EDS Glossary
Transcript of 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.
G–4 G l o s sa r y
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
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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
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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.
G–6 G l o s sa r y
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
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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
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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,
G–8 G l o s sa r y
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
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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
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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.
G–10 G l o s sa r y
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
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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
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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
G–12 G l o s sa r y
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
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G l o s sa r y G–1
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
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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.
G–14 G l o s sa r y
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
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G l o s sa r y G–1
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
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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.
G–16 G l o s sa r y
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