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Iowa State University - Geology terms Category: Education > University Date & country: 18/11/2013, USA Words: 782

aa
A Hawaiian term for a lava flow that has a rough, jagged surface. compare pahoehoe.

ablation
As applied to glacier ice, the process by which ice below the snow line is wasted by evaporation and melting.

absolute time
Geologic time expressed in years before the present.

abundant meta
l Iron, aluminum, magnesium, manganese, and titanium. Ores of the abundant metals only need to be 3

abyssal plain
Large area of extremely flat ocean floor lying near a continent and generally over 4 km in depth.

acceleration
The rate at which velocity changes, either by increasing or decreasing.

accretion
The process by which the terrestrial planets grew, increasing their mass by gradually accumulating smaller bodies, called planetesimals.

acid mine drainage
Water contamination by sulfuric acid produced by seepage through sulfur-bearing spoil and tailings from coal and metal mining

acid rain
The acidity in rain due to gases from internal combustion engines and coal- and oil-burning power plants.

active layer
The seasonally thawed zone above permafrost.

actualism
see uniformitarianism.

aftershock
An earthquake that follows and has its epicenter near a larger earthquake.

agate
A siliceous rock with alternating bands of chalcedony and variously colored chert.

alluvial fan
Land counterpart of a delta . An assemblage of sediments marking place where a stream moves from a steep gradient to a flatter gradient and suddenly loses transporting power. Typical of arid and semiarid climates but not confined to them.

alpha decay
The process of radioactive decay in which the nucleus of an atom emits an alpha particle. The new atom

alpha particle
A particle consisting of two protons and two neutrons, produced during alpha decay. Identical to the nucleus of a 4He atom.

alpine glacier
see valley glacier.

amygdaloidal
A textural term describing volcanic rocks that contain numerous amygdules.

amygdule
A gas cavity (vesicle ) in volcanic rock that has been filled with mineral matter such as calcite, chalcedony, or quartz.

andesite
A fine-grained volcanic rock of intermediate composition, consisting largely of plagioclase and one or more mafic minerals.

andesite line
The geographic boundary between rocks of the Pacific Basin, which are basaltic, and those around the rim of the basin, which are in part andesitic.

angle of incidence
The angle at which a ray of energy approaches a surface.

angle of reflection
The angle at which a reflected ray of energy leaves a surface.

angle of refraction
The angle at which a refracted ray of energy leaves a surface after passing through it.

angle of repose
The maximum angle at which loose material will come to rest when added to a pile of similar material.

angular unconformity
An unconformity in which the beds below the unconformity dip at a different angle than the beds above it.

anion
An ion with a negative electrical charge. That is, an atom that has gained one or more electrons.

anticline
A fold that is convex upward, or that had such an attitude at some stage of its development. compare syncline .

aquifer
A permeable region of rock or soil through which ground water can move.

aquitard
A material of low permeability that greatly slows the movement of ground water.

arch
Forms along a coast as wave erosion cuts through a headland.

Archean
An eon of geologic time extending from about 3.9 billion years to 2.5 billion years ago.

ar
A narrow, saw-toothed mountain ridge developed by glacier erosion in adjacent cirques .

arkose
A sedimentary rock formed by the cementation of sand-sized grains of feldspar and quartz.

artesian well
A well in which the water in the aquifer is under pressure that raises the water above the point that the well first encounters it.

assemblage
The collection of minerals that characterize a rock or a facies.

astronomic theory of glaciation
A theory based on the changing position of the Earth in its orbit around the Sun.

asymmetric rock knob or hill
Bedrock forms with a gentle slope on one side created by glacial abrasion and a steep slope on the opposite side created by glacial plucking.

atoll
A roughly circular reef with an occasional small, low, coral sand island surrounding a shallow lagoon.

atom
A building block of matter, the smallest particle that has the chemical characteristics of a particular chemical element. It contains a nucleus of protons and neutrons surrounded by a cloud of electrons.

atomic mass number
The sum of the number of protons and the number of neutrons in an atom. Approximately equal to the mass of the atom.

atomic number
The number of protons in an atom, a quantity that determines which element the atom represents. Example: all atoms of oxygen have 8 protons.

aureole
A zone surrounding an igneous intrusion, in which contact metamorphism has taken place.

authigenesis
The process by which new minerals form in a sediment or sedimentary rock during or after deposition.

axial plane
A geometric plane that intersects the trough or crest of a fold in such a way that the limbs of the fold are more or less symmetrically arranged with reference to it.

axis
The line formed by the intersection of the axial plane of a fold with a bedding plane, marking where the bed shows its maximum curvature.

back swamp
A swamp that forms in the low lying flood plain behind a levee.

back-arc basin
The region between an island arc and the continental mainland, commonly with at least some oceanic crust on its floor.

banded iron formation
(BIF) A sedimentary mineral deposit dominated by iron oxides, carbonates, or silicates that were deposited chemically from seawater. Most BIFs were formed between 2.5 and 3.5 billion years ago. Their formation is related to the rise of oxygen in the atmosphere.

bankfull stag
e A stream discharge that just fills the stream channel.

bar 1
. A mass of sand, gravel, or alluvium deposited on the bed of a stream, sea, or lake, or at the mouth of a stream 2. A unit of pressure, approximately equal to atmospheric pressure at sea level.

barchan
A crescent-shaped sand dune with horns pointing downwind.

barrier beaches or islands
Long narrow beaches separated in many places from the mainland by lagoons.

barrier reef
A coral reef separated from the mainland by a lagoon.

basalt
A dark colored extrusive igneous rock composed chiefly of calcium plagioclase and pyroxene. Extrusive equivalent of gabbro, underlies the ocean basins and comprises oceanic crust.

base flow
Ground water that enters a stream channel, maintaining stream flow at times when it is not raining.

base level
Of a stream is the point below which the stream cannot cut. A temporary base level along a stream, such as a lake may be removed by stream action . Ultimate base level is the ocean.

basin
A synclinal structure, roughly circular in its outcrop pattern, in which beds dip gently toward the center from all directions.

batholith
A large, discordant, intrusive body of igneous rock.

bauxite
A rock composed of aluminum hydroxides and impurities in the form of silica, clay, silt, and iron hydroxides. A residual weathering product, exploited as the primary ore for aluminum.

bay barrier
A beach that cuts off a bay from the sea.

beach
Temporary accumulations of sediments that collect between low and high tide marks.

beach replenishment
Rebuilding a beach by adding sand to it.

bed load
Material in motion along a stream bed.

bedding
A collective term used to signify presence of beds, or layers, in sedimentary rocks and deposits.

bedding plane
Surface separating layers of sedimentary rocks and deposits. Each bedding plane marks termination of one deposit and beginning of another of different character, such as a surface separating a sandstone bed from an overlying mudstone bed. Rock tends to breaks or separate, readily along bedding planes.

bedrock
Any solid rock exposed at the Earth

beheaded stream
The headwaters of a stream that have been captured by another stream. compare stream piracy .

berm
A small terrace in the backshore area of the coast with its terrace facing seaward.

beta decay
The process of radioactive decay in which a neutron loses a beta particle, which is physically identical to an electron. This increases the atomic number of the atom by one by turning the neutron into a proton. The atom

BIF
see banded iron formation

binding energy
The energy that holds the particles in the nucleus of an atom together. It is this energy, when released, that is used to generate nuclear power.

biogenic sediment
Sediments produced directly by the life processes of plants or animals.

biogenic sedimentary rock
A sedimentary rock composed primarily of biogenic sediments.

bioturbation
The turning and mixing of sediments by organisms.

black smoker
A vent on the seafloor from which hydrothermal fluids are emitted. Upon mixing with seawater and cooling, the fluids precipitate a cloud of fine-grained sulfide minerals that resembles a cloud of black smoke.

blind valley
A valley in karst that ends abruptly downstream at the point where its stream disappears underground as a sinking stream.

blowout
A an irregular depression excavated by wind, usually in previously deposited blown sand.

body wave
Any seismic wave that travels through the body of the Earth, rather than along its surface. compare surface wave.

bond
(ionic, covalent, Van der Waals, metallic) see chemical bond

bottomset bed
Layer of fine sediment deposited in a body of standing water beyond the edge of a growing delta and which is eventually built over by the advancing delta. Similarly bottomset beds may accumulate in the wind shadow of a sand dune and be preserved beneath it as the dune advances.

boudinage
A structure in which brittle beds bounded by more ductile ones have been divided into segments during metamorphism.

boulder train
Clusters of erratics from same source, with some distinctive characteristic that makes their common source easily recognizable.

boundary
The tectonic region in which two plates meet. compare margin.

Bowen
A series of minerals formed during crystallization of a magma, in which the formation of minerals alters the composition of the remaining magma. Mafic minerals comprise a discontinuous series, in which successive minerals form at the expense of early-formed ones. The plagioclase feldspars form in a continuous series, in which the composition of pla...

braided stream
A stream with a complex tangle of converging and diverging channels separated by sand bars or islands.

branch work cave
Cave with passage ways formed along bedding planes and with an areal pattern similar to that of surface streams.

breakwater
A protective wall built offshore and usually parallel to the shore.

breccia
A clastic rock in which the gravel-sized particles are angular in shape and make up an appreciable volume of the rock.

breeder reactor
A nuclear reactor in which 238U or 232Th, which are not easily fissionable, absorb neutrons to become atoms of 239Pu or 236U, which can later be used as fuels in fission reactors. Breeder technology is not yet feasible.

brittle
Structural behavior in which a material deforms permanently by fracturing.

brittle limi
t The stress limit beyond which a material fractures, rather than behaving in a ductile or elastic fashion.

burial metamorphism
Takes place in an environment where pressure and temperature are barely more intense than during diagenesis , typically in a deepening sequence of sediments.

calcarenite
A sandstone in which the sand-sized grains are calcite.

caldera
A large, basin-shaped volcanic depression, more or less circular in form. Typically steep-sided, found at the summit of a shield volcano .

caliche
Gravel, sand, or desert debris cemented by calcium carbonate, an accumulated product of chemical weathering in a dry climate. compare claypan, fragipan, hardpan.

calving
The breaking away of ice from the front of the glacier when it ends in a lake or an ocean. Produces icebergs.

cap rock
A comparatively impervious stratum immediately overlying an oil- or gas-bearing rock.

capacity
The total amount of material a stream is able to carry under given conditions.

carbonate rock
A rock consisting primarily of a carbonate mineral such as calcite or dolomite, the chief minerals in limestone and dolostone, respectively.