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
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
Alluvium: Unconsolidated terrestrial sediment
composed of sorted or unsorted sand, gravel, and clay that has been deposited
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
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.
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
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
Bathymetry: The study and mapping of sea-floor
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
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
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
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
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
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.
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
Diagenesis: The physical and chemical changes
undergone by a sediment during lithification and compaction, excluding erosion
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
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
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
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
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
Eutrophication: A superabundance of algal life in
a body of water; caused by an unusual influx of nitrate, phosphate, or other
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.
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
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
Fluid inclusion: A small body of fluid that is
entrapped in a crystal and has the same composition as the fluid from which the
Flume: A laboratory model of stream flow and
sedimentation consisting of a rectangular channel filled with sediment and
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
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
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.
Gabbro: A black, coarse-grained, intrusive igneous
rock, composed of calcic feldspars and pyroxene. The intrusive equivalent of
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
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
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
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,
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.
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
Horst: An elongate, elevated block of crust
forming a ridge or plateau, typically bounded by parallel, outward-dipping
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
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.
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.
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.
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
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
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
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.
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
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
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
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
Lithology: The systematic description of rocks, in terms of mineral
composition and texture.
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
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
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
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
Magnetic anomaly: The value of the local magnetic field remaining after the
subtraction of the dipole portion of the Earth's field.
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
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
Mass spectrometer: An instrument for separating ions of different mass but
equal charge (mainly isotopes in geology) and measuring their relative
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.
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
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
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
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.
Monadnock: An isolated hill or mountain rising above a peneplain.
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
Mountain: A steep-sided topographic elevation larger than a hill; also a
single prominence forming part of a ridge or mountain range.
Mudflow: A mass movement of material finer than sand, lubricated with large
amounts of water. Mudstone: The citified equivalent of mud, a fine grained
sedimentary rock similar to shale but more massive.
My.: 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.
Native metal: A natural deposit of a metallic element in pure metallic form,
neither oxidized nor combined with sulfur or other elements.
Neap tide: A tide cycle of unusually small amplitude, which occurs twice
monthly when the lunar and solar tides are opposed-that is, when the
gravitational pull of the Sun is at right angles to that of the Moon.
Nebula: An immense, diffuse body of interstellar gas and dust that has not
condensed into a star.
Nebular hypothesis: A theory of the formation of the planets that states
that a rotating nebula contracted and was then torn into fragments by
centrifugal forces, with planets condensing from the fragments.
Neutron: An electrically neutral elementary particle in the atomic nucleus
having the mass of one proton.
Neutron-activation analysis: A method of identifying isotopes of an element
by bombarding them with neutrons and observing the characteristic radioactive
decay products emitted.
Normal fault: A dip-slip fault in which the block above the fault has moved
downward relative to the block below.
Oblique-slip fault: A fault that combines some strike slip motion with some
Obsidian: Dark volcanic glass of felsic composition.
Octahedral coordination: The packing of six ions around an ion of opposite
charge to form an octahedron.
Oil field: An underground accumulation of oil and gas concentrated beneath
an impermeable trap, preventing its escape upward.
Oil shale: A dark-colored shale containing organic material that can be
crushed and heated to liberate gaseous hydrocarbons.
Old age: A stage in the geomorphic cycle, characterized by formation of a
peneplain near sea level.
Oolite: A sedimentary carbonate particle composed of spherical grains
precipitated from warm ocean water on carbonate platforms. Also a rock composed
of such particles.
Opaque mineral: A mineral which transmits no light through a thin section
under a microscope. Usually a native metal, sulfide, or metallic oxide mineral.
Ophiolite suite: An assemblage of mafic and ultra-mafic igneous rocks with
deep-sea sediments supposedly associated with divergence zones and the
Orbit: The elliptical or hyperbolic path traced by a planet or meteorite or
satellite in the presence of a more massive body.
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.
Original Horizontality, Principle of: The proposition of Steno, that all
sedimentary bedding is horizontal at the time of deposition.
Orogenic belt: A linear region, often a former geo-syncline, that has been
subjected to folding, and other deformation in a mountain-building episode.
Orogeny: The tectonic process in which large areas are folded,
thrust-faulted, metamorphosed, and subjected to plutonism. The cycle ends with
uplift and the formation of mountains.
Oscillation ripple: A ripple with a symmetrical cross section and a sharp
peak formed by waves.
Outcrop: A segment of bedrock exposed to the atmosphere.
Outgassing: The release of juvenile gases to the atmosphere and oceans by
Outwash: A glaciofluvial sediment that is deposited by meltwater streams
emanating from a glacier. Overturned fold: A fold in which a limb has tilted
past vertical so that the older strata are uppermost. Oxbow lake: A long,
broad, crescent-shaped lake formed when a stream abandons a meander and takes a
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.
Pahoehoe: A basaltic lava flow with a glassy, smooth, and undulating, or
Paleoclimate: The average state or typical conditions of climate during some
past geologic period.
Paleocurrent map: A map of depositional currents that have been inferred
from cross-bedding, ripples, or other sedimentary structures.
Paleogeographic map: A map showing the surface landforms and coastline of an
area at some time in the geologic past.
Paleomagnetism: The science of the reconstruction of the Earth's ancient
magnetic field and the positions of the continents from the evidence of remnant
magnetization in ancient rocks.
Paleontology: The science of fossils, of ancient life-forms, and their
Paleowind: A prevailing wind direction in an area, inferred from dune
structure or the distribution of volcanic ash for one particular time in
Pangaea: According to some theories, a great proto-continent from which all
present continents have broken off by the mechanism of sea-floor spreading and
Panthalassa: A hypothetical primeval ocean covering two-thirds of the world
except for the continent of Pangaea.
Parent element: An element that is transformed by radioactive decay to a
different (daughter) element.
Peat: A marsh or swamp deposit of water-soaked plant remains containing more
than 50 percent carbon.
Pedalfer: A common soil type in humid regions, characterized by an abundance
of iron oxides and clay minerals deposited in the B-horizon by leaching.
Pediment: A planar, sloping rock surface forming a ramp up to the front of a
mountain range in an arid region. It may be covered locally by thin alluvium.
Pedocal: A common soil type of arid regions, characterized by accumulation
of calcium carbonate in the A-horizon.
Pegmatite: An igneous rock with extremely large grains, more than a
centimeter in diameter. It may be of any composition but most frequently is
Pelagic sediment: Deep-sea sediments composed of fine-grained detritus that
slowly settles from surface waters. Common constituents are clay, radiolarian
ooze, and foraminiferal ooze.
Peneplain: A hypothetical extensive area of low elevation and relief reduced
to near sea level by a long period of erosion and representing the end product
of the ideal geomorphic cycle.
Perched groundwater: An isolated body of ground-water that is perched above
and separated from the main water table by an aquiclude.
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
Pothole: A semispherical hole in the bedrock of a stream bed, formed by
abrasion of small pebbles and cobbles in a strong current.
Ppm: Abbreviation for "parts per million."
Pratt isostatic compensation: The mechanism in which variations in crustal
density act to counterbalance the varying weight of topographic features. The
crust is here assumed to be of approximately uniform thickness, thus a mountain
range would be underlain by lighter rocks.
Preferred orientation: Any deviation from randomness in the distribution of
the crystallographic or grain shape axes of minerals of a rock (including flow
cleavage and foliation), produced by deformation and non-uniform stress during
crystallization in metamorphic rocks or by depositional currents in sediments.
Proto-sun: A large cloud of dust and gas gradually coalescing into a star
under the force of gravity.
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.
P-wave: The primary or fastest wave traveling away from a seismic event
through the solid rock, and consisting of a train of compressions and dilations
of the material.
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.
Quartz arenite: A sandstone containing very little except pure quartz grains
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 arenite, that may contain a moderate amount of other
detrital minerals and/or calcite cement.
Radial drainage: A system of streams running in a radial pattern away from
the center of a circular elevation, such as a volcano or dome.
Radiative transfer: One mechanism for the movement of heat, in which it
takes the form of long-wavelength infrared radiation.
Radiolarian: A class of one-celled marine animals with siliceous skeletons
that have existed in the ocean throughout the Phanerozoic Eon.
Radiolarian ooze: A siliceous deep-sea sediment composed largely of the
skeletons of radiolaria. Radiolarite: The lithified sedimentary rock formed
from radiolarian ooze.
Ray: A linear landform of the lunar surface emanating from a large crater
and extending as much as 100 kilometers outward, probably consisting of fine
ejecta thrown out by the impact of a meteorite.
Reaction series: A series of chemical reactions occurring in a cooling magma
by which a mineral formed at high temperature becomes unstable in the melt and
reacts to form another mineral (see also Discontinuous reaction series).
Recharge: In hydrology, the replenishment of ground-water by infiltration of
meteoric water through the soil.
Recrystallization: The growth of new mineral grains in a rock at the expense
of old grains, which supply the material.
Rectangular drainage: A system of streams in which each straight segment of
each stream takes one of two characteristic perpendicular directions, with
right-angle bends between. The streams are usually following two perpendicular
sets of joints.
Recumbent fold: An overturned fold with both limbs nearly horizontal.
Refraction (wave): The departure of a wave from its original direction of
travel at the interface with a material of different index of refraction
(light) or seismic wave velocity (see also Seismic refraction).
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 seawater, 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.
Respiration: The chemical reaction by which carbohydrates are oxidized and
by which all animals and plants convert their food into energy. Carbon dioxide
is released and oxygen used up.
Reversible reaction: A chemical reaction which can proceed in either
direction, depending on the concentration of reacting materials.
Rheidity: (1) The ability of a substance to yield to viscous flow under
large strains. (2) One thousand times the time required for a substance to stop
changing shape when stress is no longer applied.
Rhyolite: The fine-grained volcanic or extrusive equivalent of granite,
light brown to gray and compact. Richter magnitude scale: See Magnitude.
Ridge (mid-ocean): A major linear elevated landform of the ocean floor, from
200 to 20,000 kilometers in extent. It is not a single ridge, but resembles a
mountain range and may have a central rift valley.
Rift valley: A fault trough formed in a divergence zone or other area of
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
Rip current: A current that flows strongly away from the sea shore through
gaps in the surf zone at intervals along the shoreline.
Ripple: A very small dune of sand or silt whose long dimension is formed at
right angles to the current. River order: See Stream order.
Rock cycle: The geologic cycle, with emphasis on the rocks produced;
sedimentary rocks are metamorphosed to metamorphic rocks, or melted to create
igneous rocks, and all rocks may be uplifted and eroded to make sediments,
which lithify to sedimentary rocks.
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.
Rock glacier: A glacier-like mass of rock fragments or talus with
interstitial ice that moves downhill under the force of gravity.
Rockslide: A landslide involving mainly large blocks of detached bedrock
with little or no soil or sand. Rounding: The degree to which the edges and
corners of a particle become worn and rounded as a result of abrasion during
transportation. Expressed as angular, subrounded, well-rounded, etc.
Runoff: The amount of rain water directly leaving an area in surface
drainage, as opposed to the amount that seeps out as groundwater.
Rupture strength: The greatest stress that a material can sustain without
fracturing at one atmosphere pressure.
Saltation: The movement of sand or fine sediment by short jumps above the
ground or stream bed under the influence of a current too weak to keep it
Sandblasting: A physical weathering process in which rock is eroded by the
impact of sand grains carried by the wind, frequently leading to ventifact
formation of pebbles and cobbles.
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 shaly or prismatic minerals like
micas and amphiboles resulting from nonhydrostatic stress in metamorphism.
Scoria: Congealed lava, usually of mafic composition, with a large number of
vesicles formed by gases coming out of solution.
Sea-floor spreading: The mechanism by which new sea floor crust is created
at ridges in divergence zones and adjacent plates are moved apart to make room.
This process may continue at 0.5 to 10 centimeters/year through many geologic
Seamount: An isolated tall mountain on the sea floor that may extend more
than 1 kilometer from base to peak (see also Guyot).
Secular variation: Slow changes in the orientation of the Earth's magnetic
field that appear to be long lasting and internal in origin as opposed to rapid
fluctuations, which are external in origin.
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 discontinuity: A surface within the Earth across which P-wave or
S-wave velocities change rapidly, usually by more than +~0.2 kilometer/second.
Seismicity: The world-wide or local distribution of earthquakes in space and
time; a general term for the number of earthquakes in a unit of time.
Seismic profile: The data collected from a set of seismographs arranged in a
straight line with an artificial seismic source, especially the times of P-wave
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 surface wave: A seismic wave that follows the earth's surface only,
with a speed less than that of S-waves. There are Raleigh
waves (forward and vertical vibrations) and Love waves (transverse vibrations).
Seismic transition zone: A seismic discontinuity, found in all parts of the
Earth, at which the velocity increases rapidly with depth; especially the one
at 300 to 600 kilometers.
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.
Streak plate: A ceramic abrasive surface for streak tests.
Streaming flow: A tranquil flow slower than shooting flow.
Streamline: A curved line representing the successive positions of a
particle in a flow as time passes.
Stream order: The hierarchical number of a stream segment in dendritic
drainage: the smallest tributary streams have order one and at each junction of
streams of equal order the order of the subsequent segment is one higher.
Stress: A quantity describing the forces acting on each part of a body in
units of force per unit area. Striation: See Glacial striation.
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.
Stromatolite: A fossil form representing the growth habit of an algal mat:
concentric spherules, stacked hemispheres, or flat sheets of calcium carbonate
and trapped silt encountered in limestones.
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.
Submarine canyon: An underwater canyon in the continental shelf.
Subsidence: A gentle epeirogenic movement where a broad area of the crust
sinks without appreciable deformation.
Superposed stream: A stream that flows through resistant formations because
its course was established at a higher level on uniform rocks before
Superposition, Principle of: The principle stated by Steno that, except in
extremely deformed strata, a bed that overlies another bed is always the
Supersaturation: The unstable state of a solution that contains more solute
than its solubility allows.
Surf: The breaking or tumbling forward of water waves as they approach the
Surf zone: An offshore belt along which the waves collapse into breakers as
they approach the shore.
Suspended load: The fine sediment kept suspended in a stream because the
settling velocity is lower than the upward velocity of eddies.
Swash: The landward rush of water from a breaking wave up the slope of the
S-wave: The secondary seismic wave, traveling slower than the P-wave, and
consisting of elastic vibrations transverse to the direction of travel. It
cannot penetrate a liquid.
Swell: An oceanic water wave with a wavelength on the order of 30 meters or
more and a height of perhaps 2 meters or less that may travel great distances
from its source.
Symbiosis: The interaction of two mutually supporting species that do not
compete with or prey upon each other.
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.
Tableland: A large elevated region with a relatively low relief surface.
Tar sand: A sandstone containing the densest asphaltic components of
petroleum - the end-product of evaporation of volatile components or of some
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
Terminal moraine: A sinuous ridge of unsorted glacial till deposited by a
glacier at the line of its farthest advance.
Terrestrial planet: A planet similar in size and composition to the Earth;
especially Mars, Earth, Venus, and Mercury.
Terrestrial sediment: A deposit of sediment that accumulated above sea level
in lakes, alluvial fans, floodplains, moraines, etc., regardless of its present
Texture (rock): The rock characteristics of grain or crystal size, size
variability, rounding or angularity, and preferred orientation.
Thalweg: A sinuous imaginary line following the deepest part of a stream.
Thermal conductivity: A measure of a rock's capacity for heat conduction.
Thermal expansion: The property of increasing in volume as a result of an
increase in internal temperature.
Thermonuclear reaction: A reaction in which atomic nuclei fuse into new
elements with a large release of heat; especially a reaction that is
self-sustaining. Occasionally used to include fission reactions as well.
Thermoremnent magnetization: A permanent magnetization acquired by igneous
rocks in the presence of the Earth's magnetic field as they cool through the
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
Tidal current: A horizontal displacement of ocean water under the
gravitational influence of Sun and Moon, causing the water to pile up against
the coast at high tide and move outward at low tide.
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 stratigraphy 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.
Topset bed: A horizontal sedimentary bed formed at the top of a delta and
overlying the foreset beds.
Trace element: An element that appears in minerals in a concentration of
less than l percent (often less than 0.001 percent).
Transform fault: A strike-slip fault connecting the ends of an offset in a
mid-ocean ridge. Some pairs of plates slide past each other along transform
Transgression: A rise in sea level relative to the land which causes areas
to be submerged and marine deposition to begin in that region.
Transition element: Elements of atomic number 21 to 29, 38 to 46, and 71 to
78, whose second outermost electron shell is only partially filled.
Transpiration: The removal of water from the ground into plants, ultimately
to be evaporated into the atmosphere by them.
Transverse dune: A dune that has its axis transverse to the prevailing winds
or to a current. The upwind or upcurrent side has a gentle slope, and the
downwind side lies at the angle of repose.
Trap (oil): A sedimentary or tectonic structure that impedes the upward
movement of oil and gas and allows it to collect beneath the barrier.
Travel-time curve: A curve on a graph of travel time versus distance for the
arrival of seismic waves from distant events. Each type of seismic wave has its
Travertine: A terrestrial deposit of limestone formed in caves and around hot
springs where cooling, carbonate-saturated groundwater
is exposed to the air.
Trellis drainage: A system of streams in which tributaries tend to lie in
parallel valleys formed in steeply dipping beds in folded belts.
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.
Triple junction: A point that is common to three plates and which must also be
the meeting place of three boundary features, such as divergence zones,
convergence zones, or transform faults.
Tsunami: A large destructive wave caused by sea-floor movements in an
Tuff: A consolidated rock composed of pyroclastic fragments and fine ash. If
particles are melted slightly together from their own heat, it is a
Turbidite: The sedimentary deposit of a turbidity current, typically showing
graded bedding and sedimentary structures on the undersides of the sandstones.
Turbidity current: A mass of mixed water and sediment that flows downhill
along the bottom of an ocean or lake because it is denser than the surrounding
water. It may reach high speeds and erode rapidly (see also Density current).
Turbulent flow: A high-velocity flow in which streamlines are neither
parallel nor straight but curled into small tight eddies (compare Laminar
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.
Uniformitarianism, Principle of: The concept that the processes that have
shaped the Earth through geologic time are the same as those observable today.
Unit cell: The smallest contiguous group of atomic structural units in a
mineral that can be repeated in three directions to form a crystal.
Uplift: A broad and gentle epeirogenic increase in the elevation of a region
without a eustatic change of sea level.
Upwelling current: The upward movement of cold bottom water in the sea,
which occurs when wind or currents displace the lighter surface water.
U-shaped valley: A deep valley with steep upper walls that grade into a flat
floor, usually eroded by a glacier.
Vadose zone: The region in the ground between the surface and the water
table in which pores are not filled with water. Also called the unsaturated
Valence electron: An electron of the outermost shell of an atom; one of
those most active in bonding.
Valley glacier: A glacier that is smaller than a continental glacier or an
icecap, and which flows mainly along well-defined valleys, many with
Van der Waals bond: A bond much weaker than the ionic or covalent, which
bonds atoms by small electrostatic attraction.
Varve: A thin layer of sediment grading upward from coarse to fine and light
to dark, found in a lake bed and representing one year's deposition of glacial
Vector: A mathematical element that has a direction and magnitude, but no
fixed position. Examples are force and gravity.
Vein: A deposit of foreign minerals within a rock fracture or joint.
Ventifact: A rock that exhibits the effects of sand-blasting or
"snowblasting" on its surfaces, which become fiat with sharp edges in
Vertical exaggeration: The ratio of the horizontal scale (for example,
100,000: 1) to the vertical scale (for example, 500: 1) in an illustration.
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 magma.
Volcanic ash fall: A deposit of volcanic ash resting where it was dropped by
eruptions and winds.
Volcanic ash flow: A mixture of volcanic ash and gases that moves downhill
as a density current in the atmosphere.
Volcanic block: A pyroclastic rock fragment ranging from about fist- to
Volcanic bomb: A pyroclastic rock fragment that shows the effects of cooling
in flight in its streamlined or "bread-crust" surface.
Volcanic breccia: A pyroclastic rock in which all fragments are more than 2
millimeters in diameter.
Volcanic cone: The deposit of lava and pyroclastic materials that has
settled close to the volcano's central vent.
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 ejecta blanket: A collective term for all the pyroclastic rocks
deposited around a volcano, especially by a volcanic explosion.
Volcanic emanations: Gases, especially steam, emitted from a vent or
released from lava.
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.
V-shaped valley: A valley whose walls have a more-or-less uniform slope from
top to bottom, usually formed by stream erosion.
Wadi: A steep-sided valley containing an intermittent stream in an arid
Warping: In tectonics, refers to the gentle, regional bending of the crust,
which occurs in epeirogenic movements.
Water mass: A mass of water that fills part of an ocean or lake and is
distinguished by its uniform physical and chemical properties, such as
temperature and salinity.
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
Wave-cut terrace: A level surface formed by wave erosion of coastal bedrock
to the bottom of the turbulent breaker zone. May appear above sea level if
Wavelength: The distance between two successive peaks, or between troughs,
of a cyclic propagating disturbance.
Wave steepness: The maximum height or amplitude of a wave divided by its
Weathering: The set of all processes that decay and break up bedrock, by a combination
of physically fracturing or chemical decomposition.
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
Youth (geomorphology): A stage in the geomorphic cycle in which a landscape
has just been uplifted and is beginning to be dissected by canyons cut by young
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.
Zoned crystal: A single crystal of one mineral that has a different chemical
composition in its inner and outer parts. Formed from minerals belonging to a
solid-solution series, and caused by the changing concentration of elements in
a cooling magma that results from crystals settling out.