Glossary of Geologic Terms
This glossary of geologic terms is based on the glossary in Earth:
An Introduction to Geologic Change, by S. Judson and S.M. Richardson
(Englewood Cliffs, NJ, Prentice Hall, 1995). Where possible, definitions
conform generally, and in some cases specifically, to definitions given
in Robert L Bates and Julia A Jackson (editors), Glossary of Geology,
3rd ed., American Geological Institute, Alexandria, Virginia, 1987.
14C method A method for determining the age in years
of organic matter by calculating the amount of radioactive carbon still
remaining, as compared to the stable isotope, 12C.
40K/40Ar method A method used for the dating of potassium-bearing
rocks by using the ratio of radioactive 40K to its daughter, 40Ar.
aa A Hawaiian term for a lava flow that has a rough,
jagged surface. compare pahoehoe.
ablation As applied to glacier ice, the process by
which ice below the snow line is wasted by evaporation and melting.
absolute time Geologic time expressed in years before
abundant metal Iron, aluminum, magnesium, manganese,
and titanium. Ores of the abundant metals only need to be 3 – 5 times
as metal-rich as average rock.
abyssal plain Large area of extremely flat ocean floor
lying near a continent and generally over 4 km in depth.
acceleration The rate at which velocity changes, either
by increasing or decreasing.
accretion The process by which the terrestrial planets
grew, increasing their mass by gradually accumulating smaller bodies,
acid mine drainage Water contamination by sulfuric
acid produced by seepage through sulfur-bearing spoil and tailings from
coal and metal mining
acid rain The acidity in rain due to gases from internal
combustion engines and coal- and oil-burning power plants.
active layer The seasonally thawed zone above permafrost.
actualism see uniformitarianism.
aftershock An earthquake that follows and has its epicenter
near a larger earthquake.
agate A siliceous rock with alternating bands of chalcedony
and variously colored chert.
alluvial fan Land counterpart of a delta . An assemblage
of sediments marking place where a stream moves from a steep gradient
to a flatter gradient and suddenly loses transporting power. Typical
of arid and semiarid climates but not confined to them.
alpha decay The process of radioactive decay in which
the nucleus of an atom emits an alpha particle. The new atom’s atomic
number is lower by two and its atomic mass number is reduced by four.
alpha particle A particle consisting of two protons
and two neutrons, produced during alpha decay. Identical to the nucleus
of a 4He atom.
alpine glacier see valley glacier.
amygdule A gas cavity (vesicle ) in volcanic rock that
has been filled with mineral matter such as calcite, chalcedony, or
amygdaloidal A textural term describing volcanic rocks
that contain numerous amygdules.
andesite A fine-grained volcanic rock of intermediate
composition, consisting largely of plagioclase and one or more mafic
andesite line The geographic boundary between rocks
of the Pacific Basin, which are basaltic, and those around the rim of
the basin, which are in part andesitic.
angle of incidence The angle at which a ray of energy
approaches a surface.
angle of reflection The angle at which a reflected
ray of energy leaves a surface.
angle of refraction The angle at which a refracted
ray of energy leaves a surface after passing through it.
angle of repose The maximum angle at which loose material
will come to rest when added to a pile of similar material.
angular unconformity An unconformity in which the beds
below the unconformity dip at a different angle than the beds above
anion An ion with a negative electrical charge. That
is, an atom that has gained one or more electrons.
anticline A fold that is convex upward, or that had
such an attitude at some stage of its development. compare syncline
aphanitic A textural term meaning "fine-grained"
that applies to igneous rocks.
aquifer A permeable region of rock or soil through
which ground water can move.
aquitard A material of low permeability that greatly
slows the movement of ground water.
arch Forms along a coast as wave erosion cuts through
Archean An eon of geologic time extending from about
3.9 billion years to 2.5 billion years ago.
arête A narrow, saw-toothed mountain ridge developed
by glacier erosion in adjacent cirques .
arkose A sedimentary rock formed by the cementation
of sand-sized grains of feldspar and quartz.
artesian well A well in which the water in the aquifer
is under pressure that raises the water above the point that the well
first encounters it.
assemblage The collection of minerals that characterize
a rock or a facies.
asthenosphere The weak or "soft" zone in
the upper mantle just below the lithosphere , involved in plate movement
and isostatic adjustments. It lies 70 to 100 km below the surface and
may extend to a depth of 400 km. Corresponds to the seismic low-velocity
astronomic theory of glaciation A theory based on the
changing position of the Earth in its orbit around the Sun.
asymmetric rock knob or hill Bedrock forms with a gentle
slope on one side created by glacial abrasion and a steep slope on the
opposite side created by glacial plucking.
atoll A roughly circular reef with an occasional small,
low, coral sand island surrounding a shallow lagoon.
atom A building block of matter, the smallest particle
that has the chemical characteristics of a particular chemical element.
It contains a nucleus of protons and neutrons surrounded by a cloud
atomic mass number The sum of the number of protons
and the number of neutrons in an atom. Approximately equal to the mass
of the atom.
atomic number The number of protons in an atom, a quantity
that determines which element the atom represents. Example: all atoms
of oxygen have 8 protons.
aureole A zone surrounding an igneous intrusion, in
which contact metamorphism has taken place.
authigenesis The process by which new minerals form
in a sediment or sedimentary rock during or after deposition.
axial plane A geometric plane that intersects the trough
or crest of a fold in such a way that the limbs of the fold are more
or less symmetrically arranged with reference to it.
axis The line formed by the intersection of the axial
plane of a fold with a bedding plane, marking where the bed shows its
back-arc basin The region between an island arc and
the continental mainland, commonly with at least some oceanic crust
on its floor.
back swamp A swamp that forms in the low lying flood
plain behind a levee.
backshore Lies between high tide mark and the foot
of the beach dune or the limit of effective wave action.
banded iron formation (BIF) A sedimentary mineral deposit dominated
by iron oxides, carbonates, or silicates that were deposited chemically
from seawater. Most BIFs were formed between 2.5 and 3.5 billion years
ago. Their formation is related to the rise of oxygen in the atmosphere.
bankfull stage A stream discharge that just fills the
bar 1. A mass of sand, gravel, or alluvium deposited
on the bed of a stream, sea, or lake, or at the mouth of a stream 2.
A unit of pressure, approximately equal to atmospheric pressure at sea
barchan A crescent-shaped sand dune with horns pointing
barrier beaches or islands Long narrow beaches separated
in many places from the mainland by lagoons.
barrier reef A coral reef separated from the mainland
by a lagoon.
basalt A dark colored extrusive igneous rock composed
chiefly of calcium plagioclase and pyroxene. Extrusive equivalent of
gabbro, underlies the ocean
basins and comprises oceanic crust.
base flow Ground water that enters a stream channel,
maintaining stream flow at times when it is not raining.
base level Of a stream is the point below which the
stream cannot cut. A temporary base level along a stream, such as a
lake may be removed by stream action . Ultimate base level is the ocean.
basin A synclinal structure, roughly circular in its
outcrop pattern, in which beds dip gently toward the center from all
batholith A large, discordant, intrusive body of igneous
bauxite A rock composed of aluminum hydroxides and
impurities in the form of silica, clay, silt, and iron hydroxides. A
residual weathering product, exploited as the primary ore for aluminum.
bay barrier A beach that cuts off a bay from the sea.
beach replenishment Rebuilding a beach by adding sand
beach Temporary accumulations of sediments that collect
between low and high tide marks.
bed load Material in motion along a stream bed.
bedding A collective term used to signify presence
of beds, or layers, in sedimentary rocks and deposits.
bedding plane Surface separating layers of sedimentary
rocks and deposits. Each bedding plane marks termination of one deposit
and beginning of another of different character, such as a surface separating
a sandstone bed from an overlying mudstone bed. Rock tends to breaks
or separate, readily along bedding planes.
bedrock Any solid rock exposed at the Earth’s surface
or overlain by unconsolidated material.
beheaded stream The headwaters of a stream that have
been captured by another stream. compare stream piracy .
berm A small terrace in the backshore area of the coast
with its terrace facing seaward.
beta decay The process of radioactive decay in which
a neutron loses a beta particle, which is physically identical to an
electron. This increases the atomic number of the atom by one by turning
the neutron into a proton. The atom’s atomic mass number stays the same
because the total number of protons and
neutrons remain the same. The most common form of radioactive decay.
BIF see banded iron formation
binding energy The energy that holds the particles
in the nucleus of an atom together. It is this energy, when released,
that is used to generate nuclear power.
biogenic sediment Sediments produced directly by the
life processes of plants or animals.
biogenic sedimentary rock A sedimentary rock composed
primarily of biogenic sediments.
bioturbation The turning and mixing of sediments by
black smoker A vent on the seafloor from which hydrothermal
fluids are emitted. Upon mixing with seawater and cooling, the fluids
precipitate a cloud of fine-grained sulfide minerals that resembles
a cloud of black smoke.
blind valley A valley in karst that ends abruptly downstream
at the point where its stream disappears underground as a sinking stream.
blowout A an irregular depression excavated by wind,
usually in previously deposited blown sand.
body wave Any seismic wave that travels through the
body of the Earth, rather than along its surface. compare surface wave.
bond (ionic, covalent, Van der Waals, metallic) see
bottomset bed Layer of fine sediment deposited in a
body of standing water beyond the edge of a growing delta and which
is eventually built over by the advancing delta. Similarly bottomset
beds may accumulate in the wind shadow of a sand dune and be preserved
beneath it as the dune advances.
boudinage A structure in which brittle beds bounded
by more ductile ones have been divided into segments during metamorphism.
boulder train Clusters of erratics from same source,
with some distinctive characteristic that makes their common source
boundary The tectonic region in which two plates meet.
Bowen’s Reaction Series A series of minerals formed
during crystallization of a magma, in which the formation of minerals
alters the composition of the remaining magma. Mafic minerals comprise
a discontinuous series, in which successive minerals form at the expense
of early-formed ones. The plagioclase feldspars form in a continuous
series, in which the composition of plagioclase becomes progressively
sodium rich, but the crystal structure of the mineral does not change.
braided stream A stream with a complex tangle of converging
and diverging channels separated by sand bars or islands.
branch work cave Cave with passage ways formed along
bedding planes and with an areal pattern similar to that of surface
breakwater A protective wall built offshore and usually
parallel to the shore.
breccia A clastic rock in which the gravel-sized particles
are angular in shape and make up an appreciable volume of the rock.
breeder reactor A nuclear reactor in which 238U or
232Th, which are not easily fissionable, absorb neutrons to become atoms
of 239Pu or 236U, which can later be used as fuels in fission reactors.
Breeder technology is not yet feasible.
brittle Structural behavior in which a material deforms
permanently by fracturing.
brittle limit The stress limit beyond which a material
fractures, rather than behaving in a ductile or elastic fashion.
burial metamorphism Takes place in an environment where
pressure and temperature are barely more intense than during diagenesis
, typically in a deepening sequence of sediments.
calcarenite A sandstone in which the sand-sized grains
caldera A large, basin-shaped volcanic depression,
more or less circular in form. Typically steep-sided, found at the summit
of a shield volcano .
caliche Gravel, sand, or desert debris cemented by
calcium carbonate, an accumulated product of chemical weathering in
a dry climate. compare claypan, fragipan, hardpan.
calving The breaking away of ice from the front of
the glacier when it ends in a lake or an ocean. Produces icebergs.
cap rock A comparatively impervious stratum immediately
overlying an oil- or gas-bearing rock.
capacity The total amount of material a stream is able
to carry under given conditions.
capillary water Water in the zone of aeration held
to soil particles by surface tension of the water molecules for each
other and for the soil particles.
carbonate conservation depth The water depth below which the
calcium carbonate produced in the ocean is completely dissolved. There
is no calcium carbonate deposition below this level.
carbonate rock A rock consisting primarily of a carbonate
mineral such as calcite or dolomite, the chief minerals in limestone
and dolostone, respectively.
cataclastic metamorphism Takes place in an environment
where intense pressure due to shearing is common, as in a major fault
cation An ion that has a positive electrical charge.
That is, an atom that has lost one or more electrons.
cave A natural open space underground, large enough
for a person to enter. Most commonly occur by the dissolution of soluble
rocks, generally limestone.
cementation Process by which a binding, or cementing,
agent is precipitated in spaces among individual particles of a deposit.
Common cementing agents are calcite, quartz, and dolomite.
Cenozoic The current geologic era, which began 66.4
million years ago and continues to the present.
chain reaction A self-sustaining nuclear reaction,
made possible when neutrons released by fission of some atoms in a nuclear
reactor strike other atoms, causing them to fission as well.
chalcedony A cryptocrystalline form of quartz, microscopically
fibrous with waxy luster. May be transparent or translucent, and with
a uniform tint of white, gray, pale blue, and, less often, black.
chalk A variety of limestone made up in part of biochemically
derived calcite, in form of skeletons or skeletal fragments of microscopic
oceanic plants and animals mixed with fine-grained calcite deposits
of biochemical or inorganic-chemical origin.
chemical bond The interactions among the electrons
of atoms that hold atoms together to form chemical compounds. If electrons
cluster primarily around one atom of a pair, the bond is ionic . If
they are shared more or less equally, it is covalent . If electrons
move freely between atoms over an extended region, the bond is metallic.
A weak electrostatic bond due to uneven distribution of electrons around
atoms or groups of atoms is a Van der Waals bond.
chemical element A fundamental substance that cannot be further refined
or subdivided by chemical means. All atoms of a chemical element have
the same number of protons.
chemical remanent magnetism Acquired as magnetic minerals
form and align themselves to the global magnetic field during diagenesis
of a sedimentary deposit.
chemical sediment Sediment formed by chemical precipitation
from water. Example: halite precipitated as the result of the evaporation
of sea water.
chemical sedimentary rock A sedimentary rock made up
of chemical sediments. Example: rock salt.
chemical weathering see decomposition
chert A cryptocrystalline form of quartz, microscopically
granular. Occurs as nodules and as thin, continuous layers. Duller,
less waxy luster than
chalcedony. Occurs in limestone, dolostone, and mudstones.
chlorofluorocarbons (CFC) Gases that can be dissociated
by solar radiation, which releases chlorine, which in turn destroys
chute cutoff A narrow "short cut" across
a meander bend, formed in flood as the main stream flow is diverted
into a trough between point bars. Sometimes called simply a ‘"chute".
cinder cone A conical volcano formed by the accumulation
of pyroclastic debris around a vent.
cirque A steep-walled hollow in a mountain side, shaped
like an amphitheater, or bowl, with one side partially cut away. Place
of origin of a mountain glacier.
clastic Refers to rock or sediments made up primarily
of broken fragments of pre-existing rocks or minerals.
clay 1. The name for a family of finely-crystalline
sheet silicate minerals. 2. Fine-grained soil consisting of mineral
particles, not necessarily clay minerals, that are less than 0.074 mm
in their maximum dimension.
claypan A layer of stiff, compact, relatively impervious
clay which is not cemented. compare caliche , fragipan, hardpan.
cleavage 1. of a mineral: The tendency of a mineral
to split along planes determined by the crystal structure. 2. of a rock:
see slaty cleavage
coal Sedimentary rock composed of combustible matter
derived from the partial decomposition of plant material.
coast A narrow strip of land along the margin of the
ocean extending inland for a variable distance from low water mark.
col Mountain pass formed by enlargement of two opposing
cirques until their head walls meet and are broken down.
column Pillar formed as a stalactite and stalagmite
columnar jointing The type of jointing that breaks
rock, typically basalt, into columnar prisms. Usually the joints form
a more or less distinct hexagonal pattern.
compaction Reduction of pore space between individual
particles as the result of overlying sediments or of tectonic movements.
competence The maximum size of particle that a stream
composite volcano see stratovolcano
Comprehensive Soil Classification System (CSCS) The
classification system in most common use by North American soil scientists.
Categories are based on the chemical and physical characteristics of
a soil. compare USDA Soil Classification System .
compression Squeezing a material from opposite directions.
concordant Lying parallel to, rather than cutting across
concretion A compact mass of mineral matter, usually
spherical or disk-like in shape and embedded in a host rock of different
composition. They form by precipitation of mineral matter about a nucleus
such as a leaf, or a piece of shell of bone.
conduction Heat transport by direct transfer of energy
from one particle to another, without moving the particle to a new location.
compare convection , radiation .
cone of depression A downward distortion or dimple
in the water table that forms as a well pumps water faster than it can
flow through the aquifer.
conglomerate A clastic sedimentary rock composed of
lithified beds of rounded gravel mixed with sand.
Constancy of Interfacial Angles The statement that
the angles between congruent crystal faces on samples of a single mineral
are always identical. A consequence of, and therefore evidence for the
existence of crystalline structure in minerals.
contact metamorphism Metamorphism genetically related
to the intrusion (or extrusion) of magmas and taking place in rocks
at or near their contact with a body of igneous rock.
continental arc A belt of volcanic mountains on the
continental mainland that lie above a subduction zone. compare island
continental crust The part of the crust that directly
underlies the continents and continental shelves. Averages about 35
km in thickness, but may be over 70 km thick under largest mountain
continental deserts Located in continental interior
far from moisture-bearing winds.
continental divide A major drainage divide separating
the drainage to one ocean from another.
continental drift The theory that explained the relative positions
and shapes of continents, the formation of mountains, and other large-scale
geologic phenomena as results of the lateral movement of continents.
The crust of ocean basins was assumed to be relatively immobile. compare
plate tectonics , sea floor spreading .
continental ice glacier An ice sheet that obscures
all but the highest peaks of a large part of a continent.
continental rise The portion of the continental margin
that lies between the abyssal plain and the continental slope. The
continental rise is underlain by crustal rocks of the ocean basin.
continental shelf The portion of the continental margin
that extends as a gently sloping surface from the shoreline seaward
to a marked change in slope at the top of the continental slope . Seaward
depth averages about 130 m.
continental slope That part of the continental margin
that lies between the continental shelf and the continental rise. Slope
relatively steep, 3o – 6o. The continental slope is underlain by crustal
rocks of the continent.
convection Heat transport by moving particles, and
the thermal energy that they carry, to a new location. compare conduction
, radiation .
convection cell A cyclical pattern of movement in a
fluid body such as the ocean, the atmosphere, or the Earth’s mantle,
driven by density variations which in turn are the result of differences
in temperature from one part of the fluid to another.
convergent boundary A boundary between two plates of
the Earth’s crust that are pushing together.
co-product A mineral commodity that is recovered from
a mining operation for some other mineral product. Example: Platinum
is commonly a co-product of nickel mining.
coquina A coarse-grained, porous variety of clastic
limestone made up chiefly of shells and shell fragments.
core Innermost zone of Earth. Consists of two parts,
an outer liquid section and an inner solid section, both chiefly of
iron and nickel with about 10 percent lighter elements. It is surrounded
by the mantle.
correlation Process of establishing contemporaneity
of rocks or events in one area with rocks or events in another area.
crater 1. A steep-walled, usually conical depression
at the summit or on the flanks of a volcano, resulting from the explosive
ejection of material from a vent. 2. A bowl-shaped depression with a
raised, overturned rim produced by the impact of a meteorite or other
craton The stable portions of the continents that have
escaped orogenic activity for the last 2 billion years. Made predominantly
of granite and metamorphic rocks. compare orogen .
creep 1. The very slow, generally continuous downslope
movement of soil and debris under the influence of gravity. 2. The movement
of sand grains along the land surface.
crevasse 1. Breach in a natural levee . 2. Deep crevice
or open fracture in glacier ice.
cross-bedding see inclined bedding .
cross-cutting relationships Geologic discontinuities
that suggest relative ages: A geologic feature is younger than the feature
it cuts. Thus, a fault cutting across a rock is younger than the rock.
crust The upper part of the lithosphere , divided into
oceanic crust and continental crust .
crystal The multi-sided form of a mineral, bounded
by planar growth surfaces, that is the outward expression of the ordered
arrangement of atoms within it.
crystal settling Gravitational sinking of crystals
from the liquid in which they formed, by virtue of their greater density.
A type of igneous differentiation.
crystal structure The regular and repeated three-dimensional
arrangement of atoms or ions in a crystal.
crystalline 1. Having a crystal structure. 2. When
referring to sedimentary rocks, crystalline designates a texture in
which mineral crystals have formed in an interlocking pattern. see nonclastic.
3. As a generic term, geologists use the term
"crystalline rocks" as a rough synonym for "igneous or
cumulate An igneous rock that forms by crystal settling
Curie point The temperature above which a mineral loses
current ripple mark An asymmetric ripple mark formed
by wind or water moving generally in one direction. Steep face of ripple
faces in direction of current. compare oscillation ripple mark .
cyclothem A series of beds, of interest because they
include coal, which were associated with unstable shelf or interior
basin conditions in which alternating marine transgressions and regressions
Darcy’s law A formula describing the flow of water
through an aquifer.
daughter An atom that results from the radioactive
decay of a parent atom.
debris flow Fast-moving, turbulent mass movement with
a high content of both water and rock debris. The more rapid
debris flows rival the speed of rock slides.
decay rate The rate at which a population of radioactive
atoms decays into stable daughter atoms. Rate often expressed in terms
of half life of the parent isotope .
decomposition (chemical weathering) Weathering processes
that are the result of chemical reactions. Example: the transformation
of orthoclase to kaolinite.
deflation A process of erosion in which wind carries
off particles of dust and sand.
dehydration Any process by which water bound within
a solid material is released. Example: Gypsum (CaSO4·2H2O) becomes anhydrite
(CaSO4) by dehydration.
delta An assemblage of sediments accumulated where
a stream flows into a body of standing water and its velocity and transporting
power are suddenly reduced. . A "delta plain" is the upper
surface of a delta.
dendritic drainage A stream pattern that, when viewed
on a map or from the air, resembles the branching pattern of a
deciduous tree such as a maple or oak.
denudation The sum of the processes that result in
the wearing away or the progressive lowering of the Earth’s surface
by weathering, erosion, mass wasting, and transportation.
depositional environment The nature of the environment
in which sediments are laid down. They are immensely varied and may
range from the deep ocean to the coral reef and the glacial lake of
the high mountains. The nature of the
depositional environment may be deduced from the nature of the sediments
and rock deposited there.
depositional remanent magnetism Develops as magnetic
minerals settle through water and align themselves in the Earth’s magnetic
desert pavement A lag accumulation of pebbles or boulders
that cuts off further deflation.
desertification A process of land degradation initiated
by human activity, particularly in the zones along the margins of deserts.
detrital sedimentary rock A sedimentary rock made up
of detrital sediments.
detrital sediments Sediments made of fragments or mineral
grains weathered from pre-existing rocks.
diagenesis All the physical, chemical, and biologic
changes undergone by sediments from the time of their initial deposition,
through their conversion to solid rock, and subsequently to the brink
differential weathering Weathering that occurs at different
rates, as the result of variations in composition and mechanical resistance
of rocks, or differences in the intensity of weathering processes.
differentiation The process of developing more than
one rock type, in situ, from a common magma.
dike A tabular igneous intrusion that cuts across the
dilatancy An increase in the bulk volume of rock during
deformation. Possibly related to the migration of water into microfractures
dip The angle that a structural surface such as a bedding
plane or fault surface makes with the horizontal, measured perpendicular
to the strike and in the vertical plane.
dip pole see magnetic pole
dip slip fault A fault on which the movement is parallel
to the dip of the fault plane.
directed pressure Pressure applied predominately in
one direction, rather than uniformly.
discharge In a stream, the volume of water passing
through a channel in a given time.
disconformity An unconformity in which the beds above
the unconformity are parallel to the beds below the unconformity.
discordant Cutting across surrounding strata.
disintegration (mechanical weathering) The processes
of weathering by which physical actions such as frost wedging break
down a rock into fragments, involving no chemical change.
dissolution A chemical reaction in which a solid material
is dispersed as ions in a liquid. Example: Halite (NaCl) undergoes dissolution
when placed in water.
dissolved load Amount of material water carries in
distributary channels Stream channels that fan out from the
upstream point of the delta and carry the sediments that build the delta.
divergent boundary Boundary between two crustal plates that
are pulling apart.
dolostone A carbonate rock made up predominately of
the mineral dolomite, CaMg(C03)2.
dome An uplift or anticlinal structure, roughly circular
in its outcrop exposure, in which beds dip gently away from the center
in all directions.
drag fold A minor fold produced within a weak bed or
adjacent to a fault by the movement of surrounding rocks in opposite
drainage basin The area from which a stream and its
tributaries receives its water.
drainage divide The line that separates one drainage
basin from another.
drift Glacial deposits laid down directly by glaciers
or laid down in lakes, ocean, or streams as result of glacial activity.
dripstone Calcium carbonate deposited from solution
as water enters a cave through the zone of aeration. Forms stalactites,
stalagmites and other cave deposits.
drumlin Streamlined hill, largely of till, with blunt
end pointing into direction from which ice moved. Occur in clusters
called drumlin fields.
dry farming Farming without irrigation in drylands.
drylands A general term for semiarid and desert lands.
ductile Structural behavior in which a material deforms
permanently without fracturing.
dust bowl An area subject to dust storms, especially
south central United States.
dust devil A small, dust-bearing whirlwind.
dust storm Large volume of dust-sized particles lifted
high into the atmosphere.
Earth system System involving continuous interaction
of the solid Earth, the atmosphere, the oceans and living things.
earthflow A form of slow, but perceptible, mass movement,
with high content of water and rock debris. Lateral boundaries are well-defined
and the terminus is lobed. With increasing moisture content grades into
eccentricity of the Earth’s orbit A measure of the
circularity of the Earth’s orbit. It varies in cycles of about 100,000
and 400,000 years.
elastic Non-permanent structural deformation during
which the amount of deformation (strain) is proportional to the stress.
elastic rebound The statement that movement along a
fault is the result of an abrupt release of a progressively increasing
elastic strain between the rocks on either side of the fault.
elasticity The tendency for a body to return to its
original shape and size when a stress is removed.
electron A fundamental unit of matter, negatively charged
and disposed in a cloud surrounding the nucleus of an atom.
electron capture Nuclear decay in which a proton in
the nucleus acquires an electron from the outer cloud of the atom’s
electrons. This converts the proton to a neutron, reduces the number
of protons in the nucleus by one and atomic number of the original element
by one. Atomic mass number remains constant because the total number
of protons and neutrons is unchanged.
electron shell A characteristic energy level with which
an electron is associated. Electrons occupy discrete shells within the
cloud surrounding an atom’s nucleus. These may be thought of, loosely,
as if they represented orbits at distinct heights above the nucleus.
element see chemical element
end moraine see terminal moraine.
eon The primary division of geologic time which are,
from oldest to youngest, the Hadean, Archean, Proterozoic, and Phanerozoic
epicenter The point on the Earth’s surface that is
directly above the focus of an earthquake.
epoch A division of geologic time next shorter than
a period. Example: the Pleistocene epoch is in the Quaternary period.
equilibrium line On a glacier the line separating the
zone of accumulation from the zone of ablation .
era A division of geologic time next smaller than the
eon and larger than a period. Example: The Paleozoic era is in the Phanerozoic
eon and includes, among others, the Devonian period.
erratic A stone or boulder, glacially transported from
place of origin and left in an area of different bedrock composition.
esker A winding ridge of stratified drift . Forms in
a glacial tunnel and, when ice melts, stands as ridge up to 15 m high
and kilometers in length.
ETP curve see Milankovitch curve .
eustatic change in sea level A worldwide change in sea level,
such as caused by melting glaciers.
eutrophication The process of aging of lakes by the
addition of nutrients.
evaporite A mineral or rock deposited directly from
a solution (commonly seawater) during evaporation. For example, gypsum
and halite are evaporite minerals.
exfoliation The process by which concentric scales,
plates, or shells of rock are stripped or spall from the bare surface
of a large rock mass.
exfoliation dome A large dome-shaped form that develops
in homogeneous crystalline rocks as the result of exfoliation.
exotic river A river that is able to maintain its flow
through a desert because of water received from outside the desert.
extrusive Pertaining to igneous rocks or features formed
from lava released on the Earth’s surface.
facies see metamorphic facies , sedimentary facies
failed rift A rift emanating from a plate triple junction
along which minimal divergence has taken place.
fall When applied to mass movement of material refers
to free fall of material moving without contact with the surface.
fault The surface of rock rupture along which there
has been differential movement of the rock on either side.
fault gouge Soft, uncemented, pulverized clay-like
material found along some faults.
ferromagnesian Containing iron and magnesium, applied
to the mafic minerals. Example: olivine.
fetch Distance over which wave-forming winds blow.
field capacity see specific retention.
fiery cloud see nuée ardente .
fjord Glaciated valleys now flooded by the sea.
firn (névé) Granular ice formed by the recrystallization
of snow. Intermediate between snow and glacier ice.
fission The spontaneous or induced splitting, by particle
collision, of a heavy atomic nucleus into a pair of fragments plus some
neutrons. Controlled induced fission can be used as a source of nuclear
fission track dating Dating of minerals by fission
tracks, damage tracks left in a mineral by spontaneous alpha emissions.
fissure eruption An eruption of lava that takes place
from a fracture, usually without producing a cone.
flash flood A flood that rises and falls very rapidly.
flashy stream A stream with a high, short flood peak
and short lag time.
flint A variety of chert , often black because of included
flood Peak flow that tops the banks of a stream channel.
flood recurrence interval The number of years of record
plus 1 divided by the rank of each maximum annual flood.
floodplain Area bordering a stream over which water
spreads when the stream tops its channel banks.
flow When applied to mass movement, refers to a chaotic
movement of material in continuous contact with the ground surface,
commonly involving a moderate to high amount of water.
flow folding A fold formed in relatively fluid rocks
that have flowed toward a synclinal trough.
flowstone General term for deposits formed by dripping
and flowing water on walls and floors of caves.
fluid inclusion A tiny cavity in a crystal, commonly
1 to 100 microns in diameter, containing liquid and/or gas. Formed by
the entrapment of fluid during the growth or subsequent deformation
of the crystal.
focus The point within the Earth which is the center
of an earthquake, at which strain energy is first released and converted
to elastic wave energy.
foliation A planar structure that develops in metamorphic
rocks as a result of directed pressure.
fold and thrust mountains Mountains, characterized
by extensive folding and thrust faulting, that form at convergent plate
boundaries on continents.
foot wall block The body of rock that lies below an
inclined fault plane. compare hanging wall block
formation water The water, held in pore volume in sedimentary
rocks, that has persisted with little change in composition since it
was buried with the sediment.
foreset bed Inclined layers of sediment deposited on
the advancing margin of a growing delta or along the slip face of a
foreshock A minor tremor that precedes an earthquake.
An increase in seismicity may signal that a major release of strain
energy is about to occur.
foreshore Lies between low and high tide marks.
fossil Evidence in rock of the presence of past life,
such as a dinosaur bone, an ancient clam shell, or the footprint of
a long-extinct animal.
fossil fuel A hydrocarbon (coal or petroleum) that
can be extracted from the Earth for use as a fuel. Fossil fuels are
non-renewable energy sources.
fractional crystallization A sequence of crystallization
from magma in which the early-formed crystals are prevented from reacting
with the remaining magma, resulting in a magma with an evolving chemical
fragipan A dense layer of soil, containing silt and
sand but no organic matter and little clay, whose extreme hardness and
impermeability are due primarily to compaction. compare caliche , claypan,
free oscillation A vibration of a body such as a bell
or the Earth that continues without further influence after an initial
fringing reef A coral reef attached directly to the
frost wedging A type of disintegration in which jointed
rock is forced apart by the expansion of water as it freezes in fractures.
fusion The combination of two light nuclei to form
a heavier nucleus, with the accompanying release of energy. This is
the source of energy in a hydrogen bomb. If it could be controlled,
it could serve as an alternative to fission in nuclear power generation.
gabbro A coarse-grained igneous rock, chemically equivalent
to a basalt.
gardening The constant and slow churning of the lunar
regolith as the result of meteorite impacts.
geanticline An anticlinal structure presumed to form
in the context of geosynclinal evolution. Not in current use since the
development of plate tectonic theory.
geode Roughly spherical, hollow or partially hollow
accumulation of mineral matter. A few centimeters to nearly 0.5 m in
diameter. Outer layer of chalcedony lined with crystals that project
toward the hollow center. Crystals, often perfectly formed, usually
quartz although calcite and dolomite and – more rarely – other minerals.
Most commonly occur in limestone, and less often in shale.
geologic column The arrangement of rock units in the
proper chronological order from youngest to oldest.
geologic time scale The chronological sequence of units
of Earth time.
geology The science that deals with the study of the
planet Earth–the materials of which it is made, the processes that
act to change these materials from one form to another, and the history
recorded by these materials; the forces acting to deform the outer layers
of the Earth and create ocean basins and continents; the processes that
modify the Earth’s surface; the application of geologic knowledge to
the search for useful materials and the understanding of the relationship
of geologic processes to people.
geosyncline A downwarping of the Earth’s crust, either
elongate or basin-like, measured in scores of kilometers, in which sedimentary
and volcanic rocks accumulate to thicknesses of thousands of meters.
Not in current use since the development of plate tectonic theory.
geothermal energy Heat extracted from the Earth for
use as an power source.
geothermal gradient The rate at which temperature increases
with depth below the surface.
geyser A type of thermal spring which ejects water
intermittently with considerable force.
glaciation The formation, advance and retreat of glaciers
and the results of these activities.
glacier A mass of ice, formed by the recrystallization
of snow, that flows forward, or has flowed at some time in the past.
glacier ice Ice with interlocking crystals that makes
up the bulk of a glacier.
glass An inorganic solid in which there is no crystalline
glassy A texture of extrusive igneous rocks that develops
as the result of rapid cooling, so that crystallization is inhibited.
global warming The prediction that climate will warm
as a result of the addition to the atmosphere of humanly produced greenhouse
gneiss A coarse, foliated metamorphic rock in which
bands of granular minerals (commonly quartz and feldspars) alternate
with bands of flaky or elongate minerals (e.g., micas, pyroxenes). Generally
less than 50% of the minerals are aligned in a parallel orientation.
gneissosity The style of foliation typical of gneiss.
Gondwana The southern portion of the late Paleozoic
supercontinent known as Pangea. It means, literally "Land of the
Gonds" (a people of the Indian subcontinent). The variant Gondwanaland
found in some books, therefore, is a tautology.
gouge see fault gouge
graben see rift
graded bedding Type of bedding sedimentary deposits
in which individual beds become finer from bottom to top.
gradient Slope of a stream bed or hillside. The vertical
distance of descent over horizontal distance of slope.
granite Light colored, coarse grained, intrusive igneous
rock characterized by the minerals orthoclase and quartz with lesser
amounts of plagioclase feldspar and iron-magnesium minerals. Underlies
large sections of the continents.
granitic belt A region of granitic rock, one of two
characteristic regions within cratons .
granitization A metamorphic process by which solid
rock is converted into granite by the addition or removal of material,
without passing through a magmatic stage. compare metasomatism .
gravitational heating Planetary heating caused by the
conversion of potential energy into heat. Associated with the iron catastrophe
gravitational moisture Water in the zone of aeration
that is moving down toward the zone of saturation.
graywacke (lithic sandstone) A variety of sandstone
characterized by angular-shaped grains of quartz and feldspar, and small
fragments of dark rock all
greenhouse gases Gases (primarily water and carbon
dioxide, but also a variety of sulfur and nitrogen compounds and gaseous
hydrocarbons) that trap the Sun’s heat in the atmosphere.
greenstone An altered or metamorphosed mafic igneous
rock that owes its dark color to the presence of chlorite, epidote,
greenstone belt A region of greenstones, one of two
characteristic regions within cratons .
groin A wall built out from the shore, usually at perpendicular
to it to trap sand carried by longshore currents .
groove A broad, deep, generally straight furrow carved
in bed rock by the abrasive action of debris embedded in a moving glacier.
Larger and deeper than a glacial striation.
ground moraine Till deposited from main body of glacier
ground water table see water table .
ground water Water beneath the Earth’s surface.
guyot see seamount
habit A general term for the outward appearance of
a mineral, defined by the relative sizes and arrangement of characteristic
Hadean The oldest eon in Earth history, extending from
the origin of the Earth to about 3.9 billion years ago.
half-life The amount of time that it takes for one
half of an original population of atoms of a radioactive isotope to
hanging valley A valley whose mouth is high above the
floor of the main valley to which it is tributary. Usually, but not
always, the result of mountain glaciation.
hanging wall block The body of rock that lies above
an inclined fault plane. compare foot wall block
hardness Resistance of a mineral to scratching, determined
on a comparative basis by the Mohs scale .
hardpan A general term for a relatively hard layer
of soil at or just below the ground surface, cemented by silica, iron
oxide, calcium carbonate, or organic matter. compare caliche , claypan,
head (hydraulic head) The level to which ground water
in the zone of saturation will rise.
heat flow The amount of thermal energy leaving the
Earth per cm2/sec.
heave In mass movement the upward motion of material
by expansion as, for example, the heaving caused by freezing water.
hiatus A gap or interruption in the continuity of the
geologic record either because the record was never formed or because
it was destroyed by erosion. It represents the time interval spanned
by an unconformity .
high level nuclear waste Radioactive waste from defense
activities of the U. S. government and from spent fuel rods from nuclear
hinge fault A fault along which there is increasing
offset or separation along the strike of the fault plane, from an initial
point of no separation.
hoodoo A column or pillar of rock produced by differential
weathering in a region of sporadic heavy rainfall, commonly facilitated
by joints and by rock layers of varying hardness.
Hooke’s Law A statement of elastic deformation, that
strain is directly proportional to stress.
horn The sharp spire of rock formed as glaciers in
several cirques erode into a central mountain peak.
horst compare rift
hot spot A region of high heat flow on the Earth’s
surface, thought to lie above a mantle plume .
humus The generally dark, more or less stable part
of the organic matter in a soil, so well decomposed that the original
sources cannot be identified.
hydraulic conductivity Measure of permeability in Earth
hydraulic gradient The slope of the water table. Measured
by the difference in elevation between two points on the slope of the
water table and the distance of flow between them.
hydraulic head see head.
hydrograph Graph of variation of stream flow over time.
hydrologic system (or hydrologic cycle) The pattern
of water circulation from the ocean to the atmosphere to the land and
back to the ocean.
hydrolysis A decomposition reaction involving water,
in which hydrogen ions (H+) or hydroxyl ions (OH-) replace other ions.
The result is a new residual mineral. Example: the addition of water
to orthoclase produces kaolinite and releases K+ and silica into solution.
ice sheet A broad, mound-like mass of glacier ice that
usually spreads radially outward from a central zone.
ice shelf A floating ice sheet extending across water
from a land-based glacier.
icecap A small ice sheet.
igneous rock A rock that has crystallized from a molten
inclined bedding (cross-bedding) Bedding laid down
at an angle to the horizontal, as in many sand dunes.
inclined fold A fold whose axial plane is inclined
from the vertical, but in which the steeper of the two limbs is not
overturned. compare overturned fold.
inclusion (xenolith) A fragment of older rock caught
up in an igneous rock.
index fossil A fossil that identifies and dates the
strata in which it is typically found. To be most useful, an index fossil
must have broad, even worldwide distribution and must be restricted
to a narrow stratigraphic range.
index mineral A mineral formed under a particular set
of temperature and pressure conditions, thus characterizing a particular
degree of metamorphism.
inertia The tendency of a body to resist acceleration
. A moving body tends to keep moving at a constant speed in the same
direction, and a stationary body tends to remain in one place, unless
acted upon by an outside force.
inner core The solid innermost part of the core with
a diameter of a little over 1,200 km.
intensity A measure of the size of an earthquake in
terms of the damage it causes.
interlobate moraine Ridge formed along junction of
adjacent glacier lobes.
intrusive Pertaining to igneous rocks or features formed
by the emplacement of magma in pre-existing rock.
ion An atom that has an electrical charge, by virtue
of having gained or lost electrons. see cation, anion
ionic radius The effective distance from the center
of an ion to the edge of its electron cloud.
ionic substitution The replacement of one or more ions
in a crystal structure by others of similar size and electrical charge.
Example: Fe2+ is interchangeable with Mg2+ in most ferromagnesian minerals.
iron catastrophe The period in the Hadean eon during
which much of the iron in outer portions of the Earth migrated toward
the center of the planet, producing the core and releasing large amounts
of gravitational heat.
ironpan A hardpan in which iron oxides are the primary
island arc A curved belt of volcanic islands lying
above a subduction zone. compare continental arc.
isochemical reaction A reaction in which chemical constituents
of a rock are rearranged to form a new mineral assemblage, but no material
is added to or lost from the rock as a whole. Applied generally to diagenetic
or metamorphic environments.
isoclinal fold A fold in which the limbs are parallel.
isograd A line on a map joining points at which metamorphism
took place under similar temperature and pressure conditions, as indicated
by rocks belonging to the same metamorphic facies . Generally, the line
separates two adjacent metamorphic zones, as indicated by specific .
isotope Atoms that differ in atomic mass number , but
not in atomic number , are called isotopes. For example, oxygen (atomic
number 8) may have an atomic mass number of 16, 17, or 18, depending
on whether it has 8, 9, or 10 neutrons. It therefore has three isotopes.
isoseismal line A line on a map joining points of equal
isostasy The condition of equilibrium, comparable to
floating, of units of the lithosphere above the asthenosphere .
isostatic change in sea level A sea level change due
to change in load on Earth’s crust.
jasper A red variety of chert , its color coming from
minute particles of included hematite.
jet flow Flow in which fluid moves at high speed in
jet-like surges as does water in free fall over a falls.
jetty Similar to a groin but built to keep sand out
of a harbor entrance.
joint A surface of fracture in a rock, without displacement
parallel to the fracture.
juvenile hydrothermal fluid A hot fluid, largely water,
presumed to have been released from a magma.
kame Stratified drift deposited in depressions and
cavities in stagnant ice and left as irregular, steep sided hills when
the ice is melts.
kame terrace Stratified drift deposited between wasting
glacier and adjacent valley wall. Stands as a terrace when glacier melts.
karst A landscape that develops from the action of
ground water in areas of easily soluble rocks. Characterized by caves,
underground drainage and sinkholes.
kettle Depression in ground surface formed by the melting
of a block of glacier ice buried or partially buried by drift.
komatiite An ultramafic rock with a non-cumulate texture,
presumed to be extrusive.
laccolith A concordant igneous intrusion with a flat
floor and a convex upper surface, usually less than 8 km across and
from a few meters to a few hundred meters thick at its thickest point.
lag time The delay in the response of stream flow between
precipitation and flood peak.
lahar A mudflow composed chiefly of pyroclastic material
on the flanks of a volcano.
laminar flow Fluid flow in which flow lines are distinct,
and parallel and do not mix. compare turbulent flow .
lateral continuity The extent of a rock unit over a
considerable but definite area.
lateral moraine Moraine formed by valley glaciers along
laterite A highly weathered red soil rich in iron and
aluminum oxides. Typically formed in a tropical to temperate climate
where intense chemical weathering is common.
Laurasia The northern portion of the late Paleozoic
supercontinent called Pangea.
lava Molten rock that flows at the Earth’s surface.
lava dome A steep-sided rounded extrusion of highly
viscous lava squeezed out from a volcano and forming a dome-shaped or
bulbous mass above and around the volcanic vent. The structure generally
develops inside a volcanic crater.
lava flood (plateau basalt) A term applied to large
areas of basaltic lava presumably extruded from fissures.
lava lake A lake of lava, usually basaltic, in a volcanic
layered complex An intrusive igneous body in which
there are layers of varying mineral content.
levees Banks of sand and silt along stream bank built
by deposition in small increments during successive floods.
limb The portions of a fold that are away from the
hinge; the "sides" of the fold.
limestone A sedimentary rock composed mostly of the
mineral calcite, CaCO3.
linear dune Long, straight dune with slip faces on
lineation A general term applying to any linear feature
in a metamorphic rock.
liquefaction The transformation of a soil from a solid
to a liquid state as the result of increased pore pressure.
lithic sandstone see graywacke.
lithification The process by which an unconsolidated
deposit of sediments is converted in to solid rock. Compaction, cementation
and recrystallization are involved.
lithophile Said of an element that has a greater chemical
affinity for silicate rocks than for sulfides or for a metallic state.
lithosphere The rigid outer shell of the Earth. It
includes the crust and uppermost mantle and is on the order of 100 km
lithostatic stress The confining (non-directed) pressure
imposed by the weight of overlying rock.
littoral current see longshore current.
load Of a stream, the amount that it carries at any
loess Deposits of wind-borne dust.
longshore current (littoral current) A current that
flows parallel to the shore just inside the surf zone. Also called the
longshore drift The general movement of sediment parallel
to the shoreline. Waves generally carry sediment up the shore face at
an angle to the shoreline, but carry it straight out again, resulting
in a net longshore displacement.
Love wave A seismic surface wave that has a horizontal
(side-to-side) component but no vertical component.
low level nuclear waste (TRU) Comes largely from national
defense utilities and includes contaminated lab coasts, gloves, and
low velocity zone The seismic region within the upper
mantle that corresponds to the asthenosphere .
luster The manner in which light reflects from the
surface of a mineral, described by its quality and intensity.
mafic Referring to a generally dark-colored igneous
rock with significant amounts of one or more ferromagnesian minerals,
or to a magma with significant amounts of iron and magnesium.
magma Molten rock, containing dissolved gases and suspended
solid particles. At the Earth’s surface, magma is known as lava .
magma ocean A global-scale ocean of magma, according
to some calculations several hundred kilometers deep, thought to have
existed during the final stages of accretion as the Earth was forming.
magnetic anomaly The amount by which a measurement
of the local magnetic field intensity exceeds or falls below the intensity
of the global magnetic field.
magnetic chron Time during which magnetic polarity
is dominantly normal or dominantly reversed.
magnetic declination Angle of divergence between true
north and magnetic north. Measured in degrees east or west of true,
or geographic north.
magnetic equator Lies half way between the north and
south magnetic poles.
magnetic inclination The angle of dip of the compass
needle as it varies from horizontal at the magnetic equator to vertical
at the magnetic poles.
magnetic polarity The direction, north (normal) or
south (reversed), that a magnetic compass needle points.
magnetic polarity time scale A chronology based on
the shifting polarity of the Earth’s magnetic field.
magnetic pole The point on the Earth’s surface where
a magnetic needle points vertically downward (north magnetic pole) or
vertically upward (south magnetic pole).
magnetic stratigraphy A stratigraphic sequence based
on the magnetic polarity of the rocks.
magnetic subchron A period during a magnetic chron
when the magnetic polarity is the opposite from that of the magnetic
magnetic polarity The direction, north (normal) or
south (reversed) that a magnetic needle points.
magnitude A measure of the strength of an earthquake
based on the amount of movement recorded by a seismograph . compare
mantle That portion of the Earth below the crust and
reaching to about 2,780 km, where a transition zone of about 100 km
thickness separates it from the core.
mantle plume A hypothetical column of hot, partially
molten material that rises from an indeterminate depth in the mantle
and is thought by some geologists to provide a driving force for plate
movement. compare hot spot .
marble A metamorphic rock composed largely of calcite.
The metamorphic equivalent of limestone.
margin The tectonic region that lies at the edge of
a continent, whether it coincides with a plate boundary or not.
mass movement The downslope movement of material under
the influence of gravity.
maze cave Caves in which passageways have interconnecting
loops that form a maze-like pattern.
meander A sharp bend, loop or turn in a stream’s course.
When abandoned, called a meander scar or an oxbow.
medial moraine Formed by the merging of lateral moraines
as two valley glaciers join.
mélange (clastic wedge) A mappable body of rock characterized
by blocks and fragments of all sizes, embedded in a sheared matrix.
A tectonic mélange commonly forms in the upper portions of a subduction
zone, where crustal rock is crushed and sheared.
mesosphere A zone in the Earth between 400 and 670
km below the surface separating the upper mantle from the lower mantle.
Mesozoic An era of time during the Phanerozoic eon
lasting from 245 million years ago to 66.4 million ago.
metal porphyry deposit A mineral deposit genetically
related to a pluton of porphyritic rock, commonly granodiorite. Scarce
metals are typically enriched by the passage of hydrothermal fluids
through rocks surrounding the intrusion, with the result that a metal-rich
halo forms there.
metamorphic facies A set of metamorphic mineral assemblages,
repeatedly associated in space and time, such that there is a constant
and therefore predictable relationship between mineral composition and
chemical composition. That relationship is a consequence of conditions
of temperature and pressure under which the assemblages are stable.
metamorphic rock A rock changed from its original form
and/or composition by heat, pressure, or chemically active fluids, or
some combination of them.
metamorphic zone A mappable region in which rocks have
been metamorphosed to the same degree, as evidenced by the similarity
of mineral assemblages in them.
metamorphism The processes of recrystallization, textural
and mineralogical change that take place in the solid state under conditions
beyond those normally encountered during diagenesis.
metasomatism The metamorphic processes that occur as
a result of the passage of chemically active fluids through a rock,
adding to or removing constituents during metamorphism.
microplate see terrane
migmatite A composite rock composed of igneous and
metamorphic materials, the result of partial melting at the upper limit
Milankovitch curve (ETP curve) A graph representing
the amount of solar radiation received at the Earth’s surface at a particular
latitude and time and based on the variations in the Earth’s orbital
mineral A naturally occurring inorganic solid that
has a well-defined chemical composition and in which atoms are arranged
in an ordered fashion.
mineral deposit Any natural concentration of a valuable
material in the Earth’s crust, whether that material can be extracted
profitably or not.
Modified Mercalli Scale A commonly used scale of earthquake
Mohorovicic discontinuity (Moho) The sharp seismic
velocity discontinuity that separates the crust and the mantle.
Mohs scale The ten-point scale of mineral hardness , keyed
arbitrarily to the minerals talc, gypsum, calcite, fluorite, apatite,
orthoclase, quartz, topaz, corundum, and diamond.
molecule The smallest unit of matter that has the chemical
and physical properties of a particular chemical compound.
momentum transfer In a rock slide the forward transfer of energy
by the collision of one block of rock with the next block forward. The
process makes possible progressively more rapid movement of material
in downslope positions.
monocline A simple fold, described as a local steepening in
strata with an otherwise uniform dip.
moraine Landform made largely of till.
mountain glacier see valley glacier .
mud cracks Cracks, generally polygonal, caused by the
shrinking of a deposit of clay or silt under surface conditions.
mudflow Form of mass movement similar to a debris flow
but containing less rock material.
mudstone A fine-grained detrital sedimentary rock made
up of clay- and silt-sized particles.
mylonite A chert-like rock without cleavage but with
a banded or streaky structure produced by extreme shearing of rocks
that have been pulverized and rolled during intense dynamic metamorphism.
nappe A sheet of rock that has moved over a large horizontal
distance by thrust faulting, recumbent folding, or both, so that it
lies on rocks of markedly different age or lithologic character.
neck cutoff Occurs as a river cuts through the narrow
neck of a meander. Sometimes called simply a "cutoff."
neutron A particle in the nucleus of an atom, which
is without electrical charge and with approximately the same mass as
névé see firn.
nivation Erosion beneath and around edges of a snow
bank. Results can foreshadow a cirque.
nodule A small, irregular, knobby-surfaced rock body
that differs in composition from the rock that encloses it. Formed by
the replacement of the original mineral matter. Quartz in the form of
flint or chert is the most common component. Most common in limestone
nonclastic A term applied to sedimentary rocks that
are not composed of fragments of pre-existing rocks or minerals. The
term "crystalline" is more commonly used.
nonconformity An unconformity that separates profoundly
different rock types, such as sedimentary rocks from metamorphic rocks.
normal fault A dip-slip fault on which the hanging
wall block is offset downward relative to the foot wall block . compare
reverse fault .
normal polarity Time when the compass needle points
to the magnetic north pole.
north magnetic pole The point on the Earth where the
north-seeking end of a magnetic needle, free to swing in space, points
nuclear power Power generated by controlled fission
or (potentially) fusion reactions, the heat from which is used to produce
steam and drive turbines.
nucleus (atomic) The center of an atom, containing
both protons and (except for 1H) neutrons.
nuée ardente (fiery cloud) A dense, hot (sometimes
incandescent) cloud of volcanic ash and gas produced in a Pelean eruption.
oblique-slip fault A fault with both dip-slip and strike-slip
components of movement.
obliquity of the Earth’s ecliptic Tilt of the Earth’s
rotational axis in relation to the plane in which the Earth circles
to Sun. Cycles from about 21.5o through 24.5o and back to 21.5o every
oceanic crust That part of the crust underlying the
ocean basins. Composed of basalt and having a thickness of about 5 km.
oil shale A mudrock that will yield liquid or gaseous
hydrocarbons upon distillation.
oolite Spheroidal grains of sand size, usually composed
of calcite and thought to have formed by inorganic precipitation.
open pit mining Surficial mining, in which the valuable
rock is exposed by removal of overlying rock or soil.
ophiolite A suite of mafic and ultramafic rocks and
associated marine cherts and their metamorphic equivalents.
ore The naturally occurring material from which a mineral or
minerals of economic value can be extracted at a profit.
ore deposit A continuous well-defined mass of material of sufficient
ore content to make extraction economically feasible. compare mineral
original horizontality Refers to the condition of beds or strata
as being horizontal or nearly horizontal when first formed.
orogen Linear to arcuate in plan, intensely deformed crustal
belt associated with mountain building. compare craton .
orogeny The process of mountain building.
oscillation ripple mark A symmetric ripple mark formed by waves,
which move water back and forth. compare current ripple mark .
outer core The outermost part of the core. It is liquid, about
1,700 km thick, and separated from the inner, solid core by a transition
zone about 565 km thick.
outwash Beds of sand and gravel laid down by glacial melt water
outwash plain A plain underlain by outwash.
overbank deposits Sediments deposited from flood water
on the flood plain.
overturned fold An inclined fold in which one limb
has been tilted beyond the vertical, so that the stratigraphic sequence
within it is reversed. compare inclined fold.
oxbow An abandoned meander .
oxbow lake A lake in an abandoned meander.
oxidation The decomposition process by which iron or
other metallic elements in a rock combine with oxygen to form residual
ozone hole Decrease of ozone in the stratosphere.
P- wave (primary wave, compressional wave) A seismic
body wave that involves particle motion, alternating compression and
expansion, in the direction of wave propagation. It is the fastest seismic
wave. compare S-wave.
pahoehoe A Hawaiian term for a basaltic lava flow
with a smooth, or ropy surface. compare aa.
paleomagnetism Study of the Earth’s past magnetism
as it is recorded in the rocks.
paleosol A buried soil horizon of the geologic past.
Paleozoic An era of geologic time lasting from 570
to 245 million years ago.
Pangea A supercontinent that existed from about 300
to 200 million years ago, and included most of the continental crust
of the Earth.
parabolic dune A sand dune that is parabolic in plan
with slip face convex downwind.
parent A radioactive element whose decay produces stable
partial melting The igneous process in which a rock
begins to melt at the lower end of its melting interval, yielding a
magma with a chemical composition different from the bulk composition
of the parent rock.
pascal A unit of pressure, equal to 1/100,000 of a
pedalfer A generic term used to describe the soils
typically formed in a humid region. Characteristically have an accumulation
of iron and aluminum oxides and hydroxides.
pedocal A generic term used to describe the soils typically
found in an arid or semiarid region. Characteristically have an accumulation
of carbonates, particularly calcite.
pegmatite An extremely coarse-grained igneous rock
with interlocking crystals, usually with a bulk chemical composition
similar to granite but commonly containing rare minerals enriched in
lithium, boron, fluorine, niobium, and other scarce metals. Pegmatites
are also the source for many gem-quality precious and semiprecious stones.
pegmatitic Having the texture of a pegmatite.
pelagic ooze A deep ocean sediment consisting of at
least 30% skeletal remains of calcareous or siliceous microorganisms,
the rest being clay minerals.
Peléan eruption A type of volcanic eruption characterized
by nuées ardentes and the development of lava domes .
peneplain Low gently, rolling landscapes produced by
perched water table A water table that develops at
a higher elevation than the main water table. <peridotite An ultramafic
igneous rock, the major constituent of the mantle.
periglacial Refers to conditions in a near glacial
period In the geologic time scale a unit of time less
than an era and greater than an epoch. Example: The Tertiary period
was the earliest period in the Cenozoic era and included, among others,
the Eocene epoch.
permafrost table The depth in a permafrost region at
which the maximum temperature reaches 0o C.
permafrost Soil conditions prevailing in area whose
mean annual temperature is 0o C.
permeability The capacity of material to transmit water
or other fluids.
petroleum A general term including both oil and natural
phaneritic A textural term meaning "coarse-grained"
that applies to igneous rocks.
Phanerozoic the most recent eon of geologic time beginning
570 million years ago and continuing to the present.
phenocryst Any relatively large, conspicuous crystal
in a porphyritic igneous rock. compare porphyroblast.
phyllite A metamorphosed mudstone with a silky sheen,
more coarse-grained than a slate and less coarse-grained than a schist.
piedmont glacier A glacier that spreads out at the
foot of mountains, formed by the coalescence of two or more valley glaciers.
pillow A structure observed in certain igneous rocks
extruded into water, characterized by discontinuous, close-fitting,
pillow-shaped masses, commonly 30 to 60 cm across.
pipe A vertical conduit through the Earth’s crust below
a volcano, through which magma has passed.
pirate stream A stream that captures the headwaters
of another stream.
placer A surficial mineral deposit formed by mechanical
concentration of valuable minerals from weathered debris, usually through
the action of stream currents or of waves.
plate A rigid segment of the Earth’s lithosphere that
moves horizontally and adjoins other plates along zones of seismic activity.
Plates may include portions of both continents and ocean basins.
plate boundaries The zones of seismic activity long
which plates are in contact. These may coincide with continental margins
, but usually do not. Movement between plates is predominately horizontal,
and may be divergent, or convergent, or side-by-side.
plate tectonics A theory of global tectonics according
to which the lithosphere is divided into mobile plates. The entire lithosphere
is in motion, not simply those segments composed of continental material.
compare continental drift
plate triple junction A point from which three rifts
emanate at roughly 120 degree angles. Example: the Afar triangle in
playa A broad flat desert basin, often containing an
ephemeral playa lake.
plucking (quarrying) A process of erosion in which
the glacier pulls loose pieces of bed rock.
plume The movement of water along flow lines from a
point source of ground water pollution toward its eventual emergence
at the surface.
plunging fold A fold in which the axis is inclined
at an angle from the horizontal.
pluton An igneous intrusion.
pluvial lake A lake formed during a pluvial period.
pluvial period Time when a dryland area had greater
effective moisture than at present.
pocket beach Small, narrow beach, usually crescentic,
at head of a bay or small inlet.
point bar Accumulations of sand and gravel deposited
in slack water on inside of a winding or meandering river.
polar deserts Deserts in which most moisture is locked
up in ground ice and unavailable as liquid water.
polar glacier A glacier whose temperature throughout
is always below freezing.
polish A smooth, polished surface imparted to some
rock types by glacier abrasion.
polymetamorphism A series of events in which two or
more metamorphic episodes have left their imprint on the same rocks.
polymorphism The circumstance in which two minerals with different
crystalline structures have identical chemical compositions. Example:
Diamond and graphite.
porosity The percentage of material occupied by pore space.
porphyritic A texture of an igneous rock in which large
crystals (phenocrysts) are set in a matrix of relatively finer-grained
crystals or of glass.
porphyroblast A large crystal of a mineral such as
garnet or staurolite set in a matrix of much finer-grained minerals
in a metamorphic rock. compare phenocryst.
potentiometric surface The level to which water will
rise in an artesian system when its confining aquitard is pierced.
pothole A hole or basin cut into bedrock of a stream
by the abrasive action of pebbles and sand swirled by turbulent stream
Precambrian An informal term to include all geologic
time from the beginning of the Earth to the beginning of the Cambrian
period 570 million years ago.
precession of the equinox The wobble of the Earth as
it spins changes the direction in which its axis of rotation points.
One wobble takes about 23,000 years.
pressure melting The phenomenon causing increased melting
of ice by increase of pressure.
Principle of faunal and floral succession Groups of
animals and plants have succeeded one another in a definite and discernible
prograde A succession of metamorphic conditions, each
of which is at a higher temperature and/or pressure than the preceding
Proterozoic The geologic eon lying between the Archean
and Phanerozoic eons, beginning about 2.5 billion years ago and ending
about 0.57 billion years ago.
proton A fundamental particle of matter. Provides a
positive charge in the nucleus of an atom.
pyroclastic Pertaining to clastic material formed by
volcanic explosion or aerial expulsion from a volcanic vent.
quarrying 1. The process by which building stone, usually
in blocks or sheets, is extracted from the Earth.. 2. see plucking
quartz arenite A sandstone in which the sand grains
are predominantly quartz.
quartzite A metamorphic rock consisting largely of
interlocking quartz grains; the metamorphic equivalent of a sandstone
radial drainage A pattern in which streams radiate
outward from a high central zone.
radiation Heat transport without the intervention of
matter, as in the transport of heat from the Sun to the Earth. compare
conduction , convection .
radioactivity The spontaneous decay of the nucleus
of an element. It involves the change in the number of protons in the
nucleus and therefore creates an atom of a new element.
radiocarbon 14C derived from 14N as cosmic ray bombardment
adds a neutron to its nucleus and the nucleus emits a proton. Radiocarbon
decays back to 14N by beta decay . Half life is 5730 ± 30 years.
rain shadow deserts Deserts formed by blocking moisture-bearing
winds with mountain barriers.
ramp The planar surface sloping seaward from the foot
of the shore face.
rapids Turbulent stream water flow down a steep gradient,
but not as steep as in a waterfall.
Rayleigh wave A type of seismic surface wave in which
particles follow a backward elliptical orbit in a vertical plane.
reaction rim A peripheral zone around a mineral grain,
composed of another mineral.
recessional moraine Ridges of glacial till marking
halt and slight readvance of glacier during its general retreat.
rectangular drainage A pattern in which a stream and
its tributaries follow courses marked by nearly right angle bends.
recumbent fold A fold in which the axial plane is horizontal.
refraction 1. Bending of waves or rays of energy, e.g.
seismic waves. 2. As applies to the near shore environment, the bending
of wave crests as they approach the shore.
regional metamorphism Metamorphism affecting an extensive
region, associated with orogeny .
regolith A layer of unconsolidated fragmental rock
rejuvenation Renewed stream erosion, generally as the
result of uplift. Generates features of youthful topography on a landscape
that was previously worn down to a base level.
relative time Dating of rocks and geologic events by
their positions in chronological order without reference to number of
years before the present.
remanent magnetism Magnetism acquired by a rock as
some time in the past.
reserves That portion of the resources for a valuable
mineral commodity that can be extracted from the Earth at a profit today.
reservoir rock Any porous and permeable rock that yields
oil or natural gas.
residual (resistant) mineral A mineral that persists
in soil after weathering, either because it was resistant to weathering
or because it was formed during the weathering process.
residual soil A soil presumed to have developed in
place as the product of decomposition and disintegration of bedrock
resources The reserves of a valuable mineral commodity
plus all other mineral deposits that may eventually become available,
even those that are presumed to exist but have not yet been discovered
and those that are not economically or technologically exploitable at
the moment. The total mineral endowment ultimately available for extraction.
retrograde A succession of metamorphic conditions,
each one of which is at a lower temperature and/or pressure than the
reverse fault A dip-slip fault on which the hanging
wall block is offset upward relative to the foot wall block . compare
normal fault .
reversed polarity Time when a magnetic needle points
to the south pole.
rhyolite A fine-grained silica-rich igneous rock, the
extrusive equivalent of granite.
Richter scale A commonly used measure of earthquake
magnitude , based on a logarithmic scale. Each integral step on the
scale represents a tenfold increase in the extent of ground shaking,
as recorded on a seismograph.
rift (graben) A valley caused by extension of the Earth’s
crust. Its floor forms as a portion of the crust moves downward along
normal faults .
rip current Carries excess water in the longshore current
out through the surf zone where it dissipates.
ripple marks of oscillation Symmetrical ripple marks
formed by oscillating movement of water such as may be found along the
coast just outside the surf zone.
ripple marks Small waves produced by wind or water
moving across deposits of sand or silt.
rock An aggregate of one or more minerals in varying
rock avalanche see rockslide.
rock cleavage see cleavage
rock cycle The concept of a sequence of events involving
the formation, alteration, destruction and reformation of rocks as a
result of geologic processes and which is recurrent, returning to a
starting point. It represents a closed system. compare rock system.
rock flour Finely divided rock material ground by glacial
action and fed by streams fed by melting glaciers.
rock glacier A mass of ice-cemented rock rubble found
on slopes of some high mountains. Movement is slow, averaging 30 to
rock record The history recorded in rocks.
rockslide (rock avalanche) A slide involving a downward and
usually sudden movement of newly detached segments of
bedrock sliding or slipping over an inclined surface of weakness such
as a bedding plane, fault plane, or joint surface.
rock system The concept of a sequence of events involving
the formation, alteration, destruction and reformation of rocks as a
result of geologic processes. Unlike the rock cycle it is an open system
and does not return to a starting point.
compare rock cycle
rock varnish A thin, shiny veneer of clay minerals
and iron and manganese oxides deposited on some rocks in a desert environment.
rock waste Angular fragments of rock. Forms a talus
if abundant enough.
rockfall The sudden fall of one or more large pieces
of a rock from a cliff.
roundness The degree to which a sedimentary particle’s
corners and edges are rounded.
runoff The precipitation that runs directly off the
surface to stream or body of standing water.
S wave (secondary wave , shear wave) A seismic body
wave that involves particle motion from side to side, perpendicular
to the direction of wave propagation. S-waves are slower than P-waves
and cannot travel through a liquid. compare P-wave .
salinization. A process by which salts accumulate in
salt-water invasion Displacement of fresh surface or
ground water by the advance of salt water.
saltation A process of sediment transport in which
a particle jumps from one point to another.
sand dune An accumulation of wind driven sand into
a distinctive shape.
sand sea A large area completely, or nearly completely,
covered with sand dunes.
sandstone A clastic sedimentary rock in which the particles
are dominantly of sand size, from 0.062 mm to 2 mm in diameter.
sandstorm A blanket of wind-driven sand with an upper
surface about a meter above ground level.
sanitary land fill An artificial hill formed by the
refuse of present-day civilization.
schist A strongly foliated, coarsely crystalline metamorphic
rock, produced during regional metamorphism, that can readily be split
into slabs or flakes because more than 50% of its mineral grains are
parallel to each other.
schistosity The foliation in a schist, due largely
to the parallel orientation of micas.
seafloor spreading Process by which ocean floors spread
laterally from crests of main ocean ridges. As material moves laterally
from the ridge, new material replaces it along the ridge crest by welling
upward from the mantle. compare continental drift , plate tectonics
seamount (guyot) A volcanic mountain on the seafloor.
If flat-topped, it is a guyot.
seawall A wall at the shore and parallel to it for
protection against wave erosion
sedimentary facies An accumulation of deposits that
exhibits specific characteristics and grades laterally into other
sedimentary accumulations that were formed at the same time but exhibit
sedimentary rock Rock formed from the accumulation
of sediment, which may consist of fragments and mineral grains of varying
sizes from pre-existing rocks, remains or products of animals and plants,
the products of chemical action, or mixtures of these.
seismic gap A segment of an active fault zone that
has not experienced a major earthquake during a time period when
most other segments of the zone have. Generally regarded as having a
higher potential for future earthquakes.
seismic sea wave (tsunami) A sea wave produced by any
large-scale, short duration disturbance on the seafloor, commonly a
shallow submarine earthquake but possibly also a submarine slide or
seismic tomography A technique for three-dimensional
imaging of the Earth’s interior by using a computer to compare the
seismic records from a large number of stations. Similar
in concept to a CAT scan used for medical purposes.
seismograph An instrument that detects, magnifies,
and records vibrations of the Earth, especially earthquakes.
seismology The study of earthquakes, and of the structure
of the Earth by both natural and artificially generated seismic waves.
seismoscope An instrument that merely indicates the
occurrence of an earthquake.
self-exciting dynamo In reference to the Earth, the
suggestion that movements in the fluid core may help initiate the Earth’s
shadow zone A region 100º to 140º from the epicenter
of an earthquake in which, due to refraction from below the core-mantle
boundary, no direct seismic waves can be detected.
shale A mudstone that splits or fractures readily.
shatter cone A distinctively striated conical structure
in rock, ranging from a few centimeters to a few meters in length, believed
to have been formed by the passage of a shock wave following meteorite
shear Rock deformation involving movement past each
other of adjacent parts of the rock and parallel to the plane separating
shear strength The resistance of a body to shear stress.
shear stress The stress on an object operating parallel
to the slope on which it lies.
sheeting A type of jointing produced by pressure release
(unloading) or exfoliation .
shield volcano A volcano in the shape of a flattened
cone, broad and low, built by very fluid flows of basaltic lava.
shock lamellae Closely spaced microscopic planes, distinct
from cleavage planes, that occur in shock-metamorphosed minerals and
are regarded as important indicators of shock metamorphism.
shock metamorphism Metamorphism induced in rock by
the passage of a high-pressure shock wave acting over a period of time
from a few microseconds to a fraction of a minute. The only known natural
cause of shock metamorphism is the hypervelocity impact of a meteorite.
shore Seaward edge of coast between low tide and effective
shore face The concave section of the beach from high
tide mark down to the ramp between 5 and 20 m off shore.
shore platform A surface of erosion that slopes gently
seaward from a cliff base to the low tide mark.
shoreline The line separating land and water. Fluctuates
as water rises and falls.
sial The upper layer of the continental crust, so called
because it is rich in silica and aluminum oxide. compare sima.
sialic Enriched in sial.
silica Silicon dioxide (SiO2) as a pure crystalline
substance makes up quartz and related forms such as flint and
chalcedony. More generally, silica is the basic chemical constituent
common to all silicate minerals and magmas.
silica tetrahedron The basic structural unit of which
all silicates are composed, consisting of a silicon atom surrounded
symmetrically by four oxygen atoms. The structure, therefore, has the
form of a tetrahedron with an oxygen atom at each corner.
sill A tabular igneous intrusion that parallels the
planar structure of the surrounding rock.
sima The oceanic crust, also the lower layer of the
continental crust, so called because it is enriched in silica and magnesium
oxide. compare sial.
sinkhole Depression in ground surface caused by collapse
into a cave below.
sinking stream A stream that empties into the underground
into a cave, usually through a sinkhole.
slate A compact, fine-grained metamorphic rock that
has slaty cleavage.
slaty cleavage A style of foliation common in metamorphosed
mudstones, characterized by nearly flat, sheet-like planes of breakage,
similar in appearance to a deck of playing cards. compare cleavage
slickenside A polished and smoothly striated surface
that results from friction along a fault plane.
slide A mass movement in which material maintains continuous
contact with the surface on which it moves.
slip face Steep face on lee side of sand dune.
slump Downward and outward rotational movement of Earth
materials traveling as a unit or series of units.
smelting The process of removing metal from ore.
snow line The elevation at which snow persists throughout
snowfields Expanses of snow that lie above the snow
soil All unconsolidated materials above bedrock. Natural
earthy materials on the Earth’s surface, in places modified or even
made by human activity, containing living matter, and supporting or
capable of supporting plants out of doors.
soil horizon A layer of soil that is distinguishable
from adjacent layers by characteristic physical properties such as texture,
structure, or color, or by chemical composition.
soil moisture Ground water in the zone of aeration
soil structure The combination of soil particles into
aggregates or clusters which are separated from adjacent aggregates
by surfaces of weakness.
soil texture The physical nature of the soil, according
to its relative proportions of sand, clay, and silt.
sole mark Develops as an irregularity on the bottom
of a stratum. It is a cast of a depression on the top surface of the
immediately underlying bed.
solifluction Turbulent movement of saturated soil or
sorting The range of particle sizes in a sedimentary
deposit. A deposit with a narrow range of particle sizes is termed "well-sorted."
south magnetic pole The point on the Earth where a
north seeking magnetic needle free to swing in space points directly
specific gravity The ratio of the density of a material
to the density of water.
specific retention (field capacity) The amount of capillary
water retained in a soil after the drainage of gravitational moisture.
sphericity A descriptive term to describe how close
a particle’s shape is to a sphere.
spit A sandy bar built out from the land into a body
spoil Overburden or non-ore removed in mining or quarrying.
spreading axis (spreading center) A region of divergence on
the Earth’s surface, as at a rift .
spreading pole A rotational pole around which a plate
appears to rotate on the Earth’s surface.
spring Occurs at the intersection of the water table
with the ground surface.
stack An isolated, steep-sided, rocky mass or island
just offshore from a rocky headland, usually on a shore platform.
stalactite An icicle-shaped accumulation of dripstone
hanging from cave roof.
stalagmite A post of dripstone growing up from a cave
star dune A sand dune built by winds alternating through
several directions. Builds vertically rather than migrating and growing
stick-slip A jerky, sliding motion associated with
stock A small batholith.
stoping A process of magmatic intrusion that involves
detaching and engulfing pieces of the surrounding rock, so that the
magma moves slowly upward.
storm surge A ridge of high water associated with a
hurricane and which floods over the shore .
strain Change in the shape or volume of a body as a
result of stress.
strain rate The rate at which a body changes shape
or volume as a result of stress.
strain seismograph A seismograph that is designed to
detect deformation of the ground by measuring relative displacement
of two points.
stratification The accumulation of material in layers
stratified drift Debris washed from a glacier and laid
down in well-defined layers.
stratigraphy The succession and age relation of layered
stratovolcano (composite volcano) A volcano that is
composed of alternating layers of lava and pyroclastic material, along
with abundant dikes and sills. Viscous, intermediate lava may flow from
a central vent. Example: Mt. Fuji in Japan.
streak The color of a mineral in its powdered form,
usually obtained by rubbing the mineral against an unglazed porcelain
tile to see the mark it makes. A mineral harder than the tile must be
pulverized by crushing.
stream capture see stream piracy.
stream order A classification of the relative hierarchy
of stream segments in a drainage network.
stream piracy (stream capture) The natural diversion of the
headwaters of one stream into the channel of another stream that has
greater erosional activity and flows at a lower level.
stream terrace A relatively flat surface along a valley,
with a steep bank separating it either from the floodplain, or from
a lower terrace.
strength The ability to withstand a stress without
stress The force per unit area acting on any surface
within a solid; also, by extension, the external pressure which generates
the internal force.
striations Scratches, or small channels, gouged by
glacier action. Occur on boulders, pebbles, and bedrock. Striations
along bedrock indicate direction of ice movement.
strike The compass direction of the intersection between
a structural surface (e.g., a bedding plane or a fault surface) and
strike-slip fault (transcurrent fault) A fault on which
the movement is parallel to the fault’s strike.
strip mining Open pit mining, typically for coal.
subduction zone A narrow, elongate region in which
one lithospheric plate descends relative to another.
sublimation The process by which matter in the solid
state passes directly to the gaseous state without first becoming liquid.
subtropical deserts Deserts in zones of descending
air between 25 degrees and 30 degrees north and south latitude.
superimposed stream A stream that was established on
a new surface and then, as it cut downward, maintained its course despite
encountering different lithologies in the process.
superposition A statement of relative age in layered
rocks: In a series of sedimentary rocks that has not been overturned,
the topmost layer is always the youngest and the bottommost layer is
always the oldest.
surf Produced as a wave steepens and falls forward
as the wave nears the shore.
surface of discontinuity In sand dune formation the surface
between quiet air of the wind shadow and the rapidly moving air above.
surface wave compare body wave
surging glacier A glacier that moves rapidly (tens
of meters per day) as it breaks away from the ground surface on which
suspended load The amount of material a stream carries
suspension A method of sediment transport in which
the turbulence of a fluid is able to keep particles supported in the
suture The line of juncture where continental rocks
on two converging plates meet. Example: The region in the Himalayas
where the Eurasian and Indian-Australian plates meet.
swash and back wash Uprush of a wave onto the beach
followed by the return flow of the water down the beach slope in the
intervals between waves.
swells Persistence of wind-formed waves after wind
syncline A fold that is convex downward, or that had
such an attitude at some stage in its development. compare anticline.
taconite A bedded ferruginous chert containing at least
25% iron. A potential iron ore.
tailings Washed or milled ore that is too poor to be
talus A slope built up by the accumulation of rock
waste at the foot of a cliff or ridge.
tar A thick brown to black viscous organic liquid,
too thick to migrate easily through most porous sediment.
tar sand A sand containing tar or asphalt, from which
the hydrocarbons may potentially be extracted by distillation.
tarn A lake in the bedrock basin of a cirque.
tell An artificial hill formed by the debris of successive
temperate glacier A glacier whose temperature throughout
is at, or close to, the pressure point of ice, except in winter when
it is frozen for a few meters below the surface.
tensile fracture A fracture caused by tensional stress
in a rock.
tension A stress that tends to pull a body apart.
tephra A general term for all pyroclastic material.
terminal moraine (end moraine) Ridge of till marking
farthest extent of glacier.
terrane (microplate) A fragment of the lithosphere,
smaller than a plate, that forms a portion of an accreted terrane margin.
texture The general appearance of a rock as shown by
the size, shape, and arrangement of the materials composing it.
"The present is the key to the past" A shorthand
reference to the principle of uniformitarianism .
thermal conductivity A measure of the ability of a
material to conduct heat.
thermal gradient see geothermal gradient.
thermal spring A spring whose temperature is 6.5o C
or more above mean annual air temperature.
thermoremanent magnetism The magnetism of a mineral
that it is acquired as it cools below its Curie point.
threshold of movement The point at which a slope or
slope material crosses from a condition of stability to one of instability
and movement begins.
thrust fault A reverse fault on which the dip angle
of the fault plane is 15 degrees or less.
thrust sheet A body of rock above a large-scale thrust
tidal delta A delta formed at both sides of a tidal
tidal inlet Waterway from open ocean into a lagoon.
tidal power Power generated by harnessing the energy
of tidal motion in the ocean.
till (unstratified drift) Glacial drift composed of
rock fragments that range from clay to boulder size and randomly
arranged without bedding.
topset bed Layer of sediments deposited over surface
of a delta, nearly horizontal and covering the tops of the inclined
triangulation The method of locating an epicenter by
determining how far it lies from three widely separated seismographs.
transcurrent fault see strike-slip fault
transform boundary A plate boundary in which plates
on opposite sides of the boundary move past each other in opposite directions.
transform fault A plate boundary that ideally shows
pure strike-slip movement. Associated with the offset segments of midocean
transported soil A soil that has been moved from the
site of its parent rock.
transverse dune A long, straight dune, perpendicular
to direction of wind.
trap 1. Any barrier to the upward migration of petroleum,
allowing it to accumulate. 2. Any dark colored extrusive igneous rock.
A reference to the tendency of basalt and similar rocks to form columnar
travel – time diagram A plot of seismic wave travel
time against distance on the Earth’s surface from the epicenter of an
travertine (tufa) Variety of limestone which forms
stalactites and stalagmites and other deposits in limestone caves (dripstone)
and the mouths of hot and cold calcareous springs.
trellis drainage A drainage pattern in which a stream
and its tributaries resemble the pattern of a vine on a trellis.
trench Along, narrow, steep-walled, often arcuate depression
in the ocean floor, much deeper than the adjacent ocean and
associated with a subduction zone .
troughs and bars Linear features in unconsolidated
sediments at the foot of the shoreface, the result of breaking waves.
TRU see Low level nuclear waste.
truncated spur The beveled end of a ridge separating
two valleys where they join a larger glaciated valley. Glacier of main
valley has eroded back the end of the ridge.
tufa see travertine .
tuff A general term for all consolidated pyroclastic
rock. Not to be confused with tufa.
turbidite Sedimentary deposit settled out of turbid
water carrying particles of widely varying grade size. Characteristically
displays graded bedding.
turbulent flow Fluid flow in which the flow lines are
confused and mixed. Fluid moves in eddies and swirls. compare laminar
U-shaped valley A valley carved by glacier erosion
and whose cross-valley profile has steep sides and a nearly flat floor,
suggestive of a large letter "U".
unconformity A buried erosion surface separating two
uniformitarianism The principle that applies to geology
our assumption that the laws of nature are constant As originally
used it meant that the processes operating to change the Earth in the
present also operated in the past and at the same rate and intensity
and produced changes similar to those we see today. The meaning has
evolved and today the principle of uniformitarianism acknowledges that
past processes, even if the same as today, may have operated at different
rates and with different intensities than those of the present. The
term "actualism" is sometimes used to designate this later
unloading The release of confining pressure associated
with the removal of overlying material. May result in expansion of rock,
accompanied by the development of joints or sheeting .
unstratified drift see till
USDA Soil Classification System A classification of
soils on the basis of the processes and conditions by which they form.
compare Comprehensive Soil Classification System.
valley glacier (alpine glacier, mountain glacier )
Streams of ice that flow down valleys in mountainous areas.
valley train Outwash plain contained within valley
varve A pair sedimentary units, one coarse-grained,
the other fine-grained, interpreted as representing one year of sedimentation.
velocity Distance of travel in unit of time
velocity profile A plot of seismic velocity against
depth in the Earth.
ventifact A pebble, cobble, or boulder faceted by wind
vesicle A cavity in a lava, formed by the entrapment
of a gas bubble during solidification of the lava.
vesicular A textural term applied to an igneous rock
containing abundant vesicles, formed by the expansion of gases initially
dissolved in the lava.
viscosity The internal resistance to flow in a liquid.
volcanic ash The dust-sized, sharp-edged, glassy particles
resulting from an explosive volcanic eruption.
volcanic cinder A pyroclastic fragment, 0.5 to 2.5
cm in diameter, formed as magma spatters into the air during a volcanic
eruption and cools as it falls to Earth.
volcano A vent in the surface of the Earth, from which
lava, ash, and gases erupt, forming a structure that is roughly conical.
volcanogenic massive sulfide deposit A mineral deposit
of metallic sulfides formed directly through processes associated with
volcanism, commonly in a submarine setting.
vulnerable mineral A mineral that does not easily resist decomposition
Wadati-Benioff zone An inclined plane, roughly coincident with
a subduction zone, along which the foci of earthquakes cluster.
Waste Isolation Pilot Plant A pilot plant near Carlsbad,
New Mexico, for the storage of low level nuclear waste.
water gap A gap in a ridge or mountain through which
a stream flows.
water power Power generated through the agency of moving
water table The surface between the zone of saturation
and the zone of aeration.
waterfall The perpendicular or very steep descent of a stream.
wave base A depth equal to one half the wave length
of waves in deep water, below which stirring due to wind is negligible.
wave crest The top of a wave.
wave height The vertical distance between the crest
and adjacent trough of a wave.
wave length The distance between two successive wave
crests or troughs.
wave trough The low spot between two successive waves.
weathering The process by which Earth materials change
when exposed to conditions at or near the Earth’s surface and different
from the ones under which they formed. compare decomposition , disintegration
welded tuff A pyroclastic rock in which glassy clasts
have been fused by the combination of the heat retained by the clasts,
the weight of overlying material, and hot gases.
well An artificial intersection of the surface and the water
Wilson Cycle The opening and closing of ocean basins
through plate tectonics.
wilting point The stage at which all water available
to plants has been used.
wind farm An area in which a large number of windmills
have been erected to generate electrical power.
wind gap An abandoned water gap.
wind power Power generated by using the force of the
wind shadow An area of quiet air in lee of an obstacle.
Zone of sand accumulation in lee of sand dune.
xenolith see inclusion
X-ray diffraction The diffraction of a beam of X-rays
by the three dimensional periodic array of atoms in a crystal structure
. The identity and arrangement of atomic in the structure can be determined
by interpreting the angles at which
X-rays are scattered by the structure and the intensities of scattered
yardang Sharp, irregularly-crested ridges carved by
wind and oriented parallel to wind.
yazoo-type river A tributary stream unable to enter
a main stream because of natural levees along the main stream. It flows
in a backswamp area, parallel to the main stream until it finds an entry
to the main stream.
yield point The stress limit at which permanent deformation
takes place in a non-brittle material.
Yucca Mountain Site Site in Nevada proposed for the
storage of high level nuclear waste.
zone of ablation The area of wastage in a glacier.
zone of accumulation 1. The B horizon in a residual
soil. 2. The area in which ice accumulates in a glacier.
zone of aeration Zone immediately below the ground
surface within which pore spaces are partially filled with water and
partially filled with air.
zone of flow The zone in a glacier that flows by deforming along planes
of weakness in the ice crystals.
zone of fracture The near surface zone in a glacier
that behaves like a brittle substance.
zone of leaching The upper horizons in a soil, through
which gravitational moisture travels, removing soluble decomposition
zone of saturation The zone below the zone of aeration
in which all pore spaces are filled with water.