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Iowa State University - Glossary of geologic terms Catégorie: Géographie Date & Pays: 29/04/2015, Us. Mots: 835

caliche
Gravel, sand, or desert debris cemented bycalcium carbonate, an accumulated product of chemical weathering ina dry climate. compare claypan, fragipan, hardpan.

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

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

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

capillary water
Water in the zone of aeration heldto soil particles by surface tension of the water molecules for eachother and for the soil particles.

carbonate conservation depth
The water depth below which thecalcium carbonate produced in the ocean is completely dissolved. Thereis no calcium carbonate deposition below this level.

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

cataclastic metamorphism
Takes place in an environmentwhere intense pressure due to shearing is common, as in a major faultzone.

cation
An ion that has a positive electrical charge.That is, an atom that has lost one or more electrons.

cave
A natural open space underground, large enoughfor a person to enter. Most commonly occur by the dissolution of solublerocks, generally limestone.

cementation
Process by which a binding, or cementing,agent is precipitated in spaces among individual particles of a deposit.Common cementing agents are calcite, quartz, and dolomite.

Cenozoic
The current geologic era, which began 66.4million years ago and continues to the present.

chain reaction
A self-sustaining nuclear reaction,made possible when neutrons released by fission of some atoms in a nuclearreactor strike other atoms, causing them to fission as well.

chalcedony
Occurs in limestone, dolostone, and mudstones.

chalcedony
A cryptocrystalline form of quartz, microscopicallyfibrous with waxy luster. May be transparent or translucent, and witha uniform tint of white, gray, pale blue, and, less often, black.

chalk
A variety of limestone made up in part of biochemicallyderived calcite, in form of skeletons or skeletal fragments of microscopicoceanic plants and animals mixed with fine-grained calcite depositsof biochemical or inorganic-chemical origin.

chemical bond
The interactions among the electronsof atoms that hold atoms together to form chemical compounds. If electronscluster primarily around one atom of a pair, the bond is ionic . Ifthey are shared more or less equally, it is covalent . If electronsmove freely between atoms over an extended region, the bond is metallic.A weak electrostatic bond due to uneven distribution of electrons aroundatoms or groups of atoms is a Van der Waals bond. chemical element A fundamental substance that cannot be further refinedor subdivided by chemical means. All atoms of a chemical element havethe same number of protons.

chemical remanent magnetism
Acquired as magnetic mineralsform and align themselves to the global magnetic field during diagenesisof a sedimentary deposit.

chemical sediment
Sediment formed by chemical precipitationfrom water. Example: halite precipitated as the result of the evaporationof sea water.

chemical sedimentary rock
A sedimentary rock made upof chemical sediments. Example: rock salt.

chemical weathering
see decomposition

chert
A cryptocrystalline form of quartz, microscopicallygranular. Occurs as nodules and as thin, continuous layers. Duller,less waxy luster than

chlorofluorocarbons
(CFC) Gases that can be dissociatedby solar radiation, which releases chlorine, which in turn destroysozone.

chute cutoff
A narrow "short cut" acrossa meander bend, formed in flood as the main stream flow is divertedinto a trough between point bars. Sometimes called simply a ‘"chute".

cinder cone
A conical volcano formed by the accumulationof pyroclastic debris around a vent.



cirque
A steep-walled hollow in a mountain side, shapedlike an amphitheater, or bowl, with one side partially cut away. Placeof origin of a mountain glacier.

clastic
Refers to rock or sediments made up primarilyof broken fragments of pre-existing rocks or minerals.

clay 1
The name for a family of finely-crystallinesheet silicate minerals. 2. Fine-grained soil consisting of mineralparticles, not necessarily clay minerals, that are less than 0.074 mmin their maximum dimension.

claypan
A layer of stiff, compact, relatively imperviousclay which is not cemented. compare caliche , fragipan, hardpan.

cleavage
1. of a mineral: The tendency of a mineralto split along planes determined by the crystal structure. 2. of a rock:see slaty cleavage

co-product
A mineral commodity that is recovered froma mining operation for some other mineral product. Example: Platinumis commonly a co-product of nickel mining.

coal
Sedimentary rock composed of combustible matterderived from the partial decomposition of plant material.

coast
A narrow strip of land along the margin of theocean extending inland for a variable distance from low water mark.

col
Mountain pass formed by enlargement of two opposingcirques until their head walls meet and are broken down.

column
Pillar formed as a stalactite and stalagmitemeet.

columnar jointing
The type of jointing that breaksrock, typically basalt, into columnar prisms. Usually the joints forma more or less distinct hexagonal pattern.

compaction
Reduction of pore space between individualparticles as the result of overlying sediments or of tectonic movements.

competence
The maximum size of particle that a streamcan carry.

composite
volcano see stratovolcano

Comprehensive Soil Classification System
(CSCS) Theclassification system in most common use by North American soil scientists.Categories are based on the chemical and physical characteristics ofa soil. compare USDA Soil Classification System .

compression
Squeezing a material from opposite directions.

concordant
Lying parallel to, rather than cutting acrosssurrounding strata.

concretion
A compact mass of mineral matter, usuallyspherical or disk-like in shape and embedded in a host rock of differentcomposition. They form by precipitation of mineral matter about a nucleussuch as a leaf, or a piece of shell of bone.

conduction
Heat transport by direct transfer of energyfrom one particle to another, without moving the particle to a new location.compare convection , radiation .

cone of depression
A downward distortion or dimplein the water table that forms as a well pumps water faster than it canflow through the aquifer.

conglomerate
A clastic sedimentary rock composed oflithified beds of rounded gravel mixed with sand.

Constancy of Interfacial Angles
The statement thatthe angles between congruent crystal faces on samples of a single mineralare always identical. A consequence of, and therefore evidence for theexistence of crystalline structure in minerals.

contact metamorphism
Metamorphism genetically relatedto the intrusion (or extrusion) of magmas and taking place in rocksat or near their contact with a body of igneous rock.

continental
ice glacier An ice sheet that obscuresall but the highest peaks of a large part of a continent.

continental arc
A belt of volcanic mountains on thecontinental mainland that lie above a subduction zone. compare islandarc .

continental crus
t The part of the crust that directlyunderlies the continents and continental shelves. Averages about 35km in thickness, but may be over 70 km thick under largest mountainranges.

continental deserts
Located in continental interiorfar from moisture-bearing winds.

continental divide
A major drainage divide separatingthe drainage to one ocean from another.

continental drift
The theory that explained the relative positionsand shapes of continents, the formation of mountains, and other large-scalegeologic phenomena as results of the lateral movement of continents.The crust of ocean basins was assumed to be relatively immobile. compareplate tectonics , sea floor spreading .

continental rise
The portion of the continental marginthat lies between the abyssal plain and the continental slope. The continental rise is underlain by crustal rocks of the ocean basin.

continental shelf
The portion of the continental marginthat extends as a gently sloping surface from the shoreline seawardto a marked change in slope at the top of the continental slope . Seawarddepth averages about 130 m.

continental slope
That part of the continental marginthat lies between the continental shelf and the continental rise. Sloperelatively steep, 3o – 6o. The continental slope is underlain by crustalrocks of the continent.

convection
Heat transport by moving particles, andthe thermal energy that they carry, to a new location. compare conduction, radiation .

convection cell
A cyclical pattern of movement in afluid body such as the ocean, the atmosphere, or the Earth’s mantle,driven by density variations which in turn are the result of differencesin temperature from one part of the fluid to another.

convergent boundary
A boundary between two plates ofthe Earth’s crust that are pushing together.

coquina
A coarse-grained, porous variety of clasticlimestone made up chiefly of shells and shell fragments.

core
Innermost zone of Earth. Consists of two parts,an outer liquid section and an inner solid section, both chiefly ofiron and nickel with about 10 percent lighter elements. It is surroundedby the mantle.

correlation
Process of establishing contemporaneityof rocks or events in one area with rocks or events in another area.

crater
1. A steep-walled, usually conical depressionat the summit or on the flanks of a volcano, resulting from the explosiveejection of material from a vent. 2. A bowl-shaped depression with araised, overturned rim produced by the impact of a meteorite or otherenergetic projectile.

craton
The stable portions of the continents that haveescaped orogenic activity for the last 2 billion years. Made predominantlyof granite and metamorphic rocks. compare orogen .

creep
1. The very slow, generally continuous downslopemovement of soil and debris under the influence of gravity. 2. The movementof sand grains along the land surface.

crevasse
1. Breach in a natural levee . 2. Deep creviceor open fracture in glacier ice.

cross-bedding
see inclined bedding .

cross-cutting relationships
Geologic discontinuitiesthat suggest relative ages: A geologic feature is younger than the featureit cuts. Thus, a fault cutting across a rock is younger than the rock.

crust
The upper part of the lithosphere , divided intooceanic crust and continental crust .

crystal
The multi-sided form of a mineral, boundedby planar growth surfaces, that is the outward expression of the orderedarrangement of atoms within it.

crystal settling
Gravitational sinking of crystalsfrom the liquid in which they formed, by virtue of their greater density.A type of igneous differentiation.

crystal structure
The regular and repeated three-dimensionalarrangement of atoms or ions in a crystal.

crystalline
1. Having a crystal structure. 2. Whenreferring to sedimentary rocks, crystalline designates a texture inwhich mineral crystals have formed in an interlocking pattern. see nonclastic.3. As a generic term, geologists use the term "crystalline rocks" as a rough synonym for "igneous ormetamorphic rocks".

cumulate
An igneous rock that forms by crystal settling.

Curie point
The temperature above which a mineral losesits magnetism.

current ripple mark
An asymmetric ripple mark formedby wind or water moving generally in one direction. Steep face of ripplefaces in direction of current. compare oscillation ripple mark .

cyclothem
A series of beds, of interest because theyinclude coal, which were associated with unstable shelf or interiorbasin conditions in which alternating marine transgressions and regressionsoccurred.

daughter
An atom that results from the radioactivedecay of a parent atom.

debris flow
Fast-moving, turbulent mass movement witha high content of both water and rock debris. The more rapid debris flows rival the speed of rock slides.

decay rate
The rate at which a population of radioactiveatoms decays into stable daughter atoms. Rate often expressed in termsof half life of the parent isotope .

decomposition
(chemical weathering) Weathering processesthat are the result of chemical reactions. Example: the transformationof orthoclase to kaolinite.

deflation
A process of erosion in which wind carriesoff particles of dust and sand.

dehydration
Any process by which water bound withina solid material is released. Example: Gypsum (CaSO4·2H2O) becomes anhydrite(CaSO4) by dehydration.

delta
An assemblage of sediments accumulated wherea stream flows into a body of standing water and its velocity and transportingpower are suddenly reduced. . A "delta plain" is the uppersurface of a delta.

dendritic drainage
A stream pattern that, when viewedon a map or from the air, resembles the branching pattern of a deciduous tree such as a maple or oak.

denudation
The sum of the processes that result inthe wearing away or the progressive lowering of the Earth’s surfaceby weathering, erosion, mass wasting, and transportation.

depositional environment
The nature of the environmentin which sediments are laid down. They are immensely varied and mayrange from the deep ocean to the coral reef and the glacial lake ofthe high mountains. The nature of the depositional environment may be deduced from the nature of the sedimentsand rock deposited there.

depositional remanent magnetism
Develops as magneticminerals settle through water and align themselves in the Earth’s magneticfield.

desert pavement
A lag accumulation of pebbles or bouldersthat cuts off further deflation.

desertification
A process of land degradation initiatedby human activity, particularly in the zones along the margins of deserts.

detrital sedimentary rock
A sedimentary rock made upof detrital sediments.

detrital sediments
Sediments made of fragments or mineralgrains weathered from pre-existing rocks.

diagenesis
All the physical, chemical, and biologicchanges undergone by sediments from the time of their initial deposition,through their conversion to solid rock, and subsequently to the brinkof metamorphism.

differential weathering
Weathering that occurs at differentrates, as the result of variations in composition and mechanical resistanceof rocks, or differences in the intensity of weathering processes.

differentiation
The process of developing more thanone rock type, in situ, from a common magma.

dike
A tabular igneous intrusion that cuts across thesurrounding rock.

dilatancy
An increase in the bulk volume of rock duringdeformation. Possibly related to the migration of water into microfracturesor pores.

dip
The angle that a structural surface such as a beddingplane or fault surface makes with the horizontal, measured perpendicularto the strike and in the vertical plane.

dip pole
see magnetic pole