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Iowa State University - Glossary of geologic terms
Catégorie: Géographie
Date & Pays: 29/04/2015, Us.
Mots: 835
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14C methodA method for determining the age in yearsof organic matter by calculating the amount of radioactive carbon stillremaining, as compared to the stable isotope, 12C.
40K/40Ar methodA method used for the dating of potassium-bearingrocks by using the ratio of radioactive 40K to its daughter, 40Ar.
aa A Hawaiian term for a lava flow that has a rough,jagged surface.
compare pahoehoe.
ablation
As applied to glacier ice, the process bywhich ice below the snow line is wasted by evaporation and melting.
absolute time
Geologic time expressed in years beforethe present.
abundant meta
l Iron, aluminum, magnesium, manganese,and titanium. Ores of the abundant metals only need to be 3 – 5 timesas metal-rich as average rock.
abyssal plain
Large area of extremely flat ocean floorlying near a continent and generally over 4 km in depth.
acceleration
The rate at which velocity changes, eitherby increasing or decreasing.
accretion
The process by which the terrestrial planetsgrew, increasing their mass by gradually accumulating smaller bodies,called planetesimals.
acid mine drainage
Water contamination by sulfuricacid produced by seepage through sulfur-bearing spoil and tailings fromcoal and metal mining
acid rain
The acidity in rain due to gases from internalcombustion 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 epicenternear a larger earthquake.
agate
A siliceous rock with alternating bands of chalcedonyand variously colored chert.
alluvial fan
Land counterpart of a delta . An assemblageof sediments marking place where a stream moves from a steep gradientto a flatter gradient and suddenly loses transporting power. Typicalof arid and semiarid climates but not confined to them.
alpha decay
The process of radioactive decay in whichthe nucleus of an atom emits an alpha particle. The new atom’s atomicnumber is lower by two and its atomic mass number is reduced by four.
alpha particle
A particle consisting of two protonsand two neutrons, produced during alpha decay. Identical to the nucleusof a 4He atom.
alpine glacier
see valley glacier.
amygdaloidal
A textural term describing volcanic rocksthat contain numerous amygdules.
amygdule
A gas cavity (vesicle ) in volcanic rock thathas been filled with mineral matter such as calcite, chalcedony, orquartz.
andesite
A fine-grained volcanic rock of intermediatecomposition, consisting largely of plagioclase and one or more maficminerals.
andesite line
The geographic boundary between rocksof the Pacific Basin, which are basaltic, and those around the rim ofthe basin, which are in part andesitic.
angle of incidence
The angle at which a ray of energyapproaches a surface.
angle of reflection
The angle at which a reflectedray of energy leaves a surface.
angle of refraction
The angle at which a refractedray of energy leaves a surface after passing through it.
angle of repose
The maximum angle at which loose materialwill come to rest when added to a pile of similar material.
angular unconformity
An unconformity in which the bedsbelow the unconformity dip at a different angle than the beds aboveit.
anion
An ion with a negative electrical charge. Thatis, an atom that has gained one or more electrons.
anticline
A fold that is convex upward, or that hadsuch an attitude at some stage of its development. compare syncline.
aphanitic
A textural term meaning "fine-grained"that applies to igneous rocks.
aquifer
A permeable region of rock or soil throughwhich ground water can move.
aquitard
A material of low permeability that greatlyslows the movement of ground water.
arch
Forms along a coast as wave erosion cuts througha headland.
Archean
An eon of geologic time extending from about3.9 billion years to 2.5 billion years ago.
arête
A narrow, saw-toothed mountain ridge developedby glacier erosion in adjacent cirques .
arkose
A sedimentary rock formed by the cementationof sand-sized grains of feldspar and quartz.
artesian well
A well in which the water in the aquiferis under pressure that raises the water above the point that the wellfirst encounters it.
assemblage
The collection of minerals that characterizea rock or a facies.
asthenosphere
The weak or "soft" zone inthe upper mantle just below the lithosphere , involved in plate movementand isostatic adjustments. It lies 70 to 100 km below the surface andmay extend to a depth of 400 km. Corresponds to the seismic low-velocityzone .
astronomic theory of glaciation
A theory based on thechanging position of the Earth in its orbit around the Sun.
asymmetric rock knob or hill
Bedrock forms with a gentleslope on one side created by glacial abrasion and a steep slope on theopposite 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 particlethat has the chemical characteristics of a particular chemical element.It contains a nucleus of protons and neutrons surrounded by a cloudof electrons.
atomic mass number
The sum of the number of protonsand the number of neutrons in an atom. Approximately equal to the massof the atom.
atomic number
The number of protons in an atom, a quantitythat determines which element the atom represents. Example: all atomsof oxygen have 8 protons.
aureole
A zone surrounding an igneous intrusion, inwhich contact metamorphism has taken place.
authigenesis
The process by which new minerals formin a sediment or sedimentary rock during or after deposition.
axial plane
A geometric plane that intersects the troughor crest of a fold in such a way that the limbs of the fold are moreor less symmetrically arranged with reference to it.
axis
The line formed by the intersection of the axialplane of a fold with a bedding plane, marking where the bed shows itsmaximum curvature.
back swamp
A swamp that forms in the low lying floodplain behind a levee.
back-arc basin
The region between an island arc andthe continental mainland, commonly with at least some oceanic cruston its floor.
backshor
e Lies between high tide mark and the footof the beach dune or the limit of effective wave action.
banded iron formation
(BIF) A sedimentary mineral deposit dominatedby iron oxides, carbonates, or silicates that were deposited chemicallyfrom seawater. Most BIFs were formed between 2.5 and 3.5 billion yearsago. Their formation is related to the rise of oxygen in the atmosphere.
bankfull stage
A stream discharge that just fills thestream channel.
bar 1
A mass of sand, gravel, or alluvium depositedon 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 sealevel.
barchan
A crescent-shaped sand dune with horns pointingdownwind.
barrier beaches or islands
Long narrow beaches separatedin many places from the mainland by lagoons.
barrier reef
A coral reef separated from the mainlandby a lagoon.
basalt
A dark colored extrusive igneous rock composedchiefly of calcium plagioclase and pyroxene. Extrusive equivalent ofgabbro, 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 thestream cannot cut. A temporary base level along a stream, such as alake may be removed by stream action . Ultimate base level is the ocean.
basin
A synclinal structure, roughly circular in itsoutcrop pattern, in which beds dip gently toward the center from alldirections.
batholith
A large, discordant, intrusive body of igneousrock.
bauxite
A rock composed of aluminum hydroxides andimpurities in the form of silica, clay, silt, and iron hydroxides. Aresidual 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 collectbetween low and high tide marks.
beach replenishment
Rebuilding a beach by adding sandto it.
bed load
Material in motion along a stream bed.
bedding
A collective term used to signify presenceof beds, or layers, in sedimentary rocks and deposits.
bedding plane
Surface separating layers of sedimentaryrocks and deposits. Each bedding plane marks termination of one depositand beginning of another of different character, such as a surface separatinga sandstone bed from an overlying mudstone bed. Rock tends to breaksor separate, readily along bedding planes.
bedrock
Any solid rock exposed at the Earth’s surfaceor overlain by unconsolidated material.
beheaded stream
The headwaters of a stream that havebeen captured by another stream. compare stream piracy .
berm
A small terrace in the backshore area of the coastwith its terrace facing seaward.
beta decay
The process of radioactive decay in whicha neutron loses a beta particle, which is physically identical to anelectron. This increases the atomic number of the atom by one by turningthe neutron into a proton. The atom’s atomic mass number stays the samebecause the total number of protons and neutrons remain the same. The most common form of radioactive decay.
BIF
see banded iron formation
binding energy
The energy that holds the particlesin 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 thelife processes of plants or animals.
biogenic sedimentary rock
A sedimentary rock composedprimarily of biogenic sediments.
bioturbation
The turning and mixing of sediments byorganisms.
black smoker
A vent on the seafloor from which hydrothermalfluids are emitted. Upon mixing with seawater and cooling, the fluidsprecipitate a cloud of fine-grained sulfide minerals that resemblesa cloud of black smoke.
blind valley
A valley in karst that ends abruptly downstreamat 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 thebody of the Earth, rather than along its surface. compare surface wave.
bond
(ionic, covalent, Van der Waals, metallic) seechemical bond
bottomset bed
Layer of fine sediment deposited in abody of standing water beyond the edge of a growing delta and whichis eventually built over by the advancing delta. Similarly bottomsetbeds may accumulate in the wind shadow of a sand dune and be preservedbeneath it as the dune advances.
boudinage
A structure in which brittle beds boundedby 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 sourceeasily recognizable.
boundary
The tectonic region in which two plates meet.compare margin.
Bowen's Reaction Series
A series of minerals formedduring crystallization of a magma, in which the formation of mineralsalters the composition of the remaining magma. Mafic minerals comprisea discontinuous series, in which successive minerals form at the expenseof early-formed ones. The plagioclase feldspars form in a continuousseries, in which the composition of plagioclase becomes progressivelysodium rich, but the crystal structure of the mineral does not change.
braided stream
A stream with a complex tangle of convergingand diverging channels separated by sand bars or islands.
branch work cave
Cave with passage ways formed alongbedding planes and with an areal pattern similar to that of surfacestreams.
breakwater
A protective wall built offshore and usuallyparallel to the shore.
breccia
A clastic rock in which the gravel-sized particlesare angular in shape and make up an appreciable volume of the rock.
breeder reactor
A nuclear reactor in which 238U or232Th, which are not easily fissionable, absorb neutrons to become atomsof 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 deformspermanently by fracturing.
brittle limit
The stress limit beyond which a materialfractures, rather than behaving in a ductile or elastic fashion.
burial metamorphism
Takes place in an environment wherepressure and temperature are barely more intense than during diagenesis, typically in a deepening sequence of sediments.
calcarenite
A sandstone in which the sand-sized grainsare calcite.
caldera
A large, basin-shaped volcanic depression,more or less circular in form. Typically steep-sided, found at the summitof a shield volcano .
