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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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outer core The outermost part of the core. It is liquid, about1,700 km thick, and separated from the inner, solid core by a transitionzone about 565 km thick.
outwashBeds of sand and gravel laid down by glacial melt water
outwash plain A plain underlain by outwash.
overbank deposits Sediments deposited from flood wateron the flood plain.
overturned fold An inclined fold in which one limbhas been tilted beyond the vertical, so that the stratigraphic sequencewithin it is reversed. compare inclined fold.
oxbow An abandoned meander .
oxbow lake A lake in an abandoned meander.
oxidation The decomposition process by which iron orother metallic elements in a rock combine with oxygen to form residualoxide minerals.
ozone hole Decrease of ozone in the stratosphere.
P- wave (primary wave, compressional wave) A seismicbody wave that involves particle motion, alternating compression andexpansion, in the direction of wave propagation. It is the fastest seismicwave. compare S-wave.
P- wave (primary wave, compressional wave) A seismicbody wave that involves particle motion, alternating compression andexpansion, in the direction of wave propagation. It is the fastest seismicwave. compare S-wave.
pahoehoe A Hawaiian term for a basaltic lava flowwith a smooth, or ropy surface. compare aa.
paleomagnetism Study of the Earth’s past magnetismas it is recorded in the rocks.
paleosol A buried soil horizon of the geologic past.
Paleozoic An era of geologic time lasting from 570to 245 million years ago.
Pangea A supercontinent that existed from about 300to 200 million years ago, and included most of the continental crustof the Earth.
parabolic dune A sand dune that is parabolic in planwith slip face convex downwind.
parent A radioactive element whose decay produces stabledaughter elements.
partial melting The igneous process in which a rockbegins to melt at the lower end of its melting interval, yielding amagma with a chemical composition different from the bulk compositionof the parent rock.
pascalA unit of pressure, equal to 1/100,000 of abar.
pedalfer A generic term used to describe the soilstypically formed in a humid region. Characteristically have an accumulationof iron and aluminum oxides and hydroxides.
pedocal A generic term used to describe the soils typicallyfound in an arid or semiarid region. Characteristically have an accumulationof carbonates, particularly calcite.
pegmatite An extremely coarse-grained igneous rockwith interlocking crystals, usually with a bulk chemical compositionsimilar to granite but commonly containing rare minerals enriched inlithium, boron, fluorine, niobium, and other scarce metals. Pegmatitesare also the source for many gem-quality precious and semiprecious stones.
pegmatitic Having the texture of a pegmatite.
pelagic ooze A deep ocean sediment consisting of atleast 30% skeletal remains of calcareous or siliceous microorganisms,the rest being clay minerals.
Peléan eruption A type of volcanic eruption characterizedby nuées ardentes and the development of lava domes .
peneplain Low gently, rolling landscapes produced bylong-continued erosion.
perched water table A water table that develops ata higher elevation than the main water table. <peridotite An ultramaficigneous rock, the major constituent of the mantle.
periglacial Refers to conditions in a near glacialclimate.
period In the geologic time scale a unit of time lessthan an era and greater than an epoch. Example: The Tertiary periodwas the earliest period in the Cenozoic era and included, among others,the Eocene epoch.
permafrost Soil conditions prevailing in area whosemean annual temperature is 0o C.
permafrost table The depth in a permafrost region atwhich the maximum temperature reaches 0o C.
permeability The capacity of material to transmit wateror other fluids.
petroleum A general term including both oil and naturalgas.
phaneritic A textural term meaning "coarse-grained"that applies to igneous rocks.
Phanerozoic the most recent eon of geologic time beginning570 million years ago and continuing to the present.
phenocryst Any relatively large, conspicuous crystalin a porphyritic igneous rock. compare porphyroblast.
phyllite A metamorphosed mudstone with a silky sheen,more coarse-grained than a slate and less coarse-grained than a schist.
piedmont glacier A glacier that spreads out at thefoot of mountains, formed by the coalescence of two or more valley glaciers.
pillow A structure observed in certain igneous rocksextruded into water, characterized by discontinuous, close-fitting,pillow-shaped masses, commonly 30 to 60 cm across.
pipe A vertical conduit through the Earth’s crust belowa volcano, through which magma has passed.
pirate stream A stream that captures the headwatersof another stream.
placer A surficial mineral deposit formed by mechanicalconcentration of valuable minerals from weathered debris, usually throughthe action of stream currents or of waves.
plate A rigid segment of the Earth’s lithosphere thatmoves horizontally and adjoins other plates along zones of seismic activity.Plates may include portions of both continents and ocean basins.
plate boundaries The zones of seismic activity longwhich plates are in contact. These may coincide with continental margins, but usually do not. Movement between plates is predominately horizontal,and may be divergent, or convergent, or side-by-side.
plate tectonics A theory of global tectonics accordingto which the lithosphere is divided into mobile plates. The entire lithosphereis in motion, not simply those segments composed of continental material.compare continental drift
plate triple junction A point from which three riftsemanate at roughly 120 degree angles. Example: the Afar triangle inEast Africa.
playa A broad flat desert basin, often containing anephemeral playa lake.
plucking(quarrying) A process of erosion in whichthe glacier pulls loose pieces of bed rock.
plume The movement of water along flow lines from apoint source of ground water pollution toward its eventual emergenceat the surface.
plunging fold A fold in which the axis is inclinedat an angle from the horizontal.
pluton An igneous intrusion.
pluvial lake A lake formed during a pluvial period.
pluvial period Time when a dryland area had greatereffective moisture than at present.
pocket beach Small, narrow beach, usually crescentic,at head of a bay or small inlet.
point bar Accumulations of sand and gravel depositedin slack water on inside of a winding or meandering river.
polar deserts Deserts in which most moisture is lockedup in ground ice and unavailable as liquid water.
polar glacier A glacier whose temperature throughoutis always below freezing.
polish A smooth, polished surface imparted to somerock types by glacier abrasion.
polymetamorphism A series of events in which two ormore metamorphic episodes have left their imprint on the same rocks.
polymorphism The circumstance in which two minerals with differentcrystalline structures have identical chemical compositions. Example:Diamond and graphite.
porosity The percentage of material occupied by pore space.
porphyritic A texture of an igneous rock in which largecrystals (phenocrysts) are set in a matrix of relatively finer-grainedcrystals or of glass.
porphyroblast A large crystal of a mineral such asgarnet or staurolite set in a matrix of much finer-grained mineralsin a metamorphic rock. compare phenocryst.
potentiometric surface The level to which water willrise in an artesian system when its confining aquitard is pierced.
pothole A hole or basin cut into bedrock of a streamby the abrasive action of pebbles and sand swirled by turbulent streamflow.
Precambrian An informal term to include all geologictime from the beginning of the Earth to the beginning of the Cambrianperiod 570 million years ago.
precession of the equinox The wobble of the Earth asit spins changes the direction in which its axis of rotation points.One wobble takes about 23,000 years.
pressure meltingThe phenomenon causing increased meltingof ice by increase of pressure.
Principle of faunal and floral succession Groups ofanimals and plants have succeeded one another in a definite and discernibleorder.
prograde A succession of metamorphic conditions, eachof which is at a higher temperature and/or pressure than the precedingone.
Proterozoic The geologic eon lying between the Archeanand Phanerozoic eons, beginning about 2.5 billion years ago and endingabout 0.57 billion years ago.
proton A fundamental particle of matter. Provides apositive charge in the nucleus of an atom.
pyroclastic Pertaining to clastic material formed byvolcanic explosion or aerial expulsion from a volcanic vent.
quarrying 1. The process by which building stone, usuallyin blocks or sheets, is extracted from the Earth.. 2. see plucking
quartz arenite A sandstone in which the sand grainsare predominantly quartz.
quartzite A metamorphic rock consisting largely ofinterlocking quartz grains; the metamorphic equivalent of a sandstoneor chert.
radial drainage A pattern in which streams radiateoutward from a high central zone.
radiation Heat transport without the intervention ofmatter, as in the transport of heat from the Sun to the Earth. compareconduction , convection .
radioactivity The spontaneous decay of the nucleusof an element. It involves the change in the number of protons in thenucleus and therefore creates an atom of a new element.
radiocarbon 14C derived from 14N as cosmic ray bombardmentadds a neutron to its nucleus and the nucleus emits a proton. Radiocarbondecays back to 14N by beta decay . Half life is 5730 ± 30 years.
rain shadow deserts Deserts formed by blocking moisture-bearingwinds with mountain barriers.
ramp The planar surface sloping seaward from the footof the shore face.
rapids Turbulent stream water flow down a steep gradient,but not as steep as in a waterfall.
Rayleigh wave A type of seismic surface wave in whichparticles follow a backward elliptical orbit in a vertical plane.
reaction rim A peripheral zone around a mineral grain,composed of another mineral.
recessional moraine Ridges of glacial till markinghalt and slight readvance of glacier during its general retreat.
rectangular drainage A pattern in which a stream andits tributaries follow courses marked by nearly right angle bends.
recumbent fold A fold in which the axial plane is horizontal.
refraction 1. Bending of waves or rays of energy, e.g.seismic waves. 2. As applies to the near shore environment, the bendingof wave crests as they approach the shore.
regional metamorphism Metamorphism affecting an extensiveregion, associated with orogeny .
regolith A layer of unconsolidated fragmental rockmaterial.
rejuvenation Renewed stream erosion, generally as theresult of uplift. Generates features of youthful topography on a landscapethat was previously worn down to a base level.
relative time Dating of rocks and geologic events bytheir positions in chronological order without reference to number ofyears before the present.
remanent magnetism Magnetism acquired by a rock assome time in the past.
reserves That portion of the resources for a valuablemineral commodity that can be extracted from the Earth at a profit today.
reservoir rock Any porous and permeable rock that yieldsoil or natural gas.
residual (resistant) mineral A mineral that persistsin soil after weathering, either because it was resistant to weatheringor because it was formed during the weathering process.
residual soilA soil presumed to have developed inplace as the product of decomposition and disintegration of bedrock.
resources The reserves of a valuable mineral commodityplus all other mineral deposits that may eventually become available,even those that are presumed to exist but have not yet been discoveredand those that are not economically or technologically exploitable atthe moment. The total mineral endowment ultimately available for extraction.
