eds glossary

8/21/2019 EDS Glossary

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G–

aa flow A lava flow with a surface typi-

fied by angular, jagged blocks. Contrast

with pahoehoe flow.

ablation Reduction of a glacier by melt-

ing,evaporation, iceberg calving, or

deflation.

abrasion The mechanical wea ring awa y

of a rock by friction, rubbing, scraping, or

grinding.absolute age G eologic time measured in

a specific duration o f years (in contrast to

relative time, which involves only the

chronologic order of events).A lso called

numerical age.

abyssal Pertaining to the great depths

of the o ceans, generally 1000 fatho ms

(2000 m) or more below sea level.

abyssal hills The part o f the ocean floor

consisting of hills rising as much as 1000 m

above the surrounding floor. They a re

found seaward of most aby ssal plains and

occur in profusion in basins isolated fromcontinents by trenches, ridges, or rises.

abyssal plains Flat areas of the ocean

floor, having a slope of less than 1:1000.

Most aby ssal plains lie at the ba se of a

continental rise and are simply areas

where a byssal hills are completely cov-

ered with sediment.

accretionary prism A w edge-shaped

body of fa ulted and folded material

scraped off subducting oceanic crust and

added to an island arc or continental

margin at a subduction zone.

amphibolite A meta morphic rock con-

sisting mostly of amphibole a nd plagio-

clase feldspar.

andesite A f ine-grained igneo us rock

composed mostly of plagioclase feldspa

and from 25 to 40% pyroxene, amphi-

bole, or biotite, but no quartz or K-

feldspar. It is abundant in mountains bo

dering the Pacific Ocean, such as the

Andes Mountains of So uth America,

from which the name was derived.

angle of repose The steepest angle at

which loose grains will remain stable

without sliding dow nslope.

angular unconformity An unconformitin which the older strata dip at a differe

angle (generally steeper) than the

younger strata.

anion A negatively charged ion.

anomaly A deviation from the norm o

average.anorthosite A coarse-grained intrusive

igneous rock composed primarily of

calcium-rich plagioclase feldspar.

anticline A fo ld in which the limbs dip

awa y from the hinge. After erosion, the

oldest rocks are exposed in the central

core of the fold.

aphanitic texture A rock texture in

which individual crystals are too

small to be identified without the aid

of a microscope. In hand specimens,

aphanitic rocks appear to be dense

and structureless.

aftershock An earthq uake that follows a

larger earthquake. G enerally, many after-

shocks occur over a period of da ys or

even months after a major earthquake.

agate A va riety of cryptocrystalline

qua rtz in which colors occur in bands. It

is commonly deposited in cavities in

rocks.

A horizon The topsoil layer in a soil

profile that commonly conta ins organic

matter.

alluvial fan A f an-shaped deposit of

sediment built by a stream where it

emerges from an upland or a mounta in

range into a broad valley or plain.A llu-vial fans are common in arid and semi-

arid climates but are not restricted to

them.

alluvium A genera l term for any sedi-

mentary a ccumulations deposited bycompara tively recent action of rivers. It

thus includes sediment laid do wn in river

beds, floodplains, and alluvial fa ns.

amorphous solid A solid in which atoms

or ions are not arranged in a definite

crystal structure. Examples: glass, amber,

obsidian.

amphibole An importa nt rock-forming

mineral group of ma fic silicates. Amphi-

bole crystals are constructed from d ouble

chains of silicon-oxygen tetrahedra . E x-

ample: hornblende.

Glossary

Lava

aa flow

Continental shelf

Submarine canyon

Continental slope

Abyssal fan

Abyssal plains

Abyssal hills

Abyssal hills

Alluvial fan

Unconformity

Angular unconform

Anticl


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aquifer A permeable stratum or zone

below the E arth’s surface through which

groundwater moves.

arch An a rch-shaped landform pro-

duced by weathering and differential

erosion.

arête A narrow, sharp ridge separating

two a djacent glacial valleys.

arid A dry climate such as exists in

deserts.

arkose A sandstone conta ining at least

25% feld spar.

artesian-pressure surface The level to

which water in a n artesian system

would rise in a pipe high enough to

stop the flow .

artesian water G roundwater confined in

an aq uifer and under pressure great

enough to cause the water to rise above

the top of the aq uifer when it is tapped

by a w ell.

ash Volcanic fragments the size of d ust

particles.

ash flow A turbulent blend of unsorted

pyroclastic material (mostly fine-grained) mixed w ith high-temperature

gases ejected explosively fro m a fissure

or crater.

ash-flow tuff A rock composed of vol-

canic ash and dust, formed by deposition

and consolidatio n of ash flows.

assimilation The process by which ho t

magma incorporates or d issolves the sur-

rounding solid country rock.

asteroid A small, rocky planetary body

orbiting the Sun.A steroids are numbered

in the tens of thousands. Most are located

between the orbit of Ma rs and the orbitof Jupiter. Their diameters range dow n-

ward from 1000 km.

asthenosphere The we ak zo ne inside

Ea rth directly below the lithosphere,

from 10 to 200 km below the surfa ce.

Seismic velocities are d istinctly lower

in the asthenosphere than in adjacent

parts of E arth’s interior. The material

in the asthenosphere is therefore

G–2 G l o s sa r y

into an intricate system of closely spaced,

narrow ravines.

bajada The surface of a system of coa-

lesced a lluvial fa ns.

bar An offshore, submerged, elongate

ridge of sand or gra vel built on the

seafloor by w aves and currents.

barchan dune A crescent-shaped d une,

the tips or horns of w hich point down-wind. B archan dunes form in desert areas

where sand is scarce.

barrier island An elongate island of

sand or gravel formed parallel to a coast.

barrier reef An elongate coral reef that

trends parallel to the shore of a n island o r

a continent, separated from it by a lagoon.

basalt A dark-colored, fine-grained,

mafic volcanic rock composed of plagio-

clase (over 50%) and pyroxene. O livine

may or may not be present.base level The level below which a

stream cannot effectively erode. Sea level

is the ultimate ba se level, but lakes form

temporary ba se levels for inland dra inage

systems.

basement complex A series of igneous

and meta morphic rocks lying beneath the

oldest stratified rocks of a region. In

shields, the ba sement complex is exposed

over large area s.

basin 1(structural geology) A circular

or elliptical dow nwarp.A fter erosion, the

youngest beds are exposed in the central

part of the structure. 2(topography)

believed to be soft and yielding to

plastic flow.

asymmetric fold A fold (a nticline or

syncline) in w hich one limb d ips more

steeply than the other.

atmosphere The mixture o f ga ses sur-

rounding a planet.The E arth’s atmo-

sphere consists chiefly of oxygen a nd

nitrogen, with minor amounts of other

gases. Synonymo us with air.

atoll A ring of low cora l islands sur-

rounding a lagoon.

atom The smallest unit of an e lement.

Atoms are composed of protons, neu-

trons, and electrons.

atomic number The number of protons

in the nucleus of an a tom. It uniquely de-

fines an element.

atomic weight The mass of one at om ofan element, essentially the sum of t he

proto ns and neutrons in the nucleus of

an atom.

axis 1(crystallography) An imaginary

line passing through a crysta l around

which the parts of the crystal are sym-

metrically arranged. 2(geophysics) A

straight line about which a planet or

moon rot ates or spins.

backarc basin The area behind a

subduction-related volcanic arc where

folds and faults form.M ost oceanicbackarcs are extending.

backswamp The marshy area of a flood -

plain at some distance beyond and lower

than the na tural levees that confine the

river.

backwash The return sheet flow d own a

beach af ter a wa ve is spent.

badlands An area nearly devoid of veg-

etat ion and dissected by stream erosion

Artesian-pressuresurface

Aquifer Asymmetric fold

Atoll

Barrier island

Volcano

Reef

Barrier reef

Basement complex


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G l o ss a r y G–

A depression into w hich the surrounding

area drains.

batholith A large bo dy of intrusive ig-

neous rock exposed over an area of a t

least 100 km2.

bathymetry The mea surement of ocea n

depths and mapping of the t opography of

the ocean floor.

bauxite A mixture of various amor-

phous or crystalline hydro us aluminum

oxides and a luminum hydroxides, com-

monly formed by intense chemical

weathering in tropical and subtropical

regions. B auxite is the principal ore of

aluminum.

bay A w ide, curving recess or inlet be-

tween two capes or headlands.

baymouth bar A narrow, usually sub-

merged ridge of sand or gravel deposited

across the mouth of a bay by longshore

drift. B aymouth bars commonly are

formed b y extension of spits along em-

bayed coasts.

beach A deposit of wa ve-wa shed sedi-

ment along a coast between the landward

limit of wave action and the outermost

breakers.bed A lay er of sediment 1 cm or more

in t hickness.

bedding plane A surface separating lay-

ers of sedimentary rock.

bed load Material transported along the

botto m of a stream by rolling or sliding,

in contrast to materia l carried in suspen-

sion or in solution.

bedrock The continuous solid rock

that und erlies the regolith everywhere

and is exposed locally at the surface. An

exposure of bedrock is called an out-

crop.benioff zone A zone of earthquakes

that d ips aw ay from a deep-sea trench

and slopes beneath the adjacent conti-

nent or island arc.

B horizon The solid zone of accumula-

tion underlying the A horizon of a soil

profile. Some of the material dissolved by

leaching in the A ho rizon is deposited in

the B horizon.

calcite A mineral composed of ca lcium

carbonate (CaC O 3).

caldera A large, more or less circular

depression or ba sin associated w ith a vo

canic vent. Its diameter is many times

greater tha n that of the included vents.

Ca lderas are believed to result from sub

sidence or collapse and may or ma y not

be relat ed to explosive eruptions.calving The breaking of f of la rge block

of ice from a gla cier that terminates in a

body of water.

capacity The maximum q uantity o f sed

ment a given stream, glacier, or wind ca

carry under a given set of conditions.

carbon 14 A radioactive isotope of car

bon . It s half-life is 5730 yea rs.

carbonaceous Containing carbon.

carbonate mineral A mineral formed b

the bonding of carbonate ions (CO 32-)

with positive ions. Exa mples: calcite

(CaCO 3), dolomite [Ca Mg(CO 3)2].

carbonate rock A rock composed mos

ly of carbonate minerals. Examples: lim

stone, dolomite.

catastrophism The belief tha t geologic

history consists of ma jor cata strophic

events involving processes that were f ar

more intense than any we ob serve now .

Contrast with uniformitarianism.

cation A negatively charged ion.

cave A naturally formed subterranean

open area, chamber, or series of cham-

bers, commonly produced in limestoneby solution activity or in basalt flow s as

lava tubes.

cement Minerals precipitated fro m

groundw ater in the pore spaces of a sed

mentary ro ck and binding the rock’s pa

ticles together.

Cenozoic The era o f geologic time from

the end o f the Me sozoic Era (65 million

years ago) to t he present.

chalcedony A general term for fibrous

cryptocrystalline quartz.

chalk A variety of limestone composed

shells of microscopic oceanic organisms.chemical weathering Chemical reactio

that a ct on rocks exposed to water a nd

the atm osphere so as to change t heir

biochemical sediment A sediment made

of mat erial precipitated a s a result of bio-

logical processes, such a s shells made of

calcium carbonate.

biosphere The tota lity of life on or near

Earth’s surface.

biotite “ B lack mica.” An important

mafic silicate with silicon-oxygen tet rahe-

dra a rranged in sheets.bird-foot delta A delta with distribu-

taries extending seaward and in map

view resembling the claws of a bird.

E xample: the Mississippi D elta.

block faulting A type o f normal faulting

in which segments of the crust are b ro-

ken and displaced to different elevations

and orientations.

blowout A dune shaped like a parabola

with the concave side toward the wind.

Commonly formed along shorelines

(same as a parabo lic dune).

blueschist A fine-grained schistose

rock characterized by high-pressure,

low-temperature mineral a ssemblages

and t ypically containing the blue amphi-

bole glaucophane.

boulder A rock fragment with a diame-

ter of mo re than 256 mm (abo ut the size

of a volleyball). A bo ulder is one size

larger than a cobble.

bracketed intrusion An intrusive rock

that w as once exposed at the surface by

erosion and wa s subsequently covered by

younger sediment. The relative age of t heintrusion thus falls between, or is brack-

eted by,the a ges of the younger and

older sedimenta ry deposits.

braided stream A stream with a com-

plex of converging and d iverging chan-

nels separated by bars or islands. B raided

streams form where more sediment is

ava ilable than can be removed by the dis-

charge of the stream.

breaker A collapsing water wave.

breccia A genera l term for sediment

consisting of angula r fragments in a ma-

trix of finer particles. E xamples: sedimen-tary breccias, volcanic breccias, fault

breccias, impact breccias.

brittle Ea sily broken or fractured in

contrast to plastic flow.

butte A somewhat isolated hill, usually

capped with a resistant la yer of rock

and bordered by ta lus. A butte is an

erosional remnant of a formerly more

extensive slope.

Batholith

Bu


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unstable mineral components to more

stable forms. Oxidation, hydrolysis, car-

bonation, and direct solution are the

most common rea ctions.

chert A sedimentary rock composed of

granular cryptocrystalline silica.

C horizon The low est zone o f soil con-

sisting of pa rtly decomposed bed rock un-

derlying the B horizon. It grades down-ward into fresh, unweathered bedrock.

cinder A fra gment of volcanic ejecta

from 0.5 to 2.5 cm in diameter.

cinder cone A cone-shaped hill com-

posed of loo se volcanic fragments erupt-

ed from a central vent.

cirque An a mphitheater-shaped depres-

sion at the head of a glacial valley, exca-

vated mainly by ice plucking and frost

wedging.

clastic 1Pert aining to fragments (such

as mud, sand, and gravel) produced by

the mechanical breakdow n of rocks. 2A

sedimentary rock composed chiefly of

consolidated clastic material.

clastic texture The texture of sedimen-

tary ro cks consisting of fra gments of

minerals, rocks, and organic skeletalremains.

clay Sedimentary material composed of

fragments with a dia meter of less than1/256 mm. Cla y particles are smaller than

silt part icles.

clay minerals A gro up of hydro us sili-

cates formed by weathering of minerals

such a s feldspar, pyroxene, or amphibole.

Silicate tet rahedra are arra nged in sheets.

cleavage The tendency of a mineral to

break in a preferred plane in the crystal

lattice.


conduction Tran smission of hea t energy

by the impact of moving atoms. Contrast

with convection.

cone of depression A co nical depression

of the w ater table surrounding a well

after hea vy pumping.

conglomerate A coa rse-grained sedi-

mentary rock composed of rounded frag-

ments of pebbles, cobbles, or boulders.

contact The surface separating t wo dif-

ferent rock bo dies.

contact metamorphism Metamorphism

of a rock near its contact with a magma.

continent A large land mass composed

mostly of gra nitic rock. Co ntinents rise

abruptly above the deep-ocean floor and

include the ma rginal area s submerged

beneath sea level.

continental accretion The grow th of

continents by incorporation of deformed

sediments, arc magmas, and a ccreted ter-

ranes along their margins.

continental crust The type of crust un-

derlying the continents, including the

continental shelves. The co ntinental crust

is commonly a bout 35 to 70 km thick. Its

density is typica lly 2.7 g/cm

3

, and the ve-locities of primary seismic wa ves travel-

ing through the crust a re less than 6.2

km/sec. Co ntrast w ith ocea nic crust.

continental drift The theory that the con-

tinents move in relation to one ano ther.

continental glacier A thick ice sheet

covering large parts of a continent.

Present-day examples are found in

G reenland and Antarctica.

continental margin The zone of transi-

tion from a continent to the a djacent

ocean basin. It generally includes a conti-

nental shelf, continental slope, and conti-nental rise.

continental rise The gen tly sloping sur-

face located at the base of a continental

slope (see diagra m for ab yssal hills).

continental shelf The submerged margin

of a continenta l mass extending from the

shore to the first prominent brea k in

slope, which usually occurs at a d epth of

ab out 120 m.

coal A common fuel mineral made

mostly of carbo n resulting from the

metamo rphic decomposition of the re-

mains of terrestrial plants. Found in sedi-

mentary rock.

cobble A rock fragment with a diameter

betwe en 6.4 cm (abo ut the size of a t en-

nis ball) and 25.67 cm (about the size of a

volleyball). Cobbles are larger than peb-bles but smaller than b oulders.

climate The long-term average of pre-

cipitation, temperature, and w ind direc-

tion and orientation.

columnar jointing A system of fractures

that splits a rock body into long prisms,

or columns. It is characteristic of lava

flows and shallow intrusive igneous

flows.

comet A small icy object in orbit around

the Sun. The orbits of many come ts are

elliptical and w hen they near the Sun, the

ice sublimes to ma ke a fuzzy head and

long tail of gas and dust.

competence The maximum size of parti-

cles that a given stream, glacier, or wind

can move a t a given velocity.

composite volcano A large volcaniccone built by extrusion of a sh, lava, and

shallow intrusions. Synonymo us with

stratovolcano.

compound A substance made of tw o or

more elements bound together.

compression A system of stresses that

tends to reduce the volume of or shortena substance.

conchoidal fracture A type of fra cture

that prod uces a smooth, curved surface.

It is characteristic of q uartz and o bsidian.

concretion A spherical or ellipsoidal

nodule formed by a ccumulation of min-

eral matter after deposition of sediment.

condensation The pro cess by w hich a

vapor becomes a liquid o r a solid.

Cirque

Clastic texture

Columnar joint

Composite volcano

Water table

Cone of depression


5/21

G l o ss a r y G–

continental slope The slope that extends

from a continental shelf down to the

ocean deep.I n some areas, such as off

eastern North America, the continental

slope grades into the more gently sloping

continental rise.

convection Tran smission of hea t energy

by the rise of buoyant hot material and

sinking of cold mat erial.

convection cell The space o ccupied by a

single convection current.

convergent plate boundary A plate

boundary at which plates collide.C onver-

gent plate boundaries are sites of consider-

able geologic activity and are character-

ized by volcanism, earthquakes, and crustal

deformation.See also subduction zone.

coquina A limestone composed of an

aggrega te of shells and shell fragments.

coral A bo ttom-dwelling marine inver-

tebrate organism of the class Anthozoa.

Most build hard skeletons of calcium

carbonate.

core The central part of the E arth below

a depth o f 2900 km. The core is tho ught

to be composed mostly of iron, in con-trast to t he overlying mantle of silicate

rock.

Coriolis effect The tend ency of moving

fluids on Ea rth’s surface to be d eflected

to the right in the Northern H emisphere

and to the left in the Southern Hemi-

sphere. Ca used by Ea rth’s spin.

country rock A general term for rock

surrounding an igneous intrusion.

defined as the part of the Earth a bove th

Moho rovicic discontinuity. It represents

less than 1% of Earth’s total volume. See

also continenta l crust, oceanic crust.

cryptocrystalline texture The text ure o

rocks composed of crystals too small to

be identified with an ordinary micro-

scope.

crystal A solid, polyhedral form bound

ed by nat urally formed plane surfaces

resulting from grow th of a crystal lattice

crystal face A smooth plane formed by

growt h of the surface of a crystal.

crystal form The geomet ric shape of a

crystal. Examples: cubic, prismatic.

crystal lattice A systematic,symmetric

network of atoms within a crystal.

crystalline texture The rock t exture resu

ing from simultaneous growth of crystals.

crystallization The process of crysta l

growth. It occurs as a result of condensation from a gaseous state, precipitat ion

from a solution, or cooling of a melt.

crystal structure The orderly a rrange-

ment of at oms in a crystal.

cuesta An elongate ridge formed on th

tilted and erod ed edges of gently dippin

strata.

daughter isotope An isotope produced

by rad ioactive decay of its parent isotop

The quant ity of a da ughter isotope con-

tinually increases with time.

debris flow The rapid dow nslope movement of debris (rock, soil, and mud).

declination,magnetic The ho rizontal

angle between true north and magnetic

north at a given point on Earth’s surfac

deep-marine environment The sed ime

tary environment o f the ab yssal plains.

deep-sea fan A cone-shaped o r fan-

shaped deposit of la nd-derived sedimen

located seaward of large rivers or

covalent bond A chemical bond in

which electrons are shared between dif-

ferent atoms so that none o f the atoms

has a net charge.

crater An a brupt circular depression

formed by extrusion o f volcanic materi-

al, by collapse, or by the impact of a

meteorite.

craton The stable continenta l crust, in-

cluding the shield and stable platf orm

areas, most of which have not been af-

fected by significant tectonic activity

since the close of the Preca mbrian E ra.

creep The imperceptibly slow dow n-

slope movement of materia l as a result

of gra vity.

crevasse 1(glacial geology) A deep

crack in the upper surface of a glacier.

2(natura l levee) A break in a naturallevee.

cross-bedding Stratification inclined to

the original horizonta l surface upon

which the sediment accumulated. It is

produced by depo sition on the slope of a

dune or sand wa ve.

crosscutting relations,principle of The

principle that a rock body is younger

than any ro ck across which it cuts.

crust The outermost compositional layer,

or shell, of E arth (or any other differenti-

ated planet). The crust consists of low -

density ma terials compared to the under-

lying mantle. E arth’s crust is generally

Convection

Coquina

Crater

Creep

Dune Cross-bedding

Cross-bedding

Oceanic crustContinental crust

Cr

Crystal fa


6/21

submarine canyons. Synonymo us with

abyssal cone, abyssal fan, and submarine

cone.

deep-sea trench See trench.

deflation Ero sion of loose rock particles

by wind.

deflation basin A shallow depression

formed by wind erosion where ground-

wa ter solution activity has left unconsoli-dat ed sediment exposed at the surface.

delta A bo dy of sediment deposited at

the mouth of a river. Many a re roughly

triangular in shape.

dendritic drainage pattern A bra nching

stream pattern, resembling the branch-

ing of certain trees, such as oa ks and

maples.

density Mass per unit volume, expressed

in gra ms per cubic cent imeter (g/cm3).

density current A current that flow s as a

result of d ifferences in density. In ocea ns,

density currents are produced by differ-

ences in temperature, salinity, and turbid-

ity (the concentration of ma terial held in

suspension).

deranged drainage A distinctively dis-

ordered drainage pattern formed in a

recently glaciated area. It is characterizedby irregular direction of stream flow, few

short tributaries, swampy areas, and

many lakes.

desert climate A climate with generally

high temperatures, high rates of evapora -

tion, and low precipitation. Most deserts

lie at ab out 30 degrees north or south ofthe equator.

desertification The process of tra ns-

forming arid land into a barren desert.

Of ten induced by human a ctivities or cli-

mate change.

desert pavement A veneer of pebbles

left in place where wind has removed the

finer material.

detrital 1Pert aining to detritus. 2A

rock formed fro m detritus.


dissolution The process by which ma te-

rials are d issolved.

dissolved load The part o f a stream’s

load t hat is carried in solution.

distributary Any of the numerous

stream branches into w hich a river

divides where it reaches its delta.

divergent plate boundary A plate mar-

gin formed where t he lithosphere splitsinto plates that drift a part from one a n-

other. D ivergent plate boundaries are

area s subject to tension, where new crust

is generated by igneous activity. See also

oceanic ridge.

divide A ridge separating two adjacent

draina ge ba sins.

dolomite 1A mineral composed of

CaMg(CO 3)2. 2A sedimentary rock com-

posed primarily of the mineral dolomite.

dolostone A sedimentary rock composed

mostly of the mineral dolomite. Some-

times referred to simply as dolomite.

dome 1(structural geology) An uplift

that is circular or e lliptical in ma p view,

with beds dipping away in all directions

from a central area. 2(topography) A gen-

eral term for a ny dome-shaped landform.

downwarp A do wnward bend or subsi-dence of a part of E arth’s crust.

drainage basin The tota l area tha t

contributes wa ter to a single drainage

system.

drift A general term for sediment de-

posited directly on land by glacial ice or

deposited in lakes, oceans, or streams as a

result of glaciat ion.

drip curtain A thin sheet of d ripstone

hanging from the ceiling or wall of a cave.

dripstone A cave d eposit formed by

precipitation of ca lcium carbona te from

groundwater entering an undergroundcavern.

drumlin A smooth, glacially streamlined

hill that is elongate in the direction of ice

movement. D rumlins are generally com-

posed of till.

detritus A genera l term for loose rock

fragments produced by mechanical

weathering.

differential erosion Variation in the rate

of erosion on different rock masses. As a

result of differential erosion, resistant

rocks form steep cliffs, wherea s nonresis-

tant rocks form gentle slopes.

differential stress A cond ition in whichthe stress applied to a rock body is not

the same in all directions.

differentiated planet A planetary body

in which various elements and minerals

are separated a ccording to d ensity and

concentrated at different levels. Ear th ,

for example, is differentiated, with heavy

metals (iron and nickel) concentrated in

the core; lighter minerals in the mantle;

and still lighter materials in the crust, hy-

drosphere, and atmosphere.

differentiation See magmatic differenti-

ation a nd planetary differentiation.

dike A ta bular intrusive rock that cuts

across strata or other structural features

of the surrounding rock.

dike swarm A gro up of associated d ikes.diorite A phaneritic intrusive igneous

rock consisting mostly of intermed iate

plagioclase feldspar and pyroxene, with

some amphibole and biotite.

dip The angle betw een the horizonta l

plane and a structural surface (such as a

bedding plane, a joint, a fault , foliation, or

other planar features).

disappearing stream A stream that d is-

appears into an underground channel and

does not reappear in the same, or in an

adjacent, drainage basin. In karst regions,

streams commonly d isappear into sink-holes and follow channels through caves.

discharge Ra te of flow; the volume of

wa ter moving through a given cross sec-

tion of a stream in a given unit of time.

disconformity An unconformity in

which beds above and below a re parallel.

discontinuity A sudden or rapid change in

physical properties of rocks within Ea rth.

D iscontinuities are recognized by seismic

data. See also Mohorovicic discontinuity.

Delta

Deranged drainage

Dike Dome

Drumlin


7/21

G l o ss a r y G–

ductile Ea sily bent. Contrast with brittle.

dune A low mound o f fine-grained mater-

ial that accumulates as a result of sediment

transport in a current system. D unes have

characteristic geometric forms that a re

maintained as they migrate. Sand dunes are

commonly classified a ccording to shape.

See also barchan dune,pa rabolic dune, seif

dune, star dune, and transverse dune.

earthquake A series of elastic waves

propagated in Earth, initiated where

stress along a fault exceeds the ela stic

limit of the rock so that sudden move-

ment occurs along the fault.

eclogite A high-grade met amorphic

rock made of garnet and pyroxene and

lacking plagioclase. As a result, eclogite

has a high density.

ecology The study of relationships be-

tween o rganisms and their environments.

ejecta R ock fragments, glass, and other

material thrown out of an impact crater

or a volcano.

ejecta blanket Rock material (crushed

rock, large blocks, breccia, and dust)

ejected from an impact crat er or explo-

sion crater and de posited over the sur-

rounding a rea.

elastic deformation Temporary deforma -

tion of a substance, after which the mater-

ial returns to its original size and shape.

E xample: the bending of mica flakes.

elastic limit The maximum stress that agiven substance can withstand without

undergoing permanent deformation ei-

ther by solid flow or b y rupture.

elastic-rebound theory The theory that

earthq uakes result from energy released

by fa ulting; the sudden release of stored

strain creates earthquake waves.

electron A negatively charged subatom-

ic particle.

end moraine A ridge of t ill that accumu-

lates at the margin of a gla cier.

energy A measure of the amount of

work that can be done, usually measuredin ergs (cgs) or joules (mks).

entrenched meander A meander cut

into the underlying rock a s a result of

from a change in the volume of wa ter o

the capacity of ocean ba sins.

evaporite A rock composed of minera

derived from evaporation of mineralize

water. Examples: rock salt, gypsum.

exfoliation A w eathering process by

which concentric shells, slabs, sheets, or

flakes are successively broken loo se and

stripped aw ay fro m a rock mass.

exposure B edrock not covered with so

or regolith; outcrop.

extrusive rock A rock formed from a

mass of magma that flowed out on the

surface of Earth. Example: basalt.

faceted spur A spur or ridge that has

been beveled or truncated by fa ulting,

erosion, or glaciation.

facies A d istinctive group of chara cter

tics within part of a rock bod y (such as

composition, grain size, or fossil assem-

blages) that differ a s a group from thos

found elsewhere in the same ro ck unit.

Examples: conglomerate fa cies, shale fa

cies, and brachiopod fa cies.

fan A fa n-shaped deposit of sediment.

See also alluvial fan and d eep-sea fan.

fault A surface along which a rock bod

has broken and been displaced.fault block A rock mass bounded by

faults on at least two sides.

fault scarp A cliff produced by faulting

faunal succession,principle of The prin

ciple that f ossils in a strat igraphic se-

quence succeed one another in a d efinit

recognizable order.

feldspar A m ineral group consisting of

silicates of aluminum and one o r more o

the metals potassium, sodium, or calcium

Examples: K-feldspar, Ca -plagioclase,

and Na-plagioclase.

felsic The minerals feldspar and q uartzor an igneous or metamorphic rock mad

predominantly of feldspar and quartz.

Contrast with ma fic.

fiord A glaciated valley flooded by the

sea to form a long, narrow, steep-walled

inlet.

firn G ranular ice formed by recrystalliza

tion of snow . It is intermediate between

snow and glacial ice. Sometimes referred

to a s neve.

regional uplift or lowering of the regional

base level.

eolian Pertaining to wind.

eolian environment The sedimentary

environment of deserts, where sediment

is transported a nd deposited primarily by

wind.

eon A ma jor subdivision of geologic

time consisting of eras. E xample:Phanerozoic Eon.

epicenter The area on E arth’s surface

that lies directly above the fo cus of a n

earthquake.

epoch A division of geologic time; a

subdivision of a period. Example: Pleis-

tocene epoch.

era A division of geologic time; a subdi-

vision of an eon. Example: Mesozoic Era.

Eratosthenian Period The period of

lunar history when large craters, the rays

of which are no longer visible, such as Er-

ato sthenes, were forme d (from 3.1 billion

to 0.8 billion years ago ).

erg A large a rea covered with sand

dunes. A sand sea such as those found in

E arth’s large deserts.

erosion The processes that loosen sedi-

ment and move it from one place to a n-other on E arth’s surface.A gents of ero-

sion include water, ice, wind, and gravity.

erratic A large boulder carried by

glacial ice to an area fa r removed from its

point of origin.

escarpment A cliff o r very steep slope.

esker A long, narrow, sinuous ridge ofstratified glacial drift deposited by a

stream flowing beneat h a glacier in a tun-

nel or in a subglacial stream bed .

estuary A bay at the mouth of a river

formed b y subsidence of the sand or by a

rise in sea level. Fresh wa ter from the

river mixes with a nd dilutes seawat er in

an estuary.

eustatic change of sea level A world-

wide rise or f all in sea level resultingEntrenched meander

Focus

Epicenter

Erratic

Exfoliat


8/21

fissure An open fracture in a rock.

fissure eruption Extrusion of lava along

a f issure.

flint A popular name fo r dark-coloredchert (cryptocrystalline qua rtz).

flood basalt An extensive flow of b asalt

erupted chiefly along fissures. Synony-

mous with plateau basalt.

floodplain The flat, occasionally flooded

area bordering a stream.

fluvial Pert aining to a river or rivers.

fluvial environment The sedimentary

environment of river systems.

focus The area within Earth where an

earthquake originates.

fold A bend, or flexure, in a rock.

folded mountain belt A long, linear

zone of E arth’s crust where rocks have

been intensely deformed by horizontal

stresses and generally intruded b y ig-

neous rocks. The great f olded mo untains

of the w orld (such as the Appalachians,

the Himalayas,the R ockies, and the

Alps) are believed to have been formed

at convergent plate margins.foliation A planar fea ture in metamor-

phic rocks, produced by the secondary

growt h of minerals.Three major types

are recognized: slaty cleavage, schistosity,

and gneissic layering.

footwall The block benea th a dipping

fault surface.

foraminifer Single-celled orga nisms that

secrete calcium carb onat e shells. They

are an importa nt source of biochemical

sediment in the oceans.

forearc At a convergent plate margin, the

region between the trench and volcanicarc. The forea rc is underlain by a long sed-

imentary basin and accretionary prism.

foreshore The seaward pa rt of the

shore or beach lying between high tide

and low tide.

formation A distinctive body o f rock

that serves as a convenient unit for study

and mapping.

fossil Natura lly preserved remains or

evidence of pa st life, such as bo nes, shells,


geologic column A diagram representing

divisions of geologic time and the rock

units formed during each major period.

geologic cross section A diagram show-

ing the structure and arra ngement of

rocks as they would appear in a vertical

plane below E arth’s surface.

geologic map A ma p showing the distri-

bution of rocks at E arth’s surface.geologic time scale The time sca le de-

termined by the geolo gic column and by

radiomet ric dating of rocks.

geothermal Pertaining to the heat of the

interior of E arth.

geothermal energy E nergy useful to

human beings that can be extracted from

steam and hot water found within Earth’s

crust.

geothermal gradient The rat e at which

temperature increases with depth.

geyser A therma l spring that intermit-

tently erupts steam and bo iling wa ter.

glacial environment The sediment ary

environment of glaciers and their melt-

waters.

glacier A ma ss of ice formed from com-

pacted, recrystallized snow tha t is thick

enough to f low plastically.

glass 1A state o f matter in which a sub-

stance displays many properties of a solid

but la cks crystal structure. 2An amor-

phous igneous rock formed from a rapid-

ly cooling magma.

glassy texture The texture o f igneousrocks in which the ma terial is in the form

of natura l glass rather tha n crystal.

global change A w orldwide change, usu-

ally referring to a cha nge in climate of

the entire planet and no t of just a local

area or region.

glossopteris flora An assemblage of late

Pa leozoic fossil plants named for the

seed fern G lossopteris, one of the plants

in the assemblage.These flora are w ide-

spread in South America,A frica, Aus-

tralia, India, and Antarctica and provide

important evidence for the theory of con-tinental drift.

gneiss A coarse-grained metamorphic

rock with a characteristic type of folia-

tion (gneissic layering), resulting from a l-

ternating lay ers of light-colored a nd

dark-colored minerals.

gneissic layering The type of foliation

characterizing gneiss, resulting from a l-

ternating la yers of t he constituent silicic

and ma fic minerals.

casts, impressions, and t rails.

fossil fuel A fuel containing solar ener-

gy that w as absorbed by plants and a ni-

mals in the geolo gic past and thus is pre-

served in organic compounds in their

remains. Fossil fuels include petroleum,

natural gas, and coal.

fractional crystallization The separation

of crystals and melt that causes the resid-ual magma to progressively change its

composition. Early crystallized mafic

minerals commonly a re separa ted by

gravitational settling,so that the residual

magma is left enriched in silica, sodium,

and pot assium.

fracture An irregular break in a ro ck or

a brea k in a crystal that is not parallel to

a crystal face.

fracture zone 1(field geology) A zone

where the bedro ck is cracked and fra c-

tured. 2(tectonics) A zone of long, linear

fractures on the ocean floor, expressed

topogra phically by ridges and troughs.

Fracture zones are the topogra phic ex-

pression of transform f aults.

fringing reef A reef tha t lies alongside

the shore of a la ndmass.

frost heaving The lifting of unconsoli-dat ed materia l by the freezing of subsur-

face water.

frost wedging The forcing a part o f rocks

by the expansion of wa ter as it freezes in

fractures and pore spaces.

gabbro A dark-colored, coarse-grained

rock composed of Ca -plagioclase, pyrox-

ene, and possibly olivine, but no q uartz.

gas The state of matte r in which a sub-

stance has neither independent shape nor

independent volume. G ases can be com-

pressed a nd tend to expand indefinitely.geode A hollow nodule of rock lined with

crystals;w hen separated from the rock

body by weathering,it a ppears as a hollow,

rounded shell partly filled with crystals.

Fissure eruption

Fold

Fringing reef

Geode


9/21

G l o ss a r y G–

Gondwanaland The ancient continental

landmass that is thought to ha ve split

apart d uring Mesozoic time to form the

present-day continents of South America,

Africa, India, Australia, and Antarctica.

graben An elongate fa ult block that has

been lowered in relation to the blocks on

either side.

graded bedding A type of bedding in

which each layer is characterized by a

progressive decrea se in grain size from

the bottom of the bed to the top.

graded stream A stream that has at-

tained a state of eq uilibrium, or balance,

between erosion and deposition, so that

the velocity of the w ater is just great

enough to transport the sediment load

supplied from the drainage basin, andneither erosion no r deposition o ccurs.

gradient (stream) The slope of a stream

channel measured along the course of

the stream.

grain A particle of a mineral or rock, gen-

erally lacking well-developed crystal faces.

granite A coa rse-grained igneo us rock

composed of K-feldspar, plagioclase,

and q uartz, with small amounts of mafic

minerals.

hanging wall The surface or block of

rock that lies above an inclined fault

plane.

hardness 1(mineralogy) The mea sure

the resistance of a mineral to scratching

or a brasion. 2(water) A property of

wa ter resulting from the presence of cal

cium carbonate and magnesium carbon

ate in solution.

headland An extension of land seawar

from the general trend of the coast; a

promontory, cape,o r peninsula.

headward erosion Extension of a

stream headward, up the regional slope

of erosion.

heat flow The flow of heat fro m the in

terior of Earth.

high-grade metamorphism Metamor-

phism tha t occurs under high tempera-

ture a nd high pressure.

hinge The line where f olded beds show

maximum curvature. The line formed by

the intersection of the hinge plane with

the bedd ing surface.

hogback A na rrow, sharp ridge formed

on steeply inclined, resistant ro ck.

horizon 1 (geologic) A plane of stratifi

cation a ssumed to ha ve been originally

horizontal. 2(soil) A la yer of soil distin

guished by chara cteristic physical propeties. Soil horizons generally are

granulite A high-grade meta morphic

rock that t ypically lacks hydrous minerals

like micas and a mphibole.

gravel The coa rsest (greate r tha n 2 cm

across) clasts found in clastic sedimenta-

ry ro cks, includes cobbles and boulders.

gravity anomaly An area w here gravita-

tional a ttraction is greater or less than its

normal value.graywacke An impure sandstone con-

sisting of rock fra gments and gra ins of

quartz a nd feldspar in a matrix of clay-

size part icles.

greenhouse effect The wa rming of a

planet’s atmosphere caused when certain

gases (especially water vapor a nd carbon

dioxide) absorb of solar energy reflected

off the surface.

greenschist facies Metamorphic condi-

tions typified by low temperature and

low pressure.

greenstone A low-grade meta morphic

rock that commo nly has green minerals

such as chlorite a nd ta lc.

groundmass The ma trix of rela tively

fine-grained mat erial between the phe-

nocrysts in a po rphyritic rock.

groundwater Water be low E arth’s sur-

face; generally in pore spaces of rocks

and soil.

guyot A seamount with a flat to p.

gypsum An evaporite mineral com-

posed of calcium sulfat e with wat er

(CaSO 4• 2H 2O).

half-life The time required fo r half of a

given sample of a radioa ctive isotope to

decay to its daughter isotope.halite An evaporite mineral composed

of sodium chloride (NaCl).

hanging valley A tributary valley with

the floor lying (“ha nging”) above the

valley floor of the ma in stream or shore

to which it flows. H anging valleys com-

monly are created by deepening of the

main valley by glaciation, but they can

also be produced by faulting or rapid re-

treat of a sea cliff.

Africa

SouthAmerica

A n a r c t i c

a

A u s t r a l i a

India

Gondwanaland

HorstGraben

Graben

Graded bedding

Guyot

Hanging val

Footwall

Hanging wall

Hanging w

Hinge

Hinge plane

Hin


10/21

designated by letters (for example,A

horizon,B horizon,C horizon).

horn A sharp peak formed at the inter-

section of the headwa lls of three or more

cirques.

hornblende A va riety of the amphibole

mineral group.

hornfels A nonfoliated metamorphic

rock of uniform grain size, formed by

high-temperature metamo rphism.

Hornfelses typically are formed by

contact metamorphism around igneous

intrusions.

horst An elongate fa ult block that has

been uplifted in relation to the ad jacent

rocks.

hot spot The expression at Earth’s sur-face of a mantle plume, or column of hot,

buoya nt rock rising in the mantle be-

neath a lithospheric plate.

hummock A small, rounded or cone-

shaped, low hill or a surface of other

small, irregular shapes.A surface that is

not eq uidimensional o r ridgelike.

hydrolysis A chemical reaction wherein

hydrogen ions replace other ions in a

mineral. Co mmonly results in the pro-

duction of hydrous minerals such as clay

or complete d issolution of calcite.

hydraulic Perta ining to a fluid in motion.hydraulic head The pressure exerted by a

fluid at a given depth beneath its surface.

It is proportional to the height of the

fluid’s surface abo ve the a rea w here the

pressure is mea sured.

hydrologic system The system of mov-

ing water at Earth’s surface.

hydration Chemical combination of

wa ter with other substances.


inverted valley A valley that has been

filled with lava or other resistant ma terial

and ha s subsequently been eroded into

an elongate ridge.

ion An atom or combination of atoms

that ha s gained or lost one or more elec-

trons and thus has a net electrical

charge.

ionic bond A chemical bond formed byelectrostatic attraction between opposite-

ly charged ions.

ionic substitution The replacement of

one kind of io n in a crystalline lattice by

anot her kind that is of similar size and

electrical charge.

island arc A cha in of volcanic islands.

Island arcs are generally convex toward

the open ocean. Example: the Aleutian

Islands.

isostasy A state of eq uilibrium, resem-

bling flotation, in which segments of

E arth’s crust stand a t levels determined

by their thickness and density. Isostatic

equilibrium is attained by flow of mat eri-

al in the mantle.

isotope One of t he several forms of a

chemical element that have the same

number of proto ns in the nucleus but dif-fer in the number of neutrons and thus

differ in atomic weight.

joint A fra cture in a rock along which no

appreciable displacement has occurred.

kame A body of stratified glacial sedi-

ment.A mound or an irregular ridge

deposited by a subglacial stream as an

alluvial fan or a delta.

karst topography A landscape charac-

terized by sinks, solution valleys, and

other features produced by ground-wa ter activity.

hydrosphere The waters of E arth, as dis-

tinguished f rom the rocks (lithosphere),

the air (atmosphere), and living things

(biosphere).

hydrostatic pressure The pressure within

a fluid (such as water) at rest, exerted on

a given point within the body of t he fluid.

hydrothermal deposit A mineral depo sit

formed by hot w ater. The high tempera-ture commonly is associated w ith em-

placement of a magma.

ice sheet A thick, extensive body of

glacial ice that is not confined to va lleys.

Lo calized ice sheets are sometimes called

ice ca ps.

ice wedging A type o f mechanical

wea thering in which rocks are broken by

the expansion of wa ter as it freezes in

joints, pores, or bedding planes. Synony-

mous with frost wedging.

igneous rock R ock formed by cooling

and solidification of m olten silicate min-

erals (magma). Igneous rocks include

volcanic and plutonic rocks.

inclination,magnetic The angle b etween

the horizontal plane and a magnetic line

of force.

inclusion A rock fragment incorporated

into a yo unger igneous rock.

intermediate-focus earthquake An

earthquake with a f ocus located at a

depth between 70 and 300 km.

intermittent stream A stream throughwhich wat er flows only part of the time.

internal drainage A drainage system

that does not extend to the ocean.

interstitial Pertaining to material in the

pore spaces of a rock.P etroleum and

groundw ater are interstitial fluids. Miner-

als deposited by groundwater in a sand-

stone a re interstitial minerals.

intrusion 1Injection of a ma gma into a

preexisting rock. 2A bod y of rock result-

ing from t he process of intrusion.

intrusive rock Igneous rock that, while it

was fluid, penetrated into or betweenother rocks and solidified. It can later be

exposed at E arth’s surface aft er erosion

of the overlying rock.

Horn

Horst ors

Graben

Graben

Horst

Hydrologic system

Isostasy

Solution valley

Disappearing stream

CavesSinks

Karst topography


11/21

G l o s sa r y G–1

kettle A closed d epression in a deposit

of glacial drift formed where a block of

ice was buried or partly buried and then

melted.

laccolith A concorda nt igneous intru-

sion that has arched up the strata into

which it wa s injected, so that it forms a

pod-shaped o r lens-shaped bod y with agenerally horizontal floor.

lag deposit A residual accumulation of

coarse fragment s that remains on the sur-

face af ter finer material has been re-

moved by wind.

lagoon A shallow body of seawater sep-

arated from the open ocean by a barrier

island o r reef.

lahar A volcanic debris flow.

lamina (pl.laminae) A layer of sediment

less than 1 cm thick.

laminar flow A type of flow in which the

fluid moves in parallel lines. Co ntrast

with turbulent flow.

landform Any feature of Ea rth’s surface

having a distinct shape and origin. Land-

forms include major fea tures (such as

continents, ocean basins, plains, plateaus,

and mountain ranges) and minor features(such a s hills, valleys, slopes, drumlins,

and dunes). Collectively, the landforms of

E arth constitute the entire surface con-

figuration of the planet.

landslide A genera l term for relatively

rapid types of ma ss movement, such as

debris flows, debris slides, rockslides, and

slumps.

lateral moraine An a ccumulation of till

deposited along the side margins of a

valley glacier. It a ccumulates as a result

of ma ss movement of d ebris on the sides

of the gla cier.lateral slip Nearly horizontal movement

(shear) of blocks on either side of a verti-

cal fault.

laterite A soil that is rich in oxides of

iron and a luminum formed by deep

wea thering in tropical and subtropical

areas.

Laurasia The ancient cont inental land -

mass that is thought to have split apart to

the shore at an angle.

longshore drift The process in which se

iment is moved in a zigzag pattern along

beach by the swash and ba ckwash of

waves that approach the shore obliquely

low-grade metamorphism Metamor-

phism that is accomplished under low o

moderate temperature and low or mod-

erate pressure.luster The appeara nce of the light re-

flected from a mineral surface, describe

for example,a s dull, glassy, or metallic.

mafic A mineral or ro ck rich in iron an

magnesium silicates such as o livine a nd

pyroxene.

magma Molten rock, generally a silicat

melt with suspended crystals and dis-

solved gases.

magmatic differentiation A general term

for the processes by which magmas diffe

entiate. It includes fractional crystalliza-

tion, magma mixing, and assimilation.

magmatic segregation Separation of

crystals of certain minerals from a magm

as it cools. For example, some minerals

(including certain valuable met als) crys-

tallize while other components of the

magma are still liquid. These early forme

crystals can settle to the bottom o f a

magma chamb er and thus become con-

centrated there, forming an ore deposit.

magnetic anomaly A deviation of ob-

served magnetic inclination or intensity(as measured by a magnetometer) from

constant normal value.

magnetic reversal A co mplete 180-

degree reversal of the polarity of E arth

magnetic field.

magnetosphere A region of the extrem

upper atmosphere that is dominat ed by

the magnetic field and charged pa rticle

are tra pped in it. It a cts as a type of radi

ation shield.

magnitude A mea sure of the size of an

earthquake, usually calculated from the

common logarithm of t he largest grounmotion observed and corrected f or dis-

tance from the earthquake focus.

mantle The zone of the E arth’s interio

betwe en the base of the crust (the Moh

discontinuity) and the core.

mantle plume A buoyant mass of hot

mantle mat erial that rises to the base of

the lithosphere. Mantle plumes commo

ly produce volcanic activity and

form E urope, Asia, North America, and

G reenland.

lava Magma that reaches Ea rth’s surface.

lava dome B ulbous lava flow or viscous

plug of lava piled near its vent. Most are

made of rhyolite.

leach To dissolve a nd remove the so lu-

ble constituents of a rock or soil.

lee slope The part of a hill, dune, or rockthat is sheltered or turned aw ay fro m the

wind. Synonymo us with slip face.

levee,natural A broad, low embank-

ment built up along the banks of a river

channel during flood s.

limb The flank, or side, of a fold.

limestone A sedimentary rock com-

posed mostly of calcium carbona te

(CaCO 3).

lineament A topographic feature or

group of fea tures having a linear configu-

ration. Lineaments commonly are ex-

pressed a s ridges or depressions or a s an

alignment of features such as stream

beds, volcanoes, or vegetation.

linear dune An elongate sand dune ori-

ented in the direction of the prevailing

wind.

liquid The state o f mat ter in which a

substance flow s freely and la cks crystal

structure. U nlike a ga s, a liquid retains

the same volume independent of the

shape of its container.

lithification The processes by w hich sed-

iment is converted into sedimentary ro ck.These processes include cementation and

compaction.

lithosphere The relat ively rigid oute r

zone of Ea rth, which includes the conti-

nental crust, the oceanic crust, and the

part of the upper mantle lying above the

wea ker asthenosphere.

load The tota l amount of sediment car-

ried at a given time by a stream, glacier,

or wind.

loess U nconsolidated, wind-deposited

silt and dust.

longitudinal profile The prof ile of astream or valley draw n along its length,

from source to mouth.

longitudinal wave A seismic body w ave

in which part icles oscillate along lines in

the direction in which the wave tra vels.

Synonymous with P wa ve.

longshore current A current in t he surf

zone moving parallel to the shore. Long-

shore currents occur where w aves strike

Laccolith


12/21

structural deformation in the central part

of lithospheric plat es.

marble A m etamo rphic rock consisting

mostly of metamo rphosed limestone or

dolomite.

mare (pl. maria) Any of the relatively

smooth, low, dark areas of the Moon.The

lunar maria were fo rmed by extrusion of

lava.mass movement The tra nsfer of rock

and soil downslope by direct action of

gravity witho ut a flow ing medium (such

as a river or glacial ice).Sy nonymous

with ma ss wa sting.

matrix The rela tively fine-grained ro ck

materia l occupying the space between

larger part icles in a ro ck. See also

groundmass.

meander A broad, looping bend in a river.

medial moraine A ridge of till formed in

the middle of a valley glacier by the junc-

tion of two lateral moraines where two

valley gla ciers converge.

melange A mixture of diverse deformed

rocks formed in the accretionary prism at

a convergent plate margin.

Mercalli scale A measure of earthquakeintensity determ ined from the effects on

people and buildings, ranges from I (low)

to X II (nearly total destruction).

mesa A fla t-topped, steep-sided high-

land capped with a resistant rock forma-

tion. A mesa is smaller than a platea u but

larger than a butte.

Mesozoic The era o f geologic time from

the end of t he Pa leozoic Era (225 million

years ago) to the beginning of the C eno-

zoic Era (65 million yea rs ago).

metaconglomerate A metamorphosed

conglomerate.metallic bond A chemical bond in which

shared electrons move freely among the

atoms.

metamorphic rock Any rock formed

from preexisting rocks by solid stat e re-

crystallization driven by changes in tem-

perature and pressure and by chemical

action of fluids.

metamorphism Alteration of the miner-

als and textures of a rock by changes in


moraine A general term for a landform

composed of till.

mountain A general term for any land-

mass that sta nds abo ve its surroundings.

In the stricter geological sense, a moun-

tain belt is a highly deformed pa rt of

E arth’s crust that has been injected with

igneous intrusions and the deeper pa rts

of which have been metamo rphosed. The

topogra phy of young mounta ins is high,

but erosion can reduce old mountains to

flat lowlands.

mud crack A crack in a deposit of mud

or silt resulting from the contra ction that

accompanies drying.

mudflow A flow ing mixture of mud and

water.

mudrock A fine-grained sedimentary

rock mad e of clay a nd silt-size particles.

Shale is a finely laminated t ype of

mudrock.

multiring basin A large crater (on the

Moon they a re more tha n 300 km in

diameter) conta ining a series of concen-tric ridges and depressions. Example: the

Orientale basin on the Moon.

mylonite A foliated metamorphic

rock formed by intense shearing and

deforma tion of preexisting grains.

Formed in the transition between brittle

fracture and ductile flow.

nappe A large t hrust sheet with over-

turned fo lds.

temperature and pressure and by a ga in

or loss of chemical components.

metasomatism A change in the chemical

composition of a rock during metamor-

phism, usually caused by the transport of

ions by fluids.

meteoric water Water d erived from the

atmosphere, such as rainwater, snow, or

hail.meteorite Any part icle of solid matter

that has fallen to Earth, the Moon, or an-

other planet fro m space.

mica A gro up of silicate minerals ex-

hibiting perfect cleavage in one direction.

midocean ridge B road fra ctured swell in

the ocean ba sins. New ocea nic crust is

formed a t this type of divergent plate

boundary. Synonymous with oceanic

ridge.

migmatite A mixture of igneous and

metamo rphic rocks in which thin dikes

and stringers of granitic material interfin-

ger with metamo rphic rocks.

Milankovitch theory The theory that

cyclical climatic changes are ca used by

variations in Earth’s orbital characteris-

tics—eccentricity of the orbit a nd tilt

(obliquity) a nd precession (wobble) of

the spin a xis.

mineral A nat urally occurring inorgan-

ic solid ha ving a d efinite internal struc-

ture and a definite chemical composi-

tion tha t varies only w ithin strict limits.

Chemical composition and internalstructure determine its physical proper-

ties, including the tendency to a ssume a

particular geometric form (crystal

form).

Mohorovicic discontinuity The f irst

global seismic discontinuity below the

surface of E arth. It lies at a depth varying

from a bout 5 to 10 m beneath the o cean

floor to abo ut 35 km beneath t he conti-

nents. Commonly referred to as the

Moho.

Mohs hardness scale A scale of mineral

hardness ranging from 1 for soft m ineralsto 10 for very hard minera ls.

monocline A bend o r fold in gently dip-

ping horizontal strata.

Meander

Monocline

MedialMoraine

LateralMoraine

LateralMoraine

Moraines

Mudflow


13/21


neve G ranular ice formed by recrystal-

lization of snow. Synonymous with firn.

neutron A subato mic nuclear particle

that ha s no electrical charge but a ma ss

almost the same as a proton.

nonconformity An unconformity in

which stratified rocks rest on eroded

granitic or metam orphic rocks.

nonfoliated A metamorphic rock that

lacks any preferred orientation of its

mineral gra ins.

normal fault A steeply inclined fault in

which the hanging wall has moved dow n-

ward in relation to the footw all.

numerical age G eologic time measured

in a specific duration of yea rs (in contra st

to relative time, which involves only the

chronologic order of events). Synonym:

absolute age.

obsidian A glassy igneous rock with a

composition equivalent to that of granite.ocean basin A low part of the litho-

sphere lying between continental ma sses.

The rocks of an o cean ba sin are mostly

basalt with a veneer of o ceanic sediment.

oceanic crust The type o f crust that un-

derlies the ocean basins. It is generally

less than 8 km thick, composed predomi-

nantly of basalt and ga bbro. Its density is

about 3.0 g/cm3. The velocities of com-

pressional seismic wa ves traveling

thro ugh it exceed 6.2 km/sec. C ompa re

with continenta l crust.

oceanic ridge The continuous ridge, orbroad, fractured topographic swell, that

extends through the central part of the

Arctic, Atlantic,I ndian, and South Pacific

oceans. It is several hundred kilometers

wide, and its elevation above the ocean

floo r is 600 m or mo re. The ridge marks a

divergent plate boundary where new

oceanic lithosphere is being formed.

oil reservoir Porous rock that can

contain oil.

overturned fold A f old in which at leas

one limb has been rotated through an

angle grea ter tha n 90 degrees.

oxbow lake A lake formed in the chan

nel of an abandoned meander.

oxidation Chemical combination of ox

gen with ano ther substance.

oxide mineral A mineral lacking silico

but containing oxygen bound to a meta

Examples: hematite and magnetite.

ozone layer A zone w ithin the strato-

sphere where ozone (O 3) is abundant an

forms a protection from some of the

Sun’s harmful ultraviolet radia tion.

pahoehoe flow A lava flow w ith a billow

or ropy surface. Contrast with aa flow.

paleocurrent An a ncient current, whic

existed in the geologic past, with a direc

tion of flow that can be inferred from

cross-bedding, ripple marks, and other

sedimentary structures.

paleogeography The study of geograph

in the geologic past, including the pat-

terns of Eart h’s surface, the distribution

of land and ocean, and ancient mountai

and other landforms.

paleomagnetism The study o f a ncientmagnetic fields, as preserved in the mag

netic properties of rocks. It includes stu

ies of changes in the position of the ma g

netic poles and reversals of the magnet i

poles in the geologic past.

paleontology The study o f a ncient life

Paleozoic The era o f geologic time from

the end o f the P recambrian (600 million

years ago) to t he beginning of the Meso

zoic E ra (225 million yea rs ago).

Pangaea A former continent from

which the present continents originated

by plate movement from the Me sozoicEra to the present.

parabolic dune A d une shaped like a

parabola w ith the concave side toward

the wind. B lowout dune.

oil shale Shale tha t is rich in hydrocar-

bon derivatives. In the U nited States, the

chief oil shale is the G reen R iver Forma-

tion in the Rocky Mountain region.

oil trap Impermeab le rocks or struc-

tures that block the flow of oil and force

it to accumulate into larger bo dies.

olivine An importa nt silicate mineral

with ma gnesium and iron [(Mg,Fe)2SiO 4].oolite A limestone consisting largely of

spherical grains of calcium carbona te in

concentric spherical layers.

ooze (marine geology) Ma rine sediment

consisting of more t han 30% shell frag-

ments of microscopic organisms.

ophiolite A sequence of ro cks charac-

terized by ultrama fic rocks at the ba se

and (in ascending order) gabbro, sheeted

dikes, pillow lava s, and deep-sea sedi-

ments.The typical seq uence of ro cks con-

stituting the ocea nic crust.

ore deposit A ma ss of rock conta ining

metal (or some other comm odity like di-

amond s) of sufficient abundance to beextracted a t a profit.

organic sediment A sediment deposited

through biological means and rich in hy-

drocarb ons, such as coal.

orogenic Pertaining to deformation of a

continental margin to the extent that a

mountain range is formed.

orogenic belt A mountain belt.

orogeny A major episode of mountain

building.

outcrop An exposure of bedrock.

outlet glacier A to nguelike stream of

ice, resembling a valley glacier, that fo rmswhere a cont inental glacier encounters a

mountain system and is forced to move

through a mountain pa ss in large streams.

outwash Stratified sediment washed out

from a glacier by meltwater streams and

deposited in front of the end mora ine.

outwash plain The area beyond t he

margins of a glacier where meltwater de-

posits sand, gravel,a nd mud washed out

from the glacier.

Nonconformity

Foot wall

Hanging wallNormal fault

Oolite

Outwash pla

Overturned f


14/21

partial melting The pro cess by w hich

minerals with low melting points liquefy

within a rock body a s a result of an in-

crease in temperature or a decrease in

pressure (or both) while other minerals in

the rock are still solid. If the liquid

(magma) is removed before other compo-

nents of the parent rock have melted, the

composition of the magma can be q uitedifferent from that of t he parent rock.

Pa rtial melting is believed to be impor-

tant in the generation of basaltic magma

from peridotite at o cean ridges and in the

generation of granitic magma from

basaltic crust.

passive margin (plate t ectonics) A

lithospheric plate ma rgin at which crust is

neither created nor destroyed. Passive

plate margins generally are marked by

transform fa ults.

peat An accumulation of partly car-

bonized plant material containing ap-

proximately 60% carbon and 30% oxy-

gen. It is considered an early stage, or

rank, in the development of coal.

pebble A rock fragment with a diameter

between 2 mm (about the size of a ma tch

head) a nd 64 mm (about t he size of a

tennis ball).

pediment A ge ntly sloping erosion sur-

face formed at the ba se of a receding

mountain front o r cliff. It cuts across

bedrock and can be covered with a ve-

neer of sediment. Ped iments characteris-tically form in arid and semiarid climates.

pegmatite A very coarse grained ig-

neous rock typically with a gra nitic com-

position.pelagic sediment D eep-sea sediment

composed of fine-grained detritus that

slowly settles from surface waters. Co m-

mon constituents are clay, radiola rian

ooze, and foraminiferal ooze.

peninsula An elongate body of land ex-

tending into a body of water.

perched water table The upper surfa ce

of a local zone of saturation that lies

above the regional water ta ble.


placer A mineral deposit formed by the

sorting or washing action of wa ter. Plac-

ers are usually deposits of hea vy miner-

als, such as gold.

plagioclase A group of feldspar miner-

als with a composition range from NaAl-

Si3O 8 to CaAl2Si2O 8.

planetary differentiation The processes by

which the materials in a planetary body a reseparated according to density, so that the

originally homogeneous body is converted

into a zoned or layered (shelled) body with

a dense core, a mantle,a nd a crust.

plankton Co llective term for very

small plants and anima ls that d rift near

the surface of wa ter. Phytoplankton in-

clude bacteria, algae (including di-

atoms), and fungi. The small animals are

called zoo-plankton.

plastic deformation A permanent

change in a substance’s shape or volume

that d oes not involve failure by rupture.

plate (tectonics) A bro ad segment of

the lithosphere (including the rigid upper

mantle, plus oceanic and continental

crust) that f loats on the underlying as-

thenosphere and moves independently of

other pla tes.

plateau An extensive upland region.

plateau basalt Basalt extruded in exten-

sive,nea rly horizontal layers, which,a fter

uplift, tend to erode into great plateaus.

Synonymous with flood ba salt.

plate tectonics The theory of globa l

dynamics in which the lithosphere is

believed to be broken into individual plates

that move in response to convection in the

upper mantle. The margins of the plates are

sites of considerable geologic activity.

playa A d epression in the center of a

desert basin, the site of occasional tem-

porary lakes.

playa lake A shallow temporary lake

formed in a desert basin after rain.

peridotite A d ark-colored ultramafic

igneous rock of coarse-grained texture,

composed of olivine, pyroxene, but with

essentially no feldspar and no q uartz.

period A d ivision of geo logic time

smaller than an era and larger than an

epoch.E xample: Cretaceous Period.

permafrost Permanently frozen ground.

permanent stream A stream or reach ofa stream tha t flows continuously

throughout the yea r. Synonymous with

perennial stream.

permeability The ability of a materia l to

transmit fluids.

phaneritic texture The texture of ig-

neous rocks in which the interlocking

crystals are large enough to be seen with-

out magnification.

phenocryst A crysta l that is significantly

larger tha n the crystals surrounding it.

Phenocrysts form during an early phase

in the cooling of a magma when the

magma cools relatively slowly.

phyllite A foliated metamorphic rock

intermediate betw een slate a nd schist.

Small mica crystals give broken surfaces

a silky sheen.

physical weathering The breakdo wn of

rock into smaller fragments by physical

processes such as frost wedging. Synony-

mous with mechanical weathering.

physiographic map A ma p showing sur-

face features of E arth.

pillow lava An ellipsoidal mass of ig-

neous rock formed by extrusion of lavaunderwater.

Pediment

Pediment

Perched water tablewater table

Perched water table

Phenocryst

Groundmass(matrix)

Phenocryst

Plateau basalt

Playa lake Playa lakePlaya

Playa


15/21


Pleistocene The epoch o f geo logic time

from the end of the P liocene Epoch of

the Tertiary Period (abo ut 2 million yea rs

ago) to the beginning of the H olocene

Epoch of the Quaternary Period (about

10,000 yea rs ago). The major event dur-

ing the Pleistocene was the expansion of

continental glaciers in the Northern

H emisphere. Synonymous with glacialepoch,ice age.

plucking (glacia l geology ) The process of

glacial erosion by which large rock frag-

ments are loosened by ice wedging, be-

come frozen to the bottom surface of the

glacier, and are torn out of the bedrock

and tra nsported by t he glacier as it

moves. The process involves the freezing

of subglacial meltwater tha t seeps into

fractures and bedd ing planes in the rock.

plume See mantle plume.

plunge The inclination, with respect to

the horizontal plane, of a ny linear structur-

al element of a rock. The plunge of a fold is

the inclination of the axis of the fold.

plunging fold A fo ld with its axis in-

clined from the horizontal.

pluton Igneous rock formed beneath

Earth’s surface.

pluvial lake A lake that w as created

under former climatic conditions, at a

time when rainfa ll in the region was

more abundant than it is now.P luvial

lakes were common in arid regions dur-

ing the P leistocene.point bar A crescent-shaped a ccumula-

tion of sand and gravel deposited on the

inside of a meander bend.

polar climate The climate t hat pre-

vails at E arth’s poles, with temperatures

commonly below freezing and precipita-

tion low.polarity epoch A relat ively long period

of time during which Eart h’s magnetic

field is oriented in either the norma l

direction or the reverse direction.

polarity event A relatively brief interval

of time within a polarity epoch; during a

polarity event, the polarity of E arth’s

magnetic field is reversed with respect to

the prevailing polarity of the epoch.

polar wandering The a pparent mo ve-

primary wave See P wa ve.

proton A positively charged nuclear

particle.

pumice A light-colored volcanic rock

with a bundant vesicles in nat ural glass.

P wave (primary seismic wave) A type

of seismic wave, propaga ted like a soun

wa ve, in which the material involved in

the wave motion is alternately com-pressed and expand ed.

pyroclastic Perta ining to fragmenta l ro

material formed by vo lcanic explosions.

pyroxene A group of rock-forming sili

cate minerals composed of single chains

of silicon-oxygen tetrahedra. Compare

with amphibole, which is composed of

double chains.

quartz An importa nt rock-forming

silicate mineral composed of silicon-

oxygen tetrahedra joined in a three-

dimensional network. It is distinguishe

by its hardness, glassy luster, and con-

choidal fracture.

quartzite A sandstone recrystallized by

metamorphism.

radioactivity The spontaneous disinte-

gration of a n atomic nucleus with the

emission of energy.

radiocarbon A rad ioactive isotope of car

bon, 14C, which is formed in the atmo-

sphere and is absorbed by living organism

radiogenic heat Hea t generated by

radioactivity.

radiometric dating D etermination of

the age in yea rs of a rock or mineral b

measuring the proportions of an origi-

nal radioactive material and its decay

product.rain shadow A dry a rea lying downwin

from a high mounta in chain.

rayed crater A meteorite crater that ha

a system of rays extending like splash

marks from the crater rim.

ment of t he magnetic poles with respect

to the continents.

pole of rotation A pole of the imaginary

axis about which a tectonic plate rotates.

polymorphism The a bility of a chemical

compound to crystallize with more than

one kind of crystal structure. For example,

Al2SiO 5 may crystallize as three different

minerals, depending on the prevailingtemperature a nd pressure.

pore fluid A fluid, such as groundwater

or liquid rock materia l resulting from

partial melting, that o ccupies pore spaces

of a rock.

pore space The spaces within a rock

body t hat are unoccupied by solid mate r-

ial. Pore spaces include spaces between

grains, fractures, vesicles, and voids

formed by dissolution.

porosity The percentage of the tot al vol-

ume of a ro ck or sediment that consists

of pore space.

porphyritic texture The texture of ig-

neous rocks in which some crystals are

distinctly larger than others.

porphyry copper D eposits of copper

disseminated t hroughout a porphyritic

granitic rock.

pothole A hole formed in a stream bed

by sand and gra vel swirled around in one

spot by ed dies.

Precambrian The division of geologic

time from the formation of E arth (about

4.5 billion years a go) to the beginning of

the Ca mbrian Period of the P aleozoic era

(abo ut 600 million years ago). Also, the

rocks formed during that time. Precam-

brian t ime constitutes about 90% of

E arth’s history.

pressure ridge An elongat e uplift of the

congealing crust of a lava flow, resultingfrom the pressure of underlying and still

fluid lava.

primary sedimentary structure A struc-

ture of sedimentary ro cks (such as cross-

bedding, ripple marks, or mud cracks)

that originates contemporaneously with

the deposition of t he sediment (in con-

trast to a seconda ry structure, such as a

joint or fault, which originates after the

rock has been formed).

Point bar

Pothole

Pyroclastic fl

Rayed cra


16/21

recharge R eplenishment of the ground-

water reservoir by the addition of wa ter.

recrystallization R eorganization of ele-

ments of the o riginal minerals in a ro ck

resulting from cha nges in temperature

and pressure and from the a ctivity of

pore fluids.

reef A solid structure built of shells and

other secretions of ma rine organisms,particularly coral.

regional metamorphism Metamor-

phism of la rge areas of crust, usually

during mountain building at convergent

plate margins. Co ntrast with contact

metamorphism.

regolith The blanket of soil and loose

rock fragments overlying the bedrock.

regression A d rop in sea level causes

the shoreline to move do wnslope.

relative age The age of a rock or an

event as compared with some other rock

or event.

relative dating D etermination of the

chronologic order of a sequence of

events in relation to one ano ther without

reference to their a ges measured in years.

R elative geologic dating is based primari-

ly on superposition, fauna l succession,

and crosscutting relations.

relative time G eologic time as deter-

mined by relative dating, that is, by

placing events in chronologic order

without reference to their ages mea-

sured in years.relief The difference in altitude bet ween

the high and the low parts of an area.

reverse fault A fa ult in which the hang-

ing wall has moved upward in relation to

the footwall; a high-angle thrust fault.

rhyolite A f ine-grained vo lcanic rockcomposed of quartz, K-feldspar,a nd pla-

gioclase. It is the extrusive equivalent of

a granite.

Richter scale A loga rithmic scale for

expressing the magnitude o f an earth-

qua ke in terms of the energy dissipated

in it. A mo dified version of this scale is

commonly used.

rift system A system of faults resulting

from extension.


saltation The transporta tion of pa rticles

in a current of wind or wate r by a series

of bo uncing movements.

salt dome A do me produced in sedi-

mentary rock by the upward movement

of a bod y of salt.

saltwater encroachment D isplacement

of fresh groundwater by salt water in

coastal areas, due to the greater d ensity

of salt water.

sand Sedimentary material composed of

fragments ranging in diameter from

0.0625 to 2 mm. Sand particles are larger

than silt particles but smaller than peb-

bles. Much sand is composed of quartz

grains, because q uartz is abundant and

resists chemical and mechanical d isinte-

gration, but other ma terials, such as shell

fragments and rock fragments, can alsoform sand.

sandstone A sedimentary rock composed

mostly of san d-sized part icles, usually ce-

mented by calcite,silica, or iron oxide.

saturated The condition wherein the

pore spaces in a rock a re completely

filled w ith water.

saturated zone The zo ne in t he subsur-

face in w hich all pore spaces are filled

with water. Contrast with the overlying

unsaturated zone.

scarp A cliff produced by faulting or

erosion.schist A medium-grained or coarse-

grained meta morphic rock with strong

foliation (schistosity) resulting from par-

allel orientation of platy minerals, such as

mica, chlorite, and talc.

schistosity The type of f oliation that

characterizes schist, resulting from the

parallel arrangement of coa rse-grained

platy minerals, such as mica, chlorite,

and talc.

rift valley 1A va lley of regional extent

formed by block faulting in which

tensional stresses tend to pull the crust

apart. Synonymous with graben. 2The

down-dropped block along divergent

plate ma rgins.

rip current A current formed on the sur-

face of a bod y of wa ter by the c

eds glossary

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