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

geyser
A type of thermal spring which ejects waterintermittently with considerable force.

glaciation
The formation, advance and retreat of glaciersand the results of these activities.

glacier
A mass of ice, formed by the recrystallizationof snow, that flows forward, or has flowed at some time in the past.

glacier ice
Ice with interlocking crystals that makesup the bulk of a glacier.

glass
An inorganic solid in which there is no crystallinestructure .

glassy
A texture of extrusive igneous rocks that developsas the result of rapid cooling, so that crystallization is inhibited.

global warming
The prediction that climate will warmas a result of the addition to the atmosphere of humanly produced greenhousegases.

gneiss
A coarse, foliated metamorphic rock in whichbands of granular minerals (commonly quartz and feldspars) alternatewith bands of flaky or elongate minerals (e.g., micas, pyroxenes). Generallyless than 50% of the minerals are aligned in a parallel orientation.

gneissosity
The style of foliation typical of gneiss.

Gondwana
The southern portion of the late Paleozoicsupercontinent known as Pangea. It means, literally "Land of theGonds" (a people of the Indian subcontinent). The variant Gondwanalandfound in some books, therefore, is a tautology.

gouge
see fault gouge

graben
see rift

graded bedding
Type of bedding sedimentary depositsin which individual beds become finer from bottom to top.

gradient
Slope of a stream bed or hillside. The verticaldistance of descent over horizontal distance of slope.

granite
Light colored, coarse grained, intrusive igneousrock characterized by the minerals orthoclase and quartz with lesseramounts of plagioclase feldspar and iron-magnesium minerals. Underlieslarge sections of the continents.

granitic belt
A region of granitic rock, one of twocharacteristic regions within cratons .

granitization
A metamorphic process by which solidrock is converted into granite by the addition or removal of material,without passing through a magmatic stage. compare metasomatism .

gravitational heating
Planetary heating caused by theconversion of potential energy into heat. Associated with the iron catastrophe.

gravitational moisture
Water in the zone of aerationthat is moving down toward the zone of saturation.

graywacke
(lithic sandstone) A variety of sandstonecharacterized by angular-shaped grains of quartz and feldspar, and smallfragments of dark rock all

greenhouse gases
Gases (primarily water and carbondioxide, but also a variety of sulfur and nitrogen compounds and gaseoushydrocarbons) that trap the Sun’s heat in the atmosphere.

greenstone
An altered or metamorphosed mafic igneousrock that owes its dark color to the presence of chlorite, epidote,or amphiboles.

greenstone belt
A region of greenstones, one of twocharacteristic regions within cratons .

groin
A wall built out from the shore, usually at perpendicularto it to trap sand carried by longshore currents .

groove
A broad, deep, generally straight furrow carvedin bed rock by the abrasive action of debris embedded in a moving glacier.Larger and deeper than a glacial striation.



ground moraine
Till deposited from main body of glacierduring ablation.

ground water
Water beneath the Earth's surface.

ground water table
see water table .

guyot
see seamount

habit
A general term for the outward appearance ofa mineral, defined by the relative sizes and arrangement of characteristiccrystal faces.

Hadean
The oldest eon in Earth history, extending fromthe origin of the Earth to about 3.9 billion years ago.

half-life
The amount of time that it takes for onehalf of an original population of atoms of a radioactive isotope todecay.

hanging valley
A valley whose mouth is high above thefloor of the main valley to which it is tributary. Usually, but notalways, the result of mountain glaciation.

hanging wall block
The body of rock that lies abovean inclined fault plane. compare foot wall block

hardness
Resistance of a mineral to scratching, determinedon a comparative basis by the Mohs scale .

hardpan
A general term for a relatively hard layerof soil at or just below the ground surface, cemented by silica, ironoxide, calcium carbonate, or organic matter. compare caliche , claypan,fragipan.

head
(hydraulic head) The level to which ground waterin the zone of saturation will rise.

heat flow
The amount of thermal energy leaving theEarth per cm2/sec.

heave
In mass movement the upward motion of materialby expansion as, for example, the heaving caused by freezing water.

hiatus
A gap or interruption in the continuity of thegeologic record either because the record was never formed or becauseit was destroyed by erosion. It represents the time interval spannedby an unconformity .

high level nuclear waste
Radioactive waste from defenseactivities of the U. S. government and from spent fuel rods from nuclearreactors.

hinge fault
A fault along which there is increasingoffset or separation along the strike of the fault plane, from an initialpoint of no separation.

hoodoo
A column or pillar of rock produced by differentialweathering in a region of sporadic heavy rainfall, commonly facilitatedby joints and by rock layers of varying hardness.

Hooke's Law
A statement of elastic deformation, thatstrain is directly proportional to stress.

horn
The sharp spire of rock formed as glaciers inseveral cirques erode into a central mountain peak.

hot spot
A region of high heat flow on the Earth’ssurface, thought to lie above a mantle plume .

humus
The generally dark, more or less stable partof the organic matter in a soil, so well decomposed that the originalsources cannot be identified.

hydraulic conductivity
Measure of permeability in Earthmaterials.

hydraulic gradient
The slope of the water table. Measuredby the difference in elevation between two points on the slope of thewater table and the distance of flow between them.

hydrograph
Graph of variation of stream flow over time.

hydrologic system
(or hydrologic cycle) The patternof water circulation from the ocean to the atmosphere to the land andback to the ocean.

hydrolysis
A decomposition reaction involving water,in which hydrogen ions (H+) or hydroxyl ions (OH-) replace other ions.The result is a new residual mineral. Example: the addition of waterto orthoclase produces kaolinite and releases K+ and silica into solution.

ice sheet
A broad, mound-like mass of glacier ice thatusually spreads radially outward from a central zone.

ice shelf
A floating ice sheet extending across waterfrom a land-based glacier.

icecap
A small ice sheet.

igneous rock
A rock that has crystallized from a moltenstate.

inclined bedding
(cross-bedding) Bedding laid downat an angle to the horizontal, as in many sand dunes.

inclined fold
A fold whose axial plane is inclinedfrom the vertical, but in which the steeper of the two limbs is notoverturned. compare overturned fold.

inclusion
(xenolith) A fragment of older rock caughtup in an igneous rock.

index fossil
A fossil that identifies and dates thestrata in which it is typically found. To be most useful, an index fossilmust have broad, even worldwide distribution and must be restrictedto a narrow stratigraphic range.

index minera
l A mineral formed under a particular setof temperature and pressure conditions, thus characterizing a particulardegree of metamorphism.

inertia
The tendency of a body to resist acceleration. A moving body tends to keep moving at a constant speed in the samedirection, and a stationary body tends to remain in one place, unlessacted upon by an outside force.

inner core
The solid innermost part of the core witha diameter of a little over 1,200 km.

intensity
A measure of the size of an earthquake interms of the damage it causes.

interlobate moraine
Ridge formed along junction ofadjacent glacier lobes.

intrusive
Pertaining to igneous rocks or features formedby the emplacement of magma in pre-existing rock.

ion
An atom that has an electrical charge, by virtueof having gained or lost electrons. see cation, anion

ionic radius
The effective distance from the centerof an ion to the edge of its electron cloud.

ionic substitution
The replacement of one or more ionsin a crystal structure by others of similar size and electrical charge.Example: Fe2+ is interchangeable with Mg2+ in most ferromagnesian minerals.

iron catastrophe
The period in the Hadean eon duringwhich much of the iron in outer portions of the Earth migrated towardthe center of the planet, producing the core and releasing large amountsof gravitational heat.

ironpan
A hardpan in which iron oxides are the primarycementing agents.

island arc
A curved belt of volcanic islands lyingabove a subduction zone. compare continental arc.

isochemical reaction
A reaction in which chemical constituentsof a rock are rearranged to form a new mineral assemblage, but no materialis added to or lost from the rock as a whole. Applied generally to diageneticor metamorphic environments.

isoclinal fold
A fold in which the limbs are parallel.

isograd
A line on a map joining points at which metamorphismtook place under similar temperature and pressure conditions, as indicatedby rocks belonging to the same metamorphic facies . Generally, the lineseparates two adjacent metamorphic zones, as indicated by specific .

isoseismal line
A line on a map joining points of equalearthquake intensity.

isostasy
The condition of equilibrium, comparable tofloating, of units of the lithosphere above the asthenosphere .

isostatic change in sea level
A sea level change dueto change in load on Earth’s crust.

isotope
Atoms that differ in atomic mass number , butnot in atomic number , are called isotopes. For example, oxygen (atomicnumber 8) may have an atomic mass number of 16, 17, or 18, dependingon whether it has 8, 9, or 10 neutrons. It therefore has three isotopes.

jasper
A red variety of chert , its color coming fromminute particles of included hematite.

jet flow
Flow in which fluid moves at high speed injet-like surges as does water in free fall over a falls.

jetty
Similar to a groin but built to keep sand outof a harbor entrance.

joint
A surface of fracture in a rock, without displacementparallel to the fracture.

juvenile hydrothermal fluid
A hot fluid, largely water,presumed to have been released from a magma.

kame
Stratified drift deposited in depressions andcavities in stagnant ice and left as irregular, steep sided hills whenthe ice is melts.

kame terrace
Stratified drift deposited between wastingglacier and adjacent valley wall. Stands as a terrace when glacier melts.

karst
A landscape that develops from the action ofground water in areas of easily soluble rocks. Characterized by caves,underground drainage and sinkholes.

kettle
Depression in ground surface formed by the meltingof a block of glacier ice buried or partially buried by drift.

komatiite
An ultramafic rock with a non-cumulate texture,presumed to be extrusive.

laccolith
A concordant igneous intrusion with a flatfloor and a convex upper surface, usually less than 8 km across andfrom a few meters to a few hundred meters thick at its thickest point.

lag time
The delay in the response of stream flow betweenprecipitation and flood peak.

lahar
A mudflow composed chiefly of pyroclastic materialon the flanks of a volcano.

laminar flow
Fluid flow in which flow lines are distinct,and parallel and do not mix. compare turbulent flow .

lateral continuity
The extent of a rock unit over aconsiderable but definite area.

lateral moraine
Moraine formed by valley glaciers alongvalley sides.

laterite
A highly weathered red soil rich in iron andaluminum oxides. Typically formed in a tropical to temperate climatewhere intense chemical weathering is common.

Laurasia
The northern portion of the late Paleozoicsupercontinent called Pangea.

lava
Molten rock that flows at the Earth’s surface.

lava dome
A steep-sided rounded extrusion of highlyviscous lava squeezed out from a volcano and forming a dome-shaped orbulbous mass above and around the volcanic vent. The structure generallydevelops inside a volcanic crater.

lava flood
(plateau basalt) A term applied to largeareas of basaltic lava presumably extruded from fissures.