure of resistance to flow in a liquid.

Volcanic ash: A volcanic sediment of rock fragments, usually glass, less than 4 millimeters in diameter that is formed when escaping gases force out a fine spray of

Volcanic breccia: A pyroclastic rock in which all fragments are more than 2 millimeters in diameter.

Volcanic dome: A rounded accumulation around a volcanic vent of congealed lava too viscous to flow away quickly; hence usually rhyolite lava. Volcanic dust: See Volcanic ash.

Volcanic pipe: The vertical chamber along which magma and gas ascend to the surface; also, a formation of igneous rock that cooled in a pipe and remains after the erosion of the volcano.

Volcano: Any opening through the crust that has allowed magma to reach the surface, including the deposits immediately surrounding this vent.

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Wadi: A steep-sided valley containing an intermittent stream in an arid region.

Water table: A gently-curved surface below the ground at which the vadose zone ends and the phreatic zone begins; the level to which a well would fill with water.

Weathering: The set of all processes that decay and break up bedrock, by a combination of physically fracturing or chemical decomposition.

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Xenolith: A piece of country rock found engulfed in an intrusion.

X-ray diffraction: In mineralogy, the process of identifying mineral structures by exposing crystals to X-rays and studying the resulting diffraction pattern.

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Zeolite: A class of silicates containing H=O in cavities within the crystal structure. Formed by alteration at low temperature and pressure of other silicates, often volcanic glass.

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