Geology Encyclopedia

Geology Encyclopedia

If you would like to prepare for school subjects or simply increase your general knowledge, then enjoy our geology encyclopedia. We tried to focus only on very important terms and definitions. We also kept our terminology very brief so that you absorb the concept more quickly and easily.

Geology Glossary (Page 1)


14C METHOD: A method for determining the age in years of organic matter by calculating the amount of radioactive carbon still remaining, as compared to the stable isotope, 12C.
40K/40AR METHOD: A method used for the dating of potassium-bearing rocks by using the ratio of radioactive 40K to its daughter, 40Ar.
AA: A blocky and fragmented form of lava occurring in flows with fissured and angular surfaces.
ABLATION: As applied to glacier ice, the process by which ice below the snow line is wasted by evaporation and melting.
ABSOLUTE DATING: The process of determining a specific date (in years or some other unit of time) for an archaeological, geological or paleontological site or artifact.
ABSOLUTE TIME: Geologic time expressed in years before the present.
ABUNDANT METAL: Iron, aluminum, magnesium, manganese, and titanium. Ores of the abundant metals only need to be 3 - 5 times as metal-rich as average rock.
ABYSSAL PLAIN: Flat or very gently sloping areas of the deep ocean basin floor.
ACCELERATION: The rate at which velocity changes, either by increasing or decreasing.
ACCRETE: V. To add terranes (small land masses or pieces of crust) to another, usually larger, land mass.
ACCRETION: A process by which material is added to a tectonic plate.
ACID MINE DRAINAGE: Water contamination by sulfuric acid produced by seepage through sulfur-bearing spoil and tailings from coal and metal mining.
ACID RAIN: The acidity in rain due to gases from internal combustion engines and coal- and oil-burning power plants.
ACTIVE LAYER: The seasonally thawed zone above permafrost.
ACTUALISM: See uniformitarianism.
AFTERSHOCK: An earthquake that follows and has its epicenter near a larger earthquake.
AGATE: A siliceous rock with alternating bands of chalcedony and variously colored chert.
AGE: An interval of geologic time; a division of an epoch.
AGGRADING STREAM: One that is actively depositing sediment in its channel or floodplain because it is being supplied with more load than it can transport.
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.
ALKALINE: A highly basic substance that dissolves in water.
ALLOCHTHONOUS: A large block of rock which has been moved from its original site of formation, usually by low angle thrust faulting.
ALLUVIAL FAN: A fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain.
ALLUVIATED VALLEY: One that has been at least partially filled with sand, silt, and mud by flowing water.
ALLUVIUM: A general term for clay, silt, sand, gravel, or similar unconsolidated sorted or semi-sorted sediment deposited during comparatively recent time by a stream or other body of running water.
ALPHA DECAY: The process of radioactive decay in which the nucleus of an atom emits an alpha particle. The new atom's atomic number is lower by two and its atomic mass number is reduced by four.
ALPHA PARTICLE: A particle consisting of two protons and two neutrons, produced during alpha decay. Identical to the nucleus of a 4He atom.
ALPINE GLACIER: See valley glacier.
AMBER: Name for fossil resin or tree sap that is appreciated for its colour.
AMYGDALOIDAL: A textural term describing volcanic rocks that contain numerous amygdules.
AMYGDULE: A gas cavity (vesicle ) in volcanic rock that has been filled with mineral matter such as calcite, chalcedony, or quartz.
ANDESITE: A fine-grained volcanic rock of intermediate composition, consisting largely of plagioclase and one or more mafic minerals.
ANDESITE LINE: The geographic boundary between rocks of the Pacific Basin, which are basaltic, and those around the rim of the basin, which are in part andesitic.
ANGLE OF INCIDENCE: The angle at which a ray of energy approaches a surface.
ANGLE OF REFLECTION: The angle at which a reflected ray of energy leaves a surface.
ANGLE OF REFRACTION: The angle at which a refracted ray of energy leaves a surface after passing through it.
ANGLE OF REPOSE: The maximum angle at which loose material will come to rest when added to a pile of similar material.
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.
ANION: An ion with a negative electrical charge. That is, an atom that has gained one or more electrons.
ANTHRACITE: The most highly metamorphosed form of coal, containing 92 to 98 percent of fixed carbon. It is black, hard, and glassy.
ANTICLINE: A convex upward rock fold in which strata have been bent into an arch; the strata on each side of the core of the arch are inclined in opposite directions away from the axis or crest; the core contains older rocks than does the perimeter of the structure. .
APHANITIC: A textural term meaning "fine-grained" that applies to igneous rocks.
AQUIFER: A body of saturated rock or sediment through which water can move readily.
AQUITARD: A material of low permeability that greatly slows the movement of ground water.
ARAGONITE SEA: Contains aragonite and high-magnesium calcite as the primary inorganic carbonate precipitates.
ARCH: Forms along a coast as wave erosion cuts through a headland.
ARCHEAN: An eon of geologic time extending from about 3.9 billion years to 2.5 billion years ago.
ARCHEAN EON: The oldest eon of the Earth's history.
ARCHIPELAGO: A chain or cluster of islands.
ARENITE: A relatively clean quartz sandstone that is well sorted and contains less than 10% argillaceous material.
ARĘTE: A narrow, saw-toothed mountain ridge developed by glacier erosion in adjacent cirques .
ARGILLACEOUS: Said of rock or sediment that contains, or is composed of, clay-sized particles or clay minerals.
ARKOSE: A sedimentary rock formed by the cementation of sand-sized grains of feldspar and quartz.
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 in which the water in the aquifer is under pressure that raises the water above the point that the well first encounters it.
ASPHALT: Sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits.
ASSEMBLAGE: The collection of minerals that characterize a rock or a facies.
ASTHENOSPHERE: A region of the Earth's outer shell beneath the lithosphere. The asthenosphere is of indeterminate thickness and behaves plastically.
ASTROBLEME: A circular erosional feature that has been ascribed to the impact of a meteorite or comet.
ASTRONOMIC THEORY OF GLACIATION: A theory based on the changing position of the Earth in its orbit around the Sun.
ASYMMETRIC ROCK KNOB OR HILL: Bedrock forms with a gentle slope on one side created by glacial abrasion and a steep slope on the opposite side created by glacial plucking.
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.
ATOM: A building block of matter, the smallest particle that has the chemical characteristics of a particular chemical element. It contains a nucleus of protons and neutrons surrounded by a cloud of electrons.
ATOMIC MASS NUMBER: The sum of the number of protons and the number of neutrons in an atom. Approximately equal to the mass of the atom.
ATOMIC NUMBER: The number of protons in an atom, a quantity that determines which element the atom represents. Example: all atoms of oxygen have 8 protons.
AUREOLE: A zone surrounding an igneous intrusion, in which contact metamorphism has taken place.
AUTHIGENESIS: The process by which new minerals form in a sediment or sedimentary rock during or after deposition.
AUTHIGENIC: Formed or generated in place; specif. 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. The term, as used, often refers to a mineral (such as quartz or feldspar) formed after deposition of the original sediment.
AUTOCHTHONOUS: Rock which has not been moved from its original site of formation.
AUX VASES: The Aux Vases Sandstone is named for the Aux Vases River in Ste. Genevieve County, Missouri, and the type section consists of outcrops in the Mississippi River bluffs at the mouth of the Aux Vases River. The Aux Vases consists of sandstone, siltstone, and minor amounts of shale and, locally, dolomite and limestone. It occurs in much of the area of the Chesterian Series. It crops out along the Mississippi River Valley in St. Clair, Monroe, and Randolph Counties, being particularly well exposed in the bluffs 2-3 miles southeast of Prairie du Rocher, Randolph County. In southern Illinois it crops out principally in Union, Johnson, and Hardin Counties.
AXIAL PLANE: A geometric plane that intersects the trough or crest of a fold in such a way that the limbs of the fold are more or less symmetrically arranged with reference to it.
AXIS: The line formed by the intersection of the axial plane of a fold with a bedding plane, marking where the bed shows its maximum curvature.
BACK SWAMP: A swamp that forms in the low lying flood plain behind a levee.
BACK-ARC BASIN: The region between an island arc and the continental mainland, commonly with at least some oceanic crust on its floor.
BACKSHORE: Lies between high tide mark and the foot of the beach dune or the limit of effective wave action.
BACKWASH: The return flow of water down a beach after a wave has broken.
BANDED IRON FORMATION: (BIF) A sedimentary mineral deposit dominated by iron oxides, carbonates, or silicates that were deposited chemically from seawater. Most bifs were formed between 2.5 and 3.5 billion years ago. Their formation is related to the rise of oxygen in the atmosphere.
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: A stream discharge that just fills the stream channel.
BAR: 1. A mass of sand, gravel, or alluvium deposited on the bed of a stream, sea, or lake, or at the mouth of a stream 2. A unit of pressure, approximately equal to atmospheric pressure at sea level.
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 BEACHES OR ISLANDS: Long narrow beaches separated in many places from the mainland by lagoons.
BARRIER ISLAND: A long, narrow island parallel to the shore, composed of sand and built by wave action.
BARRIER REEF: A coral reef separated from the mainland by a lagoon.
BASALT: A dark colored extrusive igneous rock composed chiefly of calcium plagioclase and pyroxene. Extrusive equivalent of gabbro, underlies the ocean basins and comprises oceanic crust.
BASE FLOW: Ground water that enters a stream channel, maintaining stream flow at times when it is not raining.
BASE LEVEL: Lower limit of erosion of the land's surface by running water. Controlled locally and temporarily by the water level of stream mouths emptying into lakes, or more generally and semi-permanently by the level of the ocean (mean sea level).
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.
BASEMENT COMPLEX: The suite of mostly crystalline igneous and/or metamorphic rocks that generally underlies the sedimentary rock sequence.
BASEMENT ROCK: The thick foundation of ancient, and oldest metamorphic and igneous rock that forms the crust of continents, often in the form of granite.
BASIC ROCK: Any igneous rock containing mafic minerals rich in iron and magnesium, but containing no quartz and little sodium rich plagioclase feldspar.
BASIN: A synclinal structure, roughly circular in its outcrop pattern, in which beds dip gently toward the center from all directions.
BASIN AND RANGE PROVINCE: Particular type of topography that covers much of the southwestern United States and northwestern Mexico that is typified by elongate north-south trending arid valleys bounded by mountain ranges which also bound adjacent valleys.

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