Geology Glossary
(Most definitions taken from Physical Geology 11e)
(Last update - January 25, 2009)
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A §
0/00 The
symbol for parts per thousand. §
Aa A lava flow that solidifies with a spiny, rubbly
surface. § Ablation The loss of the glacial ice or snow by melting,
evaporation, or breaking off into icebergs. (Also called wastage.) §
Abrasion The grinding
away of rock by friction and impact during transportation. § Absolute age
Age given in years or some other unit
of time. (Also known as numerical age.) § Abyssal fan Great fan-shaped deposit of sediment on the deep-sea
floor at the base of many submarine canyons. §
Abyssal
plain Very flat, sediment-covered region of the deep-sea
floor, usually at the base of the continental rise. §
Accreted
terrane Terrane that did not form at its present site on a
continent. §
Accretionary
wedge (subduction complex) A wedge of thrust-faulted and folded sediment scraped off
a subducting plate by the overlying plate. § Active continental margin A margin
consisting of a continental shelf, a continental slope, and an oceanic
trench. § Actualism The principle that the same processes and natural laws
that operated in the past are those we can actually observe or infer from
observations as operating at present. Under present usage, uniformitarianism
has the same meaning as actualism for most geologists. §
Advancing
glacier Glacier with a positive budget, so that accumulation
results in the lower edges being pushed outward and downward. §
Aftershock Small
earthquake that follows a main shock. § A horizon The top layer of soil, characterized by the downward
movement of water. (Also called zone of leaching.) §
Alkali soil Soil containing
such a great quantity of sodium salts precipitated by evaporating ground
water that it is generally unfit for plant growth. §
Alluvial fan
Large,
fan-shaped pile of sediment that usually forms where a stream's velocity
decreases as it emerges from a narrow canyon onto a flat plain at the foot of
a mountain range. §
Alpine
glaciation Glaciation of a mountainous area. §
Amphibole
group Mineral group in which all members are double-chain
silicates. §
Amphibolite Amphibole
(hornblende), plagioclase schist. §
Andesite Fine-grained
igneous rock of intermediate composition. Up to half of the rock is
plagioclase feldspar with the rest being ferromagnesian minerals. §
Angle of dip
A
vertical angle measured downward from the horizontal plane to an inclined plane. §
Angular Sharp-edged;
lacking rounded edges or corners. §
Angular
unconformity An unconformity in which younger strata overlie an
erosion surface on tilted or folded layered rock. §
Anorthosite A
crystalline rock composed almost entirely of calcium-rich plagioclase
feldspar. §
Antecedent
stream A stream that maintains its original course despite
later deformation of the land. §
Anthracite Coal that
has undergone low-grade metamorphism. Burns dust-free and smokeless. §
Anticline An arched
fold in which the rock layers usually dip away from the axis of the fold. §
Aquifer A body of
saturated rock or sediment through which water can move readily. §
Arch (sea
arch) Bridge of rock left above an opening eroded in a
headland by waves. §
Archean Eon The oldest
eon of Earth's history. §
Arκte A sharp
ridge that separates adjacent glacially carved valleys. §
Arid region An area with
less than 25 centimeters of rain per year. §
Arkose A sandstone
in which more than 25% of the grains are feldspar. §
Artesian
aquifer See confined
aquifer. §
Artesian
well A well in which water rises above the aquifer. §
Artificial
recharge Groundwater recharge increased by engineering
techniques. §
Aseismic
ridge Submarine ridge with which no earthquakes are
associated. §
Ash (volcanic)
Fine
pyroclasts (less than 4 millimeters). § Assimilation
The process in which very hot magma
melts country rock and assimilates the newly molten material. §
Asteroid A small, generally
rocky, solid body orbiting the Sun and ranging in diameter from a few meters
to hundreds of kilometers. § Asthenosphere
A region of Earth's outer shell
beneath the lithosphere. The asthenosphere is of indeterminate thickness and
behaves plastically. §
Astronomical
unit (AU) A distance unit based on the
average distance of the Earth from the Sun. §
Atmosphere Gases that
envelop Earth. §
Atoll A circular
reef surrounding a deeper lagoon. §
Atom Smallest
possible particle of an element that retains the properties of that element. §
Atomic mass
number The total number of neutrons and protons in an atom. §
Atomic
number The total number of protons in an atom. §
Atomic
weight The sum of the weight of the subatomic particles in an
average atom of an element, given in atomic mass units. §
Augite Mineral of
the pyroxene group found in mafic igneous rocks. §
Aulacogen See
failed rift. §
Aureole Zone of
contact metamorphism adjacent to a pluton. §
Axial plane A plane containing
all of the hinge lines of a fold. §
Axis See
hinge line. B §
Backarc
spreading A type of seafloor spreading that moves an island arc
away from a continent, or tears an island arc in two, or splits the edge of a
continent, in each case forming new sea floor. §
Backshore Upper part
of the beach, landward of the high-water line. §
Bajada A broad,
gently sloping, depositional surface formed at the base of a mountain range
in a dry region by the coalescing of individual alluvial fans. § Bar A ridge of sediment, usually sand or gravel, that has
been deposited in the middle or along the banks of a stream by a decrease in
stream velocity. §
Barchan A
crescent-shaped dune with the horns of the crescent pointing downwind. §
Barrier
island Ridge of sand paralleling the shoreline and extending
above sea level. §
Barrier reef
A
reef separated from the shoreline by the deeper water of a lagoon. §
Basal
sliding Movement in which the entire glacier slides along as a single
body on its base over the underlying rock. §
Basalt A
fine-grained, mafic, igneous rock composed predominantly of ferromagnesian
minerals and with lesser amounts of calcium-rich plagioclase feldspar. §
Base level A
theoretical downward limit for stream erosion of Earth's surface. §
Batholith A large
discordant pluton with an outcropping area greater than 100 square
kilometers. §
Bauxite The
principal ore of aluminum; Al203 ∙ nH2O. §
Baymouth bar
A
ridge of sediment that cuts a bay off from the ocean. §
Beach Strip of
sediment, usually sand but sometimes pebbles, boulders, or mud, which extends
from the low-water line inland to a cliff or zone of permanent vegetation. §
Beach face The section
of the beach exposed to wave action. §
Bedding An arrangement of
layers or beds of rock. §
Bedding plane
A
nearly flat surface separating two beds of sedimentary rock. §
Bed load Heavy or large
sediment particles in a stream that travel near or on the stream-bed. §
Bedrock Solid rock that
underlies soil. §
Benioff zone
Distinct
earthquake zone that begins at an oceanic trench and slopes landward and
downward into Earth at an angle of about 30° to 60°. §
Bergschrund
The
crevasse that develops where a glacier is pulling away from a cirque wall. §
Berm Platform of wave-deposited
sediment that is flat or slopes slightly landward. § B horizon A soil layer characterized by the accumulation of
material leached downward from the A horizon above; also called zone of
accumulation. §
Biochemical
Precipitated
by the action of organisms. §
Bioclastic limestone A limestone consisting of fragments of shells,
corals, and algae. §
Biosphere
All
of the living or once-living material on Earth. §
Biotite Iron/magnesium-bearing
mica. §
Block Large angular
pyroclast. §
Blowout A depression on
the land surface caused by wind erosion. §
Body wave
Seismic
wave that travels through Earth's interior. §
Bomb Large spindle- or
lens-shaped pyroclast. §
Bonding Attachment of an
atom to one or more adjacent atoms. § Bottomset bed A delta deposit formed from the finest silt and clay,
which are carried far out to sea by river flow or by sediments sliding
downhill on the sea floor. §
Boulder A sediment
particle with a diameter greater than 256 millimeters. §
Bowen's reaction series The sequence in which minerals crystallize from a
cooling basaltic magma. §
Braided stream A
stream that flows in a network of many interconnected rivulets around
numerous bars. §
Breaker A wave that has
become so steep that the crest of the wave topples forward, moving faster
than the main body of the wave. § Breakwater An offshore structure built to absorb the force of
large breaking waves and provide quiet water near shore. §
Brittle strain Cracking
or rupturing of a body under stress. §
Butte A narrow pinnacle
of resistant rock with a flat top and very steep sides. C §
Calcareous
Containing
calcium carbonate. §
Calcite Mineral with the formula CaCO3. §
Caldera A volcanic
depression much larger than the original crater. §
Capacity (of stream) The total load that a stream can carry. §
Capillary action The
drawing of water upward into small openings as a result of surface tension. § Capillary fringe A thin zone near the water table in which capillary
action causes water to rise above the zone of saturation. §
Carbonaceous chondrite Stony meteorite containing chondrules and composed
mostly of serpentine and large quantities of organic materials. §
Carbonic acid
H2CO3,
a weak acid common in rain and surface waters. §
Cave (cavern)
Naturally
formed underground chamber. § Cement The solid material that precipitates in the pore
space of sediments, binding the grains together to form solid rock. §
Cementation
The
chemical precipitation of material in the spaces between sediment grains,
binding the grains together into a hard rock. §
Cenozoic Era
The
most recent of the eras; followed the Mesozoic Era. §
Chain silicate structure Silicate structure in which two of each tetrahedron's
oxygen ions are shared with adjacent tetrahedrons, resulting in a chain of
tetrahedrons. §
Chalk A very
fine-grained bioclastic limestone. § Channel (Mars) Feature on the
surface of the planet Mars that very closely resembles certain types of
stream channels on Earth. §
Chaotic terrain (Mars) Patch of jumbled and
broken angular slabs and blocks on the surface of Mars. §
Chemical sedimentary rock A rock composed of material precipitated directly
from solution. §
Chemical weathering The decomposition of rock resulting from exposure to
water and atmospheric gases. §
Chert A hard, compact,
fine-grained sedimentary rock formed almost entirely of silica. §
Chill zone
In
an intrusion, the finer-grained rock adjacent to a contact with country rock. §
Chondrule
Round
silicate grain within some stony meteorites. §
C horizon A soil layer composed of incompletely weathered
parent material. §
Cinder (volcanic) Pyroclast
approximately the size of a sand grain. Sometimes defined as between 4 and 32
millimeters in diameter. §
Cinder cone
A
volcano constructed of loose rock fragments ejected from a central vent. §
Circum-Pacific belt Major belt around the edge of the Pacific Ocean on
which most composite volcanoes are located and where many earthquakes occur. §
Cirque A steep-sided, amphitheaterlike hollow
carved into a mountain at the head of a glacial valley. §
Clastic texture An
arrangement of rock fragments bound into a rigid network by cement. §
Clay Sediment composed of particles with diameter less
than 1/256 millimeter. § Clay mineral A hydrous aluminum-silicate that occurs as a platy
grain of microscopic size with a sheet-silicate structure. §
Clay mineral group Collective
term for clay minerals. §
Cleavage The ability of a
mineral to break along preferred planes. §
Coal A sedimentary rock
formed from the consolidation of plant material. It is rich in carbon,
usually black, and burns readily. §
Coal-bed methane Gas
trapped in coal. §
Coarse-grained rock Rock in which most of the grains are larger than 1
millimeter (igneous) or 2 millimeters (sedimentary). §
Coast The land near the
sea, including the beach and a strip of land inland from the beach. §
Coastal straightening The gradual straightening of an irregular shoreline
by wave erosion of headlands and wave deposition in bays. §
Cobble A sediment
particle with a diameter of 64 to 256 millimeters. § Column A dripstone feature formed when a stalactite growing
downward and a stalagmite growing upward meet and join. § Columnar
structure Volcanic
rock in parallel, usually vertical columns, mostly six-sided; also called columnar
jointing. §
Comet Small object in space, no more than a few kilometers
in diameter, composed of frozen methane, frozen ammonia, and water-ice, with
small solid particles and dust imbedded in the ices. § Compaction A loss in overall volume and pore space of a rock as
the particles are packed closer together by the weight of overlying material. §
Competence
The
largest particle that a stream can carry. §
Composite volcano (stratovolcano) A volcano constructed of alternating layers of
pyroclastics and rock solidified from lava flows. §
Compressive stress A
stress due to a force pushing together on a body. §
Conchoidal fracture Curved fracture surfaces. §
Concordant
Parallel
to layering or earlier developed planar structures. §
Concretion
Hard,
rounded mass that develops when a considerable amount of cementing material
precipitates locally in a rock, often around an organic nucleus. §
Cone of depression A depression of the water table
formed around a well when water is pumped out; it is shaped like an inverted
cone. §
Confined aquifer (artesian aquifer) An aquifer completely filled with pressurized water
and separated from the land surface by a relatively impermeable confining
bed, such as shale. § Confining
pressure Pressure applied
equally on all surfaces of a body; also called lithostatic pressure. §
Conglomerate
A
coarse-grained sedimentary rock (grains coarser than 2 millimeters) formed by
the cementation of rounded gravel. §
Consolidation
Any
process that forms firm, coherent rock from sediment or from liquid. §
Contact Boundary surface
between two different rock types or ages of rocks. §
Contact (thermal) metamorphism Metamorphism under conditions in which high
temperature is the dominant factor. §
Continental crust The
thick, granitic crust under continents. §
Continental
drift A concept suggesting that continents move over Earth's
surface. §
Continental
glaciation The covering of a large region of a continent by a sheet
of glacial ice. §
Continental
rise A wedge of sediment that extends from the lower part of
the continental slope to the deep-sea floor. § Continental shelf A submarine platform at the edge of
a continent, inclined very gently seaward generally at an angle of less than
1°. §
Continental
slope A relatively steep slope extending from a depth of 100
to 200 meters at the edge of the continental shelf down to oceanic depths. § Contour current
A bottom current that flows
parallel to the slopes of the continental margin (along the contour rather
than down the slope). §
Contour line
A
line on a topographic map connecting points of equal elevation. § Convection (convection current) A very slow
circulation of a substance driven by differences in temperature and density
within that substance. §
Convergent
plate boundary A boundary between two plates that are moving toward
each other. §
Coquina A limestone
consisting of coarse shells. §
Core The central
zone of Earth. §
Correlation In geology,
correlation usually means determining time equivalency of rock units. Rock
units may be correlated within a region, a continent, and even between
continents. §
Country rock
Any
rock that was older than and intruded by an igneous body. §
Covalent
bonding Bonding due to the sharing of electrons by adjacent
atoms. §
Crater (of a
volcano) A basinlike depression over a vent at the summit of a
volcanic cone. §
Craton Portion of a
continent that has been structurally stable for a prolonged period of time. §
Creep Very slow,
continuous downslope movement of soil or debris. §
Crest (of
wave) The high point of a wave. §
Crevasse
Open
fissure in a glacier. §
Cross-bedding
An
arrangement of relatively thin layers of rock inclined at an angle to the
more nearly horizontal bedding planes of the larger rock unit. § Crosscutting relationship A principle
or law stating that a disrupted pattern is older than the cause of
disruption. §
Cross
section See
geologic cross section. §
Crude oil A liquid
mixture of naturally occurring hydrocarbons. §
Crust The outer
layer of rock, forming a thin skin over Earth's surface. §
Crustal
rebound The rise of Earth's crust after the removal of glacial
ice. §
Crystal A
homogeneous solid with an orderly internal atomic arrangement. § Crystal form
Arrangement of various faces on a
crystal in a definite geometric relationship to one another. § Crystalline Describing a substance in which the atoms are arranged
in a regular, repeating, orderly pattern. §
Crystalline
texture An arrangement of interlocking crystals. §
Crystallization
Crystal
development and growth. §
Crystal
settling The process whereby the minerals that crystallize at a
high temperature in a cooling magma move downward in the magma chamber
because they are denser than the magma. §
Cuesta A ridge with
a steep slope on one side and a gentle slope on the other side. §
Curie point The
temperature below which a material becomes magnetized. D §
Data What
scientists regard as facts. §
Daughter
product The isotope produced by radioactive decay. §
Debris Unconsolidated
material (soil) in which coarse-grained fragments predominate. §
Debris
avalanche Very rapid and turbulent mass wasting of debris, air,
and water. §
Debris flow Mass wasting
involving the flow of soil (unconsolidated material) in which coarse material
(gravel, boulders) is predominant. §
Decompression
melting Partial melting of hot mantle rock when it moves upward
and the pressure is reduced to the extent that the melting point drops to the
temperature of the body. §
Deflation The removal
of clay, silt, and sand particles from the land surface by wind. §
Delamination
See
lithospheric delamination. §
Delta A body of
sediment deposited at the mouth of a river when the river velocity decreases
as it flows into a standing body of water. § Dendritic pattern Drainage pattern of a river and its
tributaries that resembles the branches of a tree or veins in a leaf. §
Density Weight per
given volume of a substance. §
Deposition The settling
or coming to rest of transported material. §
Depth of focus
Distance
between the focus and the epicenter of an earthquake. §
Desert A region
with low precipitation (usually defined as less than 25 centimeters per
year). §
Desertification
The
expansion of barren deserts into once-populated regions. § Desert pavement
A thin layer of closely packed
gravel that protects the underlying sediment from deflation; also called pebble armor § Detachment fault
Major fault in a mountain belt
above which rocks have been intensely folded and faulted. §
Detrital
sedimentary rock A sedimentary rock composed of fragments of preexisting
rock. § Diapir Bodies of rock (e.g., rock salt) or magma that ascend
within Earth's interior because they are less dense than the surrounding
rock. §
Differential
stress When pressures on a body are not of equal strength in
all directions. §
Differential
weathering Varying rates of weathering resulting from some rocks in
an area being more resistant to weathering than others. §
Differentiation
Separation
of different ingredients from an originally homogeneous mixture. §
Dike A tabular,
discordant intrusive structure. §
Diorite Coarse-grained
igneous rock of intermediate composition. Up to half of the rock is
plagioclase feldspar and the rest is ferromagnesian minerals. §
Dip See angle
of dip, direction of dip. §
Dip-slip
fault A fault in which movement is parallel to the dip of the
fault surface. §
Directed
pressure See differential stress. §
Direction of
dip The compass direction in which the angle of dip is
measured. §
Discharge In a stream,
the volume of water that flows past a given point in a unit of time. §
Disconformity
A
surface that represents missing rock strata but beds above and below that
surface are parallel to one another. §
Discordant Not parallel
to any layering or parallel planes. §
Dissolved
load The portion of the total sediment load in a stream that
is carried in solution. §
Distributary
Small
shifting river channel that carries water away from the main river channel
and distributes it over a delta's surface. §
Divergent
plate boundary Boundary separating two plates moving away from each
other. §
Divide Line
dividing one drainage basin from another. §
Dolomite A
sedimentary rock composed mostly of the mineral dolomite. §
Dolomitic
marble Marble in which dolomite, rather than calcite, is the
prevalent mineral. §
Dome See structural
dome. §
Double
refraction The splitting of light into two components when it
passes through certain crystalline substances. §
Downcutting A
valley-deepening process caused by erosion of a streambed. §
Drainage
basin Total area drained by a stream and its tributaries. §
Drainage
pattern The arrangement in map view of a river and its
tributaries. §
Drawdown The lowering
of the water table near a pumped well. §
Dripstone Deposits of
calcite (and, rarely, other minerals) built up by dripping water in caves. §
Drumlin A long,
streamlined hill made of till. §
Ductile Capable of
being molded and bent under stress. §
Ductile
strain Strain in which a body is molded or bent under stress
and does not return to its original shape after the stress is removed. §
Dust
(volcanic) Finest-sized pyroclasts. E §
E horizon Soil horizon
that is the zone of leaching, characterized by the downward movement of water
and removal of fine-grained soil components. §
Earth
In
mass wasting, soil in which fine-grained particles are predominant. §
Earth
systems Study of Earth by analyzing how its components, or
subsystems, interrelate. §
Earthflow Slow-to-rapid
mass wasting in which fine-grained soil moves downslope as a very viscous
fluid. § Earthquake A trembling or shaking of the ground caused by the
sudden release of energy stored in the rocks beneath the surface. §
Earthy
luster A luster giving a substance the appearance of unglazed
pottery. §
Echo sounder
An
instrument used to measure and record the depth to the sea floor. §
Elastic
limit The maximum amount of stress that can be applied to a
body before it deforms in a permanent way by bending or breaking. §
Elastic
rebound theory The sudden release of progressively stored strain in
rocks results in movement along a fault. §
Elastic
strain Strain in which a deformed body recovers its original
shape after the stress is released. §
Electron A single,
negative electric charge that contributes virtually no mass to an atom. §
Element A substance
that cannot be broken down to other substances by ordinary chemical methods.
Each atom of an element possesses the same number of protons. §
Emergent
coast A coast in which land formerly under water has recently
been placed above sea level, either by uplift of the land or by a drop in sea
level. §
End moraine A ridge of
till piled up along the front edge of a glacier. § Environment of deposition The location
in which deposition occurs, usually marked by characteristic physical,
chemical, or biological conditions. §
Eon The largest
unit of geological time. §
Epicenter The point on
Earth's surface directly above the focus of an earthquake. §
Epoch Each period
of the standard geologic time scale is divided into epochs (e.g., Pleistocene
Epoch of the Quaternary Period). §
Equilibrium Material is
in equilibrium if it is adjusted to the physical and chemical conditions of
its environment so that it does not change or alter with time. § Equilibrium line
An irregular line marking the
highest level to which the winter snow cover on a glacier is lost during a
melt season; also called snow line. §
Era Major
subdivision of the standard geologic time scale (e.g., Mesozoic Era). §
Erosion The physical
removal of rock by an agent such as running water, glacial ice, or wind. §
Erratic An ice-transported boulder that does not derive from
bedrock near its present site. §
Esker A long,
sinuous ridge of sediment deposited by glacial meltwater. §
Estuary Drowned
river mouth. §
Etch-pitted
terrain (Mars)
A terrain on the surface of Mars characterized by small
pits. §
Evaporite Rock that
forms from crystals precipitating during evaporation of water. §
Exfoliation The
stripping of concentric rock slabs from the outer surface of a rock mass. §
Exfoliation
dome A large, rounded landform developed in a massive rock,
such as granite, by the process of exfoliation. §
Exotic
terrane Terrane that did not form at its present site on a
continent and traveled a great distance to get to its present site. §
Expansive
clay Clay that increases in volume when water is added to it. §
Extension Strain
involving an increase in length. Extension can cause crustal thinning and
faulting. §
Extrusive
rock Any igneous rock that forms at Earth's surface, whether
it solidifies directly from a lava flow or is pyroclastic. F §
Faceted A rock
fragment with one or more flat surfaces caused by erosive action. §
Failed rift
(aulacogen) An inactive, sediment-filled rift that forms above a
mantle plume. The rift becomes inactive as two other rifts widen to form an
ocean. §
Fall The
situation in mass wasting that occurs when material free-falls or bounces
down a cliff. §
Fault A fracture
in bedrock along which movement has taken place. §
Fault-block
mountain range A range created by uplift along
normal or vertical faults. §
Faunal
succession A principle or law stating that fossil species succeed
one another in a definite and recognizable order; in general, fossils in
progressively older rock show increasingly greater differences from species
living at present. §
Feldspar Group of
most common minerals of Earth's crust. All feldspars contain silicon,
aluminum, and oxygen and may contain potassium, calcium, and sodium. §
Felsic rock Silica-rich
igneous rock with a relatively high content of potassium and sodium. § Ferromagnesian mineral Iron/magnesiumbearing
mineral, such as augite, hornblende, olivine, or biotite. §
Fine-grained
rock A rock in which most of the mineral grains are less than
1 millimeter across (igneous) or less than 1/16 millimeter (sedimentary). §
Fiord A coastal
inlet that is a glacially carved valley, the base of which is submerged. §
Firn A compacted mass
of granular snow, transitional between snow and glacier ice. §
Firn limit See equilibrium
line. §
Fissility The ability
of a rock to split into thin layers. §
Flank
eruption An eruption in which lava erupts out of a vent on the
side of a volcano. §
Flash flood Flood of
very high discharge and short duration; sudden and local in extent. §
Flood plain A broad
strip of land built up by sedimentation on either side of a stream channel. §
Flow A type of
movement that implies that a descending mass is moving downslope as a viscous
fluid. §
Flowstone Calcite
precipitated by flowing water on cave walls and floors. §
Focus The point
within Earth from which seismic waves originate in an earthquake. §
Fold Bend in
layered bedrock. §
Fold and
thrust belt A portion of a major mountain belt characterized by
large thrust faults, stacked one upon another. Layered rock between the
faults was folded when faulting was taking place. §
Fold axis See hinge
line. §
Foliation Parallel
alignment of textural and structural features of a rock. §
Footwall The
underlying surface of an inclined fault plane. §
Foreland
basin A sediment-filled basin on a continent, landward of a
magmatic arc, and caused indirectly by ocean-continent convergence. §
Foreset bed A sediment layer
in the main part of a delta, deposited at an angle to the horizontal. §
Foreshock Small
earthquake that precedes a main shock. §
Foreshore The zone
that is regularly covered and uncovered by the rise and fall of tides. §
Formation A body of
rock of considerable thickness that has a recognizable unity or similarity
making it distinguishable from adjacent rock units. Usually composed of one
bed or several beds of sedimentary rock, although the term is also applied to
units of metamorphic and igneous rock. A convenient unit for mapping,
describing, or interpreting the geology of a region. §
Fossil Traces of
plants or animals preserved in rock. §
Fossil
assemblage Various different species of fossils in a rock. §
Fracture The way a
substance breaks where not controlled by cleavage. §
Fracture
zone Major line of weakness in Earth's crust that crosses the
mid-oceanic ridge at approximately right angles. §
Fracturing Cracking or
rupturing of a body under stress. §
Framework
silicate structure Crystal structure in which all four oxygen ions of a
silica tetrahedron are shared by adjacent ions. §
Fretted
terrain (Mars)
Flat lowland with some scattered high plateaus on the
surface of Mars. §
Fringing
reef A reef attached directly to shore. See barrier reef. §
Frost action
Mechanical
weathering of rock by freezing water. §
Frost
heaving The lifting of rock or soil by the expansion of freezing
water. §
Frost
wedging A type of frost action in which the expansion of
freezing water pries a rock apart. G §
Gabbro A mafic,
coarse-grained igneous rock composed predominantly of ferromagnesian minerals
and with lesser amounts of calcium-rich plagioclase feldspar. §
Gaining stream A
stream that receives water from the zone of saturation. §
Geode Partly hollow,
globelike body found in limestone or other cavernous rock. §
Geologic cross section A representation of a portion of Earth in a vertical
plane. §
Geologic map
A
map representing the geology of a given area. §
Geologic resources Valuable materials of geologic
origin that can be extracted from Earth. §
Geology The scientific study of the planet Earth. §
Geophysics
The
application of physical laws and principles to a study of Earth. §
Geosphere
Solid
Earth system. The rock and other inorganic material that make up the bulk of
the planet. §
Geothermal energy Energy
produced by harnessing naturally occurring steam and hot water. §
Geothermal gradient Rate of temperature increase associated with
increasing depth beneath the surface of Earth (normally about 25°C per
kilometer). §
Geyser A type of hot
spring that periodically erupts hot water and steam. §
Geyserite
A
deposit of silica that forms around many geysers and hot springs. § Glacier A large, long-lasting mass of ice, formed on land by
the compaction and recrystallization of snow, which moves because of its own
weight. §
Glacier ice
The
mosaic of interlocking ice crystals that form a glacier. §
Glassy (or vitreous) luster A luster that gives a substance a glazed,
porcelainlike appearance. §
Gneiss A metamorphic
rock composed of light and dark layers or lenses. § Gneissic The texture of a metamorphic rock in which minerals
are separated into light and dark-layers or lenses. §
Goethite The commonest mineral
in the limonite group; Fe203 nH2O. §
Gondwanaland
The
southern part of Pangaea (see definition) that formed South America,
Africa, India, Australia, and Antarctica. §
Graben A downdropped
block bounded by normal fault. §
Graded bed
A
single bed with coarse grains at the bottom of the bed and progressively
finer grains toward the top of the bed. § Graded stream A stream that exhibits a delicate balance between its
transporting capacity and the sediment load available to it. §
Granite A felsic, coarse-grained,
intrusive igneous rock containing quartz and composed mostly of potassium-
and sodium-rich feldspars. §
Gravel Rounded particles coarser than 2 millimeters in
diameter. §
Gravitational collapse and spreading When part of a mountain belt becomes too high, it
moves vertically downward forcing rock at depth to spread out laterally. §
Gravity The force of
attraction that two bodies exert on each other that is proportional to the
product of their masses and inversely proportional to the square of the
distance from the centers of the two bodies. §
Gravity anomaly A
deviation from the average gravitational attraction between Earth and an
object. See negative gravity anomaly, positive gravity anomaly. § Gravity meter An instrument that measures the gravitational
attraction between Earth and a mass within the instrument. §
Graywacke
A
sandstone with more than 15% fine-grained matrix between the sand grains. §
Greenhouse effect The
trapping of heat by a planet's atmosphere, making the planet warmer than
would otherwise be expected. Generally, the greenhouse effect operates if
visible sunlight passes freely through a planet's atmosphere, but the
infrared radiation produced by the warm surface cannot escape readily into space. §
Groin Short wall built
perpendicular to shore to trap moving sand and widen a beach. §
Ground moraine A
blanket of till deposited by a glacier or released as glacier ice melted. § Ground water The water that lies beneath the ground surface,
filling the cracks, crevices, and pore space of rocks. §
Guyot Flat-topped
seamount. H §
Hadean Eon
The
oldest eon. §
Half-life
The
time it takes for a given amount of a radioactive isotope to be reduced by
one-half. §
Hanging valley A
smaller valley that terminates abruptly high above a main valley. §
Hanging wall
The
overlying surface of an inclined fault plane. §
Hardness The relative ease
or difficulty with which a smooth surface of a mineral can be scratched; commonly
measured by Mohs scale. §
Headland Point of land
along a coast. §
Headward erosion The
lengthening of a valley in an uphill direction above its original source by
gullying, mass wasting, and sheet erosion. §
Heat engine
A
device that converts heat energy into mechanical energy. §
Heat flow
Gradual
loss of heat (per unit of surface area) from Earth's interior out into space. §
Heavy crude
Dense,
viscous petroleum that flows slowly or not at all. §
Hematite A type of iron
oxide that has a brick-red color when powdered; Fe203. §
Highland (Moon) A rugged region of the lunar surface representing an
early period in lunar history when intense meteorite bombardment formed
craters. §
Hinge line
Line
about which a fold appears to be hinged. Line of maximum curvature of a
folded surface. §
Hinge plane
See
axial
plane. §
Hogback A sharp-topped
ridge formed by the erosion of steeply dipping beds. §
Holocene Epoch The
youngest epoch which began around 10,000 years ago and is continuing
presently. §
Horn A sharp peak
formed where cirques cut back into a mountain on several sides. §
Hornblende
Common
amphibole frequently found in igneous and metamorphic rocks. §
Hornfels A fine-grained,
unfoliated metamorphic rock. §
Horst An up-raised
block bounded by normal faults. §
Hot spot An area of
volcanic eruptions and high heat flow above a rising mantle plume. §
Hot spring
Spring
with a water temperature warmer than human body temperature. §
Hydraulic action The
ability of water to pick up and move rock and sediment. §
Hydrologic cycle The
movement of water and water vapor from the sea to the atmosphere, to the
land, and back to the sea and atmosphere again. §
Hydrology
The
study of water's properties, circulation, and distribution. §
Hydrosphere
The
water on or near Earth's surface. §
Hydrothermal rock Rock
deposited by precipitation of ions from solution in hot water. §
Hydrothermal vein Quartz
or other minerals that have been deposited in a crack by hot fluids. §
Hypocenter
Synonym
for the focus of an earthquake. §
Hypothesis
A
tentative theory. I §
Iceberg Block of
glacier-derived ice floating in water. §
Ice cap A glacier
covering a relatively small area of land but not restricted to a valley. §
Icefall A chaotic jumble
of crevasses that split glacier ice into pinnacles and blocks. §
Ice sheet
A
glacier covering a large area (more than 50,000 square kilometers) of land. §
Igneous rock
A
rock formed or apparently formed from solidification of magma. §
Incised meander A
meander that retains its sinuous curves as it cuts vertically downward below
the level at which it originally formed. §
Inclusion
A
fragment of rock that is distinct from the body of igneous rock in which it
is enclosed. §
Inclusion, principle of Fragments included in a host rock are older than the
host rock. §
Index fossil
A
fossil from a very short-lived species known to have existed during a
specific period of geologic time. §
Inner planet
A
planet orbiting in the inner part of the Solar System. Sometimes taken to
mean Mercury, Venus, Earth, and Mars. §
Intensity
A
measure of an earthquake's size by its effect on people and buildings. §
Intermediate rock Rock
with a chemical content between felsic and mafic compositions. §
Intrusion (intrusive structure) A body of intrusive rock classified on the basis of
size, shape, and relationship to surrounding rocks. §
Intrusive rock Rock
that appears to have crystallized from magma emplaced in surrounding rock. §
Ion An electrically
charged atom or group of atoms. § Ionic bonding Bonding due to the attraction between positively
charged ions and negatively charged ions. §
Iron meteorite A
meteorite composed principally of iron-nickel alloy. §
Island arc
A
curved line of islands. §
Isoclinal fold A
fold in which the limbs are parallel to one another. § Isolated silicate
structure Silicate minerals
that are structured so that none of the oxygen atoms are shared by silica
tetrahedrons. §
Isostasy The balance or
equilibrium between adjacent blocks of crust resting on a plastic mantle. §
Isostatic adjustment Concept of vertical movement of sections of Earth's
crust to achieve balance or equilibrium. §
Isotherm A line along
which the temperature of rock (or other material) is the same. §
Isotopes Atoms (of the
same element) that have different numbers of neutrons but the same number of
protons. § Isotopic dating Determining the age of a rock or mineral through its
radioactive elements and decay products (previously and somewhat inaccurately
called radiometric or radioactive dating). J §
Jetty Rock wall protruding above sea level, designed to
protect the entrance of a harbor from sediment deposition and storm waves;
usually built in pairs. §
Joint A fracture or crack
in bedrock along which essentially no displacement has occurred. §
Joint set
Joints
oriented in one direction approximately parallel to one another. K §
Kame Low mound or
irregular ridge formed of outwash deposits on a stagnating glacier. § Kame and kettle
topography Irregular, bumpy
landscape of hills and depressions associated with many moraines. §
Karst topography An
area with many sinkholes and a cave system beneath the land surface and
usually lacking a surface stream. §
Kettle A depression caused
by the melting of a stagnant block of ice that was surrounded by sediment. §
Kimberlite
An
ultramafic rock that contains olivine along with mica, garnet, or both.
Diamonds are found in some kimberlite bodies. L §
Laccolith
A concordant
intrusive structure, similar to a sill, with the central portion thicker and
domed upward. Laccoliths are not common and are not discussed in this
textbook. §
Laminar flow
Slow,
smooth flow, with each drop of water traveling a smooth path parallel to its
neighboring drops. § Laminated terrain
(Mars) Area
where series of alternating light and dark layers can be seen on the surface
of Mars. §
Lamination
A
thin layer in sedimentary rock (less than 1 centimeter thick). §
Landform A
characteristically shaped feature of Earth's surface, such as a hill or a
valley. § Lapilli (plural) Pyroclasts in the 2-64 millimeter size range
(singular, lapillus). §
Landslide
The
general term for a slowly to very rapidly descending mass of rock or debris. §
Lateral continuity Principle
that states that an original sedimentary layer extends laterally until it
tapers or thins at its edges. §
Lateral erosion Erosion
and undercutting of stream banks caused by a stream swinging from side to side
across its valley floor. §
Lateral moraine A
low, ridgelike pile of till along the side of a glacier. § Laterite Highly leached soil that forms in regions of tropical
climate with high temperatures and very abundant rainfall. §
Lava Magma on Earth's
surface. §
Lava flow
Flow
of lava from a crater or fissure. §
Lava tube
Tunnel-like
cave within a lava flow. It forms during the late stages of solidification of
a mafic lava flow. § Left-lateral
fault A strike-slip
fault in which the block seen across the fault appears displaced to the left. §
Limb Portion of a fold
shared by an anticline and a syncline. §
Limestone
A
sedimentary rock composed mostly of calcite. §
Limonite A type of iron
oxide that is yellowish-brown when powdered; Fe203
nH2O. § Liquefaction A type of ground failure in which water-saturated
sediment turns from a solid to a liquid as a result of shaking, often caused
by an earthquake. §
Lithification
The
consolidation of sediment into sedimentary rock. §
Lithosphere
The
rigid outer shell of Earth, 70 to 125 or more kilometers thick. §
Lithospheric delamination The detachment of part of the mantle portion of the
lithosphere beneath a mountain belt. §
Lithostatic pressure Confining pressure due to the weight of overlying
rock. §
Loam Soil containing
approximately equal amounts of sand, silt, and clay. §
Loess A fine-grained
deposit of wind-blown dust. §
Longitudinal dune (seif) Large, symmetrical ridge of sand parallel to the wind
direction. §
Longitudinal profile A line showing a stream's slope, drawn along the
length of the stream as if it were viewed from the side. §
Longshore current A
moving mass of water that develops parallel to a shoreline. §
Longshore drift Movement
of sediment parallel to shore when waves strike a shoreline at an angle. §
Losing stream
Stream
that loses water to the zone of saturation. §
Love waves
A
type of surface seismic wave that causes the ground to move side to side in a
horizontal plane perpendicular to the direction the wave is traveling. §
Low-velocity zone Mantle
zone at a depth of about 100 kilometers where seismic waves travel more
slowly than in shallower layers of rock. §
Luster The quality and
intensity of light reflected from the surface of a mineral. M §
Mafic rock
Silica-deficient
igneous rock with a relatively high content of magnesium, iron, and calcium. §
Magma Molten rock,
usually mostly silica. The liquid may contain dissolved gases as well as some
solid minerals. §
Magmatic arc
A
line of batholiths or volcanoes. Generally the line, as seen from above, is
curved. §
Magmatic underplating See
underplating. §
Magnetic anomaly A
deviation from the average strength of Earth's magnetic field. See negative magnetic anomaly, positive magnetic anomaly. §
Magnetic field Region
of magnetic force that surrounds Earth. § Magnetic pole An area where the strength of the magnetic field is
greatest and where the magnetic lines of force appear to leave or enter
Earth. § Magnetic reversal
A change in Earth's magnetic field
between normal polarity and reversed polarity. In normal polarity, the north
magnetic pole, where magnetic lines of force enter Earth, lies near the
geographic North Pole. In reversed polarity, the south magnetic pole, where
lines of force leave Earth, lies near the geographic North Pole (the magnetic
poles have exchanged positions). §
Magnetite
An
iron oxide that is attracted to a magnet. §
Magnetometer
An
instrument that measures the strength of Earth's magnetic field. §
Magnitude
A measure
of the energy released during an earthquake. §
Major mountain belt A long chain (thousands of kilometers) of mountain
ranges. §
Mantle A thick shell of
rock that separates Earth's crust above from the core below. §
Mantle diapir
A
body of mantle rock, hotter than its surroundings, that ascends because it is
less dense than the surrounding rock. §
Mantle plume
Narrow
column of hot mantle rock that rises and spreads radially outward. §
Marble A coarse-grained
rock composed of interlocking calcite (or dolomite) crystals. § Maria (Moon) Lava
plains on Moon's surface (singular, mare). §
Marine terrace A
broad, gently sloping platform that may be exposed at low tide. §
Mass wasting (or mass movement) Movement, caused by gravity, in which bedrock, rock debris,
or soil moves downslope in bulk. §
Matrix Fine-grained
material found in the pore space between larger sediment grains. §
Meander A pronounced
sinuous curve along a stream's course. §
Meander cutoff A new, shorter channel across the narrow neck of a
meander. §
Meander scar An abandoned meander filled with sediment and
vegetation. §
Mechanical
weathering
The physical disintegration of rock into
smaller pieces. §
Medial moraine A single long ridge of till on a glacier, formed by
adjacent lateral moraines joining and being carried downglacier. §
Mediterranean-Himalayan
belt
(Mediterranean belt) A major
concentration of earthquakes and composite volcanoes that runs through the
Mediterranean Sea, crosses the Mideast and the Himalaya, and passes through
the East Indies. §
Melt Liquid rock resulting from melting in a laboratory. §
Mercalli scale See modified Mercalli
scale. §
Mesa A broad, flat-topped hill bounded by cliffs and
capped with a resistant rock layer. §
Mesozoic Era The era that followed the Paleozoic Era and preceded
the Cenozoic Era. §
Metallic bonding Bonding, as in metals, whereby atoms are closely
packed together and electrons move freely among atoms. §
Metallic luster Luster giving a substance the appearance of being
made of metal. §
Metamorphic
facies
Metamorphic rocks that contain the same
set of pressure or temperature sensitive minerals are regarded as belonging
to the same facies, implying that they formed under broadly similar pressure and
temperature conditions. §
Metamorphic rock A rock produced by metamorphism. §
Metamorphism The transformation of preexisting rock into
texturally or mineralogically distinct new rock as a result of high
temperature, high pressure, or both but without the rock melting in the
process. §
Metasomatism Metamorphism coupled with the introduction of ions
from an external source. §
Meteor Fragment that passes through Earth's atmosphere,
heated to incandescence by friction; sometimes incorrectly called
"shooting" or "falling" stars. §
Meteorite Meteor that strikes Earth's surface. §
Meteoroid Small solid particles of stone and/or metal orbiting
the sun. §
Mica group Group
of minerals with a sheet-silicate structure. §
Microcline
(potassium) feldspar A feldspar with
the formula KAlSi3O8. §
Mid-oceanic ridge A giant mountain range that lies under the ocean and
extends around the world. §
Migmatite Mixed igneous and metamorphic rock. §
Milky Way galaxy The galaxy to which the Sun belongs. Seen from Earth,
the galaxy is a pale, milky band in the night sky. §
Mineral A crystalline substance that is naturally occurring
and is chemically and physically distinctive. §
Mineraloid A substance that is not crystalline but otherwise
would be considered a mineral. § Model In science, a
model is an imagegraphic, mathematical, or verbalthat is consistent with
the known data. §
Modified Mercalli
scale
Scale expressing intensities of
earthquakes (judged on amount of damage done) in Roman numerals ranging from
Ito XII. §
Mohoroviĉić
discontinuity
The boundary separating the crust from
the mantle beneath it (also called Moho). §
Mohs' hardness
scale
Scale on which ten minerals are
designated as standards of hardness. §
Molecule The smallest possible unit of a substance that has
the properties of that substance. § Moment magnitude An
earthquake magnitude calculated from the strength of the rock, surface area
of the fault rupture, and the amount of rock displacement along the fault. §
Monocline A local steeping in a gentle regional dip; a steplike
fold in rock. §
Moraine A body of till either being carried on a glacier or
left behind after a glacier has receded. §
Mountain range A group of closely spaced mountains or parallel
ridges. §
Mud Term loosely used for silt and clay, usually wet. §
Mud crack Polygonal crack formed in very fine-grained sediment
as it dries. §
Mudflow A flowing mixture of debris and water, usually moving
down a channel. §
Mudstone A fine-grained sedimentary rock that lacks shale's
laminations and fissility. §
Muscovite Transparent or white mica that lacks iron and
magnesium. N §
Natural gas A gaseous mixture of naturally occurring
hydrocarbons. §
Natural levee Low ridges of flood-deposited sediment formed on
either side of a stream channel, which thin away from the channel. §
Nebula A large volume of interstellar gas and dust. §
Nebular
Hypotheses
The hypothesis that the Solar System
formed from a rotating cloud of gas and dust, the solar nebula. §
Negative gravity
anomaly
Less than normal gravitational
attraction. §
Negative magnetic
anomaly
Less than average strength of Earth's
magnetic field. §
Neutron A subatomic particle that contributes mass to an atom
and is electrically neutral. §
Nonconformity An unconformity in which an erosion surface on plutonic or metamorphic rock has been covered
by younger sedimentary or volcanic rock. § Nonmetallic luster
Luster that gives a substance the appearance of being made of something other
than metal (e.g., glassy). §
Nonrenewable
resource
A resource that forms at extremely slow
rates compared to its rate of consumption. §
Normal fault A fault in which the hanging-wall block moved down
relative to the footwall block. §
Nucleus Protons and neutrons form the nucleus of an atom.
Although the nucleus occupies an extremely tiny fraction of the volume of the
entire atom, practically all the mass of the atom is concentrated in the
nucleus. §
Numerical age Age given in years or some other unit of time. O §
Oblique-slip
fault
A fault with both strike-slip and
dip-slip components. §
Obsidian Volcanic glass. §
Oceanic crust The thin, basaltic crust under oceans. §
Oceanic trench A narrow, deep trough parallel to the edge of a continent
or an island arc. §
O Horizon - Dark-colored
soil layer that is rich in organic material and forms just below surface
vegetation. §
Oil See crude oil. §
Oil field An area underlain by one or more oil pools. §
Oil pool Underground accumulation of oil. oil sand
Asphalt-cemented sand deposit. §
Oil shale Shale with a high content of organic matter from
which oil may be extracted by distillation. §
Oil trap A set of conditions that hold petroleum in a
reservoir rock and prevent its escape by migration. §
Olivine A ferromagnesian mineral with the formula (Fe, Mg)2SiO4. §
Oolite (ooid) A small sphere of calcite precipitated from seawater. §
Oolitic limestone A limestone formed from oolites. §
Opal A mineraloid composed of silica and water. §
Open fold A fold with gently dipping limbs. §
Open-pit mine Mine in which ore is exposed at the surface in a
large excavation. §
Ophiolite A distinctive rock sequence found in many mountain
ranges on continents. §
Ore Naturally occurring material that can be profitably
mined. §
Ore mineral A mineral of commercial value. §
Organic
sedimentary rock
Rock composed mostly of the remains of
plants and animals. §
Original
horizontality
The deposition of most water-laid sediment
in horizontal or near-horizontal layers that are essentially parallel to
Earth's surface. §
Orogeny An episode of intense deformation of the rocks in a
region, generally accompanied by metamorphism and plutonic activity. §
Orthoclase
(potassium) feldspar A feldspar with
the formula KAlSi3O8. §
Outcrop A surface exposure of bare rock, not covered by soil
or vegetation. § Outer planet A planet whose
orbit lies in the outer part of the Solar System. Jupiter, Saturn, Uranus,
Neptune, and Pluto are outer planets. §
Outwash Material deposited by debris-laden meltwater from a
glacier. §
Overburden The upper part of a sedimentary deposit. Its weight
causes compaction of the lower part. §
Overturned
fold A fold in which both limbs dip in the same direction. §
Oxbow lake A
crescent-shaped lake occupying the abandoned channel of a stream meander that
is isolated from the present channel by a meander cutoff and sedimentation. P §
Pahoehoe A lava flow
characterized by a ropy or billowy surface. § Paired terraces
Stream
terraces (see definition) that occur at the same
elevation on each side of a river. §
Paleomagnetism
A
study of ancient magnetic fields. §
Paleozoic
Era The era that followed the Precambrian and began with the
appearance of complex life, as indicated by fossils. §
Pangaea A
supercontinent that broke apart 200 million years ago to form the present
continents. §
Parabolic
dune A deeply curved dune in a region of abundant sand. The
horns point upwind and are often anchored by vegetation. §
Parent rock Original
rock before being metamorphosed. §
Partial
melting Melting of the components of a rock with the lowest
melting temperatures. §
Passive
continental margin A margin that includes a continental shelf, continental
slope, and continental rise that generally extends down to an abyssal plain
at a depth of about 5 kilometers. §
Paternoster
lakes A series of rock-basin lakes carved by glacial erosion. §
Peat A brown,
lightweight, unconsolidated or semi-consolidated deposit of plant remains. §
Pebble A sediment
particle with a diameter of 2 to 64 millimeters. §
Pediment A gently
sloping erosional surface cut into the solid rock of a mountain range in a
dry region; usually covered with a thin veneer of gravel. §
Pegmatite Extremely
coarse-grained igneous rock. § Pelagic sediment
Sediment made up of fine-grained
clay and the skeletons of microscopic organisms that settle slowly down
through the ocean water. §
Perched
water table A water table separated from the main water table
beneath it by a zone that is not saturated. §
Peridotite Ultramafic
rock composed of pyroxene and olivine. §
Period Each era of
the standard geologic time scale is subdivided into periods (e.g., the
Cretaceous Period). §
Permafrost Ground that
remains permanently frozen for many years. §
Permeability
The
capacity of a rock to transmit a fluid such as water or petroleum. §
Petrified
wood A material that forms as the organic matter of buried
wood is either filled in or replaced by inorganic silica carried in by ground
water. §
Petroleum Crude oil
and natural gas. (Some geologists use petroleum as a synonym for oil.) §
Phanerozoic Eon of
geologic time. Includes all time following the Precambrian. §
Phenocryst Any of the
large crystals in porphyritic igneous rock. § Phyllite A metamorphic rock in which clay minerals have
recrystallized into microscopic micas, giving the rock a silky sheen. §
Physical
continuity Being able to physically follow a rock unit between two
places. § Physical geology
A large division of geology
concerned with Earth materials, changes of the surface and interior of Earth,
and the forces that cause those changes. §
Pillow
structure Rocks, generally basalt, formed in pillow-shaped masses
fitting closely together; caused by underwater lava flows. § Placer mine Surface
mines in which valuable mineral grains are extracted from stream bar or beach
deposits. §
Plagioclase
feldspar A feldspar containing sodium, calcium, or both, in
addition to aluminum, silicon, and oxygen. §
Planet A body in
orbit around a star. §
Planetesimal
Small,
planet-like body. §
Plastic Capable of
being molded and bent under stress. §
Plastic flow
Movement
within a glacier in which the ice is not fractured. §
Plate A large,
mobile slab of rock making up part of Earth's surface. § Plateau Broad, flat-topped area elevated above the surrounding
land and bounded, at least in part, by cliffs. §
Plateau
basalts Layers of basalt flows that have built up to great
thicknesses. §
Plate
tectonics A theory that Earth's surface is divided into a few
large, thick plates that are slowly moving and changing in size. Intense
geologic activity occurs at the plate boundaries. §
Playa A very flat
surface underlain by hard, mud-cracked clay. §
Playa lake A shallow
temporary lake (following a rainstorm) on a flat valley floor in a dry
region. §
Pleistocene
Epoch An epoch of the Quaternary Period characterized by
several glacial ages. §
Plunging
fold A fold in which the hinge line (or axis) is not
horizontal. §
Pluton An igneous
body that crystallized deep underground. §
Plutonic
rock Igneous rock formed at great depth. §
Pluvial lake
A
lake formed during an earlier time of abundant rainfall. §
Point bar A stream bar (see definition) deposited on the inside of a curve in the
stream, where the water velocity is low. §
Polarity See magnetic
reversal. §
Polar
wandering An apparent movement of the Earth s poles. § Polymorphs Substances having the same chemical composition but
different crystal structures (e.g., diamond and graphite). §
Pore space The total
amount of space taken up by openings between sediment grains. §
Porosity The
percentage of a rock's volume that is taken up by openings. §
Porphyritic
rock An igneous rock in which large crystals are enclosed in
a matrix (or ground mass) of much finer-grained minerals or obsidian. §
Positive
gravity anomaly Greater than normal gravitational attraction. §
Positive
magnetic anomaly Greater than average strength of the Earth s magnetic
field. §
Potassium
feldspar A feldspar with the formula KAISi3O8. §
Pothole Depression
eroded into the hard rock of a streambed by the abrasive action of the
stream's sediment load. §
Precambrian The vast
amount of time that preceded the Paleozoic Era. §
Precambrian
shield A complex of old Precambrian metamorphic and plutonic
rocks exposed over a large area. §
Pressure
release A significant type of mechanical weathering that causes
rocks to crack when overburden is removed. §
Prograde
metamorphism Metamorphism in which progressively greater pressure and
temperature act on a rock type with increasing depth in Earth's crust. §
Proterozoic Eon of
Precambrian time. §
Proton A subatomic particle
that contributes mass and a single positive electrical charge to an atom. §
Pumice A frothy volcanic glass. §
P wave A
compressional wave (seismic wave) in which rock vibrates parallel to the
direction of wave propagation. §
P-wave
shadow zone The region on Earth's surface, 103° to 142° away from an
earthquake epicenter, in which P waves from the earthquake are absent. §
Pyroclast
Fragment
of rock formed by volcanic explosion. §
Pyroclastic
debris Rock fragments produced by volcanic explosion. §
Pyroclastic
flow Turbulent mixture of pyroclastics and gases flowing down
the flank of a volcano. §
Pyroxene
group Mineral group, all members of which are single-chain
silicates. Q §
Quartz Mineral with
the formula SiO2. §
Quartzite A rock composed
of sand-sized grains of quartz that have been welded together during
metamorphism. §
Quartz
sandstone A sandstone in which more than 90% of the grains are
quartz. §
Quaternary
Period The youngest geologic period; includes the present time. R §
Radial
pattern A drainage pattern in which streams diverge outward like
spokes of a wheel. §
Radioactive
decay The spontaneous nuclear disintegration of certain
isotopes. §
Radioactivity
The
spontaneous nuclear disintegration of atoms of certain isotopes. §
Radon A radioactive gas
produced by the radioactive decay of uranium. §
Rain shadow
A
region on the downwind side of mountains that has little or no rain because
of the loss of moisture on the upwind side of the mountains. §
Rampart crater (Mars) Meteorite crater that is surrounded by material that
appears to have flowed from the point of impact. §
Rayed crater (Moon) Crater with bright streaks radiating from it on the
Moon's surface. § Rayleigh waves A type of surface seismic wave that behaves like a
rolling ocean wave and causes the ground to move in an elliptical path. §
Receding glacier A
glacier with a negative budget, which causes the glacier to grow smaller as
its edges melt back. §
Recent (Holocene) Epoch The present epoch of the Quaternary Period. §
Recessional moraine An end moraine built during the retreat of a glacier. §
Recharge The addition of
new water to an aquifer or to the zone of saturation. §
Reclamation
Restoration
of the land to usable condition after mining has ceased. §
Recrystallization The
development of new crystals in a rock, often of the same composition as the
original grains. § Rectangular
pattern A drainage
pattern in which tributaries of a river change direction and join one another
at right angles. § Recumbent fold A fold overturned to such an extent that the limbs
are essentially horizontal. §
Reef A resistant ridge
of calcium carbonate formed on the sea floor by corals and coralline algae. §
Regional (dynamothermal) metamorphism Metamorphism that takes place at considerable depth
underground. §
Regolith Loose,
unconsolidated rock material resting on bedrock. §
Relative time
The
sequence in which events took place (not measured in time units). §
Relief The vertical
distance between points on Earth's surface. §
Reserves The discovered
deposits of a geologic material that are economically and legally feasible to
recover under present circumstances. §
Reservoir rock A
rock that is sufficiently porous and permeable to store and transmit
petroleum. § Residual clay Fine-grained particles left behind as insoluble
residue when a limestone containing clay dissolves. §
Residual soil
Soil
that develops directly from weathering of the rock below. § Resources The total amount of a geologic material in all its deposits,
discovered and undiscovered. See reserves. §
Reverse fault
A
fault in which the hanging-wall block moved up relative to the footwall
block. §
Rhyolite A fine-grained,
felsic, igneous rock made up mostly of feldspar and quartz. §
Richter scale
A numerical
scale of earthquake magnitudes. § Ridge push The concept that oceanic plates diverge as a result
of sliding down the sloping lithosphere-asthenosphere boundary. §
Rift valley
A
tensional valley bounded by normal faults. Rift valleys are found at diverging
plate boundaries on continents and along the crest of the mid-oceanic ridge. § Right-lateral
fault A strike-slip
fault in which the block seen across the fault appears displaced to the
right. §
Rigid zone
Upper
part of a glacier in which there is no plastic flow. §
Rille (Moon) Elongate trenched or cracklike valley on the lunar
surface. §
Rip current
Narrow
currents that flow straight out to sea in the surf zone, returning water
seaward that has been pushed ashore by breaking waves. §
Ripple mark
Any
of the small ridges formed on sediment surfaces exposed to moving wind or
water. The ridges form perpendicularly to the motion. §
Rock Naturally formed,
consolidated material composed of grains of one or more minerals. (There are
a few exceptions to this definition.) §
Rock avalanche A
very rapidly moving, turbulent mass of broken-up bedrock. §
Rock-basin lake A
lake occupying a depression caused by glacial erosion of bedrock. §
Rock cycle
A
theoretical concept relating tectonism, erosion, and various rock-forming
processes to the common rock types. §
Rockfall Rock falling
freely or bouncing down a cliff. §
Rock flour
A
powder of fine fragments of rock produced by glacial abrasion. §
Rock gypsum
An
evaporite composed of gypsum. §
Rock salt
An
evaporite composed of halite. §
Rockslide
Rapid
sliding of a mass of bedrock along an inclined surface of weakness. §
Rotational slide In
mass wasting, movement along a curved surface in which the upper part moves
vertically downward while the lower part moves outward. Also called a slump. §
Rounded knobs (glacial) Bedrock that is more resistant to glacial erosion
stands out as rounded knobs, usually elongated parallel to the direction of
glacier flow. These are also known as roche moutonnιes (French for
"rock sheep"). §
Rounding The grinding away
of sharp edges and corners of rock fragments during transportation. §
Rubble Angular
sedimentary particles coarser than 2 millimeters in diameter. S §
Saltation
A mode
of transport that carries sediment downcurrent in a series of short leaps or
bounces. §
Sand Sediment composed
of particles with a diameter between 1/16 and 2 millimeters. §
Sand dune
A
mound of loose sand grains heaped up by the wind. § Sandstone A medium-grained sedimentary rock (grains between
1/16 and 2 millimeters) formed by the cementation of sand grains. §
Saturated zone A
subsurface zone in which all rock openings are filled with water. § Scale The relationship between distance on a map and the
distance on the terrain being represented by that map. §
Schist A metamorphic
rock characterized by coarse-grained minerals oriented approximately
parallel. §
Schistose
The
texture of a rock in which visible platy or needle-shaped minerals have grown
essentially parallel to each other under the influence of directed pressure. §
Scientific method A
means of gaining knowledge through objective procedures. §
Scoria A basalt that is
highly vesicular. §
Sea cave A cavity eroded
by wave action at the base of a sea cliff. §
Sea cliff
Steep
slope that retreats inland by mass wasting as wave erosion undercuts it. § Seafloor
metamorphism Metamorphism of
rock along a mid-oceanic ridge caused by circulating hot water. §
Seafloor spreading The
concept that the ocean floor is moving away from the mid-oceanic ridge and
across the deep-ocean basin, to disappear beneath continents and island arcs. §
Seamount Conical mountain
rising 1,000 meters or more above the sea floor. §
Seawall A wall
constructed along the base of retreating cliffs to prevent wave erosion. § Sediment Loose, solid particles that can originate by (1)
weathering and erosion of preexisting rocks, (2) chemical precipitation from solution,
usually in water, and (3) secretion by organisms. §
Sedimentary breccia A coarse-grained sedimentary rock (grains coarser
than 2 millimeters) formed by the cementation of angular rubble. §
Sedimentary facies Significantly
different rock types occupying laterally distinct parts of the same layered
rock unit. § Sedimentary rock Rock that has formed from lithification of any type
of sediment, precipitation from solution, of
(3)
consolidation of the remains of plants or animals. §
Sedimentary structure A feature found within sedimentary rocks, usually
formed during or shortly after deposition of the sediment and before
lithification. §
Seismic gap
A
segment of a fault that has not experienced earthquakes for a long time; such
gaps may be the site of large future quakes. §
Seismic profiler An
instrument that measures and records the subbottom structure of the sea
floor. §
Seismic reflection The
return of part of the energy of seismic waves to Earth's surface after the
waves bounce off a rock boundary. §
Seismic refraction The
bending of seismic waves as they pass from one material to another. §
Seismic sea wave See
tsunami. §
Seismic wave
A
wave of energy produced by an earthquake. §
Seismogram
Paper
record of Earth vibration. § Seismograph A seismometer with a recording device that produces a
permanent record of Earth motion. §
Seismometer
An
instrument designed to detect seismic waves or Earth motion. §
Serpentine
A
magnesium silicate mineral. Most asbestos is a variety of serpentine. §
Shale A fine-grained
sedimentary rock (grains finer than 1/16 millimeter in diameter) formed by
the cementation of silt and clay (mud). Shale has thin layers (laminations)
and an ability to split (fissility) into small chips. §
Shear force
In
mass wasting, the component of gravitational force that is parallel to an
inclined surface. § Shearing Movement in which parts of a body slide relative to
one another and parallel to the forces being exerted. §
Shear strength In
mass wasting, the resistance to movement or deformation of material. §
Shear stress
Stress
due to forces that tend to cause movement or strain parallel to the direction
of the forces. §
Sheet erosion
The
removal of a thin layer of surface material, usually topsoil, by a flowing
sheet of water. §
Sheet joints
Cracks
that develop parallel to the outer surface of a large mass of expanding rock,
as pressure is released during unloading. §
Sheet-silicate structure Crystal structure in which each silica tetrahedron
shares three oxygen ions. §
Sheetwash
Water
flowing down a slope in a layer. §
Shield volcano Broad,
gently sloping cone constructed of solidified lava flows. §
Silica A term used for
oxygen plus silicon. §
Silicate A substance that
contains silica as part of its chemical formula. §
Silica tetrahedron See silicon-oxygen
tetrahedron. § Silicic rock or
magma Silica-rich
igneous rock or magma with a relatively high content of potassium and sodium. §
Silicon-oxygen tetrahedron Four-sided, pyramidal object that visually represents
the four oxygen atoms surrounding a silicon atom; the basic building block of
silicate minerals. Also called a silica tetrahedron or a silicon tetrahedron. §
Sill A tabular
intrusive structure concordant with the country rock. §
Silt Sediment composed
of particles with a diameter of 1/256 to 1/16 millimeter. §
Siltstone
A
sedimentary rock consisting mostly of silt grains. §
Sinkhole A closed
depression found on land surfaces underlain by limestone. §
Sinter A deposit of
silica that forms around some hot springs and geysers. §
Slab pull
The
concept that subducting plates are pulled along by their dense leading edges. §
Slate A fine-grained
rock that splits easily along flat, parallel planes. §
Slaty Describing a rock
that splits easily along nearly flat and parallel planes. §
Slaty cleavage The
ability of a rock to break along closely spaced parallel planes. § Slide In mass wasting,
movement of a relatively coherent descending mass along one or more
well-defined surfaces. § Slip face The steep, downwind slope of a dune; formed from
loose, cascading sand that generally keeps the slope at the angle of repose
(about 34°). § Slump In mass wasting,
movement along a curved surface in which the upper part moves vertically
downward while the lower part moves outward. Also called a rotational slide. §
Snow line
See equilibrium line § Soil A layer of weathered, unconsolidated material on top
of bedrock; often also defined as containing organic matter and being capable
of supporting plant growth. In mass wasting, soil means unconsolidated material, regardless
of particle size or composition (also called engineering soil). §
Soil horizon
Any
of the layers of soil that are distinguishable by characteristic physical or
chemical properties. §
Solar nebula
The
rotating disk of gas and dust from which the Sun and planets formed. §
Solar system
The
Sun, planets, their moons, and other bodies that orbit the Sun. §
Solar wind
The
outflow of low-density, hot gas from the Sun's upper atmosphere. It is
partially this wind that creates the tail of a comet by blowing dust and gas
away from the comet's immediate surroundings. §
Solid solution The
substitution of atoms of one element for those of another element in a
particular mineral. §
Solifluction
Flow
of water-saturated soil over impermeable material. §
Solution Usually slow but
effective process of weathering and erosion in which rocks are dissolved by
water. § Sorting Process of selection and separation of sediment
grains according to their grain size (or grain shape or specific gravity). §
Source area
The
locality that eroded to provide sediment to form a sedimentary rock. §
Source rock
A
rock containing organic matter that is converted to petroleum by burial and
other postdepositional changes. §
Spatter cone
A
small, steep-sided cone built from lava spattering out of a vent. §
Specific gravity The
ratio of the mass of a substance to the mass of an equal volume of water,
determined at a specified temperature. §
Speleothem
Dripstone
deposit of calcite that precipitates from dripping water in caves. §
Spheroidally weathered boulder Boulder that has been rounded by weathering from an
initial blocky shape. §
Spit A fingerlike
ridge of sediment attached to land but extending out into open water. §
Spreading axis (or spreading center) The crest of the mid-oceanic ridge, where sea floor
is moving away in opposite directions on either side. §
Spring A place where
water flows naturally out of rock onto the land surface. §
Stable Describing a mineral
that will not react with or convert to a new mineral or substance, given
enough time. §
Stack A small rock
island that is an erosional remnant of a headland left behind as a
wave-eroded coast retreats inland. §
Stalactite
Iciclelike
pendant of dripstone formed on cave ceilings. §
Stalagmite
Cone-shaped
mass of dripstone formed on cave floors, generally directly below a
stalactite. §
Standard geologic time scale A worldwide relative scale of geologic time
divisions. §
Star A massive, gaseous
body held together by gravity and generally emitting light. Normal stars
generate energy by nuclear reactions in their interiors. §
Static pressure See confining
pressure. §
Stock A small
discordant pluton with an outcropping area of less than 100 square
kilometers. §
Stony-iron meteorite A meteorite composed of silicate minerals and
iron-nickel alloy in approximately equal amounts. §
Stony meteorite A
meteorite made up mostly of plagioclase and iron-magnesium silicates. § Stoping Upward movement of a body of magma by fracturing of
overlying country rock. Magma engulfs the blocks of fractured country rock as
it moves upward. §
Storm surge
High
sea level caused by the low pressure and high winds of hurricanes. §
Strain Change in size
(volume) or shape of a body (or rock unit) in response to stress. §
Stratigraphy
The
field of geology concerning layered rocks and their interrelationships. §
Stratovolcano
See composite
volcano. §
Streak Color of a pulverized
substance; a useful property for mineral identification. §
Stream A moving body of
water, confined in a channel and running downhill under the influence of
gravity. §
Stream capture See stream piracy. §
Stream channel A
long, narrow depression, shaped and more or less filled by a stream. §
Stream discharge Volume
of water that flows past a given point in a unit of time. §
Stream-dominated delta A delta with fingerlike distributaries formed by the
dominance of stream sedimentation; also called a birdfoot delta. §
Stream gradient Downhill
slope of a stream's bed or the water surface, if the stream is very large. §
Stream headwaters The
upper part of a stream near the source. §
Stream mouth
The
place where the stream enters the sea, a large lake, or a larger stream. § Stream piracy The natural diversion of the headwaters of one stream
into the channel of another. §
Stream terrace Steplike
landform found above a stream and its flood plain. §
Stream velocity The
speed at which water in a stream travels. §
Stress A force acting on
a body, or rock unit, which tends to change the size or shape of that body,
or rock unit. Force per unit area within a body. §
Striations
(1)
On minerals, extremely straight, parallel lines; (2) Glacial straight scratches
in rock caused by abrasion by a moving glacier. §
Strike The compass
direction of a line formed by the intersection of an inclined plane (such as
a bedding plane) with a horizontal plane. §
Strike-slip fault A
fault in which movement is parallel to the strike of the fault surface. §
Strip mine
A
mine in which the valuable material is exposed at the surface by removing a
strip of overburden. §
Structural basin A
structure in which the beds dip toward a central point. §
Structural dome A
structure in which beds dip away from a central point. §
Structural geology The
branch of geology concerned with the internal structure of bedrock and the
shapes, arrangement, and interrelationships of rock units. §
Structural [or oil] trap See
oil trap. §
Subduction
The
sliding of the sea floor beneath a continent or island arc. §
Subduction complex See accretionary
wedge. §
Subduction zone Elongate
region in which subduction takes place. §
Submarine canyon V-shaped
valleys that run across the continental shelf and down the continental slope. §
Submergent coast A
coast in which formerly dry land has been recently drowned, either by land
subsidence or a rise in sea level. §
Subsidence
Sinking
or downwarping of a part of the Earth s surface. §
Superposed stream A
river let down onto a buried geologic structure by erosion of overlying
layers. § Superposition A principle or law stating that within a sequence of
undisturbed sedimentary rocks, the oldest layers are on the bottom, the
youngest on the top. §
Surf Breaking waves. §
Surface wave
A
seismic wave that travels on Earth's surface. §
Suspect terrane A
terrane that may not have formed at its present site. §
Suspended load Sediment
in a stream that is light enough in weight to remain lifted indefinitely
above the bottom by water turbulence. § S-wave A seismic wave
propagated by a shearing motion, which causes rock to vibrate perpendicular
to the direction of wave propagation. §
S-wave shadow zone The region on Earth's surface (at any distance more
than 103° from an earthquake epicenter) in which S waves from the earthquake
are absent. §
Swelling clay
See expansive clay. §
Syncline A fold in which
the layered rock usually dips toward an axis. T §
Talus An accumulation
of broken rock at the base of a cliff. §
Tarn See rock-basin lake. § Tectite Small, rounded
bits of glass formed from rock melting and being thrown into the air due to a
meteorite impact. §
Tectonic forces Forces
generated from within Earth that result in uplift, movement, or deformation
of part of Earth's crust. §
Tectonostratigraphic terrane See
terrane. §
Temporal - of or pertaining to time. §
Tensional stress A
stress due to a force pulling away on a body. §
Tephra See pyroclastic debris. §
Terminal moraine An
end moraine marking the farthest advance of a glacier. §
Terminus The lower edge of
a glacier. §
Terrain - The character, natural features, and configuration of land.
(Dictionary definition) §
Terrane - any rock formation or series of formations or the area in
which a particular formation or group of rocks is predominant. (dictionary
definition) §
Terrane (tectonostratigraphic terrane) A region in which the geology is markedly different
from that in adjoining regions. §
Terrigenous sediment Land-derived sediment that has found its way to the
sea floor. § Texture A rock's appearance with respect to the size, shape,
and arrangement of its grains or other constituents. §
Theory An explanation
for observed phenomena that has a high possibility of being true. §
Theory of glacial ages At times in the past, colder climates prevailed
during which significantly more of the land surface of Earth was glaciated
than at present. §
Thermal metamorphism See
contact
(thermal) metamorphism. §
Thrust fault
A
reverse fault in which the dip of the fault plane is at a low angle to
horizontal. §
Tidal delta
A
submerged body of sediment formed by tidal currents passing through gaps in
barrier islands. §
"Tidal wave" An incorrect name for a tsunami. §
Tide-dominated delta A delta formed by the reworking of sand by strong
tides. §
Till Unsorted and unlayered rock debris carried by a
glacier. §
Tillite Lithified till. §
Time-transgressive rock unit An apparently continuous rock layer in which
different portions formed at different times. § Tink test
- A simple field method of distinguishing between shale (a sedimentary rock
composed of Clay the Mineral) and slate (a metamorphic rock composed of
interlocking grains of minerals with basal cleavage). Shale, being composed
of piles of loose debris, goes "thunk" when dropped, while slate,
being harder and more tightly inter-grown, goes "tink." (borrowed
from GeoMan) §
Tombolo A bar of marine
sediment connecting a former island or stack to the mainland. §
Topographic map A
map on which elevations are shown by means of contour lines. §
Topset bed
In a
delta, a nearly horizontal sediment bed of varying grain size formed by
distributaries shifting across the delta surface. §
Trace fossil
Trail,
track or burrow resulting from animal movement preserved in sedimentary rock. §
Traction Movement by
rolling, sliding, or dragging of sediment fragments along a stream bottom. §
Transform fault The
portion of a fracture zone between two offset segments of a mid-oceanic ridge
crest. §
Transform plate boundary Boundary between two plates that are sliding past
each other. § Translational
slide In mass wasting,
movement of a descending mass along a plane approximately parallel to the
slope of the surface. §
Transportation The
movement of eroded particles by agents such as rivers, waves, glaciers, or
wind. §
Transported soil Soil
not formed from the local rock but from parent material brought in from some other
region and deposited, usually by running water, wind, or glacial ice. §
Transverse dune A
relatively straight, elongate dune oriented perpendicular to the wind. §
Travel-time curve A
plot of seismic-wave arrival times against distance. §
Travertine
A
porous deposit of calcite that often forms around hot springs. §
Trellis pattern A
drainage pattern consisting of parallel main streams with short tributaries
meeting them at right angles. §
Trench See oceanic trench. §
Trench suction The
concept that overlying plates move horizontally toward oceanic trenches as
subducting plates sink at an angle steeper than their dip. §
Tributary
Small
stream flowing into a large stream, adding water to the large stream. §
Trough (of wave) The
low point of a wave. §
Truncated spur Triangular
facet where the lower end of a ridge has been eroded by glacial ice. §
Tsunami Huge (not usually
tall) ocean wave produced by displacement of the sea floor; also called
seismic sea wave. §
Tufa A deposition of
calcite that forms around a spring, lake, or percolating ground water. §
Tuff A rock formed
from fine-grained pyroclastic particles (ash and dust). §
Turbidity current A
flowing mass of sediment-laden water that is heavier than clear water and
therefore flows downslope along the bottom of the sea or a lake. §
Turbulent flow Eddying,
swirling flow in which water drops travel along erratically curved paths that
cross the paths of neighboring drops. U §
Ultramafic rock Rock
composed entirely or almost entirely of ferromagnesian minerals. §
Unconfined aquifer A
partially filled aquifer exposed to the land surface and marked by a rising
and falling water table. § Unconformity A surface that represents a break in the geologic record,
with the rock unit immediately above it being considerably younger than the
rock beneath. §
Unconsolidated In
referring to sediment grains, loose, separate, or unattached to one another. §
Underplating
The
pooling of magmas at the base of the continental crust. §
Uniformitarianism Principle
that geologic processes operating at present are the same processes that
operated in the past. The principle is stated more succinctly as, "The
present is the key to the past." See
actualism. §
Universe The largest
astronomical structure we know of. The Universe contains all matter and
radiation and encompasses all space. §
Unloading
The
removal of a great weight of rock. §
Unpaired terraces Stream terraces (see definition) that
do not have the same elevation on opposite sides of a river. §
Unsaturated zone A
subsurface zone in which rock openings are generally unsaturated and filled
partly with air and partly with water; above the saturated zone. §
U-shaped valley Characteristic
cross-profile of a valley carved by glacial erosion. V §
Valley glacier A
glacier confined to a valley. The ice flows from a higher to a lower
elevation. §
Varve Two thin layers
of sediment, one dark and the other light in color, representing one year's
deposition in a lake. §
Vein See hydrothermal
vein. §
Vent The opening in
Earth's surface through which a volcanic eruption takes place. § Ventifact Boulder, cobble, or pebble with flat surfaces caused
by the abrasion of wind-blown sand. §
Vertical exaggeration An artificial steepening of slope angles on a
topographic profile caused by using a vertical scale that differs from the
horizontal scale. §
Vesicle A cavity in
volcanic rock caused by gas in a lava. §
Viscosity
Resistance
to flow. §
Vitreous luster See
glassy
luster. §
Volcanic breccia Rock
formed from large pieces of volcanic rock (cinders, blocks, bombs). §
Volcanic dome
A
steep-sided, dome- or spine-shaped mass of volcanic rock formed from viscous
lava that solidifies in or immediately above a volcanic vent. §
Volcanic neck
An intrusive
structure that apparently represents magma that solidified within the throat
of a volcano. § Volcanism Volcanic activity, including the eruption of lava and
rock fragments and gas explosions. § Volcano A hill or mountain constructed by the extrusion of
lava or rock fragments from a vent. W §
Wastage See ablation. §
Water table
The
upper surface of the zone of saturation. §
Wave crest
See
crest. §
Wave-cut platform A
horizontal bench of rock formed beneath the surf zone as a coast retreats
because of wave erosion. §
Wave-dominated delta A delta formed by the reworking of sand by wave
action. §
Wave height
The
vertical distance between the crest (the high point of a wave) and the trough
(the low point). §
Wavelength
The
horizontal distance between two wave crests (or two troughs). §
Wave refraction Change
in direction of waves due to slowing as they enter shallow water. §
Wave trough
See
trough. §
Weathering
The
group of processes that change rock at or near Earth's surface. §
Welded tuff
A
rock composed of pyroclasts welded together. §
Well A hole, generally cylindrical and usually walled or
lined with pipe, that is dug or drilled into the ground to penetrate an
aquifer below the zone of saturation. §
Wilson cycle
The
cycle of splitting of a continent, opening of an ocean basin, followed by
closing of the basin and collision of the continents. §
Wind ripple
Small,
low ridge of sand produced by the saltation of wind-blown sand. §
Wrinkle ridge (Moon) Wrinkle on lunar
maria surface. X §
Xenolith Fragment of rock
distinct from the igneous rock in which it is enclosed. Z §
Zone of ablation That portion of a glacier in which
ice is lost. § Zone of
accumulation (1) That portion of
a glacier with a perennial snow cover; (2) See B horizon (a soil
layer). §
Zone of leaching See
A
horizon (a soil layer). §
Zone of saturation See
saturated
zone. §
Zoning Orderly variation
in the chemical composition within a single crystal. |