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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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lava lake A lake of lava, usually basaltic, in a volcaniccaldera.
layered complex An intrusive igneous body in whichthere are layers of varying mineral content.
levees Banks of sand and silt along stream bank builtby deposition in small increments during successive floods.
limb The portions of a fold that are away from thehinge; the "sides" of the fold.
limestone A sedimentary rock composed mostly of themineral calcite, CaCO3.
linear dune Long, straight dune with slip faces oneach side.
lineation A general term applying to any linear featurein a metamorphic rock.
liquefaction The transformation of a soil from a solidto a liquid state as the result of increased pore pressure.
lithic sandstone see graywacke.
lithification The process by which an unconsolidateddeposit of sediments is converted in to solid rock. Compaction, cementationand recrystallization are involved.
lithophile Said of an element that has a greater chemicalaffinity for silicate rocks than for sulfides or for a metallic state.Example: Aluminum.
lithosphere The rigid outer shell of the Earth. Itincludes the crust and uppermost mantle and is on the order of 100 kmin thickness.
lithostatic stress The confining (non-directed) pressureimposed by the weight of overlying rock.
littoral current see longshore current.
load Of a stream, the amount that it carries at anyone time.
loess Deposits of wind-borne dust.
longshore current(littoral current) A current thatflows parallel to the shore just inside the surf zone. Also called thelittoral current.
longshore drift The general movement of sediment parallelto the shoreline. Waves generally carry sediment up the shore face atan angle to the shoreline, but carry it straight out again, resultingin a net longshore displacement.
Love wave A seismic surface wave that has a horizontal(side-to-side) component but no vertical component.
low level nuclear waste(TRU) Comes largely from nationaldefense utilities and includes contaminated lab coasts, gloves, andlaboratory equipment.
low velocity zone The seismic region within the uppermantle that corresponds to the asthenosphere .
luster The manner in which light reflects from thesurface of a mineral, described by its quality and intensity.
mafic Referring to a generally dark-colored igneousrock with significant amounts of one or more ferromagnesian minerals,or to a magma with significant amounts of iron and magnesium.
magma Molten rock, containing dissolved gases and suspendedsolid particles. At the Earth’s surface, magma is known as lava .
magma ocean A global-scale ocean of magma, accordingto some calculations several hundred kilometers deep, thought to haveexisted during the final stages of accretion as the Earth was forming.
magnetic anomaly The amount by which a measurementof the local magnetic field intensity exceeds or falls below the intensityof the global magnetic field.
magnetic chron Time during which magnetic polarityis dominantly normal or dominantly reversed.
magnetic declination Angle of divergence between truenorth and magnetic north. Measured in degrees east or west of true,or geographic north.
magnetic equator Lies half way between the north andsouth magnetic poles.
magnetic inclination The angle of dip of the compassneedle as it varies from horizontal at the magnetic equator to verticalat the magnetic poles.
magnetic polarity The direction, north (normal) orsouth (reversed) that a magnetic needle points.
magnetic polarity The direction, north (normal) orsouth (reversed), that a magnetic compass needle points.
magnetic polarity time scale A chronology based onthe shifting polarity of the Earth’s magnetic field.
magnetic pole The point on the Earth’s surface wherea magnetic needle points vertically downward (north magnetic pole) orvertically upward (south magnetic pole).
magnetic stratigraphy A stratigraphic sequence basedon the magnetic polarity of the rocks.
magnetic subchron A period during a magnetic chronwhen the magnetic polarity is the opposite from that of the magneticchron.
magnitude A measure of the strength of an earthquakebased on the amount of movement recorded by a seismograph . compareRichter scale.
mantle That portion of the Earth below the crust andreaching to about 2,780 km, where a transition zone of about 100 kmthickness separates it from the core.
mantle plume A hypothetical column of hot, partiallymolten material that rises from an indeterminate depth in the mantleand is thought by some geologists to provide a driving force for platemovement. compare hot spot .
marble A metamorphic rock composed largely of calcite.The metamorphic equivalent of limestone.
margin The tectonic region that lies at the edge ofa continent, whether it coincides with a plate boundary or not.
mass movement The downslope movement of material underthe influence of gravity.
maze cave Caves in which passageways have interconnectingloops that form a maze-like pattern.
meander A sharp bend, loop or turn in a stream’s course.When abandoned, called a meander scar or an oxbow.
medial moraine Formed by the merging of lateral morainesas two valley glaciers join.
mélange(clastic wedge) A mappable body of rock characterizedby blocks and fragments of all sizes, embedded in a sheared matrix.A tectonic mélange commonly forms in the upper portions of a subductionzone, where crustal rock is crushed and sheared.
mélange(clastic wedge) A mappable body of rock characterizedby blocks and fragments of all sizes, embedded in a sheared matrix.A tectonic mélange commonly forms in the upper portions of a subductionzone, where crustal rock is crushed and sheared.
mesosphere A zone in the Earth between 400 and 670km below the surface separating the upper mantle from the lower mantle.
Mesozoic An era of time during the Phanerozoic eonlasting from 245 million years ago to 66.4 million ago.
metal porphyry deposit A mineral deposit geneticallyrelated to a pluton of porphyritic rock, commonly granodiorite. Scarcemetals are typically enriched by the passage of hydrothermal fluidsthrough rocks surrounding the intrusion, with the result that a metal-richhalo forms there.
metamorphic facies A set of metamorphic mineral assemblages,repeatedly associated in space and time, such that there is a constantand therefore predictable relationship between mineral composition andchemical composition. That relationship is a consequence of conditionsof temperature and pressure under which the assemblages are stable.
metamorphic rock A rock changed from its original formand/or composition by heat, pressure, or chemically active fluids, orsome combination of them.
metamorphic zone A mappable region in which rocks havebeen metamorphosed to the same degree, as evidenced by the similarityof mineral assemblages in them.
metamorphism The processes of recrystallization, texturaland mineralogical change that take place in the solid state under conditionsbeyond those normally encountered during diagenesis.
metasomatism The metamorphic processes that occur asa result of the passage of chemically active fluids through a rock,adding to or removing constituents during metamorphism.
microplate see terrane
migmatite A composite rock composed of igneous andmetamorphic materials, the result of partial melting at the upper limitof metamorphism.
Milankovitch curve(ETP curve) A graph representingthe amount of solar radiation received at the Earth’s surface at a particularlatitude and time and based on the variations in the Earth’s orbitalmotions.
mineralA naturally occurring inorganic solid thathas a well-defined chemical composition and in which atoms are arrangedin an ordered fashion.
mineral deposit Any natural concentration of a valuablematerial in the Earth’s crust, whether that material can be extractedprofitably or not.
Modified Mercalli Scale A commonly used scale of earthquakeintensity.
Mohorovicic discontinuity(Moho) The sharp seismicvelocity discontinuity that separates the crust and the mantle.
Mohs scale The ten-point scale of mineral hardness , keyedarbitrarily to the minerals talc, gypsum, calcite, fluorite, apatite,orthoclase, quartz, topaz, corundum, and diamond.
molecule The smallest unit of matter that has the chemicaland physical properties of a particular chemical compound.
momentum transfer In a rock slide the forward transfer of energyby the collision of one block of rock with the next block forward. Theprocess makes possible progressively more rapid movement of materialin downslope positions.
monocline A simple fold, described as a local steepening instrata with an otherwise uniform dip.
moraine Landform made largely of till.
mountain glacier see valley glacier .
mud cracks Cracks, generally polygonal, caused by theshrinking of a deposit of clay or silt under surface conditions.
mudflow Form of mass movement similar to a debris flowbut containing less rock material.
mudstone A fine-grained detrital sedimentary rock madeup of clay- and silt-sized particles.
mylonite A chert-like rock without cleavage but witha banded or streaky structure produced by extreme shearing of rocksthat have been pulverized and rolled during intense dynamic metamorphism.
nappe A sheet of rock that has moved over a large horizontaldistance by thrust faulting, recumbent folding, or both, so that itlies on rocks of markedly different age or lithologic character.
neck cutoff Occurs as a river cuts through the narrowneck of a meander. Sometimes called simply a "cutoff."
neutron A particle in the nucleus of an atom, whichis without electrical charge and with approximately the same mass asa proton.
névé see firn.
névé see firn.
nivation Erosion beneath and around edges of a snowbank. Results can foreshadow a cirque.
nodule A small, irregular, knobby-surfaced rock bodythat differs in composition from the rock that encloses it. Formed bythe replacement of the original mineral matter. Quartz in the form offlint or chert is the most common component. Most common in limestoneand dolostone.
nonclastic A term applied to sedimentary rocks thatare not composed of fragments of pre-existing rocks or minerals. Theterm "crystalline" is more commonly used.
nonconformity An unconformity that separates profoundlydifferent rock types, such as sedimentary rocks from metamorphic rocks.
normal fault A dip-slip fault on which the hangingwall block is offset downward relative to the foot wall block . comparereverse fault .
normal polarity Time when the compass needle pointsto the magnetic north pole.
north magnetic pole The point on the Earth where thenorth-seeking end of a magnetic needle, free to swing in space, pointsdirectly down.
nuclear powerPower generated by controlled fissionor (potentially) fusion reactions, the heat from which is used to producesteam and drive turbines.
nucleus(atomic) The center of an atom, containingboth protons and (except for 1H) neutrons.
nuée ardente(fiery cloud) A dense, hot (sometimesincandescent) cloud of volcanic ash and gas produced in a Pelean eruption.
oblique-slip fault A fault with both dip-slip and strike-slipcomponents of movement.
obliquity of the Earth’s ecliptic Tilt of the Earth’srotational axis in relation to the plane in which the Earth circlesto Sun. Cycles from about 21.5o through 24.5o and back to 21.5o every41,000 years.
oceanic crust That part of the crust underlying theocean basins. Composed of basalt and having a thickness of about 5 km.
oil shale A mudrock that will yield liquid or gaseoushydrocarbons upon distillation.
oolite Spheroidal grains of sand size, usually composedof calcite and thought to have formed by inorganic precipitation.
open pit mining Surficial mining, in which the valuablerock is exposed by removal of overlying rock or soil.
ophiolite A suite of mafic and ultramafic rocks andassociated marine cherts and their metamorphic equivalents.
ore The naturally occurring material from which a mineral orminerals of economic value can be extracted at a profit.
ore deposit A continuous well-defined mass of material of sufficientore content to make extraction economically feasible. compare mineraldeposit .
original horizontality Refers to the condition of beds or strataas being horizontal or nearly horizontal when first formed.
orogen Linear to arcuate in plan, intensely deformed crustalbelt associated with mountain building. compare craton .
orogeny The process of mountain building.
oscillation ripple mark A symmetric ripple mark formed by waves,which move water back and forth. compare current ripple mark .
