Dictionary of Geology Terms - For Home Use Only 1 A 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 black, 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.
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Dictionary of Geology Terms - For Home Use Only
1
A
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 black, 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.
Dictionary of Geology Terms - For Home Use Only
2
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.
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3
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.
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4
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.
Dictionary of Geology Terms - For Home Use Only
5
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.
Dictionary of Geology Terms - For Home Use Only
6
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.
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7
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.
Dictionary of Geology Terms - For Home Use Only
8
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.
Faunal succession: The evolutionary sequence of life forms, especially as recorded by the fossil
remains in a stratigraphic sequence.
Felsic: An adjective used to describe a light-colored igneous rock poor in iron and magnesium
content, abundant in feldspars and quartz.
Fiord: A former glacial valley with steep walls and a U-shaped profile now occupied by the sea.
Fissure: An extensive crack, break, or fracture in the rocks.
Fissure vein: A cleft or crack in the rock material of the earth's crust, filled with mineral matter
different from the walls and precipitated therin from aqueous solution.
Flood basalt: A plateau basalt extending many kilometers in flat, layered flows originating in
fissure eruptions.
Flood plain: A level plain of stratified alluvium on either side of a stream; submerged during
floods and built up silt and sand carried out of the main channel.
Flood tide: The part of the tide cycle during which the water is rising or leveling off at high
water.
Flow cleavage: In a metamorphic rock, the parallel arrangement of all planar or linear crystals as
a result of rock flowage during metamorphism.
Dictionary of Geology Terms - For Home Use Only
9
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.
Focus (earthquake): The point at which the rupture occurs; synonymous with hypocenter.
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.
Foraminifera: A class of oceanic protozoa most of which have shells composed of calcite.
Foraminiferal ooze: A calcareous sediment composed of the shells of dead Foraminifera.
Forset bed: One of the inclined beds found in crossbedding; also an inclined bed deposited on the
outer front of a delta.
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.
Fossil fuel: A general term for combustible geologic deposits of carbon in reduced (organic)
form and of biological origin, including coal, oil, natural gas, oil shales, and tar sands.
Free oscillation: The ringing or periodic deformation of the whole Earth at characteristic low
frequencies after a major earthquake.
Friction breccia: A breccia formed in a fault zone or volcanic pipe by the relative motion of two
rock bodies.
Fringing reef: A coral reef that is directly attached to a landmass not made of coral.
Fumarole: A small vent in the ground from which volcanic gases and heated groundwater
emerge, but not lava.
G
Gabbro: A black, coarse-grained, intrusive igneous rock, composed of calcic feldspars and
pyroxene. The intrusive equivalent of basalt.
Dictionary of Geology Terms - For Home Use Only
10
Geochronology: The science of absolute dating and relative dating of geologic formations and
events, primarily through the measurement of daughter elements produced by radioactive decay
in minerals.
Geologic cycle: The sequence through which rock material passes in going from its sedimentary
form, through diastrophism and deformation of sedimentary rock, then through metamorphism
and eventual melting and magma formation, then through volcanism and plutonism to igneous
rock formation, and finally through erosion to form new sediments.
Geomorphic cycle: An idealized model of erosion wherein a plain is uplifted epeirogenically,
then dissected by rapid streams (youth), then rounded by d0wnslope movements into a landscape
of steep hills (maturity), and finally reduced to a new peneplain at sea level (old age).
Geomorphology: The science of surface landforms and their interpretation on the basis of
geology and climate.
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.
Geotherm: A curving surface within Earth along which the temperature is constant.
Geyser: A hot spring that throws hot water and steam into the air. The heat is thought to result
from the contact of groundwater with magma bodies.
Glacial rebound: Epeirogenic uplift of the crust that takes place after the retreat of a continental
glacier, in response to earlier subsidence under the weight of the ice.
Glacial striations: Scratches left on bedrock and boulders by overriding ice, and showing the
direction of motion.
Glacial valley: A valley occupied or formerly occupied by a glacier, typically with a U-shaped
profile.
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.
Glacier surge: A period of unusually rapid movement of one glacier, sometimes lasting more
than a year.
Glass: A rock formed when magma is too rapidly cooled (quenched) to allow crystal growth.
Glassiness: The content of extent of glass in an igneous rock.
Gneiss: A coarse-grained regional metamorphic rock that shows compositional banding and
parallel alignment of minerals.
Dictionary of Geology Terms - For Home Use Only
11
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.
Graded stream: A stream whose smooth profile is unbroken by resistant ledges, lakes, or
waterfalls, and which maintains exactly the velocity required to carry the sediment provided to it.
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.
Granular snow: Snow that has been metamorphosed into small granules of ice.
Granulite: A metamorphic rock with coarse interlocking grains and little or no foliation.
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.
Greenhouse effect: The heating of the atmosphere by the absorption of infrared energy re-
emitted by the Earth as it receives light energy in the visible band from the Sun.
Greenschist: A metamorphic schist containing chlorite and epidote (which are green) and formed
by low-temperature, low-pressure metamorphism.
Ground moraine: A glacial deposit of till with no marked relief, interpreted as having been
transported at the base of the ice.
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.
Gully: A small steep-sided valley or erosional channel from 1 meter to about 10 meters across.
Guyot: A flat-topped submerged mountain or seamount found in the ocean.
Gyre: The circular rotation of the waters of each major sea, driven by prevailing winds and the
Coriolis effect.
Dictionary of Geology Terms - For Home Use Only
12
H
Half-life: The time required for half of a homogeneous sample of radioactive material to decay.
Hanging valley: A former glacial tributary valley that enters a larger glacial valley above its
base, high up on the valley wall.
Hard water: Water that contains sufficient dissolved calcium and magnesium to cause a
carbonate scale to form when the water is boiled or to prevent the sudsing of soap.
Heat conduction: The transfer of the rapid vibrational energy of atoms and molecules, which
constitutes heat energy, through the mechanism of atomic or molecular impact.
Heat engine: A device that transfers heat from a place of high temperature to a place of lower
temperature and does mechanical work in the process.
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.
Hogback: A formation similar to a Cuesta in that it is a ridge formed by slower erosion of hard
strata, but having two steep, equally inclined slopes.
Hooke's Law: The principle that the stress within a solid is proportional to the strain. It holds
only for strains of a few percent or less.
Hornfels: A high-temperature, low-pressure metamorphic rock of uniform grain size showing no
foliation. 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.
Hot spring: A spring whose waters are above both human body and soil temperature as a result
of plutonism at depth.
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.
Hydraulic conductivity: A measure of the permeability of a rock or soil: the volume of flow
through a unit surface in unit time with unit hydraulic pressure difference as the driving force.
Hydrocarbon: An organic chemical compound made up of carbon and hydrogen atoms arranged
in chains or rings.
Dictionary of Geology Terms - For Home Use Only
13
Hydrologic cycle: The cyclical movement of water from the ocean to the atmosphere, through
rain to the surface, through runoff and groundwater to streams, and back to the sea.
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.
Hypocenter: The point below the epicenter at which an earthquake actually begins; the focus.
Hypsometric diagram: A graph that shows in any way the relative amounts of the Earth's surface
at different elevations with regard to sea level.
I
Igneous rock: A rock formed by congealing rapidly or slowly from a molten state.
Ignimbrite: An igneous rock formed by the lithification of volcanic ash and volcanic breccia.
Inclination: The angle between a line in the Earth's magnetic field and the horizontal plane; also
a synonym for dip.
Index of refraction: The ratio of the speed of light in a vacuum to the speed in a material; this
ratio determines the amount that light is refracted as it passes into a crystal.
Infiltration: The movement of groundwater or hydrothermal water into rock or soil through joints
and pores.
Interfacial angle: The angle between two crystal faces of a crystal, characteristic of a mineral's
symmetry.
Interior drainage: A system of streams that converge in a closed basin and evaporate without
reaching the sea.
Intermontane basin: A basin between mountain ranges, often formed over a graben.
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.
Ion: An atom or group of atoms that has gained or lost electrons and so has a net electric charge.
Dictionary of Geology Terms - For Home Use Only
14
Ionic bond: A bond formed between atoms by electrostatic attraction between oppositely charged
ions.
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.
Isograd: A line or curved surface connecting rocks that have undergone an equivalent degree of
metamorphism.
Isostasy: The mechanism whereby areas of the crust rise or subside until the mass of their
topography is buoyantly supported or compensated by the thickness of crust below, which
"floats" on the denser mantle. The theory that continents and mountains are supported by low-
density crustal "roots."
Isotope: One of several forms of one element, all having the same number of protons in the
nucleus, but differing in their number of neutrons and thus atomic weight.
Isotope geology: The study of the relative abundances of isotopes in rocks to determine their
ages (see geo-chronology) or conditions of formation.
Isotropic substance: One in which the magnitude of a physical property, such as transmission of
light is independent of crystallographic direction.
J
Joint: A large and relatively planar fracture in a rock across which there is no relative
displacement of the two sides.
Juvenile gas: Gases that come to the surface for the first time from the deep interior.
K
Kerogen: A mixture of organic substances found in many fine-grained sedimentary rocks and a
major constituent of oil shale.
Kettle: A small hollow or depression formed in glacial deposits when outwash was deposited
around a residual block of ice that later melted.
Kilobar: A unit of pressure equal to 1000 bars.
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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L
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.
Lahar: A mudflow of unconsolidated volcanic ash, dust, breccia, and boulders mixed with rain or
the water of a lake displaced by a lava flow.
Laminar flow: A flow regime in which particle paths are straight or gently curved and parallel.
Landslide: The rapid downslope movement of soil and rock material, often lubricated by
groundwater, over a basal shear zone; also the tongue of stationary material deposited by such an
event.
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.
Lava tube: A sinuous, hollow tunnel formed when the outside of a lava flow cools and solidifies
and the molten material passing through it is drained away.
Leaching: The removal of elements from a soil by dissolution in water moving downward in the
ground.
Left-lateral fault: A strike-slip fault on which the displacement of the far block is to the left when
viewed from either side.
Levee: A low ridge along a stream bank, formed by deposits left when floodwater decelerates on
leaving the channel; also an artificial barrier to floods built in the same form.
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.
Lithification: The processes that convert a sediment into a sedimentary rock.
Lithology: The systematic description of rocks, in terms of mineral composition and texture.
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Lithosphere: The outer, rigid shell of the Earth, situated above the asthenosphere and containing
the crust, continents, and plates.
Lode: An unusually large vein or set of veins containing ore minerals.
Longitudinal dune: A long dune parallel to the direction of the prevailing wind.
Longitudinal profile: A cross section of a stream from its mouth to its head, showing elevation
versus distance to the mouth.
Longshore current: A current that moves parallel to a shore and is formed from the momentum of
breaking waves that approach the shore obliquely.
Longshore drift: The movement of sediment along a beach by swash and backwash of waves that
approach the shore obliquely.
Lopolith: A large laccolith that is bowl-shaped and depressed in the center, possibly by
subsidence of an emptied magma chamber beneath the intrusion.
Lowland: Land of general low relief at the lower levels of regional elevation.
Low-velocity zone: A region in the Earth, especially a planar layer that has lower seismic-wave
velocities than the region immediately above it.
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.
M
Maar volcano: A volcanic crater without a cone, believed to have been formed by an explosive
eruption of trapped gases.
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.
Magma chamber: A magma-filled cavity within the lithosphere.
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.
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Magnetic coupling: The transfer of momentum between celestial bodies, especially dust and gas
clouds, through magnetic forces.
Magnetic north pole: (1) The point where the Earth's surface intersects the axis of the dipole that
best approximates the Earth's field. (2) The point where the Earth's magnetic field dips vertically
downward.
Magnetic stratigraphy: The study and correlation of polarity epochs and events in the history of
the Earth's magnetic field as contained in magnetic rocks.
Magnetometer: An instrument for measuring either one orthogonal component or the entire
intensity of the Earth's magnetic field at various points.
Magnitude: A measure of earthquake size, determined by taking the common logarithm base 10)
of the largest ground motion observed during the arrival of a P-wave or seismic surface wave and
applying a standard correction for distance to the epicenter.
Manganese nodule: A small, rounded concretion found on the deep ocean floor that may contain
as much as 20 percent manganese and smaller amounts of iron, copper, and nickel oxides and
hydroxides.
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 movement: A downhill movement of soil or fractured rock under the force of gravity.
Mass spectrometer: An instrument for separating ions of different mass but equal charge (mainly
isotopes in geology) and measuring their relative quantities.
Maturity: A stage in the geomorphic cycle in which maximum relief and well-developed
drainage are both present.
Meander: Broad, semicircular curves in a stream that develop as the stream erodes the outer bank
of a curve and deposits sediment against the inner bank.
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.
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Medical geology: The application of geologic science to problems of health, especially those
relating to mineral sources of toxic or nutritious elements and natural dispersal of toxic
pollutants.
Mesophere: The lower mantle.
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.
Meteorite: A stony or metallic object from inter-planetary space that penetrates the atmosphere
to impact on the surface.
Micrometeorite: A meteorite less than 1 millimeter in diameter.
Microseism: A weak vibration of the ground that can be detected by seismographs and which is
caused by waves, wind, or human activity, but not by an earthquake.
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.
Miogeosyncline: A Geosyncline that is situated near a craton and receives chemical and well-
sorted elastic sediments from the continent.
Mohorovic discontinuity: The boundary between crust and mantle, marked by a rapid increase in
seismic wave velocity to more than 8 kilometers per second. Depth: 5 to 45 kilometers.
Abbreviated "Moho" or "M-discontinuity."
Mohs scale of hardness: An empirical, ascending scale of mineral hardness with talc as 1,