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. §
Arête – 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 image—graphic, mathematical, or verbal—that 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 |