The loss of snow and ice from a glacier, caused primarily by
A form of mechanical weathering that occurs when loose
fragments or particles of rocks and minerals that are being transported,
as by water or air, collide with each other or scrape the surfaces of
The fixing of a geological structure or event in time, as by counting
A mass of sediment and oceanic lithosphere that is
transferred from a subducting plate to the less dense,
overriding plate with which it converges.
The increase in a glacier's volume, caused primarily by
Rain that contains such acidic compounds as sulfuric acid and nitric
acid, which are produced by the combination of atmospheric water with
oxides released when hydrocarbons are burned. Acid rain is
widely considered responsible for damaging forests, crops, and
human-made structures, and for killing aquatic life.
See zone of aeration.
A ground tremor caused by the repositioning of rocks after an earthquake.
Aftershocks may continue to occur for as long as two years after the
initial earthquake. The intensity of an earth quake's aftershocks
decreases over time.
The process by which a stream's gradient steepens due to
increased deposition of sediment.
A metal that is manufactured by combining two or more molten metals. An
alloy is always harder than its component metals. Bronze is an alloy of
copper and tin.
A triangular deposit of sediment left by a stream that has lost
velocity upon entering a broad, relatively flat valley.
A deposit of sediment left by a stream on the stream's channel
or flood plain.
A mountain glacier that is confined by highlands.
The dark, aphanitic, extrusive rock that has a silica content
of about 60% and is the second most abundant volcanic rock. Andesites
are found in large quantities in the Andes Mountains.
The geographic boundary between the basalts and gabbros of
the Pacific Ocean basin and the andesites at the subductive
margins of the surrounding continents.
angle of repose
The maximum angle at which a pile of unconsolidated material can remain
A hard, jet-black coal that develops from lignite and bituminous
coal through metamorphism, has a carbon content of 92% to
98%, and contains little or no gas. Anthracite burns with an extremely
hot, blue flame and very little smoke, but it is difficult to ignite and
both difficult and dangerous to mine.
A convex fold in rock, the central part of which contains the
oldest section of rock. See also syncline.
Of or being an igneous rock containing grains that are so small
as to be barely visible to the naked eye.
An impermeable body of rock that may absorb water slowly but does not
A permeable body of rock or regolith that both stores
and transports groundwater.
A layer of rock having low permeability that stores groundwater but
delays its flow.
A sharp ridge of erosion-resistant rock formed between adjacent cirque
The ratio of a region's potential annual evaporation, as determined by
its receipt of solar radiation, to its average annual precipitation.
A small, deep, usually dry channel eroded by a short-lived or
intermittent desert stream.
Of, being, or concerning an aquifer in which water rises to the
surface due to pressure from overlying water.
A layer of soft but solid, mobile rock comprising the lower part of the
upper mantle from about 100 to 350 kilometers beneath the
Earth's surface. See also lithosphere.
A circular reef that encloses a relatively shallow lagoon and
extends from a very great depth to the sea surface. An atoll forms when
an oceanic island ringed by a barrier reef sinks below sea
The smallest particle that retains all the chemical properties of
a given element.
1. The sum of protons and neutrons in an atom's
nucleus. 2. The combined mass of all the particles in a given atom.
The number of protons in the nucleus of a given atom. Elements
are distinguished from each other by their atomic numbers.
A section of rock that surrounds an intrusion and shows the
effects of contact metamorphism.
A depression landward of a volcanic arc in a subduction
zone, which is lined with trapped sediment from the volcanic arc
and the plate interior. See also forearc basin.
The process by which the overriding plate in a subduction zone
becomes stretched to the point of rifting, so that magma can
then rise into the gap created by the rift. Backarc spreading typically
occurs when the subducting plate sinks more rapidly than the overriding
plate moves forward.
The portion of a beach that extends from the high-tide line
inland to the sea cliff or vegetation line. Swash reaches the
backshore only during major storms.
The section of a flood plain where deposits of fine silts and
clays settle after a flood. Backswamps usually lie behind a stream's natural
Water that returns into an ocean or large lake after hitting the shore
banded iron formation
A rock that is made up of alternating light silica-rich layers and
dark-colored layers of iron-rich minerals, which were deposited in
marine basins on every continent about 2 billion years ago.
A crescent-shaped dune that forms around a small patch of
vegetation, lies perpendicular to the prevailing wind direction, and has
a gentle, convex windward slope and a steep, concave leeward
slope. Barchan dunes typically form in arid, inland deserts with
stable wind direction and relatively little sand.
A ridge of sand that runs parallel to the main coast but is separated
from it by a bay or lagoon. Barrier islands range from
10 to 100 kilometers in length and from 2 to 5 kilometers in width. A
barrier island may be as high as 6 meters above sea level.
A long, narrow reef that runs parallel to the main coast but is
separated from it by a wide lagoon.
The process by which a glacier undergoes thawing at its base,
producing a film of water along which the glacier then flows. Basal
sliding primarily affects glaciers in warm climates or mid-latitude
The dark, dense, aphanitic, extrusive rock that has a silica
content of 40% to 50% and makes up most of the ocean floor. Basalt is
the most abundant volcanic rock in the Earth's crust.
Of, containing, or composed of basalt.
The lowest level to which a stream can erode the channel
through which it flows, generally equal to the prevailing global sea
A round or oval depression in the Earth's surface, containing the
youngest section of rock in its lowest, central part.
A massive discordant pluton with a surface area greater than
100 square kilometers, typically having a depth of about 30 kilometers.
Batholiths are generally found in elongated mountain - ranges after the
country rock above them has eroded.
A recess in a shoreline, or an inlet between two headlands.
A narrow ridge of sand that stretches completely across the mouth of a bay.
(Also called bay bar and bay barrier.)
The part of a coast that is washed by waves or tides, which
cover it with sediments of various sizes and composition, such
as sand or pebbles.
1. The process by which swash and backwash move sediments
along a beach face. 2. The sediments so moved. Beach
drift typically consists of sand, gravel, shell fragments, and pebbles.
See also longshore drift.
he portion of a foreshore that lies nearest to the sea - and
regularly receives the swash of breaking waves. The beach face
is the steepest part of the foreshore.
A layer of sediment or sedimentary rock that can be
distinguished from the surrounding layers by such features as chemical
composition and grain size.
A body of coarse particles that move along the bottom of a stream.
The division of sediment or sedimentary rock into
parallel layers (beds) that can be distinguished from each
other by such features as chemical composition and grain size.
The solid mass of rock that makes up the Earth's crust.
A region where the subduction of oceanic plates causes
earthquakes, the foci of which are deeper the farther inland
A low, narrow layer or mound of sediment deposited on backshore
by storm waves.
A renewable fuel derived from a living organism or the
byproduct of a living organism. Biomass fuels include wood, dung,
methane gas, and grain alcohol.
Any of a group of solid and semi-solid hydrocarbons that can be
converted into liquid form by beating. Bitumens can be refined to
produce such commercial products as gasoline, fuel oil, and asphalt.
A shiny black coal that develops from deeply buried lignite through
heat and pressure, and that has a carbon content of 80% to 93%, which
makes it a more efficient heating fuel than lignite.
A valley formed by and containing sinkholes and disappearmg
streams, and therefore dry except during periods of such heavy
rainfalls that the sinkholes cannot immediately drain the entire
accumulation of water.
A type of seismic wave that transmits energy from an
earthquake's focus through the Earth's interior in all
directions. See also surface wave .
To combine, by means of chemical reaction, with another atom to form a
compound. When an atom bonds with another, it either loses, gains, or
shares electrons with the other atom.
A large, smooth, round or dome-shaped inselberg.
The sequence of igneous rocks formed from a mafic magma, assuming
mineral crystals that have already formed continue to react with the
liquid magma and so evolve into new minerals, thereby creating the next
rock in the sequence.
A network of converging and diverging streams separated from
each other by narrow strips of sand and gravel.
A wall built seaward of a coast to intercept incoming waves and so
protect a harbor or shore. Breakwaters are typically built parallel to
A clastic rock composed of particles more than 2 millimeters in
diameter and marked by the angularity of its component grains and rock
A nuclear reactor that manufactures more fission able isotopes than it
consumes. Breeder reactors use the widely available, nonfissionable
uranium isotope U-238, together with small amounts of fissionable U-235,
to produce a fissionable isotope of plutonium, Pu-239.
Rupture of rock, a type of permanent strain caused by
relatively low stress.
A form of regional metamorphism that acts on rocks covered by 5
to 10 kilometers of rock or sediment, caused by heat from the Earth's
interior and lithosta tic pressure.
A vast depression at the top of a volcanic cone, formed when an
eruption substantially empties the reservoir of magma beneath
the cone's summit. Eventually the summit collapses inward, creating a
caldera. A caldera may be more than 15 kilometers in diameter and more
than 1000 meters deep.
A white soil horizon consisting of calcium carbonate, typical
of arid and semi-arid areas. Brief heavy rains dissolve calcium
carbonate in the upper layers of soil and transport it downward; the
rainwater then evaporates rapidly, leaving the calcium carbonate to form
a new, solid layer of soil.
The ability of a given stream to carry sediment, measured
as the maximum quantity it can transport past a given point on the
channel bank in a given amount of time. See also competence.
The lowest part of the zone of aeration, marked by the rising
of water from the water table due to the attraction of the
water molecules to mineral surfaces and other molecules, and to pressure
from the zone of saturation below.
A form of radiometric dating that relies on the 5730-year
half-life of radioactive carbon-14, which decays into nitrogen-14, to
determine the age of rocks in which carbon-14 is present. Carbon-14
dating is used for rocks from 100 to 100,000 years old.
The hypothesis that a series of immense, brief, worldwide upheavals
changed the Earth's crust greatly and can account for the development of
mountains, valleys, and other features of the Earth. See also uniformitarianism.
One of several minerals containing one central carbon atom with strong covalent
bonds to three oxygen atoms and typically having ionic bonds
to one or more positive ions.
A naturally formed opening beneath the surface of the Earth, generally
formed by dissolution of carbonate bedrock. Caves may also form
by erosion of coastal bedrock, partial melting of glaciers, or
solidification of lava into hollow tubes.
The diagenetic process by which sediment grains are bound
together by precipitated minerals originally dissolved during
the chemical weathering of preexisting rocks.
The latest era of the Phanerozoic Eon, following the Mesozoic
Era and continuing to the present time, and marked by the
presence of a wide variety of mammals, including the first hominids.
Sediment that is composed of previously dissolved minerals that
have either precipitated from evaporated water or been extracted from
water by living organisms and deposited when the organisms died or
discarded their shells.
The process by which chemical reactions alter the chemical composition
of rocks and minerals that are unstable at the Earth's surface and
convert them into more stable substances; weathering that
changes the chemical makeup of a rock or mineral. See also mechanical
A member of a group of sedimentary rocks that consist primarily
of microscopic silica crystals. Chert may be either organic or
inorganic, but the most common forms are inorganic.
A pyroclastic cone composed primarily of cinders.
Glassy, porous, pyroclastic rock fragments.
A deep, semi-circular basin eroded out of a mountain by an alpine
A small alpine glacier that forms inside a cirque,
typically near the head of a valley.
Being or pertaining to a sedimentary rock composed primarily
from fragments of preexisting rocks or fossils.
A mineral particle of any composition that is less than 1/256 of a
millimeter in diameter. Not to be confused with clay minerals.
One of a group of hydrous silicate minerals, such as kaolinite
and smectite, the extremely small particle size of which imparts the
ability to adsorb water. Clay minerals are the stable endproducts of the
chemical weathering of feldspars.
The tendency of certain minerals to break along distinct planes in their
crystal structures where the bonds are weakest. Cleavage is
tested by striking or hammering a mineral, and is classified by the
number of surfaces it produces and the angles between adjacent surfaces.
A member of a group of easily combustible, organic sedimentary rocks
composed mostly of plant remains and containing a high proportion of
The area of dry land that borders on a body of water.
A karst environment marked by numerous closely spaced,
irregular depressions and steep, conical hills.
A high mountain pass that forms when part of an arete erodes.
The diagenetic process by which the volume or thickness of
sediment is reduced due to pressure from overlying layers of sediment.
The ability of a given stream to carry sediment, measured
as the diameter of the largest particle that the stream can transport.
See also capacity.
An electrically neutral substance that consists of two or more elements
combined in specific, constant proportions. A compound typically
has physical characteristics different from those of its constituent
Stress that reduces the volume or length of a rock, as that
produced by the convergence of plate margins.
Of or being a pluton that lies parallel to the surrounding layers of
rock. See also discordant.
cone of depression
An area in a water table along which water has descended into a
well to replace water drawn out, leaving a gap shaped like an inverted
See lithostatic pressure.
A clastic rock composed of particles more than 2 millimeters in
diameter and marked by the roundness of its component grains and rock
Metamorphism that is caused by heat from a magmatic intrusion.
The convergence of two continental plates, resulting in the
formation of mountain ranges.
The hypothesis, proposed by Alfred Wegener, that today's continents
broke off from a single supercontinent and then plowed through the ocean
floors into their present positions. This explanation of the shapes and
locations of Earth's current continents evolved into the theory of plate
continental ice sheet
An unconfined glacier that covers much or all of a continent.
The cycle of movement in the asthenosphere that causes the
plates of the lithosphere to move. Heated material in the
asthenosphere becomes less dense and rises toward the solid lithosphere,
through which it cannot rise further and therefore begins to move
horizontally, dragging the lithosphere along with it and pushing forward
the cooler, denser material in its path. The cooler material eventually
sinks down lower into the mantle, becoming heated there and rising up
again, continuing the cycle. See also plate tectonics.
The coming together of two lithospheric plates. Convergence causes subduction
when one or both plates is oceanic, and mountain formation when
both plates are continental. See also divergence.
The innermost layer of the Earth, consisting primarily of pure metals
such as iron and nickel. The core is the densest layer of the Earth, and
is divided into the outer core, which is believed to be liquid, and the
inner core, which is believed to be solid. See also crust and mantle.
The process of determining that two or more geographically distant rocks
or rock strata originated in the same time period.
1. The preexisting rock into which a magma intrudes. 2. The
preexisting rock surrounding a pluton.
The combination of two or more atoms by sharing electrons so as to
achieve chemical stability under the octet rule. Atoms that
form covalent bonds generally have outer energy levels containing three,
four, or five electrons. Covalent bonds are generally stronger than
See volcanic crater.
The slowest form of mass movement, measured in millimeters or
centimeters per year and occurring on virtually all slopes.
A bed made up of particles dropped from a moving cur- rent, as
of wind or water, and marked by a downward slope that indicates the
direction of the current that deposited them.
The outermost layer of the Earth, consisting of relatively low- density
rocks. See also core and mantle.
A mineral in which the systematic internal arrangement of atoms is
outwardly reflected as a latticework of repeated three- dimensional
units that form a geometric solid with a surface consisting of
1. The geometric pattern created by the systematic internal arrangement
of atoms in a mineral. 2. The systematic internal arrangement of atoms
in a mineral. See also crystal.
Marked by the systematic internal arrangement of atoms.
1. A broad flow of ocean water that maintains a stable direction and
differs from the surrounding water in such features as temperature and
salinity. 2. The water in such a flow.
An isotope that forms from the radioactive decay of a
parent isotope. A daughter isotope may or may not be of the same
element as its parent. If the daughter isotope is radioactive, it will
eventually become the parent isotope of a new daughter isotope. The last
daughter isotope to form from this process will be stable and
The sudden, extremely rapid mass movement downward of entire
layers of regolith along very steep slopes. Debris avalanches
are generally caused by heavy rains.
1. The rapid, downward mass movement of particles coarser than
sand, often including boulders one meter or more in diameter, at a rate
ranging from 2 to 40 kilometers per hour. Debris flows occur along
fairly steep slopes. 2. The material that descends in such a flow.
The process by which wind erodes bedrock by picking up and
transporting loose rock particles.
Any of the processes by which a rock changes its shape, form, or volume.
The process by which a stream's gradient becomes less steep,
due to the erosion of sediment from the stream bed.
Such erosion generally follows a sharp reduction in the amount of
sediment entering the stream.
An alluvial fan having its apex at the mouth of a stream.
A method of absolute dating that uses the number of tree rings
found in a cross section of a tree trunk or branch to determine the age
of the tree.
A region with an average annual rainfall of 10 inches or less and sparse
vegetation, typically having thin, dry, and crumbly soil. A desert has
an aridity index greater than 4.0.
A closely packed layer of rock fragments concentrated in a layer along
the Earth's surface by the deflation of finer particles.
A thin, shiny red-brown or black layer, principally composed of iron
manganese oxides, that coats the surfaces of many exposed desert rocks.
The process through which a desert takes over a formerly nondesert area.
When a region begins to undergo desertification, the new conditions
typically include a significantly lowered water table, a
reduced supply of surface water, increased salinity in natural waters
and soils, progressive destruction of native vegetation, and an
accelerated rate of erosion.
Sediment that is composed of transported solid fragments of
preexisting igneous, sedimentary, or metamorphic rocks.
The set of processes that cause physical and chemical changes in
sediment after it has been deposited and buried under another layer of
sediment. Diagenesis may culminate in lithification.
A discordant pluton that is substantially wider than it is
thick. Dikes are often steeply inclined or nearly vertical. See also sill.
The expansion of a rock's volume caused by stress and deformation.
Any of a group of dark, phaneritic, intrusive rocks that are
the plutonic equivalents of andesite.
The angle formed by the inclined plane of a geological structure and the
horizontal plane of the Earth's surface.
A fault in which two sections of rock have moved apart
vertically, parallel to the dip of the fault plane.
Force exerted on a rock along one plane, flattening the rock in that
plane and lengthening it in the perpendicular plane.
A surface stream that drains rapidly and completely into a sinkhole.
Of or being a pluton that lies perpendicular or oblique to the
surrounding layers of rock. See also concordant.
A form of chemical weathering in which water molecules,
sometimes in combination with acid or another compound in the
environment, attract and remove oppositely charged ions or ion groups
from a mineral or rock.
A body of sediment carried by a stream in the form of ions
that have dissolved in the water.
One of a network of small streams carrying water and sediment
from a trunk stream into an ocean.
The process by which two lithospheric plates separated by rifting move
farther apart, with soft mantle rock rising between them and forming new
oceanic lithosphere. See also convergence.
A sedimentary rock composed primarily of dolomite, a mineral
made up of calcium, magnesium, carbon, and oxygen. Dolostone is thought
to form when magnesium ions replace some of the calcium ions in hmestone,
to which dolostone is similar in both appearance and chemical
A round or oval bulge on the Earth's surface, containing the oldest
section of rock in its raised, central part. See also basin.
The area from which water flows into a stream. Also called a watershed.
An area of raised, dry land separating two adjacent drainage basins.
The arrangement in which a stream erodes the channels of its
network of tributaries.
A long, spoon-shaped hill that develops when pressure from an overriding
glacier reshapes a moraine. Drumlins range in height
from 5 to 50 meters and in length from 400 to 2000 meters. They slope
down in the direction of the ice flow.
See plastic deformation.
A usually asymmetrical mound or ridge of sand that has been transported
and deposited by wind. Dunes form in both arid and humid climates.
A form of regional metamorphism that acts on rocks caught
between two converging plates and is initially caused by directed
pressure from the plates, which causes some of the rocks to rise
and others to sink, sometimes by tens of kilometers. The rocks that fall
then experience further dynamothermal metamorphism, this time caused by
heat from the Earth's interior and lithostatic pressure from
1. The flow of a dry, highly viscous mass of clay-like or silty
regolith, typically moving at a rate of one or two meters per
hour. 2. The material that descends in such a flow.
A movement within the Earth's crust or mantle, caused
by the sudden rupture or repositioning of underground rocks as they
A tide that lowers the water surface of an ocean and moves the
shoreline farther seaward.
The mapping of ocean topography based on the time required for sound
waves to reach the sea floor and return to the research ship that emits
A temporary stress-induced change in the shape or volume of a rock,
after which the rock returns to its original shape and volume.
See yield point.
A negatively charged particle that orbits rapidly around the nucleus
of an atom. See also proton.
A form of matter that cannot be broken down into a chemically simpler
form by heating, cooling, or chemical reactions. There are 106 known
elements, 92 of them natural and 14 synthetic. Elements are represented
by one- or two-letter abbreviations. See also atom, atomic
The path of a given electron's orbit around a nucleus, marked by a
constant distance from the nucleus.
The point on the Earth's surface that is located directly above the focus
of an earthquake.
The point in a glacier where overall gain in volume equals
overall loss, so that the net volume remains stable. The equilibrium
line marks the border between the zone of accumulation and the
zone of ablation.
The process by which particles of rock and soil are loosened, as by weathering,
and then transported elsewhere, as by wind, water, ice, or gravity.
A ridge of sediment that forms under a glacier's zone of
ablation, made up of sand and gravel deposited by meltwater.
An esker may be less than 100 meters or more than 500 kilometers long,
and may be anywhere from 3 to over 300 meters high.
An inorganic chemical sediment that precipitates when
the salty water in which it had dissolved evaporates.
An igneous rock formed from lava that has flowed out
onto the Earth's surface, characterized by rapid solidification and
grains that are so small as to be barely visible to the naked eye.
The fastest form of mass movement, occurring when rock or
sediment breaks off from a steep or vertical slope and descends at a
rate of 9.8 meters per second. A fall can be extremely dangerous.
A fracture dividing a rock into two sections that have visibly
moved relative to each other.
A section of rock separated from other rock by one or more faults.
A mountain containing tall horsts interspersed with much lower
grabens and bounded on at least one side by a high-angle normal
The metamorphism that acts on rocks grinding past one another
along a fault and is caused by directed pressure and frictional
Any of a group of light-colored, silicate, rock-forming minerals most
often found in plutonic igneous rocks and metamorphic rocks
and often containing potassium, sodium, or calcium. Feldspar
constitutes 60% of the Earth's crust.
Of or being a light-colored, igneous rock with a silica content
of 70% or higher. Felsic rocks are generally rich in potassium
feldspars, aluminum, and quartz.
Firmly packed snow that has survived a summer melting sea son. Firn has
a density of about 0.4 grams per cubic centimeter. Ultimately, firn
turns into glacial ice.
The division of the nucleus of a radioactive atom,
which causes the release of several subatomic particles. The fission
of a given element always occurs at a constant rate. (See also nuclear
A form of absolute dating that relies on the constant rate of
fission to determine the age of a crystal, by counting the fission
tracks left in a given area of the crystal.
Marks left in the latticework of a mineral crystal by subatomic
particles released during the fission of a radioactive
atom trapped inside the crystal.
A deep, steep-walled, U-shaped valley formed by erosion by a glacier
and submerged with seawater.
The flat land that surrounds a stream and becomes sub merged
when the stream overflows its banks.
A tide that raises the water surface of an ocean and moves the
shoreline farther inland.
Emission of visible light by a substance, such as a mineral, that is
currently exposed to ultraviolet light and absorbs radiation from it.
The light appears in the form of glowing, distinctive colors. The
emission ends when the exposure to ultraviolet light ends.
The precise point within the Earth's crust or mantle where
begin to rupture or move in an earthquake.
A bend that develops in an initially horizontal layer of rock, -~
usually caused by plastic deformation. Folds occur most
frequently in sedimentary rocks.
A mountain consisting of folds, which developed from extremely
thick layers of sediment, and thrust fault blocks, and
containing both igneous and metamorphic rocks. Fold-
and-thrust mountains may be several thousand kilometers high and a few
hundred kilometers wide. The Alps, the Appalachians, the Carpathians,
the Himalayas, and the Urals are all fold-and-thrust mountains.
The arrangement of a set of minerals in parallel, sheet-like layers that
lie perpendicular to the flattened plane of a rock. Occurs in metamorvhic
rocks on which directed pressure has been exerted.
The section of rock that lies below the fault plane in a dip-
slip fault. See also hanging wall.
A depression in the sea floor located between an accretionary wedge
and a volcanic arc in a subduction zone, and lined
with trapped sediment. See also backarc basin.
A minor, barely detectable earthquake, generally pre ceding a
full-scale earthquake with approximately the same focus. Major
quakes may follow a cluster of foreshocks by as little as a few seconds
or as much as several weeks.
The portion of a beach that lies nearest to the sea, extending
from the low-tide line to the high-tide line.
A remnant, an imprint, or a trace of an ancient organism, pre served in
the Earth's crust.
A nonrenewable energy source, such as oil, gas, or coal, that derives
from the organic remains of past life. Fossil fuels consist primarily of
The process by which a magma produces crystals that then
separate from the original magma, so that the chemical composition of
the magma changes with each generation of crystals, producing igneous
of different compositions. The silica content of the
magma becomes proportionately higher after each crystallization.
(n) A crack or break in a rock. (v) To break in random
places instead of cleaving. Said of minerals.
A reef that forms against or near an island or continental coast
and grows seaward, sloping sharply towards the sea floor. Fringing
reefs usually range from 0.5 to 1.0 or more kilometers in width.
A form of mechanical weathering caused by the freezing of water
that has entered a pore or crack in a rock. The water expands as it
freezes, widening the cracks or pores and often loosening or dislodging
rock fragments. As the ice forms, it attracts more water, increasing the
effects of frost wedging.
A source of energy, especially a combustible substance that can be
burned for heat or power, or matter used in nuclear fission.
Any of a group of dark, dense, phaneritic, intrusive rocks that
are the plutonic equivalent to basalt.
The study of the relationship between the history of the Earth and time.
geologic time scale
The division of all of Earth history into blocks of time distinguished
by geologic and evolutionary events, ordered sequentially and arranged
into eons made up of eras, which are in turn made up of periods, which
are in turn made up of epochs.
The scientific study of the Earth, its origins and evolution, the
materials that make it up, and the processes that act on it.
The branch of geology that studies the physics of the Earth,
using the physical principles underlying such phenomena as seismic
waves, heat flow, gravity, and magnetism to
investigate planetary properties.
A natural spring marked by the intermittent escape of hot water
Produced by, transported by, or concerning a glacier.
The process by which a glacier erodes the underlying bedrock through
contact between the bedrock and rock fragments embedded in the base of
the glacier. See also glacial quarrying.
A load of rock material transported and deposited by a glacier. Glacial
drift is usually deposited when the glacier begins to melt.
A rock or rock fragment transported by a glacier and deposited on
bedrock of different composition. Glacial erratics range from a few
millimeters to several yards in diameter.
The process by which a glacier erodes the under lying bedrock by
loosening and ultimately detaching blocks of rock from the bedrock and
attaching them instead to the glacier, which then bears the rock
fragments away. See also glacial abrasion.
Drift that is deposited directly from glacial ice and therefore
not sorted. Also called till. See also glacial drift.
A moving body of ice that forms on land from the accumulation and
compaction of snow, and that flows downslope or out ward due to gravity
and the pressure of its own weight.
A coarse-grained, foliated metamorphic rock marked by bands of
light-colored minerals such as quartz and feldspar that alternate with
bands of dark-colored minerals. This alternation develops through metamorphic
A block of rock that lies between two faults and has moved
downward to form a depression between the two adjacent fault blocks. See
A bed formed by the deposition of sediment in relatively still
water, marked by the presence of particles that vary in size, density,
and shape. The particles settle in a gradual slope with the coarsest
particles at the bottom and the finest at the top.
A stream maintaining an equilibrium between the processes of erosion and
deposition, and therefore between aggradation and degradation.
The vertical drop in a stream's elevation over a given horizontal
distance, expressed as an angle.
A pink-colored, felsic, plutonic rock that contains potassium
and usually sodium feldspars, and has a quartz content of about
10%. Granite is commonly found on continents but virtually absent from
the ocean basins.
1. The force of attraction exerted by one body in the universe on
another. Gravity is directly proportional to the product of the masses
of the two attracted bodies. 2. The force of attraction exerted by the
Earth on bodies on or near its surface, tending to pull them toward the
The difference between an actual measurement of gravity at a
given location and the measurement predicted by theoretical calculation.
A structure that juts out into a body of water perpendicular to the shoreline
and is built to restore an eroding beach by intercepting longshore
drift and trapping sand.
A seamount, the top of which has been flattened by weathering,
wave action, or stream erosion.
The time necessary for half of the atoms of a parent isotope to
decay into the daughter isotope.
The section of rock that lies above the fault plane in a dip-slip
fault See also footwall.
The degree of resistance of a given mineral to scratching, indicating
the strength of the bonds that hold the mineral's atoms together. The
hardness of a mineral is measured by rubbing it with substances of known
A cliff that projects out from a coast into deep water.
The study of the history, origin, and evolution of the Earth and all of
its life forms and geologic structures.
The second epoch of the Quaternary Period, beginning
approximately 10,000 years ago and continuing to the present time. See
also Pleistocene Epoch.
A spit that curves sharply at its coastal end.
A high mountain peak that forms when the walls of three or more cirques
A hard, dark-colored, dense metamorphic rock that forms from
the intrusion of magma into shale or basalt.
A block of rock that lies between two faults and has moved
upward relative to the two adjacent fault blocks. See also graben.
An area in the upper mantle, ranging from 100 to 200 km in
width, from which magma rises in a plume to form volcanoes.
A hot spot may endure for ten million years or more.
The extent to which a given substance allows water to flow through it,
determined by such factors as sorting and grain size and shape.
The difference in potential between two points, divided by the
lateral distance between the points.
The erosion of a stream bed by water pressure.
A molecule that is entirely made up of hydrogen and carbon.
An intermolecular bond formed with hydrogen.
The perpetual movement of water among the mantle, oceans, land, and
atmosphere of the Earth.
A form of chemical weathering in which ions from water replace
equivalently charged ions from a mineral, especially a silicate.
A mineral deposit formed by the precipitation of metallic ions
from water ranging in temperature from 500 to 7000 C.
The chemical alteration of preexisting rocks that is caused by the
action of hot water.
A tentative explanation of a given set of data that is expected to
remain valid after future observation and experimentation. See also theory.
A period during which the Earth is substantially cooler than usual and a
significant portion of its land surface is covered by glaciers.
Ice ages generally last tens of millions of years.
An alpine glacier that covers the peak of a mountain.
A rock made from molten (melted) or partly molten material that has
cooled and solidified.
The fossil of an organism known to have existed for a
relatively short period of time, used to date the rock in which it is
See metamorphic index mineral.
A steep ridge or hill left when a mountain has eroded and found in an
otherwise flat, typically desert plain.
The act or process by which two or more groups of atoms or molecules
combine due to weak positive or negative charges that develop at various
points within each group of atoms due to uneven distribution of their
electrons. The side of molecule where electrons are more likely to be
found will have a slight negative charge, and the side where they are
less likely to be found will have a slight positive charge. Such charged
regions attract oppositely charged regions of nearby molecules, forming
relatively weak bonds.
The rearrangement of the planes within ice crystals, due to pressure
from overlying ice and snow, that causes the downward or outward flow of
The entrance of magma into preexisting rock.
An igneous rock formed by the entrance of magma into
An atom that has lost or gained one or more electrons, thereby becoming
The combination of an atom that has a strong tendency to lose electrons
with an atom that has a strong tendency to gain electrons, such that the
former transfers one or more electrons to the latter and each achieves
chemical stability under the octet rule. The atom that loses
electrons acquires a positive electric charge and the atom that gains
electrons acquires a negative electric charge, so that - the resulting
compound is electrically neutral.
The act or process of forming of an ionic bond.
The replacement of one type of ion in a mineral by another that is
similar to the first in size and charge.
The sequence of events resulting in the separation of the Earth's matter
into concentric zones of differing densities. This sequence began when
the temperature of the Earth at depths of 400 to 800 kilometers below
the surface rose to the melting print of iron. Molten iron then
gravitated toward the Earth's center, and its movement raised the
Earth's temperature to approximately 20,000 C. This led other substances
to start melting. The densest matter then sank toward the Earth's
center, while lighter matter rose toward the surface. The iron
catastrophe took place between a few hundred million and one billion
years after the Earth formed.
The equilibrium maintained between the gravity tending to depress and
the buoyancy tending to raise a given segment of the lithosphere as
it floats above the asthenosphere.
One of two or more forms of a single element; the atoms of each isotope
have the same number of protons but different numbers of neutrons in
their nuclei. Thus, isotopes have the same atomic number but
differ in atomic mass.
A structure built along the bank of a stream channel or tidal outlet to
direct the flow of a stream or tide and keep the sediment moving so that
it cannot build up and fill the channel. Jetties are typically built in
parallel pairs along both banks of the channel. Jetties that are built
perpendicular to a coast tend to interrupt long- shore
drift and thus widen beaches.
A fracture dividing a rock into two sections that have not
visibly moved relative to each other. See also fault.
The steam that accompanies volcanic eruptions.
A topography characterized by caves, sinkholes, disappearing
streams, and underground drainage. Karst forms when groundwater
dissolves pockets of limestone, dolomite, or gypsum in bedrock.
A solid, waxy, organic substance that forms when pressure and heat from
the Earth act on the remains of plants and animals. Kerogen converts to
various liquid and gaseous hydrocarbons at a depth of seven or
more kilometers and a temperature between 500 and 1,000 C.
A large concordant pluton that is shaped like a dome or a
mushroom. Laccoliths tend to form at relatively shallow depths and are
typically composed of granite. The country rock above them
often erodes away completely.
A shallow body of water separated from the sea by a reef or barrier
A flow of pyroclastic material mixed with water. A
lahar is often produced when a snow-capped volcano erupts and hot
pyroclastics melt a large amount of snow or ice.
Magma that comes to the Earth's surface through a volcano or
Of, located on, or being the side of a dune, hill, or ridge that is
sheltered from the wind. See also windward.
A protective barrier built along the banks of a stream to pre vent
flooding. See also natural levee.
Plant-like colonies of fungi and algae that grow on the exposed surface
of rocks. Lichen grows at a constant rate within a single geographic
A method of absolute dating that uses the size of lichen colonies
on a rock surface to determine the surface's age. Lichenometry is used
for rock surfaces less than about 9,000 years old.
A soft, brownish coal that develops from peat through
bacterial action, is rich in kerogen,and has a carbon content of
70%, which makes it a more efficient heating fuel than peat.
A sedimentary rock composed primarily of calcium carbonate. 10%
to 15% of all sedimentary rocks are limestones. Limestone is usually
organic, but it may also be inorganic.
The conversion of moderately cohesive, unconsolidated sediment into
a fluid, water-saturated mass.
The conversion of loose sediment into solid sedimentary
A layer of solid, brittle rock comprising the outer 100 kilometers of
the Earth, encompassing both the crust and the outer most part of the
upper mantle. See also asthenosphere.
The force exerted on a rock buried deep within the Earth by overlying
rocks. Because lithostatic pressure is exerted equally from all sides of
a rock, it compresses the rock into a smaller, denser form without
altering the rock's shape.
A load of silt that is produced by the erosion of outwash and
transported by wind. Much loess found in the Mississippi Valley, China,
and Europe is believed to have been deposited during the Pleistocene
One of a series of long, narrow dunes lying parallel both to
each other and to the prevailing wind direction. Longitudinal dunes
range from 60 meters to 100 kilometers in length and from 3 to 50 meters
An ocean current that flows close and almost parallel to the shoreline
and is caused by the rush of waves toward the shore.
1. The process by which a current moves sediments along
surf zone. 2. The sediments so moved. Longshore drift typically
consists of sand, gravel, shell fragments, and pebbles. See also beach
An area within the Earth's upper mantle in which both P
waves and S waves travel at markedly slower velocities
than in the outermost part of the upper mantle. The low-velocity zone
occurs in the range between 100 and 350 kilometers of depth.
1. The reflection of light on a given mineral's surface, classified by
intensity and quality. 2. The appearance of a given mineral as
characterized by the intensity and quality with which it reflects light.
Molten (melted) rock that forms naturally within the Earth. Magma may be
either a liquid or a fluid mixture of liquid, crystals, and dissolved
The region within which the magnetism of a given substance or
particle affects other substances.
The process by which the Earth's magnetic north pole and its magnetic
south pole reverse their positions over time.
The property, possessed by certain materials, to attract or repel
similar materials. Magnetism is associated with moving electricity.
The middle layer of the Earth, lying just below the crust and
consisting of relatively dense rocks. The mantle is divided into two
sections, the upper mantle and the lower mantle; the lower mantle has
greater density than the upper mantle. See also core and crust.
A coarse-grained, non foliated metamorphic rock derived from limestone
marine magnetic anomaly
An irregularity in magnetic strength along the ocean floor that reflects
sea-floor spreading during periods of magnetic reversal.
massive sulfide deposit
An unusually large deposit of sulfide minerals.
The process by which such Earth materials as bedrock, loose sediment,
and soil are transported down slopes by gravity.
A stream that traverses relatively flat land in fairly evenly
spaced loops and separated from each other by narrow strips of flood
A form of mechanical weathering in which successive layers of a
large plutonic rock break loose and fall when the erosion of
overlying material permits the rock to expand upward. The thin slabs of
rock that break off fall parallel to the exposed surface of the rock,
creating the long, broad steps that can be found on many mountains.
The process by which a rock or mineral is broken down into smaller
fragments without altering its chemical makeup; weathering that
affects only physical characteristics. See also chemical weathering.
A body of rock that forms along the inner wall of an ocean trench and
is made up of fragments of lithosphere and oceanic sediment
that have undergone metamorphism.
Water formed from the melted ice of a glacier.
A scale designed to measure the degree of intensity of earthquakes,
ranging from I for the lowest intensity to XII for the highest. The
classifications are based on human perceptions.
The intermediate era of the Phanerozoic Eon, fol lowing the Paleozoic
Era and preceding the Cenozoic Era, and marked by the
dominance of marine and terrestrial reptiles, and the appearance of
birds, mammals, and flowering plants.
The act or process by which two or more atoms of electron-donating
elements pack so closely together that some of their electrons begin to
wander among the nuclei rather than orbiting the nucleus of a single
atom. Metallic bonding is responsible for the distinctive properties of
The process by which minerals from a chemically uniform rock separate
from each other during metamorphism and form individual layers
within a new metamorphic rock.
1. A group of minerals customarily found together in metamorphic
rocks and indicating a particular set of temperature and pressure
conditions at which metamorphism occurred. 2. A set of metamorphic
rocks characterized by the presence of such a group of minerals.
A measure used to identify the degree to which a metamorphic
rock has changed from its parent rock. A metamorphic
grade provides some indication of the circumstances under which the
metamorphism took place.
metamorphic index mineral
One of a set of minerals found in metamorphic rocks and used as
indicators of the temperature and pressure conditions at which the
metamorphism occurred. A metamorphic index mineral is stable only within
a narrow range of temperatures and pressures and the metamorphism that
produces it must take place within that range.
A rock that has undergone chemical or structural changes. Heat,
pressure, or a chemical reaction may cause such changes.
The process by which conditions within the Earth, below the zone of diagenesis,
alter the mineral content, chemical composition, and structure of
solid rock without melting it. Igneous, sedimentary, and metamorphic
rocks may all undergo metamorphism.
The precipitation of condensed water from clouds as rain, snow, sleet,
A section of continental lithosphere that has broken off from a
larger, distant continent, as by rifting.
An underwater mountain range that develops between the margins of two
lithospheric plates, formed by rifting.
A rock that incorporates both metamorphic and igneous materials.
A naturally occurring, usually inorganic, solid consisting of either a
single element or a compound, and having a definite chemical
composition and a systematic internal arrangement of atoms.
A naturally occurring, usually inorganic, solid consisting of either a
single element or a compound, and having a definite chemical
composition but lacking a systemic internal arrangement of atoms. See
An area of rock throughout which a given metamorphic index mineral is
found, presumed to have undergone metamorphism under uniform temperature
and pressure conditions.
Moho (abbreviation for Mohorovicic)
The seismic discontinuity between the base of the Earth's crust
and the top of the mantle. P waves passing through the
Moho change their velocity by approximately one kilometer per second,
with the higher velocity occurring in the mantle and the lower in the
The smallest particle that retains all the chemical and physical
properties of a given compound, consisting of a stable group of
A single, large mass of glacial till that accumulates,
typically at the edge of a glacier.
A fracture that develops at the top of a layer of fine grained, muddy
sediment when it is exposed to the air, dries out, and then shrinks.
The rapid flow of typically fine-grained regolith mixed with
water. There may be as much as 60% water in a mudflow.
An arch-shaped stretch of bedrock remaining in a karst region
when the surrounding bedrock has dissolved.
One of a pair of ridges of sediment deposited along both banks
of a stream during successive floods.
A place where groundwater flows to the surface and issues freely from
A particle that is found in the nucleus of an atom, has
a mass approximately equal to that of a proton, and has no
Being a metamorphic rock that does not show foliation.
A dip-slip fault marked by a generally steep dip along
which the hanging wall has moved downward relative to the foot
The division of the nuclei of isotopes of certain
heavy elements, such as uranium and plutonium, effected by
bombardment with neutrons. Nuclear fission causes the release
of energy, additional neutrons, and an enormous quantity of heat.
Nuclear fission is used in nuclear power plants and nuclear weapons. A
byproduct of nuclear fission is toxic radioactive waste. See also nuclear
The combination of the nuclei of certain extremely light elements,
especially hydrogen, effected by the application of high
temperature and pressure. Nuclear fusion causes the release of an
enormous amount of heat energy, comparable to that released by nuclear
fission. The principle byproduct of nuclear fusion is helium.
The central part of an atom, containing most of the atom's mass
and having a positive charge due to the presence of protons.
A sometimes glowing cloud of gas and pyroclastics erupted from
a volcano and moving swiftly down its slopes. Also called a pyroclastic
A deep, linear, relatively narrow depression in the sea floor, formed by
the subduction of oceanic plates.
A scientific law stating that all atoms, except those of hydrogen and
helium, require eight electrons in the outermost energy level in
order to maintain chemical stability.
A mixture of unconsolidated sand and clay that contains a semi-solid bitumen.
A brown or black clastic source rock containing kerogen.
The group of sediments, sedimentary rocks, and mafic and
ultramafic igneous rocks that make up the oceanic lithosphere.
A mineral deposit that can be mined for a profit.
Mountain formation, as caused by volcanism, subduction, plate
divergence, folding, or the movement of fault blocks. Also
The circular movement of water up and down, with little or no change in
position, as a wave passes.
A load of sediment, consisting of sand and gravel, that is
deposited by meltwater in front of a glacier.
A crescent-shaped body of standing water formed from a single loop that
was cut off from a meandering stream, typically by a flood that
allowed the stream to flow through its flood plain and bypass
The process of combining with oxygen ions. A mineral that is exposed to
air may undergo oxidation as a form of chemical weathering.
One of several minerals containing negative oxygen ions bonded to one or
more positive metallic ions.
An ancient, buried soil whose composition may reflect a climate
significantly different from the climate now prevalent in the area where
the soil is found.
1. The fixed orientation of a rock's crystals, based on the Earth's magnetic
field at the time of the rock's formation, that remains constant
even when the magnetic field changes. 2. The study of such phenomena as
indicators of the Earth's magnetic history.
The earliest era of the Phanerozoic Eon, marked by the presence
of marine invertebrates, fish, amphibians, insects, and land plants.
A horseshoe-shaped dune having a concave wind ward slope
and a convex leeward slope. Parabolic dunes tend to form along
sandy ocean and lake shores. They may also develop from transverse
dunes through deflation.
A radioactive isotope that changes into a different isotope
when its nucleus decays. See also daughter isotope.
The source from which a given soil is chiefly derived, generally
consisting of bedrock or sediment.
The preexisting rock from which a metamorphic rock forms.
The incomplete melting of a rock composed of minerals with differing
melting points. When partial melting occurs, the minerals with higher
melting points remain solid while the minerals whose melting points have
been reached turn to magma.
passive continental margin
A border that lies between continental and oceanic lithosphere, but
is not a plate margin. It is marked by lack of seismic and
A soft brown mass of compressed, partially decomposed vegetation that
forms in a water-saturated environment and has a carbon content of 50%.
Dried peat can be burned as fuel.
A broad surface at the base of a receding mountain. The pediment
develops when running water erodes most of the mass of the mountain.
A coarse-grained igneous rock with exceptionally large crystals,
formed from a magma that contains a high proportion of
perched water table
A saturated area that lies within a zone of aeration.
An igneous rock composed primarily of the iron-magnesium silicate
olivine and having a silica content of less than 40%.
Permanently frozen regolith, ranging in thickness from 30
centimeters to over 1,000 meters.
The capability of a given substance to allow the passage of a fluid.
Permeability depends upon the size of and the degree of connection among
a substance's pores.
Any of a group of naturally occurring substances made up of hydrocarbons.
These substances may be gaseous, liquid, or semisolid.
Of or being an igneous rock containing components large enough
to be seen with the unaided eye.
The eon that started 570 million years ago, when numerous fossils of sea
shells began to be formed, and that continues to the present time.
Emission of visible light by a substance, such as a mineral, that is
exposed to ultraviolet light and absorbs radiation from it. The light
appears in the form of glowing, distinctive colors. The emission
continues after the exposure to ultraviolet light ends.
A foliated metamorphic rock that develops from slate and
is marked by a silky sheen and medium grain size.
A deposit of heavy or durable minerals, such as gold or diamonds,
typically found where the flow of water abruptly slows.
A permanent strain that entails no rupture.
One of the large, thin, rigid units making up the Earth's lithosphere.
Plates may be continental, oceanic, or both.
The theory that the Earth's lithosphere consists of large,
rigid plates that move horizontally in response to the flow of the asthenosphere
beneath them, and that interactions among the plates at their
borders cause most major geologic activity, including the creation of
oceans, continents, mountains, volcanoes, and earth quakes.
A dry lake basin found in a desert.
The first epoch of the Quaternary Period, beginning two to
three million years ago and ending approximately 10,000 years ago. See
also Holocene Epoch.
An upward flow of hot material from the Earth's mantle into the
An intrusive rock, as distinguished from the preexisting country
rock that surrounds it.
An intrusive rock formed inside the Earth.
A lake that formed from rainwater falling into a land locked basin
during a glacial period marked by greater precipitation than is
found in the region in prior or subsequent periods.
A low ridge of sediment that forms along the inner bank of a meandering
A mineral that is identical to another mineral in chemical composition
but differs from it in crystal structure.
The percentage of a soil, rock, or sediment's volume that is made up of
Of or being an igneous rock containing some large grains within
a smaller-grained matrix.
porphyry copper deposit
A crystallized rock, typically porphyritic, having hairline
fractures that contain copper and other metals.
A form of radiometric dating that relies on the extremely long
half-life of radioactive isotopes of potassium, which decay into
isotopes of argon, to determine the age of rocks in which argon is
present. Potassium-argon dating is used for rocks between 100,000 and 4
billion years old.
The combined influence of gravity and water pressure on
groundwater flow at a given depth.
The level to which the water in an artesian aquifer would rise
if unaffected by friction with the surrounding rocks and sediments.
To separate from solution in solid form. Minerals may precipitate
because of cooling, evaporation, or loss of acidity.
1. The process by which a substance becomes precipitated. 2.
Water that falls from the atmosphere to Earth's surface in the form of
rain, snow, sleet, or hail.
A coast shaped primarily by nonmarine processes, such as glacial
erosion or biological processes.
The scienhfic law stating that a pluton is always
younger than the rock that surrounds it.
principle of faunal
The scientific law stating that an organism is always simpler
than those that evolved later and more - complex than those that evolved
principle of inclusions
The scientific law stating that rock fragments contained within
a larger body of rock are always older than the surrounding body of
principle of original horizontality
The scientific law stating that sediments settling out
from bodies of water are deposited horizontally or nearly horizontally
in layers that lie parallel or nearly parallel to the Earth's surface.
The scientific law stating that in any unaltered sequence of
rock strata, each stratum is younger than the one beneath it and older
than the one above it, so that the youngest stratum will be at the top
of the sequence and the oldest at the bottom.
The scientific law stating that the geological processes taking
place in the present operated similarly in the past and can therefore be
used to explain past geologic events.
A characteristic that distinguishes one substance from another.
A positively charged particle that is found in the nucleus of
an atom and has a mass approximately 1836 times that of an electron.
P wave (abbreviation for primary wave)
A body wave that causes the compression of rocks when
its energy acts upon them. When the P wave moves past a rock, the rock
expands beyond its original volume, only to be compressed again by the
next P wave. P waves are the fastest of all seismic waves. See
also S wave.
P-wave shadow zone
The region that extends from 1030 to 1430 from the epicenter of
an earthquake and is marked by the absence of P waves.
The P-wave shadow zone is due to the refraction of seismic waves in
the liquid outer core. See also S-wave shadow zone.
Being or pertaining to rock fragments formed in a volcanic eruption.
A usually steep, conic volcano composed almost entirely of an
accumulation of loose pyroclastic material. Pyroclastic cones
are usually less than 450 meters high. Because no lava binds
the pyroclastics, pyroclastic cones erode easily.
A volcanic eruption of viscous, gas-rich magma. Pyroclastic
eruptions tend to produce a great deal of solid volcanic fragments
rather than fluid lava.
A rapid, extremely hot, downward stream of pyroclastics, air,
gases, and ash ejected from an erupting volcano. A pyroclastic
flow may be as hot as 8000C or more and may move at speeds higher than
150 kilometers per hour.
pyroclastics (used only in the
Particles and chunks of igneous rock ejected from a volcanic
vent during an eruption.
An extremely durable, nonfoliated metamorphic rock derived from
pure sandstone and consisting primarily of quartz.
An ice age that began approximately 1.6 million years ago and
continues to the present time.
The second period of the Cenozoic Era, beginning two to three
million years ago and continuing to the present time.
Sediment that sets off a sudden mudflow by changing
rapidly from solid to liquid form, as after an earthquake, an explosion,
The process of spontaneously emitting protons and neutrons
that transforms one isotope into another.
The process of using relative proportions of parent to daughter
in radioactive decay to determine the age of a given rock
or rock stratum.
rain shadow effect
The result of the process by which moist air on the windward side
of a mountain rises and cools, causing precipitation and leaving the leeward
side of the mountain dry.
The diagenetic process by which unstable minerals in buried
sediment are transformed into stable ones.
A ridge that forms in clear, moderately salty seawater near the shoreline
and is composed of the carbonate remains of algae, sponges, and
Metamorphism that affects rocks over vast geographic areas
stretching for thousands of square kilometers.
The unconsolidated material that covers almost all of the Earth's land
surface and is composed of soil, sediment, and fragments from
the bedrock beneath it.
The fixing of a geologic structure or event in a chronological sequence
relative to other geologic structures or events. See also absolute
A known resource that can be exploited for profit with
available technology under existing political and economic conditions.
A permeable rock containing oil or gas.
A mineral or fuel deposit, known or not yet discovered, that may be or
become available for human exploitation.
A dip-slip fault marked by a hanging wall that has
moved upward relative to the footwall. Reverse faults are often
caused by the convergence of lithospheric plates.
Any of a group of felsic igneous rocks that are the extrusive
equivalents of granite.
A logarithmic scale that measures the amount of energy released during
an earthquake on the basis of the amplitude of the highest peak
recorded on a seismogram. Each unit increase in the Richter
scale represents a 10-fold increase in the amplitude recorded on the
seismogram and a 30-fold increase in energy released by the earthquake.
Theoretically the Richter scale has no upper limit, but the yield
point of the Earth's rocks imposes an effective limit between 9.0
The tearing apart of a plate to form a depression in the
Earth's crust and often eventually separating the plate into
two or more smaller plates.
A strong, rapid, and brief current that flows out to sea moving
perpendicular to the shoreline.
A pattern of wavy lines formed along the top of a bed by wind,
water currents, or waves.
A pile of large, angular boulders built seaward of the shoreline in
order to prevent erosion by waves or currents. See also seawall.
A naturally formed aggregate of usually inorganic materials from within
A series of events through which a rock changes, over time,
between igneous, sedimentary and metamorphic forms.
One of the twenty or so minerals contained in the rock that composes the
Earth's crust and mantle.
A form of radiometric dating that relies on the 47-billion-year
half-life of radioactive isotopes of rubidium, which decay into isotopes
of strontium, to determine the age of rocks in which strontium is
present. Rubidium-strontium dating is used for rocks that are at least
10 million years old, deep-Earth plutonic rocks, and Moon rocks.
1. A particle of rock or mineral material, coarser than silt,
that has been transported from its place of origin, as by water or wind.
A particle of sand is usually between 1/16 and two millimeters in
diameter. Sands are frequently composed of quartz. 2. A loosely
connected body of such particles.
sandstone A clastic
rock composed of particles
that range in diameter from 1/16 millimeter to 2 millimeters in
diameter. Sandstones make up about 25% of all sedimentary rocks.
See zone of saturation.
scarp The steep cliff face that is
formed by a slump.
scientific law 1. A natural phenomenon that has
been proven to occur invariably whenever certain conditions are met.
2. A formal statement describing such a phenomenon and the
conditions under which it occurs. Also called law.
Techniques that involve gathering all available data on a subject,
forming an hypothesis to explain the data, con ducting
experiments to test the hypothesis, and modifying or con firming the
hypothesis as necessary to account for the experimental results.
A coarse-grained, strongly foliated metamorphic rock that
develops from phyllite and splits easily into flat, parallel
A steep, isolated island of rock, separated from a head land by
the action of waves, as when the overhanging section of a sea arch
The formation and growth of oceans that occurs following rifting and
is characterized by eruptions along mid-ocean ridges, forming
new oceanic lithosphere, and expanding ocean basins. See also
A conical underwater mountain formed by a volcano and rising
1000 meters or more from the sea floor.
A wall of stone, concrete, or other sturdy material, built along the shoreline
to prevent erosion even by the strongest and highest of waves.
See also riprap.
A coast shaped primarily by erosion or deposition by sea
currents and waves.
The process by which a metal deposit becomes concentrated when other
minerals are eliminated from the deposit, as through dissolution,
precipitation or weathering.
A collection of transported fragments or precipitated materials that
accumulate, typically in loose layers, as of sand or mud.
The continental, oceanic, or coastal surroundings in which sediment
1. A set of characteristics that distinguish a given section of
sedimentary rock from nearby sections. Such characteristics include
mineral content, grain size, shape, and density. 2. A section of
sedimentary rock so characterized.
A rock made from the consolidation of solid fragments, as of
other rocks or organic remains, or by precipitation of
minerals from solution.
A physical characteristic of a detrital sediment that
reflects the conditions under which the sediment was deposited.
Of, concerning, subject to, or produced by an earthquake.
A surface marking the boundary between two layers of the Earth
differing in composition. Seismic waves passing through a
seismic discontinuity undergo an abrupt change in velocity.
A locked fault segment that has not experienced seismic activity for a
long time. Because stress tends to accumulate in seismic
gaps, they often become the sites of major earthquakes.
The mapping of rocks lying along and beneath the ocean floor by
recording the reflections and refractions of seismic waves.
The process whereby a computer first synthesizes data on the
velocities of seismic waves from thousands of recent
earthquakes in order to make a series of images depicting successive
planes within the Earth, and then uses these images to construct a
three-dimensional representation of the Earth's interior.
One of a series of progressive disturbances that reverberate through
the Earth to transmit the energy released from an earthquake.
A visual record produced by a seismograph and showing the
arrival times and magnitudes of various seismic waves.
A machine for measuring the intensity of earthquakes by
recording the seismic waves that they generate.
The study of earthquakes and the structure of the Earth,
based on data from seismic waves.
A sedimentary rock composed of detrital sediment particles
less than 0.004 millimeters in diameter. Shales tend to be red, brown,
black, or gray, and usually originate in relatively still waters.
shearing stress Stress that slices rocks into parallel blocks that slide in
opposite directions along their adjacent sides. Shearing stress may be
caused by transform motion.
A low, broad, gently sloping, dome-shaped structure that forms over
time as repeated eruptions eject basaltic lava through one or
more vents and the lava solidifies in approximately the same
volume all around.
The metamorphism that results when a meteorite strikes rocks
at the Earth's surface. The meteoric impact generates tremendous
pressure and extremely high temperatures that cause minerals to
shatter and recrystallize, producing new minerals which cannot arise
under any other circumstances.
The boundary between a body of water and dry land.
A compound consisting of silicon and oxygen.
One of several rock-forming minerals that contain silicon, oxygen, and
usually one or more other common elements.
A four-sided geometric form created by the tight bonding of four
oxygen atoms to each other, and also to a single silicon atom that
lies in the middle of the form.
A concordant pluton that is substantially wider than it is
thick. Sills form within a few kilometers of the Earth's surface. See
1. A particle of rock or mineral material, finer than sand but
coarser than clay, that has been transported from its place of
origin, typically by wind or water. A particle of silt is usually
between 1/16 and 1/256 of a millimeter in diameter. 2. A loosely
connected body of such particles.
A circular, often funnel-shaped depression in the ground that forms
when soluble rocks dissolve.
A coarse-grained, nonfoliated metamorphic rock containing silicates
that are rich in calcium.
A fine-grained, foliated metamorphic rock that develops from
shale and tends to break into thin, flat sheets.
The mass movement of a single, intact mass of rock, soil, or
unconsolidated material along a weak plane, such as a fault,
fracture, or bedding plane. A slide may involve as
little as a minor displacement of soil or as much as the displacement
of an entire mountainside.
The steep leeward slope of a dune.
A weak plane in a rock mass from which material is likely to break off
in a slide.
1. A downward and outward slide occurring along a concave slip
plane. 2. The material that breaks off in such a slide.
The lowest point at which snow remains year-round.
The top few meters of regolith, generally including some
organic matter derived from plants.
A layer of soil that can be distinguished from the surrounding soil by
such features as chemical composition, color, and texture.
A vertical strip of soil stretching from the surface down to the bedrock
and including all of the successive soil horizons.
A form of creep in which soil flows downslope at 0.5 to 15
centimeters per year. Solifluction occurs in relatively cold regions
when the brief warmth of summer thaws only the upper meter or two of regolith,
which becomes waterlogged because the underlying ground remains
frozen and therefore the water cannot drain down into it.
A rock in which hydrocarbons originate.
The process by which a given transport medium separates out
certain particles, as on the basis of size, shape, or density.
The ratio of the weight of a particular volume of a given substance to
the weight of an equal volume of pure water.
A mineral deposit of calcium carbonate that precipitates from solution
in a cave.
The process by which chemical weathering, especially by
water, decomposes the angles and edges of a rock or boulder, leaving a
rounded form from which concentric layers are then stripped away as
the weathering continues.
A narrow, fingerlike ridge of sand that extends from land into open
An icicle-like mineral formation that hangs from the ceiling of a cave
and is usually made up of travertine, which
precipitates as water rich in dissolved limestone drips down from the
cave's ceiling. See also stalagmite.
A cone-shaped mineral deposit that forms on the floor of a cave and
is usually made up of travertine, which precipitates as water
rich in dissolved limestone drips down from the cave's ceiling. See
A dune with three or four arms radiating from its usually
higher center so that it resembles a star in shape. Star dunes form
when winds blow from three or four directions, or when the wind
direction shifts frequently.
A cone-shaped volcano built from alternating layers of pyroclastics
and viscous andesitic lava. Stratovolcanos tend to be
very large and steep.
stratum (plural strata)
A layer of sedimentary rock that is visibly distinct from the
The color of a mineral in its powdered form. This color is usually
determined by rubbing the mineral against an unglazed porcelain slab
and observing the mark made by it on the slab.
The change in the shape or volume of a rock that results from stress.
A body of water found on the Earth's surface and confined to a narrow
topographic depression, down which it flows and transports rock
particles, sediment, and dissolved particles. Rivers, creeks, brooks,
and runs are all streams.
The volume of water to pass a given point on a stream bank per unit of
time, usually expressed in cubic meters of water per second.
A level plain lying above and running parallel to a stream bed. A
stream terrace is formed when a stream's bed erodes to a substantially
lower level, leaving its flood plain high above it.
The force acting on a rock or another solid to deform it, measured in
kilograms per square centimeter or pounds per square inch.
One of a group of usually parallel scratches engraved in bedrock by a
glacier or other geological agent.
1. The horizontal line marking the intersection between the inclined
plane of a solid geological structure and the Earth's surface. 2. The
compass direction of this line, measured in degrees from true north.
A fault in which two sections of rock have moved horizontally
in opposite directions, parallel to the line of the fracture that
divided them. Strike-slip faults are caused by shearing stress.
The scientific study of the geological processes that deform the
Earth's crust and create mountains.
The sinking of an oceanic plate edge as a result of convergence
with a plate of lesser density. Subduction often causes earthquakes
and creates volcano chains.
The lowering of the Earth's surface, caused by such factors as
compaction, a decrease in groundwater, or the pumping of oil.
One of several minerals containing positive sulfur ions bonded to
negative oxygen ions.
One of several minerals containing negative sulfur ions bonded to one
or more positive metallic ions.
One of a series of seismic waves that transmits energy from
an earthquake's epicenter along the Earth's surface. See also
The area running from the shoreline to the farthest point in
the sea where waves begin to break.
To flow more rapidly than usually. Said of a glacier.
A body of fine, solid particles, typically of sand, clay, and silt,
that travels with stream water without coming into contact with the
The area where two continental plates have joined together through continental
collision. Suture zones are marked by extremely high mountain
ranges, such as the Himalayas and the Alps.
The rush of water onto a beach after a wave breaks.
S wave (abbreviation for secondary wave)
A body wave that causes the rocks along which it passes to
move up and down perpendicular to the direction of its own movement.
See also P wave.
S-wave shadow zone
The region within an arc of 1540 directly opposite an earthquake's epicenter
that is marked by the absence of S waves. The S-wave
shadow zone is due to the fact that S waves can not penetrate the
liquid outer core. See also P-wave shadow zone.
A concave fold, the central part of which contains the
youngest section of rock. See also anticline.
A pile of rock fragments lying at the bottom of the cliff or steep slope
from which they have broken off.
A deep, typically circular lake that forms when a cirque glacier melts.
The almost constant movement of certain fault blocks that
allows strain energy to be released without major earthquakes.
tension Stress that stretches or extends rocks, so that they become
thinner vertically and longer laterally. Tension may be caused by divergence
tephra (plural noun) Pyroclastic materials that fly from an erupting volcano through
the air before cooling, and range in size from fine dust to massive
The outer margin of a glacier.
A comprehensive explanation of a given set of data that has been
repeatedly confirmed by observation and experimentation and has gained
general acceptance within the scientific community but has not yet been
decisively proven. See also hypothesis and scientific law.
A form of mechanical weathering in which heat causes a
mineral's crystal structure to enlarge.
A vertical column of upwelling mantle material 100 to 250
kilometers in diameter, that rises from beneath a continent or ocean and
can be perceived at the Earth's surface as a hot spot. Thermal
plumes carry enough energy to move a plate, and they may be found both
at plate boundaries and plate interiors.
A reverse fault marked by a dip of 450 or less.
A turbulent, abrupt, wall-like wave that is caused by a flood tide.
1. The cycle of alternate rising and falling of the surface of an ocean
or large lake, caused by the gravitational pull of the Sun and
especially Moon in interaction with the Earth's rotation. Tides occur on
a regular basis, twice every day on most of the Earth. 2. A single rise
or fall within this cycle.
See glacial till.
The set of physical features, such as mountains, valleys, and the shapes
of landforms, that characterizes a given landscape.
The seismic discontinuity located in the upper mantle just
beneath the asthenosphere and characterized by a marked
increase in the velocity of seismic waves.
The movement of two adjacent lithospheric plates in opposite directions
along a parallel line at their common edge. Transform motion often
Of, concerning, or being the movement of water over a significant
distance in the direction of a wave.
A natural agent, such as water, air, or ice, that moves a particle or
particles from one location on the Earth's surface to another.
One of a series of dunes having an especially steep slip
face and a gentle windward slope and standing
perpendicular to the prevailing wind direction and parallel to each
other. Transverse dunes typically form in arid and semi-arid regions
with plentiful sand, stable wind direction, and scarce vegetation. A
transverse dune may be as much as 100 kilometers long, 200 meters high,
and three kilometers wide.
travertine Crystalline deposits of calcium carbonate precipitated from
solution, often found in caves.
A stream that supplies water to a larger stream.
A large stream into which tributaries carry water and sediment.
tsunami (plural tsunami)
A vast sea wave caused by the sudden dropping or rising of a section of
the sea floor following an earth- quake. Tsunami may be as much
as 30 meters high and 200 kilometers long, may move as fast as 250
kilometers an hour, and may continue to occur for as long as a few days.
See volcanic tuff.
The hypothesis that current geologic processes, such as the slow erosion
of a coast under the impact of waves, have been occurring in a similar
manner throughout the Earth's history and that these processes can
account for past geologic events. See also catastrophism.
A boundary separating two or more rocks of markedly different ages,
marking a gap in the geologic record.
A mountain consisting of a broad area of the Earth's crust that
has moved gently upward without much apparent deformation, and usually
containing sedimentary, igneous, and metamorphic rocks.
A form of radiometric dating that relies on the extremely long
half-life of radioactive isotopes of uranium, which decay into
isotopes of lead, to determine the age of rocks in which uranium and
lead are present.
An alpine glacier that flows through a preexisting stream
van der Waals bond
A relatively weak kind of intermolecular bond that forms when
one side of a molecule develops a slight negative charge
because a number of electrons have temporarily moved to that
side of the molecule, and this negative charge attracts the nuclei
of the atoms of a neighboring molecule, while the side of
the molecule with fewer electrons develops a slight positive charge that
attracts the electrons of the atoms of neighboring molecules.
A pair of sediment beds deposited by a lake on its floor,
typically consisting of a thick, coarse, light-colored bed deposited in
the summer and a thin, fine-grained, dark-colored bed deposited in the
winter. Varves are most often found in lakes that freeze in the winter.
The number and nature of varves on the bottom of a lake provides
information about the lake's age and geologic events that affected the
An opening in the Earth's surface through which lava, gases,
and hot particles are expelled. Also called volcanic vent and volcano.
A stone that has been flattened and sharpened by wind abrasion.
Ventifacts are commonly found strewn across a desert floor.
A fluid's resistance to flow. Viscosity increases as temperatures
A chain of volcanoes fueled by magma that rises from an
underlying subducting plate.
A cone-shaped mountain that forms around a vent from the debris
of pyroclastics and lava ejected by numerous eruptions
A steep, bowl-shaped depression surrounding a vent. A volcanic
crater forms when the walls of a vent collapse inward following an
A bulb-shaped solid that forms over a vent when lava so
viscous that it cannot flow out of the volcanic crater cools
and hardens. When a volcanic dome forms, it traps the volcano's gases
beneath it. They either escape along a side vent of the volcano or build
pressure that causes another eruption and shatters the volcanic dome.
See extrusive rock.
A solid rock made up of tephra that have consolidated and
become cemented together. Also called tuff.
The set of geological processes that result in the expulsion of lava,
pyroclastics, and gases at the Earth's surface.
The solid structure created when lava, gases, and hot particles escape
to the Earth's surface through vents. Volcanoes are usually
conical. A volcano is "active" when it is erupting or has erupted
recently. Volcanoes that have not erupted recently but are considered
likely to erupt in the future are said to be "dormant." A volcano that
has not erupted for a long time and is not expected to erupt in the
future is "extinct."
See drainage basin.
The surface that lies between the zone of aeration and the
underlying zone of saturation.
The process by which a wave approaching the shore changes direction due
to slowing of those parts of the wave which enter shallow water first,
causing a sharp decrease in the angle at which the wave approaches until
the wave is almost parallel to the coast.
A relatively level surface formed when waves erode the base of a cliff,
causing the overlying rock to fall into the surf. A wave-cut bench
stands above the water and extends seaward from what remains of the
The process by which exposure to atmospheric agents, such as air or
moisture, causes rocks and minerals to break down.
This process takes place at or near the Earth's surface. Weathering
entails little or no movement of the material that it loosens from the
rocks and minerals. See also erosion.
A lake, marsh, or swamp that supports wildlife and replenishes the
The process by which wind erodes bedrock through contact
between the bedrock and rock particles carried by the wind.
Of, located on, or being the side of a dune, hill, or ridge facing into
the wind. See also leeward.
A preexisting rock embedded in a newer igneous rock. Xenoliths
are formed when a rising magma incorporates the preexisting
rock. If the preexisting rock does not melt, it will not be assimilated
into the magma and will therefore remain distinct from the new igneous
rock that surrounds it.
The scattering of X-rays passed through a mineral sample so as to form a
pattern peculiar to the given mineral.
The maximum stress that a given rock can withstand without
becoming permanently deformed.
zone of ablation
The part of a glacier in which there is greater over all loss
than gain in volume. A zone of ablation can be identified in the summer
by an expanse of bare ice. See also zone of accumulation.
zone of accumulation
The part of a glacier in which there is greater overall gain
than loss in volume. A zone of accumulation can be identified by a
blanket of snow that survives summer melting. See also zone of
zone of aeration
A region below the Earth's surface that is marked by the presence of
both water and air in the pores of rocks and soil. Also called aeration
zone of saturation
A region that lies below the zone of aeration and is marked by
the presence of water and the absence of air in the pores of rocks and