A-Horizon: the Uppermost Layer of a Soil, Containing Organic Material and Leached Minerals
Dictionary of Geologic Terms
- A -
Aa: A blocky and fragmented form of lava occurring in flows with fissured and angular surfaces.
A-horizon: The uppermost layer of a soil, containing organic material and leached minerals.
Algal mat: A layered communal growth of algae observed in fossils an in present day tidal zones associated with carbonate sedimentation.
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: Unconsolidated terrestrial sediment composed of sorted or unsorted sand, gravel, and clay that has been deposited by water.
Angle of repose: The steepest slope angle in which particular sediment will lie without cascading down.
Angstrom: A length of 10 to the minus tenth meter or one hundred millionth of a centimeter.
Angular unconformity: An unconformity in which the bedding planes of the rocks above and below are not parallel.
Anthracite: The most highly metamorphosed form of coal, containing 92 to 98 percent of fixed carbon. It is #006600, hard, and glassy.
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.
Arroyo: A steep-sided and flat-bottomed gulley in an arid region that is occupied by a stream only intermittently, after rains.
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.
Astrobleme: A circular erosional feature that has been ascribed to the impact of a meteorite or comet.
Atmosphere (unit): A unit of pressure equal to 101,325 newtons per square meter, or about 14.7 pounds per square inch.
Atoll: A continuous or broken circle of coral reef and low coral islands surrounding a central lagoon.
- B -
Backwash: The return flow of water down a beach after a wave has broken.
Banded iron ore: A sediment consisting of layers of chert alternating with bands of ferric iron oxides (hematite and limonite) in valuable concentrations.
Bankfull stage: The height of water in a stream that just corresponds to the level of the surrounding floodplain.
Bar: A unit of pressure equal to 10 to the sixth dynes/square centimeter; approximately one atmosphere.
Bar (stream): An accumulation of sediment, usually sandy, which forms at the borders or in the channels of streams or offshore from a beach.
Barchan: A crescent-shaped sand dune moving across a clean surface with its convex face upwind and its concave slip face downwind.
Bar-finger sand: An elongated lens of sand deposited during the growth of a distributary in a delta. The bar at the distributary mouth is the growing segment of the bar finger.
Barrier island: A long, narrow island parallel to the shore, composed of sand and built by wave action.
Basalt: A fine-grained, dark, mafic igneous rock composed largely of plagioclase feldspar and pyroxene.
Base-level: The level below which a stream cannot erode; usually sea level sometimes locally the level of a lake or resistant formation.
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.
Bathymetry: The study and mapping of sea-floor topography.
Bauxite: A rock composed primarily of hydrous aluminum oxides and formed by weathering in tropical areas with good drainage; a major ore of aluminum.
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.
Bed-load: The sediment that a stream moves along the bottom of its channel by rolling and bouncing.
Beta-particle: An electron emitted with high energy and velocity from a nucleus undergoing radioactive decay.
B-horizon: The intermediate layer in a soil, situated below the A-horizon and consisting of clays and oxides. Also called the zone of accumulation.
Biochemical precipitate: A sediment, especially of limestone or iron, formed from elements extracted from seawater by living organisms.
Bituminous coal: A soft coal formed by an intermediate degree of metamorphism and containing 15 to 20 percent volatiles. The most common grade of coal.
Block fault: A structure formed when the crust is divided into blocks of different elevation by a set of normal faults.
Blowout: A shallow circular or elliptical depression in sand or dry soil formed by wind erosion.
Bolson: In arid regions, a basin filled with alluvium and intermittent playa lakes and having no outlet.
Bond: The force that holds together two atoms in a compound. It may be derived from the sharing of electrons (covalent) or from electrostatic attraction between ions.
Butte : A steep sided and flat topped hill formed by erosion of flat laying strata where remnants of a resistant layer protect the softer rocks underneath.
- C -
Caldera: A large, circular depression in a volcanic terrain, typically originating in collapse, explosion, or erosion.
Carbonate ion: The anion group CO3 with a charge of minus two.
Carbonate platform: A submarine or intertidal shelf whose elevation is maintained by active shallow water carbonate deposition.
Carbonate rock: A rock composed of carbonate minerals, especially limestone and dolomite.
Carbonic acid: The weak acid H2CO3 formed by the dissolution of CO2 in water.
Cataclastic rock: A breccia of powdered rock formed by crushing and shearing during tectonic movements.
Cation: Any ion with a positive electric charge.
Central vent: The largest vent of a volcano, situated at the center of its cone.
Chemical sediment: One that is formed at or near its place of deposition by chemical precipitation, usually from seawater.
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 silica, partially hydrous, found in concretions and beds.
C-horizon: The lowest layer of soil, consisting of fragments of rock and their chemically weathered products.
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.
Coal: The metamorphic product of stratified plant remains. It contains more than 50 percent carbon compounds and burns readily.
Coastal plain: A low plain of little relief adjacent to the ocean and covered with gently dipping sediments.
Composite cone: The volcanic cone of a stratovolcano, composed of both cinders and lava flows.
Contact metamorphism: Mineralogical and textural changes and deformation of rock resulting from the head and pressure of an igneous intrusion in the near vicinity.
- D -
Datum plane: An artificially established, well surveyed horizontal plane against which elevations, depths, tides, etc. are measured (for example mean sea-level).
Daughter element: Also "daughter product". An element that occurs in a rock as end product of the radioactive decay of another element.
Debris avalanche: A fast downhill mass movement of soil and rock.
Declination: At any place on Earth, the angle between the magnetic and rotational poles.
Deflation: The removal of clay and dust from dry soil by strong winds.
Delta: A body of sediment deposited in an ocean or lake at the mouth of a stream.
Delta kame: A deposit having the form of a steep, flat topped hill, left at the front of a retreating continental glacier.
Dendritic drainage: A stream system that branches irregularly and resembles a branching tree in plan.
Density: The mass per unit volume of a substance, commonly expressed in grams/ cubic centimeter.
Density current: A subaqueous current that flows on the bottom of a sea or lake because entering water is denser due to temperature or suspended sediments.
Deposition: A general term for the accumulation of sediments by either physical or chemical sedimentation.
Deposition remnant magnetization: A weak magnetization created in sedimentary rocks by the rotation of magnetic crystals into line with the ambient field during settling.
Desert pavement: A residual deposit produced by continued deflation, which removes the fine grains of a soil and leaves a surface covered with closely packed cobbles.
Detrital sediment: A sediment deposited by a physical process.
Diagenesis: The physical and chemical changes undergone by a sediment during lithification and compaction, excluding erosion and metamorphism.
Diatom: A one celled plant that has a siliceous framework and grows in oceans and lakes.
Diatomite: A siliceous chert-like sediment formed from the hard parts of diatoms.
Diatom ooze: A fine muddy sediment consisting of the hard parts of diatoms.
Diatreme: A volcanic vent filled with breccia by the explosive escape of gases.
Differentiated planet: One that is chemically zoned because heavy materials have sunk to the center and light materials have accumulated in a crust.
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.
Divide: A ridge of high ground separating two drainage basins emptied by different streams.
Dome: In structural geology, a round or elliptical upwarp of strata resembling a short anticline.
Drainage basin: A region of land surrounded by divides and crossed by streams that eventually converge to one river or lake.
Drift (glacial): A collective term for all the 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.
Dry wash: An intermittent streambed in an arroyo or canyon that carries water only briefly after a rain.
Dune: An elongated mound of sand formed by wind or water.
- E -
Earthflow: A detachment of soil and broken rock and its subsequent downslope movement at slow or moderate rates in a stream- or tongue like form.
Earthquake: The violent oscillatory motion of the ground caused by the passage of seismic waves radiating from a fault along which sudden movement has taken place.
Ebb tide: The part of the tide cycle during which the water level is falling.
Echo-sounder: An oceanographic instrument that emits sound pulses into the water and measures its depth by the time elapsed before they return.
Ecliptic: The plane that contains the Earth's orbit around the Sun.
Eclogite: An extremely high-pressure metamorphic rock containing garnet and pyroxene.
Ecology: The science of the life cycles, populations, and interactions of various biological species as controlled by their physical environment, including also the effect of life forms upon the environment.
Elastic limit: The maximum stress that can be applied to a body without resulting in permanent strain.
Elastic rebound theory: A theory of fault movement and earthquake generation that holds that faults remain locked while strain energy accumulates in the country rock, and then suddenly slip and release this energy.
Electron: A negatively charged particle with negligible mass orbiting around the nucleus of an atom.
Elevation: The vertical height of one point on the Earth above a given datum plane, usually sea level.
Elliptical orbit: An orbit with the shape of a geometrical ellipse. All orbits are elliptical or hyperbolic, with the Sun occupying one focus.
Eolian: Pertaining to or deposited by wind.
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.
Epicenter: The point on the Earth's surface directly above the focus or hypocenter of an Earthquake.
Epoch: One subdivision of a geologic period, often chosen to correspond to a stratigraphic series. Also used for 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.
Eugeosyncline: The seaward part of a geosyncline; characterized by clastic sediments and volcanism.
Eustatic change: Sea level changes that affect the whole Earth.
Eutrophication: A superabundance of algal life in a body of water; caused by an unusual influx of nitrate, phosphate, or other nutrients.
Evaporite: A chemical sedimentary rock consisting of minerals precipitated by evaporating waters, especially salt and gypsum.
Exfoliation: A physical weathering process in which sheets of rock are fractured and detached from an outcrop.
Exobiology: The study of life outside the Earth.
Extinction angle: The angle between a crystallographic direction, such as a face or cleavage plane, and the direction in which all light is blocked by a pair of crossed polarizers.
- F -
Facies: The set of all characteristics of a sedimentary 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-block mountain: A mountain or range formed as a horst when it was elevated between parallel normal faults.
Fault plane: The plane that best approximates the fracture surface of a fault.