7/23/2019 Dictionary of Geologic Terms http://slidepdf.com/reader/full/dictionary-of-geologic-terms 1/35 Dictionary of Geologic Terms A B C D E F G H I J K L M N O P Q R S T U V W X Y Z # - A - Aa: A blocky and fragmented form of lava occurring in flows with fissured and angular surfaces. A-horizon: The uppermost layer of a soil, containing organic material and leached minerals. Algal mat: A layered communal growth of algae observed in fossils an in present day tidal zones associated with carbonate sedimentation. Alkali metal: A strongly basic metal like potassium or sodium. Alluvial fan: A low, cone shaped deposit of terrestrial sediment formed where a stream undergoes an abrupt reduction of slope. Alluvium: Unconsolidated terrestrial sediment composed of sorted or unsorted sand, gravel, and clay that has been deposited by water. Angle of repose: The steepest slope angle in which particular sediment will lie without cascading down. Angstrom: A length of ! 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 "# to "$ percent of fi%ed carbon. &t is black, hard, and glassy. A'uifer: A permeable formation that stores and transmits groundwater in sufficient 'uantity to supply wells. Arkose: A variety of sandstone containing abundant feldspar and 'uartz, fre'uently 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.
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3aldera: A large, circular depression in a volcanic terrain, typically originating in
collapse, e%plosion, or erosion.
3arbonate ion: The anion group 34* with a charge of minus two.
3arbonate platform: A submarine or intertidal shelf whose elevation is maintained by
active shallow water carbonate deposition.
3arbonate rock: A rock composed of carbonate minerals, especially limestone and
dolomite.
3arbonic acid: The weak acid 5#34* formed by the dissolution of 34# in water.
3ataclastic rock: A breccia of powdered rock formed by crushing and shearing duringtectonic movements.
3ation: Any ion with a positive electric charge.
3entral vent: The largest vent of a volcano, situated at the center of its cone.
3hemical sediment: 4ne that is formed at or near its place of deposition by chemical
precipitation, usually from seawater.
3hemical weathering: The total set of all chemical reactions that act on rock e%posedto water and atmosphere and so change it minerals to stable forms.
3hert: A sedimetary form of amorphous or e%tremely fine-grained silica, partially
hydrous, found in concretions and beds.
3-horizon: The lowest layer of soil, consisting of fragments of rock and their
chemically weathered products.
3inder cone: A steep, conical hill built up about a volcanic vent and composed of
coarse pyroclasts e%pelled from the vent by escaping gases.
3ir'ue: 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.
3lastic rock: A sedimentary rock formed from mineral particles (clasts) that were
mechanically transported.
3lay: Any of a number of hydrous aluminosilicate minerals formed by weathering and
hydration of other silicates1 also, any mineral fragment smaller than 0#++ mm.
3oal: The metamorphic product of stratified plant remains. &t contains more than +! percent carbon compounds and burns readily.
3oastal plain: A low plain of little relief ad/acent to the ocean and covered with gently
dipping sediments.
3omposite cone: The volcanic cone of a stratovolcano, composed of both cinders and
lava flows.
3ontact metamorphism: 6ineralogical and te%tural changes and deformation of rock
resulting from the head and pressure of an igneous intrusion in the near vicinity.
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7atum plane: An artificially established, well surveyed horizontal plane against which
elevations, depths, tides, etc. are measured (for e%ample mean sea-level).
7aughter element: Also 8daughter product8. An element that occurs in a rock as end
product of the radioactive decay of another element.
7ebris avalanche: A fast downhill mass movement of soil and rock.
7eclination: At any place on 9arth, the angle between the magnetic and rotational
poles.
7eflation: The removal of clay and dust from dry soil by strong winds.
7elta: A body of sediment deposited in an ocean or lake at the mouth of a stream.
7elta kame: A deposit having the form of a steep, flat topped hill, left at the front of a
retreating continental glacier.
7endritic drainage: A stream system that branches irregularly and resembles a
branching tree in plan.
7ensity: The mass per unit volume of a substance, commonly e%pressed in grams0
cubic centimeter.
7ensity current: A suba'ueous current that flows on the bottom of a sea or lake
because entering water is denser due to temperature or suspended sediments.
7eposition: A general term for the accumulation of sediments by either physical or
chemical sedimentation.
7eposition remnant magnetization: A weak magnetization created in sedimentary
rocks by the rotation of magnetic crystals into line with the ambient field during
settling.
7esert pavement: A residual deposit produced by continued deflation, which removesthe fine grains of a soil and leaves a surface covered with closely packed cobbles.
9ugeosyncline: The seaward part of a geosyncline1 characterized by clastic sediments
and volcanism.
9ustatic change: ;ea level changes that affect the whole 9arth.
9utrophication: A superabundance of algal life in a body of water1 caused by anunusual influ% of nitrate, phosphate, or other nutrients.
9vaporite: A chemical sedimentary rock consisting of minerals precipitated by
evaporating waters, especially salt and gypsum.
9%foliation: A physical weathering process in which sheets of rock are fractured and
detached from an outcrop.
9%obiology: The study of life outside the 9arth.
9%tinction angle: The angle between a crystallographic direction, such as a face orcleavage plane, and the direction in which all light is blocked by a pair of crossed
polarizers.
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=acies: 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.
=ault: A planar or gently curved fracture in the 9arths crust across which there has
been relative displacement.
=ault-block mountain: A mountain or range formed as a horst when it was elevated
between parallel normal faults.
=ault plane: The plane that best appro%imates the fracture surface of a fault.
=aunal succession: The evolutionary se'uence of life forms, especially as recorded by
the fossil remains in a stratigraphic se'uence.
=elsic: An ad/ective used to describe a light-colored igneous rock poor in iron and
magnesium content, abundant in feldspars and 'uartz.
=iord: A former glacial valley with steep walls and a U-shaped profile now occupied
by the sea.
=lood basalt: A plateau basalt e%tending many kilometers in flat, layered flows
originating in fissure eruptions.
=lood plain: A level plain of stratified alluvium on either side of a stream1 submergedduring floods and built up silt and sand carried out of the main channel.
=lood tide: The part of the tide cycle during which the water is rising or leveling off at
high water.
=low cleavage: &n a metamorphic rock, the parallel arrangement of all planar or linear
crystals as a result of rock flowage during metamorphism.
=luid 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.
=lume: A laboratory model of stream flow and sedimentation consisting of a
rectangular channel filled with sediment and running water.
=ocus (earth'uake): The point at which the rupture occurs1 synonymous with
hypocenter.
=old: A planar feature, such as a bedding plane, that has been strongly warped,
presumably by deformation.
=oliation: Any planar set of minerals or banding of mineral concentrations including
cleavage, found in a metamorphic rock.
=oraminifer: A class of oceanic protozoa most of which have shells composed of
calcite.
=oraminiferal ooze: A calcareous sediment composed of the shells of dead
=oraminifera.
=orset bed: 4ne of the inclined beds found in crossbedding1 also an inclined bed
deposited on the outer front of a delta.
=ormation: 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.
=ossil: 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.
=ossil 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.
=ree oscillation: The ringing or periodic deformation of the whole 9arth at
characteristic low fre'uencies after a ma/or earth'uake.
=riction breccia: A breccia formed in a fault zone or volcanic pipe by the relative
motion of two rock bodies.
=ringing reef: A coral reef that is directly attached to a landmass not made of coral.
=umarole: A small vent in the ground from which volcanic gases and heated
groundwater emerge, but not lava.
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>abbro: A black, coarse-grained, intrusive igneous rock, composed of calcic feldspars
and pyro%ene. The intrusive e'uivalent of basalt.
>eochronology: 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.
>eologic cycle: The se'uence through which rock material passes in going from its
sedimentary form, through diastrophism and deformation of sedimentary rock, thenthrough metamorphism and eventual melting and magma formation, then through
volcanism and plutonism to igneous rock formation, and finally through erosion to
form new sediments.
>eomorphic cycle: An idealized model of erosion wherein a plain is uplifted
epeirogenically, then dissected by rapid streams (youth), then rounded by d!wnslope
movements into a landscape of steep hills (maturity), and finally reduced to a new
peneplain at sea level (old age).
>eomorphology: The science of surface landforms and their interpretation on the basis of geology and climate.
>eosyncline: A ma/or downwarp in the 9arths crust, usually more than !!!
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.
>eotherm: A curving surface within 9arth along which the temperature is constant.
>eyser: 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.
>lacial rebound: 9peirogenic 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.
>lacial striations: ;cratches left on bedrock and boulders by overriding ice, and
showing the direction of motion.
>lacial valley: A valley occupied or formerly occupied by a glacier, typically with a
>lacier: A mass of ice and surficial snow that persists throughout the year and flows
downhill under its own weight. The size range is from !! meters to !,!!!
kilometers.
>lacier surge: A period of unusually rapid movement of one glacier, sometimes
lasting more than a year.
>lass: A rock formed when magma is too rapidly cooled ('uenched) to allow crystal
growth.
>lassiness: The content of e%tent of glass in an igneous rock.
>neiss: A coarse-grained regional metamorphic rock that shows compositional
banding and parallel alignment of minerals.
>raben: A downthrown block between two normal faults of parallel strike but
converging dips1 hence a tensional feature. ;ee also horst.
>raded 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.
>raded stream: A stream whose smooth profile is unbroken by resistant ledges, lakes,
or waterfalls, and which maintains e%actly the velocity re'uired to carry the sediment
provided to it.
>ranite: A coarse-grained, intrusive igneous rock composed of 'uartz, orthoclasefeldspar, sodic plagioclase feldspar, and micas. Also sometimes a metamorphic
product.
>ranitization: The formation of metamorphic granite from other rocks by
recrystallization with or without complete melting.
>ranular snow: ;now that has been metamorphosed into small granules of ice.
>ranulite: A metamorphic rock with coarse interlocking grains and little or no
foliation.
>ravel: The coarsest of alluvial sediments, containing mostly particles larger than #
mm in size and including cobbles and boulders.
>ravity anomaly: The value of gravity left after subtracting from a gravity
measurement the reference value based on latitude, and possibly the free-air and
ouguer corrections.
>ravity survey: The measurement of gravity at regularly spaced grid points with
repetitions to control instrument drift.
>reenhouse effect: The heating of the atmosphere by the absorption of infrared energyre-emitted by the 9arth as it receives light energy in the visible band from the ;un.
2othole: A semispherical hole in the bedrock of a stream bed, formed by abrasion of
small pebbles and cobbles in a strong current.
2pm: Abbreviation for 8parts per million.8
2ratt isostatic compensation: The mechanism in which variations in crustal density actto counterbalance the varying weight of topographic features. The crust is here
assumed to be of appro%imately uniform thickness, thus a mountain range would be
underlain by lighter rocks.
2referred orientation: Any deviation from randomness in the distribution of the
crystallographic or grain shape a%es 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.
2roto-sun: A large cloud of dust and gas gradually coalescing into a star under the
force of gravity.
2roven reserves: 7eposits of fossil fuels whose location and e%tent are known, as
opposed to potential but unproved (Ediscovered8) deposits.
2umice: A form of volcanic glass, usually of silicic composition, so filled with
vesicles that it resembles a sponge and is very light.
2-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.
2yroclastic rock: A rock formed by the accumulation of fragments of volcanic rock
scattered by volcanic e%plosions.
2yroclastic te%ture: The unsorted, angular, and un-rounded te%ture of the fragments in
a pyroclastic rock.
2yro%ene granulite: A coarse-grained contact metamorphic rock containing pyro%ene,
formed at high temperatures and low pressures.
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Fuartz arenite: A sandstone containing very little e%cept pure 'uartz grains and
cement.
Fuartzite: () A very hard, clean, white metamorphic rock formed from a 'uartz
arenite sandstone. (#) A 'uartz arenite containing so much cement that it resembles (
Transition element: 9lements of atomic number # to #", *$ to <, and to $,
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 a%is 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. 9ach type of seismic wave has its own curve.
Travertine: A terrestrial deposit of limestone formed in caves and around hot springswhere cooling, carbonate-saturated groundwater is e%posed 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 floor1 interpreted as marking the
line along which a plate bends down into a subduction zone.
Triple /unction: A point that is common to three plates and which must also be the
meeting place of three boundary features, such as divergence zones, convergencezones, or transform faults.
Tsunami: A large destructive wave caused by sea-floor movements in an earth'uake.
Tuff: A consolidated rock composed of pyroclastic fragments and fine ash. &f particles
are melted slightly together from their own heat, it is a 8welded tuff.8
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 mi%ed water and sediment that flows downhill along the bottom of an ocean or lake because it is denser than the surrounding water. &t may
reach high speeds and erode rapidly (see also 7ensity current).
Turbulent flow: A high-velocity flow in which streamlines are neither parallel nor
straight but curled into small tight eddies (compare @aminar flow).
I-shaped valley: A valley whose walls have a more-or-less uniform slope from top to
bottom, usually formed by stream erosion.
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?adi: A steep-sided valley containing an intermittent stream in an arid region.
?arping: &n tectonics, refers to the gentle, regional bending of the crust, which occurs
in epeirogenic movements.
?ater 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.
?ater table: A gently-curved surface below the ground at which the vadose zone endsand the phreatic zone begins1 the level to which a well would fill with water.
?ave-cut terrace: A level surface formed by wave erosion of coastal bedrock to the
bottom of the turbulent breaker zone. 6ay appear above sea level if uplifted.
?avelength: The distance between two successive peaks, or between troughs, of a
cyclic propagating disturbance.
?ave steepness: The ma%imum height or amplitude of a wave divided by its
wavelength.
?eathering: The set of all processes that decay and break up bedrock, by a
combination of physically fracturing or chemical decomposition.
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Jenolith: A piece of country rock found engulfed in an intrusion.
J-ray diffraction: &n mineralogy, the process of identifying mineral structures by
e%posing crystals to J-rays and studying the resulting diffraction pattern.