APPENDIX B Rock Symbols
APPENDIX B Rock Symbols
The symbols used in this book for types of rocks are shown below:
In this book we have adopted consistent colors and style for depicting magma and layers in the upper mantle and crust.
Rising magma
Oceanic crust
Lithosphere
Continental crust
Asthenosphere
GLOSSARY
A fine-grained gray or green volcanic rock mixture of humus and leached and weathered min-
A horizon The uppermost layer of soil composed of a
andesite
intermediate in composition between basalt and erals. (syn: topsoil)
granite, consisting of about equal amounts of pla-
aa A lava flow that has a jagged, rubbly, broken sur- gioclase feldspar and mafic minerals. face.
angle of repose The maximum slope or angle at which ablation area (See zone of ablation.)
loose material remains stable. abrasion The mechanical wearing and grinding of rock
angular unconformity An unconformity in which surfaces by friction and impact.
younger sediment or sedimentary rocks rest on the absolute age Age, or time measured in years.
eroded surface of tilted or folded older rocks.
abyssal fan
A large, fan-shaped accumulation of sedi- anion An ion that has a negative charge. ment deposited at the bases of many submarine
A stream that was established be- canyons adjacent to the deep-sea floor. (syn: subma-
antecedent stream
fore local uplift started and cut its channel at the rine fan)
same rate the land was rising.
abyssal plain
A fold in rock that resembles an arch; the the ocean floor between the mid-oceanic ridge and
A flat, level, largely featureless part of
anticline
fold is convex upward, and the oldest rocks are in the continental rise.
the middle.
accessory mineral
A porous and permeable body of rock that can usually found only in small amounts.
A mineral that is common, but
aquifer
yield economically significant quantities of ground
accreted terrain
A landmass that originated as an is-
water.
land arc or a microcontinent and was later added
A division of geologic time 3.8 to 2.5 onto a continent.
Archean Eon
billion years ago.
accumulation area (See zone of accumulation.)
arête
A sharp, narrow ridge between adjacent valleys
acid precipitation
A condition in which natural pre-
formed by glacial erosion.
cipitation becomes acidic after reacting with air pol-
A feldspar-rich sandstone formed adjacent to lutants. Often called acid rain.
arkose
granite cliffs.
active continental margin
artesian aquifer An inclined aquifer that is bounded acterized by subduction of an oceanic lithospheric
A continental margin char-
top and bottom by layers of impermeable rock so the plate beneath a continental plate. (syn: Andean mar-
water is under pressure. (syn: confined aquifer) gin)
artesian well
A well drilled into an artesian aquifer in
active volcano
which the water rises without pumping and in some pected to erupt.
A volcano that is erupting or is ex-
cases spurts to the surface.
albedo The reflectivity of a surface. A mirror or bright asbestos An industrial name for a group of minerals snowy surface reflects most of the incoming light
that crystallize as thin fibers. The two most common and has a high albedo, whereas a rough flat road sur-
types are fibrous varieties of the minerals chrysotile face has a low albedo.
and amphibole.
alluvial fan
A fan-like accumulation of sediment cre- asbestosis An often lethal lung disease most com- ated where a steep stream slows down rapidly as it
monly found among asbestos miners and others who reaches a relatively flat valley floor.
work with asbestos.
alpine glacier
A submarine mountain chain with lit- terrain.
A glacier that forms in mountainous
aseismic ridge
tle or no earthquake activity.
amphibole
A group of double chain silicate minerals. ash (volcanic) Fine pyroclastic material less than 2 Hornblende is a common amphibole.
mm in diameter.
Andean margin
A mixture of volcanic ash, larger pyroclastic by subduction of an oceanic lithospheric plate be-
A continental margin characterized
ash flow
particles, and gas that flows rapidly along the Earth’s neath a continental plate. (syn: active continental
surface as a result of an explosive volcanic eruption. margin)
(syn: nuée ardente)
G-2 GLOSSARY
asteroid One of the many small celestial bodies in or- base level The deepest level to which a stream can bit around the Sun. Most asteroids orbit between
erode its bed. The ultimate base level is usually sea Mars and Jupiter.
level, but this is seldom attained. asthenosphere The portion of the upper mantle be-
basement rocks The older granitic and related meta- neath the lithosphere. It consists of weak, plastic
morphic rocks of the Earth’s crust that make up the rock and extends from a depth of about 100 kilo-
foundations of continents.
A circular or elliptical synclinal structure, com- the Earth.
meters to about 350 kilometers below the surface of
basin
monly filled with sediment.
atmosphere
A large plutonic mass of intrusive rock with oxygen, that envelops the Earth.
A mixture of gases, mostly nitrogen and
batholith
more than 100 square kilometers of surface exposed.
atoll
A gray, yellow, or reddish brown rock com- is bounded on the outside by the deep water of the
A circular coral reef that surrounds a lagoon and
bauxite
posed of a mixture of aluminum oxides and hydrox- open sea.
ides. It is the principal ore of aluminum. atom The fundamental unit of elements, consisting of
baymouth bar
A spit that extends partially or com-
a small, dense, positively charged center called a nu- pletely across the entrance to a bay. cleus surrounded by a diffuse cloud of negatively
beach Any strip of shoreline washed by waves or tides. charged electrons.
Most beaches are covered by sediment.
aulacogen
A tectonic trough on a craton bounded by beach drift The concerted movement of sediment normal faults and commonly filled with sediment.
along a beach caused by waves striking the shore at An aulacogen forms when one limb of a continental
an angle.
A level portion of old beach elevated axial plane An imaginary plane that runs through the
rift becomes inactive shortly after it forms.
beach terrace
above the modern beach by uplift of the shoreline or axis and divides a fold as symmetrically as possible
fall of sea level.
into two halves. bed The floor of a stream channel. Also the thinnest
B horizon The soil layer just below the A horizon, layer in sedimentary rocks, commonly ranging in called the subsoil, where ions leached from the A
thickness from a centimeter to a meter or two. horizon accumulate.
bed load That portion of a stream’s load that is trans-
back arc basin
ported on or immediately above the stream bed. side of the magmatic arc from the trench, either in
A sedimentary basin on the opposite
bedding Layering that develops as sediment is de- an island arc or in an Andean continental margin.
posited.
backshore The upper zone of a beach that is usually bedrock The solid rock that underlies soil or regolith. dry but is washed by waves during storms.
Benioff zone An inclined zone of earthquake activity
bajada
A broad depositional surface extending out- that traces the upper portion of a subducting plate in ward from a mountain front and formed by merging
a subduction zone.
alluvial fans. bioclastic sediment Clastic sediment composed of banks The rising slopes bordering the two sides of a
fragments of organisms such as clams, oysters, coral, stream channel.
etc.
bar An elongate mound of sediment, usually composed biogeochemical cycle The movement of nutrients of sand or gravel, in a stream channel or along a
through the atmosphere, biosphere, hydrosphere, coastline.
and solid Earth in response to physical, biological,
barchan dune
A crescent-shaped dune, highest in the
and chemical processes.
center, with the tips facing downwind. biomass energy Electricity or other forms of energy
barrier island
A long, narrow, low-lying island that produced by combustion of plant fuels. extends parallel to the shoreline and is separated
bioremediation Use of microorganisms to decompose from the mainland by a lagoon.
a ground-water contaminant. basal slip Movement of the entire mass of a glacier
biosphere The thin zone near the Earth’s surface that along the bedrock.
is inhabited by life.
basalt
A dark-colored, very fine-grained, mafic, vol- biotite Black, rock-forming mineral of the mica group. canic rock composed of about half calcium-rich pla-
A general term for solid and semi-solid hy- gioclase feldspar and half pyroxene.
bitumen
drocarbons that are fusible and soluble in carbon
basalt plateau
bisulfide. The term includes petroleum, asphalt, nat- flows that were extruded rapidly to cover a large re-
A sequence of horizontal basalt lava
ural mineral waxes, and asphaltites. gion of the Earth’s surface. (syn: flood basalt, lava
A saucer- or trough-shaped depression cre- plateau)
blowout
ated by wind erosion.
GLOSSARY G-3
body waves Seismic waves that travel through the in- cavern An underground cavity or series of chambers terior of the Earth.
created when ground water dissolves large amounts
boulder
A rounded rock fragment larger than a cobble of rock, usually limestone. (syn: cave) (diameter greater than 256 cm).
cementation The process by which clastic sediment
Bowen’s reaction series
is lithified by precipitation of a mineral cement any early-formed mineral crystallizing from a cool-
A series of minerals in which
among the grains of the sediment. ing magma reacts with the magma to form minerals
Cenozoic era The most recent era; 65 million years lower in the series.
ago to the present.
braided stream
A very fine-grained, soft, white to gray bioclas- of branching and reuniting shallow channels sepa-
A stream that divides into a network
chalk
tic limestone made of the shells and skeletons of rated by mid-channel bars.
marine microorganisms.
breccia
A coarse-grained sedimentary rock composed chemical bond The linkage between atoms in mole- of angular, broken fragments larger than 2 mm in di-
cules and between molecules and ions in crystals. ameter cemented in a fine-grained matrix of sand or
chemical weathering The chemical decomposition of silt.
rocks and minerals by exposure to air, water, and brittle fracture The rupture that occurs when a rock
other chemicals in the environment. breaks sharply.
chert
A hard, dense, sedimentary rock composed of
butte
A flat-topped mountain, with several steep cliff microcrystalline quartz. (syn: flint) faces. A butte is smaller and more tower-like than a
A small grain, composed largely of olivine mesa.
chondrule
and pyroxene, found in stony meteorites.
A small volcano, as high as 300 meters, partly weathered bedrock grading downward into
C horizon The lowest soil layer, composed mainly of
cinder cone
made up of loose pyroclastic fragments blasted out unweathered parent rock.
of a central vent.
calcite
A common rock-forming mineral, CaCO 3 .
cinders (volcanic) Glassy pyroclastic volcanic frag-
caldera
A large circular depression caused by an ex-
ments 4 to 32 mm in size.
plosive volcanic eruption.
cirque
A steep-walled semicircular depression eroded
caliche
A hard soil layer formed when calcium car- into a mountain peak by a glacier. bonate precipitates and cements the soil.
clastic sediment Sediment composed of fragments of
calving
A process in which large chunks of ice break weathered rock that have been transported and de- off from tidewater glaciers to form icebergs.
posited at the Earth’s surface. cap rock An impermeable rock, usually shale, that
clastic sedimentary rocks Rocks composed of lithi- prevents oil or gas from escaping upward from a
fied clastic sediment.
reservoir. clay Any clastic mineral particle less than 1/256 millime- capacity The maximum quantity of sediment that a
ter in diameter. Also a group of layer silicate minerals. stream can carry.
A fine-grained clastic sedimentary rock capillary action The action by which water is pulled
claystone
composed predominantly of clay minerals and small upward through small pores by electrical attraction
amounts of quartz and other minerals of clay size. to the pore walls.
cleavage The tendency of some minerals to break along
capillary fringe
A zone above the water table in which certain crystallographic planes. the pores are filled with water due to capillary
climate The composite pattern of long-term weather action.
conditions that can be expected in a given region. carbonate rocks Rocks such as limestone and
A flammable organic sedimentary rock formed dolomite, made up primarily of carbonate minerals.
coal
from partially decomposed plant material and com-
carbonatite
A carbonate rock of magmatic origin
posed mainly of carbon.
composed mostly of calcite or dolomite. cobbles Rounded rock fragments in the 64- to 256-mm
carbonization
A process in which a fossil forms when size range, larger than pebbles and smaller than the volatile components of the soft tissues are driven
boulders.
off, leaving behind a thin film of carbon.
column
A dripstone or speleothem formed when a sta-
cast
A fossil formed when sedimentary rock or min- lactite and a stalagmite meet and fuse together. eral matter fills a natural mold.
columnar joints The regularly spaced cracks that com-
catastrophism
A principle that states that catastrophic monly develop in lava flows, forming five- or six- events have been important in Earth history and
sided columns.
modify the path of slow change. comet An interplanetary body, composed of loosely
cation
A positively charged ion. bound rock and ice, that forms a bright head and an
G-4 GLOSSARY
extended fuzzy tail when it enters the inner portion continental rise An apron of sediment between the of the solar system.
continental slope and the deep sea floor.
compaction
A shallow, nearly level area of con- ing sediment compresses deeper sediment, decreas-
A process whereby the weight of overly-
continental shelf
tinental crust covered by sediment and sedimentary ing pore space and causing weak lithification.
rocks that is submerged below sea level at the edge
competence
A measure of the largest particles that a of a continent between the shoreline and the conti- stream can transport.
nental slope.
composite volcano
continental slope The relatively steep (3º to 6º) un- nate layers of unconsolidated pyroclastic material
A volcano that consists of alter-
derwater slope between the continental shelf and the and lava flows. (syn: stratovolcano)
continental rise.
compressive stress Stress that acts to shorten an ob- continental suture The junction created where two ject by squeezing it.
continents collide and weld into a single mass of concordant Pertaining to an igneous intrusion that is
continental crust.
parallel to the layering of country rock.
control rod
A column of neutron-absorbing alloys
cone of depression
that is placed among fuel rods to control nuclear fis- ter table formed when water is pumped out of a well
A cone-like depression in the wa-
sion in a reactor.
A current in a fluid or plastic ma- confining stress Stress produced when rock or sedi-
more rapidly than it can flow through the aquifer.
convection current
terial, formed when heated materials rise and cooler ment is buried.
materials sink.
conformable The condition in which sedimentary lay-
A boundary where two ers were deposited continuously without interruption.
convergent plate boundary
lithospheric plates collide head-on.
conglomerate
A bioclastic limestone consisting of coarse rock, composed of rounded fragments larger than 2
A coarse-grained clastic sedimentary
coquina
shell fragments cemented together. mm in diameter, cemented in a fine-grained matrix
core The innermost region of the Earth, probably con- of sand or silt.
sisting of iron and nickel.
contact
A boundary between two different rock types correlation Demonstration of the age equivalence of or between rocks of different ages.
rocks or geologic features from different locations. contact metamorphic ore deposit An ore deposit
A system of analysis that at- formed by contact metamorphism.
cost-benefit analysis
tempts to weigh the cost of an act or policy, such as contact metamorphism Metamorphism caused by
pollution control, directly against the economic ben- heating of country rock, and/or addition of fluids,
efits.
from a nearby igneous intrusion. country rock The older rock intruded by a younger continental crust The predominantly granitic portion
igneous intrusion or mineral deposit. of the crust, 20 to 80 kilometers thick, that makes up
A chemical bond in which two or more the continents.
covalent bond
atoms share electrons to produce the effect of filled continental drift The theory proposed by Alfred
outer electron shells.
Wegener that continents were once joined together
A bowl-like depression at the summit of the and later split and drifted apart. The continental drift
crater
volcano.
A segment of continental crust, usually in the tectonics theory.
theory has been replaced by the more complete plate
craton
interior of a continent, that has been tectonically sta-
continental glacier
ble for a long time, commonly a billion years or cover of ice over areas of 50,000 square kilometers
A glacier that forms a continuous
longer.
or more and spreads outward under the influence of creep The slow movement of unconsolidated material its own weight. (syn: ice sheet)
downslope under the influence of gravity. continental margin The region between the shoreline
crest (of a wave) The highest part of a wave. of a continent and the deep ocean basins, including
A fracture or crack in the upper 40 to 50 me- the continental shelf, continental slope, and conti-
crevasse
ters of a glacier.
nental rise. Also the region where thick, granitic cross-bedding An arrangement of small beds lying at continental crust joins thinner, basaltic oceanic crust.
an angle to the main sedimentary layering.
continental margin basin
cross-cutting relationship (See principle of cross- sion or other thick accumulation of sediment and
A sediment-filled depres-
cutting relationships.)
sedimentary rocks near the margin of a continent. crust The Earth’s outermost layer, about 5 to 80 kilo- continental rifting The process by which a continent
meters thick, composed of relatively low-density sil- is pulled apart at a divergent plate boundary.
icate rocks.
GLOSSARY G-5
crystal
A rock that is the medium- to coarse-grained arranged in a regular, orderly, periodically repeated
A solid element or compound whose atoms are
diorite
plutonic equivalent of andesite. array.
dip The angle of inclination of bedding, measured
crystal face
A planar surface that develops if a crystal
from the horizontal.
grows freely in an uncrowded environment. directed stress Stress that acts most strongly in one crystal habit The shape in which individual crystals
direction.
grow and the manner in which crystals grow to- discharge The volume of water flowing downstream gether in aggregates.
per unit time. It is measured in units of m 3 /sec.
crystal settling
A type of unconformity in which the solidify first from a cooling magma settle to the bot-
A process in which the crystals that
disconformity
sedimentary layers above and below the unconfor- tom of a magma chamber because the solid miner-
mity are parallel.
als are more dense than liquid magma. discordant Pertaining to a dike or other feature that Curie point The temperature below which rocks can
cuts across sedimentary layers or other kinds of lay- retain magnetism.
ering in country rock.
current
A large low-grade ore de- rection.
A continuous flow of water in a concerted di-
disseminated ore deposit
posit in which generally fine-grained metal-bearing daughter isotope An isotope formed by radioactive
minerals are widely scattered throughout a rock body decay of another isotope.
in sufficient concentration to make the deposit eco-
debris flow
A type of mass wasting in which particles
nomical to mine.
move as a fluid and more than half of the particles dissolution The process by which soluble rocks and are larger than sand.
minerals dissolve in water or water solutions. deflation Erosion by wind.
dissolved load The portion of a stream’s sediment load deformation Folding, faulting and other changes in
that is carried in solution.
shape of rocks or minerals in response to mechani-
A channel that flows outward from the cal forces, such as those that occur in tectonically
distributary
main stream channel, such as is commonly found in active regions.
deltas.
delta The nearly flat, alluvial, fan-shaped tract of land divergent plate boundary The boundary or zone at the mouth of a stream.
where lithospheric plates separate from each other.
dendritic drainage pattern
(syn: spreading center, rift zone) utaries which branches like the veins in a leaf. It of-
A pattern of stream trib-
docking The accretion of island arcs or microconti- ten indicates uniform underlying bedrock.
nents onto a continental margin. deposition The laying-down of sediment by any nat-
dolomite A common rock-forming mineral, ural agent.
CaMg(CO 3 ) 2 .
depositional environment Any setting in which sed-
A circular or elliptical anticlinal structure. iment is deposited.
dome
a volcano that is not now erupting depositional remanent magnetism Remanent mag-
dormant volcano
but has erupted in the past and will probably do so netism resulting from mechanical orientation of
again.
magnetic mineral grains during sedimentation. downcutting Downward erosion by a stream.
desert
A region with less than 25 cm of rainfall a year. drainage basin The region that is ultimately drained Also defined as a region that supports only a sparse
by a single river.
plant cover.
drainage divide
A ridge or other topographically
desert pavement
higher region that separates adjacent drainage ated as wind erodes fine sediment, leaving larger
A continuous cover of stones cre-
basins.
rocks behind. drift (glacial) All rock or sediment transported and de-
desertification
A process by which semiarid land is posited by a glacier or by glacial meltwater. converted to desert, often by improper farming or by
A deposit formed in a cavern when calcite climate change.
dripstone
precipitates from dripping water. differential weathering The process by which certain
drumlin An elongate hill formed when a glacier flows rocks weather more rapidly than adjacent rocks, usu-
over and reshapes a mound of till or stratified ally resulting in an uneven surface.
drift.
dike
A mound or ridge of wind-deposited sand. structure of country rock.
A sheet-like igneous rock that cuts across the
dune
earthflow
A flowing mass of fine-grained soil parti-
dike swarm
A group of dikes that form in parallel or cles mixed with water. Earthflows are less fluid than radial sets.
mudflows.
G-6 GLOSSARY
earthquake
A sudden motion or trembling of the Earth mentary rock that formed when dissolved ions were caused by the abrupt release of slowly accumulated
concentrated by evaporation of water. elastic energy in rocks.
evolution The change in the physical and genetic char- echo sounder An instrument that emits sound waves
acteristics of a species over time. and then records them after they reflect off the sea
exfoliation Weathering in which concentric plates or floor. The data is then used to record the topography
shells split from the main rock mass like the layers of the sea floor.
of an onion.
effluent stream
extensional stress Tectonic stress in which rocks are ground water because its channel lies below the wa-
A stream that receives water from
pulled apart.
ter table. (syn: gaining stream)
external mold
A fossil cavity created in sediment by
elastic deformation
a shell or other hard body part that bears the im- an object returns to its original size and shape when
A type of deformation in which
pression of the exterior of the original. stress is removed.
externality An environmental cost not directly associ- elastic limit The maximum stress that an object can
ated with manufacturing. Examples include the costs withstand without permanent deformation.
of acid rain and purifying polluted water.
electron
A volcano that is expected never to cloud of negative charge around an atom.
A fundamental particle which forms a diffuse
extinct volcano
erupt again.
element
A substance that cannot be broken down into extrusive rock An igneous rock formed from material other substances by ordinary chemical means. An el-
that has erupted onto the surface of the Earth. ement is made up of the same kind of atoms.
eustatic sea level change Global sea level change
emergent coastline
caused by changes in water temperature, changes in der water but has been exposed either because the
A coastline that was recently un-
the volume of the mid-oceanic ridge, or growth or land has risen or sea level has fallen.
melting of glaciers.
end moraine
A type of mass wasting in which rock or regolith minus, of a glacier.
A moraine that forms at the end, or ter-
fall
falls freely or bounces down the face of a cliff. eon The longest unit of geologic time. The most re-
A fracture in rock along which one rock has cent, the Phanerozoic Eon, is further subdivided into
fault
moved relative to rock on the other side. eras and periods.
A continuous, slow movement of solid epicenter The point on the Earth’s surface directly
fault creep
rock along a fault, resulting from a constant stress above the focus of an earthquake.
acting over a long time.
epidemiology The study of the distribution of sick- fault zone An area of numerous closely spaced faults. ness in a population.
faunal succession (See principle of faunal succession.) epoch The smallest unit of geologic time. Periods are
A common group of aluminum silicate rock- divided into epochs.
feldspar
forming minerals that contain potassium, sodium, or
era
A geologic time unit. Eons are divided into eras,
calcium.
and in turn eras are subdivided into periods. fetch The distance that the wind has travelled over the erosion The removal of weathered rocks and minerals
ocean without interruption. by moving water, wind, ice, and gravity.
firn Hard, dense snow that has survived through one
erratic
A boulder that was transported to its present summer melt season. Firn is transitional between location by a glacier, deposited at some distance
snow and glacial ice.
from its original outcrop, and generally resting on a fissility Fine layering along which a rock splits easily. different type of bedrock.
fission (nuclear) The spontaneous or induced splitting
esker
A long snake-like ridge formed by deposition by particle collision of a heavy nucleus into a pair in a stream that flowed on, within, or beneath a
of nearly equal fission fragments plus some neu- glacier.
trons. Fission releases large amounts of energy (see
estuary
A shallow bay that formed when a broad river
fusion).
valley was submerged by rising sea level or a sink-
A long, deep, narrow arm of the sea bounded by ing coast.
fjord
steep walls, generally formed by submergence of a
eutrophic lake
glacially eroded valley. (Also spelled fiord.) dissolved nitrates, phosphates, and other plant
A lake characterized by abundant
A rapid, intense, local flood of short dura- nutrients, and by a seasonal deficiency of oxygen in
flash flood
tion, commonly occurring in deserts. bottom water. Such lakes are commonly shallow.
flood basalt Basaltic lava that erupts gently in great evaporation The transformation of a liquid into a gas.
volume to cover large areas of land and form a basalt
evaporite deposit
A chemically precipitated sedi-
plateau.
GLOSSARY G-7
flood plain That portion of a river valley adjacent to geologic structure Any feature formed by rock de- the channel; it is built by sediment deposited during
formation, such as a fold or a fault. Also, the com- floods and is covered by water during a flood.
bination of all such features of an area or region. flow Mass wasting in which individual particles move
A chronological arrangement of downslope as a semi-fluid, not as a consolidated
geologic time scale
geologic time subdivided into units. mass.
geology The study of the Earth, including the mate- focus The initial rupture point of an earthquake.
rials that it is made of, the physical and
A bend in rock. chemical changes that occur on its surface and in its foliation Layering in rock created by metamorphism.
fold
interior, and the history of the planet and its life footwall The rock beneath an inclined fault.
forms.
forearc basin
A sedimentary basin between the sub- geothermal energy Energy derived from the heat of duction complex and the magmatic arc in either an
the Earth.
island arc or the Andean continental margin. geothermal gradient The rate at which temperature foreshock Small earthquakes that precede a large
increases with depth in the Earth. quake by a few seconds to a few weeks.
A type of hot spring that intermittently erupts foreshore The zone that lies between the high and low
geyser
jets of hot water and steam. Geysers occur when tides; the intertidal region.
ground water comes in contact with hot rock.
formation
A smooth polish on bedrock created tary, igneous, or metamorphic rock that can be rec-
A lithologically distinct body of sedimen-
glacial polish
when fine particles transported at the base of a gla- ognized in the field and can be mapped.
cier abrade the bedrock.
fossil Any preserved trace, imprint, or remains of a glacial striations Parallel grooves and scratches in plant or animal.
bedrock that form as rocks are dragged along at the fossil fuel Fuels formed from the partially decayed re-
base of a glacier.
A massive, long-lasting accumulation of com- used fossil fuels are petroleum, coal, and natural gas.
mains of plants and animals. The most commonly
glacier
pacted snow and ice that forms on land and moves fractional crystallization Crystallization from a
downslope or outward under its own weight. magma in which early-formed crystals are prevented
A foliated rock with banded appearance formed from reacting with the magma, resulting in the evo-
gneiss
by regional metamorphism. lution of a final magma that is enriched in silica and
Gondwanaland The southern part of Wegener’s other components of granite.
Pangea, which was the late Paleozoic superconti- fracture (a) The manner in which minerals break other
nent. (syn: Gondwana)
than along planes of cleavage. (b) A crack, joint, or
A wedge-shaped block of rock that has dropped fault in bedrock.
graben
downward between two normal faults.
frost wedging
A type of bedding in which larger crack in rock and the expansion wedges the rock
A process in which water freezes in a
graded bedding
particles are at the bottom of each bed, and the par- apart.
ticle size decreases towards the top.
fuel rod
A stream with a smooth concave pro- pellets used to fuel a nuclear reactor.
A 2-meter-long column of fuel-grade uranium
graded stream
file. A graded stream is in equilibrium with its sed- fusion (of atomic nuclei) The combination of two
iment supply; once a stream becomes graded, the light nuclei to form a heavier nucleus. Fusion re-
rate of channel erosion becomes equal to the rate at leases large amounts of energy. (See fission.)
which the stream deposits sediment in its channel. gabbro Igneous rock that is mineralogically identical
Thus, there is no net erosion or deposition, and the to basalt but that has a medium- to coarse-grained
stream profile no longer changes. texture because of its plutonic origin.
gradient The vertical drop of a stream over a specific
gaining stream
A stream that receives water from
distance.
ground water because its channel lies below the wa-
A medium- to coarse-grained felsic, plutonic ter table. (syn: effluent stream)
granite
rock made predominantly of potassium feldspar and
gem
a mineral that is prized primarily for its beauty.
quartz.
Any precious or semiprecious stone, especially when gravel Unconsolidated sediment consisting of rounded cut or polished for ornamental use.
particles larger than 2 millimeters in diameter.
geologic column
A poorly sorted sandstone, commonly dark shows the sequence of rocks at a given place or re-
A composite columnar diagram that
graywacke
in color and consisting mainly of quartz, feldspar, gion arranged to show their position in the geologic
and rock fragments with considerable quantities of time scale.
silt and clay in its pores.
G-8 GLOSSARY
greenhouse effect An increase in the temperature of
hydrogeologist
A scientist who studies ground water
a planet’s atmosphere caused by infrared-absorbing and related aspects of surface water. gases in the atmosphere.
hydrologic cycle The constant circulation of water
groin
A narrow wall built perpendicular to the shore to among the sea, the atmosphere, and the land. trap sand transported by currents and waves.
hydrolysis
A weathering process in which water reacts
ground moraine
with a mineral to form a new mineral with water in- glacier deposits till in a relatively thin layer over a
A moraine formed when a melting
corporated into its crystal structure. broad area.
hydrosphere The collection of all water at or near the ground water Water contained in soil and bedrock.
Earth’s surface.
All subsurface water. hydrothermal metamorphism Changes in rock that
guyot
A flat-topped seamount. are primarily caused by migrating hot water and by
gypsum
ions dissolved in the hot water. (syn: hydrothermal It commonly forms in evaporite deposits.
A mineral with the formula (CaSO 4 ⭈ 2H 2 O).
alteration)
Hadean Eon The earliest time in the Earth’s history,
A sheet-like mineral deposit that from about 4.6 billion years ago to 3.8 billion years
hydrothermal vein
fills a fault or other fracture, precipitated from hot ago.
water solutions.
half-life The time it takes for half of the nuclei of a ra-
A time of extensive glacial activity, when alpine dioactive isotope in a sample to decompose.
ice age
glaciers descended into lowland valleys and conti-
halite
A mineral, NaCl. (syn: common salt) nental glaciers spread over the higher latitudes.
hanging valley
A glacier that forms a continuous cover of lies high above the floor of the main valley.
A tributary glacial valley whose mouth
ice sheet
ice over areas of 50,000 square kilometers or more hanging wall The rock above an inclined fault.
and spreads outward under the influence of its own hardness The resistance of the surface of a mineral to
weight. (syn: continental glacier) scratching.
A large chunk of ice that breaks from a gla- headward erosion The lengthening of a valley in an
iceberg
cier into a body of water.
upstream direction. igneous rock Rock that solidified from magma. heat flow The amount of heat energy leaving the Earth
incised meander
A stream meander that is cut below
per cm 2 /sec, measured in calories/cm 2 /sec.
the level at which it originally formed, usually caused
horn
A sharp, pyramid-shaped rock summit formed by
by rejuvenation.
glacial erosion of three or more cirques into a moun-
A fossil that dates the layers in which it is tain peak.
index fossil
found. Index fossils are abundantly preserved in
hornblende
A rock-forming mineral. The most com- rocks, widespread geographically, and existed as a mon member of the amphibole group.
species or genus for only a relatively short time.
hornfels
A fine-grained rock formed by contact meta- industrial mineral Any rock or mineral of economic morphism.
value exclusive of metal ores, fuels, and gems.
horst
A stream that lies above the water ward and is bounded by two faults.
A block of rock that has moved relatively up-
influent stream
table. Water percolates from the stream channel
hot spot
A persistent volcanic center thought to be lo- downward into the saturated zone. (syn: losing cated directly above a rising plume of hot mantle
stream)
rock. intermediate rocks Igneous rocks with chemical and
hot spring
A spring formed where hot ground water mineral compositions between those of granite and flows to the surface.
basalt.
humus The dark organic component of soil composed
A fossil that forms when the inside of of litter that has decomposed sufficiently so that
internal mold
a shell fills with sediment or precipitated minerals. the origin of the individual pieces cannot be deter-
internal processes Earth processes and movements mined.
that are initiated within the Earth––for example, for- hydraulic action The mechanical loosening and re-
mation of magma, earthquakes, mountain building, moval of material by flowing water.
and tectonic plate movement.
hydride
A compound of hydrogen and one or more intertidal zone The part of a beach that lies between metals. Hydrides can be heated to release hydrogen
the high and low tide lines. gas for use as a fuel.
A sedimentary basin located hydroelectric energy Electricity produced by turbines
intracratonic basin
within a craton.
that harness the energy of water dropping downward
A rock formed when magma solidifies through a dam.
intrusive rock
within bodies of preexisting rock.
GLOSSARY G-9
ion An atom with an electrical charge.
lateral moraine
A moraine that forms on or adjacent
ionic bond
A chemical bond in which cations and an- to the sides of a mountain glacier. ions are attracted by their opposite electronic
A highly weathered soil rich in oxides of iron charges, and thus bond together.
laterite
and aluminum that usually develops in warm, moist ionic substitution The replacement of one ion by an-
tropical or temperate regions. other in a mineral; usually the two ions are of simi-
Laurasia The northern part of Wegener’s Pangea, lar size and charge.
which was the late Paleozoic supercontinent.
island arc
A gently curving chain of volcanic islands lava Fluid magma that flows onto the Earth’s surface in the ocean formed by convergence of two plates,
from a volcano or fissure. Also, the rock formed by each bearing ocean crust, and the resulting subduc-
solidification of the same material. tion of one plate beneath the other.
A sequence of horizontal basalt lava flows isostasy The condition in which the lithosphere floats
lava plateau
that were extruded rapidly to cover a large region of on the asthenosphere as an iceberg floats on water.
the Earth’s surface. (syn: flood basalt, basalt plateau) isostatic adjustment The rising and settling of por-
leaching The dissolution and downward movement of tions of the lithosphere to maintain equilibrium as
soluble components of rock and soil by percolating they float on the plastic asthenosphere.
water.
isotopes Atoms of the same element that have the limb The side of a fold in rock. same number of protons but different numbers of
A sedimentary rock consisting chiefly of neutrons.
limestone
calcium carbonate.
joint
A fracture that occurs without movement of rock lithification The conversion of loose sediment to solid on either side of the break.
rock.
Jovian planets The outer planets—Jupiter, Saturn, lithosphere The cool, rigid, outer layer of the Earth, Uranus, and Neptune—which are massive and are
about 100 kilometers thick, which includes the crust composed of a high proportion of the lighter ele-
and part of the upper mantle. ments.
litter Leaves, twigs, and other plant or animal mate-
kame
A small mound or ridge of layered sediment de- rial that has fallen to the surface of the soil but is posited by a stream at the margin of a melting gla-
still recognizable.
cier or in a low place on the surface of a glacier. loam Soil that contains a mixture of sand, clay, and silt
kaolinite
A common clay mineral, Al 2 Si 2 O 5 (OH) 4 .
and a generous amount of organic matter.
karst topography
A homogenous, unlayered deposit of windblown limestone or other soluble rock and characterized by
A type of topography formed over
loess
silt, usually of glacial origin. caverns, sinkholes, and underground drainage.
A long, symmetrical dune oriented kerogen The solid bituminous mineraloid substance in
longitudinal dune
parallel with the direction of the prevailing wind. oil shales that yields oil when the shales are dis-
A current flowing parallel and tilled.
longshore current
close to the coast that is generated when waves strike
kettle
A depression in outwash created by melting of
a shore at an angle.
A stream that lies above the water table. ceding glacier.
a large chunk of ice left buried in the drift by a re-
losing stream
Water percolates from the stream channel downward
key bed
A thin, widespread, easily recognized sedi- into the saturated zone. (syn: influent stream) mentary layer that can be used for correlation.
a surface seismic wave that produces side- kimberlite An alkalic peridotite containing phe-
Love wave
to-side motion.
nocrysts of olivine and phlogopite in a groundmass luster The quality and intensity of light reflected from of calcite, olivine, and phlogopite. The name is de-
the surface of a mineral.
rived from Kimberley, South Africa, where the rock mafic rock Dark-colored igneous rock with high mag- contains diamonds.
nesium and iron content, and composed chiefly of L wave An earthquake wave that travels along the sur-
iron- and magnesium-rich minerals. face of the Earth, or along a boundary between lay-
magma Molten rock generated within the Earth. ers within the Earth. (syn: surface wave)
magmatic arc
A narrow, elongate band of intrusive
lagoon
A protected body of water separated from the and volcanic activity associated with subduction. sea by a reef or barrier island.
magnetic reversal
A change in the Earth’s magnetic
lake
a large, inland body of standing water that occu- field in which the north magnetic pole becomes the pies a depression in the land surface.
south magnetic pole, and vice versa.
landslide
A general term for the downslope movement magnetometer An instrument that measures the of rock and regolith under the influence of gravity.
Earth’s magnetic field.
G-10 GLOSSARY
manganese nodule
mid-channel bar An elongate lobe of sand and gravel rock found on the ocean floor.
A manganese-rich, potato-shaped
formed in a stream channel.
mantle
A continuous submarine mountain the crust and above the core. The mantle extends
A mostly solid layer of the Earth lying beneath
mid-oceanic ridge
chain that forms at the boundary between divergent from the base of the crust to a depth of about 2900
tectonic plates within oceanic crust. kilometers.
A rock composed of both igneous and mantle convection The convective flow of solid rock
migmatite
metamorphic-looking materials. It forms at very high in the mantle.
metamorphic grades when rock begins to partially
mantle plume
A rising vertical column of mantle
melt to form magma.
rock.
mineral
A naturally occurring inorganic solid with a
marble
A metamorphic rock consisting of fine- to characteristic chemical composition and a crystalline coarse-grained recrystallized calcite and/or dolomite.
structure.
maria Dry, barren, flat expanses of volcanic rock on
A local enrichment of one or more the Moon, first thought to be seas.
mineral deposit
minerals.
mass wasting The movement of earth material down- mineral reserve The known supply of ore in the slope primarily under the influence of gravity.
ground.
meander One of a series of sinuous curves or loops in
A process of fossilization in which the the course of a stream.
mineralization
organic components of an organism are replaced by mechanical weathering The disintegration of rock
minerals.
into smaller pieces by physical processes. Mohoroviˇci ´c discontinuity (Moho) The boundary be-
medial moraine
A moraine formed in or on the mid- tween the crust and the mantle, identified by a change dle of a glacier by the merging of lateral moraines
in the velocity of seismic waves. as two glaciers flow together.
Mohs hardness scale
A standard numbered from 1 to
mesa
10, to measure and express the hardness of minerals smaller than a plateau and larger than a butte.
A flat-topped mountain or a tableland that is
based on a series of ten fairly common minerals, Mesozoic era The portion of geologic time roughly
each of which is harder than those lower on the 245 to 65 million years ago. Dinosaurs rose to promi-
scale.
A scale used to measure is marked by the extinction of the dinosaurs.
nence during this era. The end of the Mesozoic era
moment magnitude scale
and express the energy released during an earth-
metallic bond
A chemical bond in which the metal
quake.
atoms are surrounded by a matrix of outer-level elec-
A fold with only one limb. trons that are free to move from one atom to another.
monocline
moraine
A mound or ridge of till deposited directly by
metamorphic facies
A set of all metamorphic rock
glacial ice.
types that formed under similar temperature and
A number of mountain ranges pressure conditions.
mountain chain
grouped together in an elongate zone. metamorphic grade The intensity of metamorphism
A series of mountains or mountain that formed a rock; the maximum temperature and
mountain range
ridges that are closely related in position, direction, pressure attained during metamorphism.
age, and mode of formation.
metamorphic rock
A rock formed when igneous, sed-
mud Wet silt and clay.
imentary, or other metamorphic rocks recrystallize mud cracks Irregular, usually polygonal fractures that in response to elevated temperature, increased pres-
develop when mud dries. The patterns may be pre- sure, chemical change, and/or deformation.
served when the mud is lithified. metamorphism The process by which rocks and min-
mudflow Mass wasting of fine-grained soil particles erals change in response to changes in temperature,
mixed with a large amount of water. pressure, chemical conditions, and/or deformation.
A non-fissile rock composed of clay and metasomatism Metamorphism accompanied by the
mudstone
silt.
introduction of ions from an external source.
mummification
A process in which the remains of an
meteorite
A fallen meteoroid. animal are preserved by dehydration.
meteoroid
A mixture of naturally occurring light orbit. Many meteoroids are asteroids or comet frag-
A small interplanetary body in an irregular
natural gas
hydrocarbons composed mainly of methane ments.
(CH 4 ).
mica
A ridge or embankment of flood- crystal habit and perfect cleavage. Muscovite and bi-
A layer silicate mineral with a distinctive platy
natural levee
deposited sediment along both banks of a stream otite are common micas.
channel.
GLOSSARY G-11
neutron
A natural material that is sufficiently enriched in ton but no electrical charge.
A subatomic particle with the mass of a pro-
ore
one or more minerals to be mined profitably.
nonconformity
A type of unconformity in which lay- original horizontality (principle of) ( See principle of ered sedimentary rocks lie on igneous or metamor-
original horizontality.)
phic rocks. orogeny The process of mountain building; all tec- nonfoliated The lack of layering in metamorphic rock.
tonic processes associated with mountain building. non-point source pollution Pollution that is gener-
orographic lifting Lifting of air that occurs when air ated over a broad area, such as that originating from
flows over a mountain.
fertilizers and pesticides spread over fields.
orthoclase
A common rock-forming mineral; a vari-
nonrenewable resource
ety of potassium feldspar, (KAlSi 3 O 8 ). tion of new deposits occurs much more slowly than
A resource in which forma-
outwash Sediment deposited by streams beyond the consumption.
glacial terminus.
normal fault
A broad, level surface composed of moved downward relative to the footwall.
A fault in which the hanging wall has
outwash plain
outwash.
normal polarity
A crescent-shaped lake formed where a that of the Earth’s modern magnetic field.
A magnetic orientation the same as
oxbow lake
meander is cut off from a stream and the ends of the nucleus The small, dense, central portion of an atom
cut-off meander become plugged with sediment. composed of protons and neutrons. Nearly all of the
oxidation The loss of electrons from a compound or mass of an atom is concentrated in the nucleus.
element during a chemical reaction. In the weather-
nuée ardente
A swiftly flowing, often red-hot cloud ing of common minerals, oxidation usually occurs of gas, volcanic ash, and other pyroclastics formed
when a mineral reacts with molecular oxygen. by an explosive volcanic eruption. (syn: ash flow)
ozone hole The unusually low concentration of ozone O horizon The uppermost soil layer, consisting mostly
in the upper atmosphere, first discovered in 1985. of litter and humus with a small proportion of min-
P wave (Also called a compressional wave.) A seismic erals.
wave that causes alternate compression and expan-
obsidian
A black or dark-colored glassy volcanic rock,
sion of rock.
A basaltic lava flow with a smooth, billowy, oceanic crust The 7- to 10-kilometer-thick layer of
usually of rhyolitic composition.
pahoehoe
or “ropy” surface.
sediment and basalt that underlies the ocean basins. paleoclimatology The study of ancient climates.
oceanic island
A seamount, usually of volcanic origin, paleomagnetism The study of natural remnant mag- that rises above sea level.
netism in rocks and of the history of the Earth’s Ogallala aquifer The aquifer that extends for almost
magnetic field.
1000 kilometers from the Rocky Mountains east- paleontology The study of life that existed in the past. ward beneath portions of the Great Plains.
Paleozoic era The part of geologic time 538 to 245
oil
A naturally occurring liquid or gas composed of a million years ago. During this era invertebrates, complex mixture of hydrocarbons. (syn: petroleum)
fishes, amphibians, reptiles, ferns, and cone-bearing
oil shale
A kerogen-bearing sedimentary rock that
trees were dominant.
A supercontinent, identified and named by oil trap Any rock barrier that accumulates oil or gas
yields liquid or gaseous hydrocarbons when heated.
Pangea
Alfred Wegener, that existed from about 300 to 200 by preventing its upward movement.
million years ago and included most of the conti-
oligotrophic lake
nental crust of the Earth. In this book we refer to water with low concentrations of nitrates, phos-
A lake characterized by nearly pure
three supercontinents: Pangea I (about 2.0 billion to phates, and other plant nutrients. Oligotrophic lakes
1.3 billion years ago), Pangea II (1 billion to 700 have low productivities and sustain relatively few
million years ago), and Pangea III (300 million to organisms, although lakes of this type typically con-
200 million years ago).
tain a few huge trout or similar game fish and are
A crescent-shaped dune with tips commonly deep.
parabolic dune
pointing into the wind.
olivine
A common rock-forming mineral in mafic and parent rock Any original rock before it is changed by ultramafic rocks with a composition that varies be-
metamorphism or other geological processes.
tween Mg 2 SiO 4 and Fe 2 SiO 4 .
partial melting The process in which a silicate rock
ooid
A small rounded accretionary body in sedimen- only partly melts as it is heated, to form magma that tary rock, generally formed of concentric layers of
is more silica-rich than the original rock. calcium carbonate around a nucleus such as a sand
A margin character- grain.
passive continental margin
ized by a firm connection between continental
G-12 GLOSSARY
and oceanic crust, where little tectonic activity
A metamorphic rock with a silky appearance occurs.
phyllite
and commonly wrinkled surface, intermediate in paternoster lake One of a series of lakes, strung out
grade between slate and schist. like beads and connected by short streams and wa-
phytoplankton All floating plants, such as diatoms. terfalls, created by glacial erosion.
pillow lava Lava that solidified under water, forming
peat
A loose, unconsolidated, brownish mass of par- spheroidal lumps like a stack of pillows. tially decayed plant matter; a precursor to coal.
pipe
A vertical conduit below a volcano, through which
pebble
A sedimentary particle between 2 and 64 mil- magmatic materials passed. It is usually filled with limeters in diameter, larger than sand and smaller
solidified magma and/or brecciated rock. than a cobble.
placer deposit
A surface mineral deposit formed by
pedalfer
A soil type that forms in humid environments, the mechanical concentration of mineral particles characterized by abundant iron and aluminum ox-
(usually by water) from weathered debris. ides and a concentration of clay in the B horizon.
planetesimal One of many small rocky spheres that
pediment
A gently sloping erosional surface that formed early in the history of the solar system and forms along a mountain front uphill from a bajada,
later coalesced to form the planets. usually covered by a patchy veneer of gravel only a
plankton Floating and drifting aquatic organisms. few meters thick.
plastic deformation
A type of deformation in which
pedocal
A soil formed in arid and semiarid climates the material changes shape permanently without characterized by an accumulation of calcium car-
fracture.
A relatively rigid independent segment of the pegmatite An exceptionally coarse-grained igneous
bonate and other minerals in the B horizon.
plate
lithosphere that can move independently of other rock, usually with the same mineral content as granite.
plates.
pelagic sediment Muddy ocean sediment that con-
A boundary between two lithospheric sists of a mixture of clay and the skeletons of mi-
plate boundary
plates.
A theory of global tectonics in peneplain According to a model popular in the first
croscopic marine organisms.
plate tectonics theory
which the lithosphere is segmented into several half of this century, streams erode mountain ranges
plates that move about relative to one another by ultimately to form a large, low, nearly featureless
floating on and gliding over the plastic astheno- surface called a peneplain. However, the theory fails
sphere. Seismic and tectonic activity occur mainly at to consider tectonic rejuvenation, and peneplains do
the plate boundaries.
A large elevated area of comparatively flat perched water table The top of a localized lens of
not actually exist.
plateau
land.
ground water that lies above the main water table, platform The part of a continent covered by a thin formed by a layer of impermeable rock or clay.
layer of nearly horizontal sedimentary rocks overly-
peridotite
A coarse-grained plutonic rock composed ing older igneous and metamorphic rocks of the mainly of olivine; it may also contain pyroxene, am-
craton.
phibole, or mica but little or no feldspar. The upper
playa
A dry desert lake bed.
part of the mantle is thought to be composed mostly playa lake An intermittent desert lake. of peridotite.
Pleistocene epoch
A span of time from roughly 2
period
A geologic time unit longer than an epoch and million to 8000 years ago, characterized by several shorter than an era.
advances and retreats of glaciers.
permafrost
A process in which glacial ice erodes rock by soil which lies from about a half meter to a few me-
A layer of permanently frozen soil or sub-
plucking
loosening particles and then lifting and carrying ters beneath the surface in arctic environments.
them downslope.
permeability
A fold with a dipping or plunging axis. can travel through a porous material.
A measure of the ease with which fluid
plunging fold
pluton An igneous intrusion.
permineralization Fossilization that occurs when plutonic rock An igneous rock that forms deep (a kilo- mineral matter is deposited in cavities or pores.
meter or more) beneath the Earth’s surface.
petroleum
A naturally occurring liquid composed of
pluvial lake
A lake formed during a time of abundant
a complex mixture of hydrocarbons. precipitation. Many pluvial lakes formed as conti- Phanerozoic Eon The most recent 538 million years
nental ice sheets melted.
of geologic time, represented by rocks that contain
A stream deposit located on the inside of a evident and abundant fossils.
point bar
growing meander.
phenocryst
A large, early-formed crystal in a finer point source pollution Pollution which arises from a
GLOSSARY G-13
polarity The magnetically positive (north) or negative principle of superposition The principle that states (south) character of a magnetic pole.
that in any undisturbed sequence of sediment or sed-
polymorph
A mineral that crystallizes with more than imentary rocks, the age becomes progressively one crystal structure.
younger from bottom to top. pore space The open space between grains in rock,
Proterozoic Eon The portion of geological time from sediment, or soil.
2.5 billion to 538 million years ago. porosity The proportion of the volume of a material
A dense, massive, positively charged particle that consists of open spaces.
proton
found in the nucleus of an atom. porphyry Any igneous rock containing larger crystals
pumice Frothy, usually rhyolitic magma solidified into (phenocrysts) in a relatively fine-grained matrix.
a rock so full of gas bubbles that it can float on water.
porphyry copper deposit
pyroclastic rock Any rock made up of material ejected igneous rock that contains disseminated copper sul-
A large body of porphyritic
explosively from a volcanic vent. fide minerals, usually mined by surface mining
A rock-forming silicate mineral group that methods.
pyroxene
consists of many similar minerals. Members of the
pothole
A smooth, rounded depression in bedrock in pyroxene group are major constituents of basalt and
a stream bed, caused by abrasion when currents cir-
gabbro.
A rock-forming silicate mineral, SiO 2 . Quartz Precambrian All of geologic time before the Paleozoic
culate stones or coarse sediment.
quartz
is a widespread and abundant component of conti- era, encompassing approximately the first 4 billion
nental rocks but is rare in the oceanic crust and years of Earth’s history. Also, all rocks formed dur-
mantle.
ing that time. quartz sandstone Sandstone containing more than 90
precautionary principle
A guideline that recom-
percent quartz.
mends that environmental precautions be taken
A metamorphic rock composed mostly of without absolute proof that the perturbation is
quartzite
quartz, formed by recrystallization of sandstone. harmful.
A drainage pattern formed precipitation (a) A chemical reaction that produces a
radial drainage pattern
when a number of streams originate on a mountain solid salt, or precipitate, from a solution. (b) Any
and flow outward like the spokes on a wheel. form in which atmospheric moisture returns to the
radioactivity The natural spontaneous decay of unsta- Earth’s surface—rain, snow, hail, and sleet.
ble nuclei.
preservation
A process in which an entire organism radiometric age dating The process of measuring the or a part of an organism is preserved with very lit-
absolute age of geologic material by measuring the tle chemical or physical change.
concentrations of radioactive isotopes and their de- pressure-release fracturing The process by which
cay products.
rock fractures as overlying rock erodes away and the
A radioactive gas formed by radioactive decay pressure diminishes.
radon
of uranium that commonly accumulates in some ig- pressure release melting The melting of rock and
neous and sedimentary rocks. the resulting formation of magma caused by a drop
A desert formed on the lee side in pressure at constant temperature.
rain shadow desert
of a mountain range.
primary (P) wave
A surface seismic wave with an up- nate compression and expansion of rock. P waves
A seismic wave formed by alter-
Rayleigh wave
and-down rolling motion.
A moraine that forms at the ter- principle of crosscutting relationships The principle
travel faster than any other seismic waves.
recessional moraine
minus of a glacier as the glacier stabilizes tem- that a dike or other feature cutting through rock must
porarily during retreat.
be younger than the rock. recharge The replenishment of an aquifer by the ad- principle of faunal succession The principle that fos-
dition of water.
sil organisms succeed one another in a definite and
A drainage pattern in recognizable sequence, so that sedimentary rocks of
rectangular drainage pattern
which the main stream and its tributaries are of ap- different ages contain different fossils, and rocks of
proximately the same length and intersect at right the same age contain identical fossils. Therefore, the
angles.
relative ages of rocks can be identified from their
A wave-resistant ridge or mound built by corals or fossils.
reef
other marine organisms.
principle of original horizontality The principle that reflection The return of a wave that strikes a surface. most sediment is deposited as nearly horizontal beds,
refraction The bending of a wave that occurs when and therefore most sedimentary rocks started out
the wave changes velocity as it passes from one
G-14 GLOSSARY
regional burial metamorphism Metamorphism of a tectonic activity including extensive volcanism that broad area of the Earth’s crust caused by elevated
borders the Pacific Ocean along the continental mar- temperatures and pressures resulting from simple
gins of Asia and the Americas. burial.
A current created when water flows back regional dynamothermal metamorphism Meta-
rip current
toward the sea after a wave breaks against the shore. morphism accompanied by deformation affecting an
(syn: undertow)
extensive region of the Earth’s crust. ripple marks Small, nearly parallel ridges and troughs regional metamorphism Metamorphism that is
formed in loose sediment by wind or water currents broadly regional in extent, involving very large ar-
and waves. They may then be preserved when the eas and volumes of rock. Includes both regional dy-
sediment is lithified.
namothermal and regional burial metamorphism. risk assessment The analysis of risk and the imple- regolith The loose, unconsolidated, weathered mater-
mentation of policy based on that analysis. ial that overlies bedrock.
roche moutonnée An elongate, streamlined bedrock
rejuvenated stream
A stream that has had its gradi-
hill sculpted by a glacier.
A naturally formed solid that is an aggregate of tonic uplift or a drop of sea level.
ent steepened and its erosive ability renewed by tec-
rock
one or more different minerals. relative age Age expressed as the order in which rocks
A type of mass wasting in which a formed and geological events occurred, but not mea-
rock avalanche
segment of bedrock slides over a tilted bedding plane sured in years.
or fracture. The moving mass usually breaks into relief The vertical distance between a high and a low
fragments. (syn: rockslide)
point on the Earth’s surface. rock cycle The sequence of events in which rocks are remediation (of a contaminated aquifer) The treat-
formed, destroyed, altered, and reformed by geolog- ment of a contaminated aquifer to remove or de-
ical processes.
compose a pollutant. rock flour Finely ground, silt-sized rock fragments remote sensing The collection of information about
formed by glacial abrasion. an object by instruments that are not in direct con-
A type of slide in which a segment of tact with it.
rockslide
bedrock slides along a tilted bedding plane or frac- replacement Fossilization in which the original or-
ture. The moving mass usually breaks into frag- ganic material is replaced by new minerals.
ments. (syn: rock avalanche) reserves Known geological deposits that can be ex-
rounding The sedimentary process in which sharp, an- tracted profitably under current conditions.
gular edges and corners of grains are smoothed. reservoir rock Porous and permeable rock in which
rubble Angular particles with diameters greater than liquid petroleum or gas accumulates.
2 millimeters.
reverse fault
A fault in which the hanging wall has runoff Water that flows back to the oceans in surface moved up relative to the footwall.
streams.
reversed polarity Magnetic orientations in rock which
A seismic wave consisting of a shearing mo- are opposite to the present orientation of the Earth’s
S wave
tion in which the oscillation is perpendicular to the field. Also, the condition in which the Earth’s mag-
direction of wave travel. S waves travel more slowly netic field is opposite to its present orientation.
than P waves.
rhyolite
A process whereby salts accumulate in sitionally equivalent to granite.
A fine-grained extrusive igneous rock compo-
salinization
soil when water, especially irrigation water, evapo-
Richter scale
A numerical scale of earthquake magni-
rates from the soil.
tude measured by the amplitude of the largest wave
A weathering process in which salty wa- on a standardized seismograph.
salt cracking
ter migrates into the pores in rock. When the water
rift
A zone of separation of tectonic plates at a diver- evaporates, the salts crystallize, pushing grains apart. gent plate boundary.
saltation Sediment transport in which particles bounce rift valley An elongate depression that develops at a
and hop along the surface.
divergent plate boundary. Examples include conti- sand Sedimentary grains that range from 1/16 to 2 nental rift valleys and the rift valley along the cen-
millimeters in diameter.
ter of the mid-oceanic ridge system. sandstone Clastic sedimentary rock composed pri- rift zone The boundary or zone where lithospheric
marily of lithified sand.
plates rift or separate from each other. (syn: diver- saturated zone The region below the water table gent plate boundary, spreading center)
where all the pores in rock or regolith are filled with ring of fire The belt of subduction zones and major
water.
GLOSSARY G-15
scarp
A line of cliffs created by faulting or by erosion. finely layered structure composed predominantly of
schist
A strongly foliated metamorphic rock that has a
clay minerals.
well developed parallelism of minerals such as shear stress Stress that acts in parallel but opposite di- micas.
rections.
sea arch An opening created when a cave is eroded all
shear wave (See S wave.)
the way through a narrow headland.
sheet flood
A broad, thin sheet of flowing water that
sea stack
A pillar of rock left when a sea arch col- is not concentrated into channels, typically in arid lapses or when the inshore portion of a headland
regions.
A large region of exposed basement rocks that sea-floor spreading The hypothesis that segments of
erodes faster than the tip.
shield
are commonly of Precambrian age. oceanic crust are separating at the mid-oceanic ridge.
shield volcano
A large, gently sloping volcanic moun-
seamount
A submarine mountain, usually of volcanic tain formed by successive flows of basaltic magma. origin, that rises 1 kilometer or more above the sur-
A rock such as granite and rhyolite that rounding sea floor.
sialic rock
contains large proportions of silicon and aluminum. secondary recovery Production of oil or gas as a re-
silica Silicon dioxide, SiO 2 . Includes quartz, opal, sult of artificially augmenting the reservoir energy
chert, and many other varieties. by injection of water or other fluids. Secondary re-
A mineral whose crystal structure contains sil- covery methods are usually applied after substantial
silicate
icate tetrahedra. All rocks composed principally of depletion of the reservoir.
silicate minerals.
sediment Solid rock or mineral fragments transported
A pyramid-shaped structure of a and deposited by wind, water, gravity, or ice, pre-
silicate tetrahedron
silicon ion bonded to four oxygen ions, (SiO 4 ) 4- . cipitated by chemical reactions, or secreted by or-
A tabular or sheetlike igneous intrusion that lies ganisms, and that accumulate as layers in loose, un-
sill
parallel to the grain or layering of country rock. consolidated form.
silt All sedimentary particles from 1/256 to 1/16 mil-
sedimentary rock
A rock formed when sediment is
limeter in size.
A rock composed of lithified silt. sedimentary structure Any structure formed in sed-
lithified.
siltstone
A circular depression in karst topography imentary rock during deposition or by later sedi-
sinkhole
caused by the collapse of a cavern roof or by disso- mentary processes; for example, bedding.
lution of surface rocks.
seismic gap An immobile region of a fault bounded by
A compact, fine-grained, low-grade metamorphic moving segments.
slate
rock with slaty cleavage that can be split into slabs
seismic profiler
and thin plates, intermediate in grade between shale graphic profile of the ocean floor and to reveal lay-
A device used to construct a topo-
and phyllite.
ering in sediment and rock beneath the sea floor. slaty cleavage Metamorphic foliation aligned in a
seismic tomography
plane perpendicular to the direction of maximum data from many earthquakes and recording stations
A technique whereby seismic
tectonic compressive stress. are analyzed to provide a three-dimensional view of
slide Any type of mass wasting in which the rock or the Earth’s interior.
regolith initially moves as coherent blocks over a seismic wave All elastic waves that travel through
fracture surface.
rock, produced by an earthquake or explosion. slip The distance that rocks on opposite sides of a fault seismogram The record made by a seismograph.
have moved.
seismograph An instrument that records seismic slip face The steep lee side of a dune that is at the an- waves.
gle of repose for loose sand so that the sand slides seismology The study of earthquake waves and the in-
or slips.
terpretation of these data to elucidate the structure of
A type of mass wasting in which the rock and the interior of the Earth.
slump
regolith move as a consolidated unit with a back- semiarid Any zone that receives between 25 and 50
ward rotation along a concave fracture. centimeters of rainfall annually. Semiarid zones sur-
A type of clay mineral that contains the abun- round most deserts.
smectite
dant elements weathered from feldspar and silicate
serpentinite
A rock composed largely of serpentine-
rocks.
group minerals, usually chrysotile and antigorite, snowline The boundary on a glacier between perma- commonly derived from alteration of peridotite or
nent glacial ice and seasonal snow. Above the snow- sea floor basalt.
line, winter snow does not melt completely during
shale
A fine-grained clastic sedimentary rock with summer, while below the snowline it does.
G-16 GLOSSARY
soil The upper layers of regolith that support plant streak The color of a fine powder of a mineral usually growth.
obtained by rubbing the mineral on an unglazed
soil horizon
A layer of soil that is distinguishable from
porcelain streak plate.
other horizons because of differences in appearance
A moving body of water confined in a channel and in physical and chemical properties.
stream
and flowing downslope.
soil-moisture belt The relatively thin, moist surface stream piracy The natural diversion of the headwaters layer of soil above the unsaturated zone beneath it.
of one stream into the channel of another.
solar cell
A device that produces electricity directly stream terrace An abandoned flood plain above the from sunlight.
level of the present stream. solar energy Energy derived from the Sun. Current
stress The force per unit area exerted against an technologies allow us to use solar energy in three
object.
ways: passive solar heating, active solar heating, and striations Parallel scratches in bedrock caused by electricity production by solar cells.
rocks embedded in the base of a flowing glacier.
solar wind
A stream of ions and electrons shot into strike The compass direction of the line produced by space by violent storms occurring in the outer re-
the intersection of a tilted rock or structure with a gions of the Sun’s atmosphere.
horizontal plane.
solifluction The slow mass wasting of water-saturated
A fault on which the motion is paral- soil that commonly occurs over permafrost.
strike-slip fault
lel with its strike and is primarily horizontal.
sorting
A process in which flowing water or wind subduction The process in which a lithospheric plate separates sediment according to particle size, shape,
descends beneath another plate and dives into the as- or density.
thenosphere.
source rock The geologic formation in which oil or subduction complex Rock and sediment scraped onto gas originates.
an island arc or continental margin during conver- specific gravity The weight of a substance relative to
gence and subduction.
the weight of an equal volume of water. subduction zone (or subduction boundary) The re- speleothems Any mineral deposit formed in caves by
gion or boundary where a lithospheric plate descends the action of water.
into the asthenosphere.
spheroidal weathering Weathering in which the edges sublimation The process by which a solid transforms and corners of a rock weather more rapidly than the
directly into a vapor or a vapor transforms directly flat faces, giving rise to a rounded shape.
into a solid without passing through the liquid phase.
spit
A deep, V-shaped, steep-walled shore into a body of water.
A long ridge of sand or gravel extending from
submarine canyon
trough eroded into a continental shelf and slope. spreading center The boundary or zone where litho-
A large, fan-shaped accumulation of spheric plates rift or separate from each other. (syn:
submarine fan
sediment deposited at the bases of many submarine divergent plate boundary, rift zone)
canyons adjacent to the deep sea floor. (syn: abyssal
spring
A place where ground water flows out of the
fan)
A coastline that was recently stalactite An icicle-like dripstone deposited from
Earth to form a small stream or pool.
submergent coastline
above sea level but has been drowned either because drops of water that hang from the ceiling of a
the land has sunk or sea level has risen. cavern.
subsidence Settling of the Earth’s surface which can
stalagmite
A deposit of mineral matter that forms on occur as either petroleum or ground water is re- the floor of a cavern by the action of dripping water.
moved by natural processes. stock An igneous intrusion with an exposed surface
supergene (ore) An ore deposit that has been en- area of less than 100 square kilometers.
riched by weathering processes that leach metals strain The deformation (change in size or shape) that
from a metal deposit, carry them downward, and re- results from stress.
precipitate them to form more highly concentrated stratification The arrangement of sedimentary rocks
ore.
A stream that has downcut stratified drift Sediment that was transported by a
in strata or beds.
superposed stream
through several rock units and maintained its course glacier and then transported, sorted, and deposited
as it encountered older geologic structures and rocks. by glacial meltwater.
superposition (principle of) ( See principle of super-
stratovolcano
A steep-sided volcano formed by an al-
position.)
ternating series of lava flows and pyroclastic erup- surf The chaotic turbulence created when a wave breaks tions. (syn: composite volcano)
near the beach.
GLOSSARY G-17
surface mine
A sedimentary rock formed of lithified till. face for the purpose of recovering mineral or fuel re-
A hole excavated into the Earth’s sur-
tillite
A sedimentary structure consisting of sources.
trace fossil
tracks, burrows, or other marks made by an organism. surface processes All processes that sculpt the Earth’s
traction Sediment transport in which particles are surface, such as erosion, transport, and deposition.
dragged or rolled along a stream bed, beach, or desert surface wave An earthquake wave that travels along
surface.
A strike-slip fault between two offset tween layers within the Earth. (syn: L wave)
the surface of the Earth or along a boundary be-
transform fault
segments of a mid-oceanic ridge. suspended load That portion of a stream’s load that is
A boundary between two carried for a considerable time in suspension, free
transform plate boundary
lithospheric plates where the plates are sliding hori- from contact with the stream bed.
zontally past one another.
suture The junction created when two continents or transpiration Direct evaporation from the leaf sur- other masses of crust collide and weld into a single
faces of plants.
mass of continental crust. transport The movement of sediment by flowing wa-
syncline
A fold that arches downward and whose cen-
ter, ice, wind, or gravity.
ter contains the youngest rocks.
transverse dune
A relatively long, straight dune that
tactite
A rock formed by contact metamorphism of is oriented perpendicular to the prevailing wind. carbonate rocks. It is typically coarse-grained and
A drainage pattern character- rich in garnet.
trellis drainage pattern
ized by a series of fairly straight parallel streams talus slope An accumulation of loose angular rocks at
joined at right angles by tributaries. the base of a cliff that has fallen mainly as a result
A long, narrow depression of the sea floor of frost wedging.
trench
formed where a subducting plate sinks into the
tarn
A small lake at the base of a cirque.
mantle.
tectonics
A branch of geology dealing with the broad tributary Any stream that contributes water to another architecture of the outer part of the Earth; specifi-
stream.
cally the relationships, origins, and histories of ma- trough The lowest part of a wave. jor structural and deformational features.
A triangular-shaped rock face that terminal moraine An end moraine that forms when a
truncated spur
forms when a valley glacier cuts off the lower por- glacier is at its greatest advance.
tion of an arête.
terminus The end or foot of a glacier.
A large sea wave produced by a submarine terrestrial planets The four Earth-like planets closest
tsunami
earthquake or a volcano, characterized by long wave- to the sun—Mercury, Venus, Earth, and Mars—which
length and great speed.
are composed primarily of rocky and metallic mate-
A general term for all consolidated pyroclastic rials.
tuff
rocks.
terrigenous sediment Sea-floor sediment derived di-
A rapidly flowing submarine cur- rectly from land.
turbidity current
rent laden with suspended sediment, that results from tertiary recovery Production of oil or gas by artifi-
mass wasting on the continental shelf or slope. cially augmenting the reservoir energy, as by
A pattern in which water flows in an injection of steam or detergents. Tertiary recovery
turbulent flow
irregular and chaotic manner. It is typical of stream methods are usually applied after secondary
flow.
A glacially eroded valley with a char- thermoremanent magnetism The permanent mag-
recovery methods have been used.
U-shaped valley
acteristic U-shaped cross section. netism of rocks and minerals that results from cool-
ultimate base level The lowest possible level of down- ing through the Curie point.
cutting of a stream, usually sea level.
thrust fault
A type of reverse fault with a dip of 45º ultramafic rock Rock composed mostly of minerals or less over most of its extent.
containing iron and magnesium–– for example, peri-
tidal current
A current caused by the tides.
dotite.
tide The cyclic rise and fall of ocean water caused by
A gap in the geological record, such as the gravitational force of the Moon and, to a lesser
unconformity
an interruption of deposition of sediments, or a break extent, of the Sun.
between eroded igneous and overlying sedimentary
tidewater glacier
strata, usually of long duration. the sea.
A glacier that flows directly into
A mine consisting of subterranean till Sediment deposited directly by glacial ice and that
underground mine
passages that commonly follow ore veins or coal has not been resorted by a stream.
seams.
G-18 GLOSSARY
undertow
A current created by water flowing back to- wash An intermittent stream channel found in a desert. ward the sea after a wave breaks. (syn: rip current)
water table The upper surface of a body of ground uniformitarianism The principle that states that geo-
water at the top of the zone of saturation and below logical change occurs over long periods of time, by
the zone of aeration.
a sequence of almost imperceptible events. In addi- wave height The vertical distance from the crest to the tion, processes and scientific laws operating today
trough of a wave.
also operated in the past and thus past geologic wave period The time interval between two crests (or events can be explained by forces observable today.
two troughs) as a wave passes a stationary observer. unit cell The smallest group of atoms that perfectly
A cliff created when a rocky coast is describes the arrangement of all atoms in a crystal,
wave-cut cliff
eroded by waves.
and repeats itself to form the crystal structure.
wave-cut platform
A flat or gently sloping platform
unsaturated zone
created by erosion of a rocky shoreline. table that may be moist but is not saturated; it lies
A subsurface zone above the water
wavelength The distance between successive wave above the zone of saturation. (syn: zone of aeration)
crests (or troughs).
upper mantle The part of the mantle that extends weather The condition of the atmosphere, including from the base of the crust downward to about 670
temperature, precipitation, cloudiness, humidity, and kilometers beneath the surface.
wind, at any given time and place.
upwelling
A rising ocean current that transports water weathering The decomposition and disintegration of from the depths to the surface.
rocks and minerals at the Earth’s surface by me-
valley train
A long and relatively narrow strip of out- chanical and chemical processes. wash deposited in a mountain valley by the streams
A hard, tough glass-rich pyroclastic rock flowing from an alpine glacier.
welded tuff
formed by cooling of an ash flow that was hot enough Van der Waals forces Weak electrical forces that bond
to deform plastically and partly melt after it stopped molecules together. They result from an uneven dis-
moving; it often appears layered or streaky. tribution of electrons around individual molecules,
wetlands Known as swamps, bogs, marshes, sloughs, so that one portion of a molecule may have a greater
mud flats, and flood plains, wetlands develop where density of negative charge while another portion has
the water table intersects the land surface. Some are
a partial positive charge. water soaked or flooded throughout the entire year;
varve
A pair of light and dark layers that was deposited others are dry for much of the year and wet only in a year’s time as sediment settled out of a body of
during times of high water. Still others are wet only still water. Most commonly formed in sediment de-
during exceptionally wet years and may be dry for posited in a glacial lake.
several years at a time.
vent
A powerful technique for the study fragments erupt.
A volcanic opening through which lava and rock
X-ray diffraction
of crystal structure in which the regular, periodic ventifact Cobbles and boulders found in desert envi-
arrangement of atoms in a crystal splits an x-ray ronments which have one or more faces flattened
beam into many separate beams, the pattern of which and polished by windblown sand.
reflects the crystal structure.
vesicle
A bubble formed by expanding gases in vol- zone of ablation The lower portion of a glacier where canic rocks.
more snow melts in summer than accumulates in viscosity The property of a substance that offers inter-
winter so that there is a net loss of glacial ice. nal resistance to flow.
zone of accumulation The upper portion of a glacier
volcanic bomb
A small blob of molten lava hurled out where more snow accumulates in winter than melts of a volcanic vent that acquired a rounded shape
in summer, and snow accumulates from year to year. while in flight.
zone of aeration
A subsurface zone above the water
volcanic neck
A vertical pipe-like intrusion formed by table that may be moist but is not saturated; it lies the solidification of magma in the vent of a volcano.
above the zone of saturation. (syn: unsaturated zone)
volcanic rock
A subsurface zone below the wa- erupted, cooled, and solidified within a kilometer or
A rock that formed when magma
zone of saturation
ter table in which the soil and bedrock are com- less of the Earth’s surface.
pletely saturated with water.
volcano
A hill or mountain formed from lava and rock zooplankton Animal forms of plankton, e.g., jellyfish. fragments ejected through a volcanic vent.
They consume phytoplankton.