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Aa A type of lava flow that has a
jagged, blocky surface.
Ablation A general term for the
loss of ice and snow from a glacier.
Abrasion The grinding and
scraping of a rock surface by the
friction and impact of rock particles
carried by water, wind, or ice.
Abyssal plain Very level area of
the deep-ocean floor, usually lying
at the foot of the continental rise.
Accretionary wedge A large
wedge-shaped mass of sediment
that accumulates in subduction
zones. Here sediment is scraped
from the subducting oceanic plate
and accreted to the overriding
crustal block.
Active continental margin
Usually narrow and consisting of
highly deformed sediments, they
occur where oceanic lithosphere is
being subducted beneath the
margin of a continent.
Active layer The zone above the
permafrost that thaws in summer
and refreezes in winter.
Aerosols Tiny solid and liquid
particles suspended in the
atmosphere.
Aftershock A smaller earthquake
that follows the main earthquake.
Alluvial channel A stream
channel in which the bed and
banks are composed largely of
unconsolidated sediment (alluvium)
that was previously deposited in the
valley.
Alluvial fan A fan-shaped deposit
of sediment formed when a stream’ s
slope is abruptly reduced.
Alluvium Unconsolidated
sediment deposited by a stream.
Alpine glacier See Valley glacier.
Andean-type plate margin A
type of convergent boundary in
which a slab of oceanic crust is
subducting beneath a continental
margin.
Andesitic Igneous rocks having a
mineral makeup between that of
granite and basalt, after the common
volcanic rock andesite. See also
Intermediate.
Angle of repose The steepest
angle at which loose material
remains stationary without sliding
downslope.
Angular unconformity An
unconformity in which the older
strata dip at an angle different from
that of the younger beds.
Anthracite A hard, metamorphic
form of coal that burns clean and
hot.
Anticline A fold in sedimentary
strata that resembles an arch.
Aphanitic texture A texture of
igneous rocks in which the crystals
are too small for individual minerals
to be distinguished with the
unaided eye.
Aquifer Rock or sediment
through which groundwater moves
easily.
Aquitard An impermeable bed
that hinders or prevents
groundwater movement.
Archean eon The first eon of
Precambrian time; the eon
preceding the Proterozoic. It
extends between 4.5 and 2.5 billion
years ago.
Arête A narrow, knifelike ridge
separating two adjacent glaciated
valleys.
Arkose A feldspar-rich sandstone.
Artesian well A well in which the
water rises above the level where it
was initially encountered.
Asthenosphere A subdivision of
the mantle situated below the
lithosphere. This zone of weak
material exists below a depth of
about 100 kilometers and in some
regions extends as deep as 700
kilometers. The rock within this
zone is easily deformed.
Atmosphere The gaseous portion
of a planet; the planet’ s envelope of
air. One of the traditional
subdivisions of Earth’ s physical
environment.
Atoll A continuous or broken ring
of coral reef surrounding a central
lagoon.
Atom The smallest particle that
exists as an element.
Atomic mass unit A mass unit
equal to exactly one-twelfth the
mass of a carbon-12 atom.
Atomic number The number of
protons in the nucleus of an atom.
Atomic weight The average of the
atomic masses of isotopes of a given
element.
Aureole A zone or halo of contact
metamorphism found in the host
rock surrounding an igneous
intrusion.
Back swamp A poorly drained
area on a floodplain, resulting when
natural levees are present.
Backshore The inner portion of
the shore, lying landward of the
high-tide shoreline. It is usually dry,
being affected by waves only during
storms.
Bajada An apron of sediment
along a mountain front created by
the coalescence of alluvial fans.
Banded iron formations Finely
layered iron- and silica-rich (chert)
layers deposited mainly during the
Precambrian.
Bar Common term for sand and
gravel deposits in a stream channel.
Barchan dune A solitary sand
dune shaped like a crescent with its
tips pointed downwind.
Barchanoid dunes Dunes
forming scalloped rows of sand
oriented at right angles to the wind.
This form is intermediate between
isolated barchans and extensive
waves of transverse dunes.
Barrier island A low, elongated
ridge of sand that parallels the coast.
Basal slip A mechanism of glacial
movement in which the ice mass
slides over the surface below.
Basalt An aphanitic igneous rock
of mafic composition.
Basaltic Term used to describe
igneous rocks that contain abundant
dark (ferromagnesian) minerals and
about 50 percent silica.
Base level The level below which
a stream cannot erode.
Basin A circular downfolded
structure.
Batholith A large mass of igneous
rock that formed when magma was
emplaced at depth, crystallized, and
was subsequently exposed by
erosion.
Baymouth bar A sandbar that
completely crosses a bay, sealing it
off from the main body of water.
Beach An accumulation of
sediment found along the landward
margin of the ocean or a lake.
Beach drift The transport of
sediment in a zigzag pattern along a
beach. It is caused by the uprush of
Glossary
Aa A type of lava flow that has a
jagged, blocky surface.
Ablation A general term for the
loss of ice and snow from a glacier.
Abrasion The grinding and
scraping of a rock surface by the
friction and impact of rock particles
carried by water, wind, or ice.
Abyssal plain Very level area of
the deep-ocean floor, usually lying
at the foot of the continental rise.
Accretionary wedge A large
wedge-shaped mass of sediment
that accumulates in subduction
zones. Here sediment is scraped
from the subducting oceanic plate
and accreted to the overriding
crustal block.
Active continental margin
Usually narrow and consisting of
highly deformed sediments, they
occur where oceanic lithosphere is
being subducted beneath the
margin of a continent.
Active layer The zone above the
permafrost that thaws in summer
and refreezes in winter.
Aerosols Tiny solid and liquid
particles suspended in the
atmosphere.
Aftershock A smaller earthquake
that follows the main earthquake.
Alluvial channel A stream
channel in which the bed and
banks are composed largely of
unconsolidated sediment (alluvium)
that was previously deposited in the
valley.
Alluvial fan A fan-shaped deposit
of sediment formed when a stream’ s
slope is abruptly reduced.
Alluvium Unconsolidated
sediment deposited by a stream.
Alpine glacier See Valley glacier.
Andean-type plate margin A
type of convergent boundary in
which a slab of oceanic crust is
subducting beneath a continental
margin.
Andesitic Igneous rocks having a
mineral makeup between that of
granite and basalt, after the common
volcanic rock andesite. See also
Intermediate.
Angle of repose The steepest
angle at which loose material
remains stationary without sliding
downslope.
Angular unconformity An
unconformity in which the older
strata dip at an angle different from
that of the younger beds.
Anthracite A hard, metamorphic
form of coal that burns clean and
hot.
Anticline A fold in sedimentary
strata that resembles an arch.
Aphanitic texture A texture of
igneous rocks in which the crystals
are too small for individual minerals
to be distinguished with the
unaided eye.
Aquifer Rock or sediment
through which groundwater moves
easily.
Aquitard An impermeable bed
that hinders or prevents
groundwater movement.
Archean eon The first eon of
Precambrian time; the eon
preceding the Proterozoic. It
extends between 4.5 and 2.5 billion
years ago.
Arête A narrow, knifelike ridge
separating two adjacent glaciated
valleys.
Arkose A feldspar-rich sandstone.
Artesian well A well in which the
water rises above the level where it
was initially encountered.
Asthenosphere A subdivision of
the mantle situated below the
lithosphere. This zone of weak
material exists below a depth of
about 100 kilometers and in some
regions extends as deep as 700
kilometers. The rock within this
zone is easily deformed.
Atmosphere The gaseous portion
of a planet; the planet’ s envelope of
air. One of the traditional
subdivisions of Earth’ s physical
environment.
Atoll A continuous or broken ring
of coral reef surrounding a central
lagoon.
Atom The smallest particle that
exists as an element.
Atomic mass unit A mass unit
equal to exactly one-twelfth the
mass of a carbon-12 atom.
Atomic number The number of
protons in the nucleus of an atom.
Atomic weight The average of the
atomic masses of isotopes of a given
element.
Aureole A zone or halo of contact
metamorphism found in the host
rock surrounding an igneous
intrusion.
Back swamp A poorly drained
area on a floodplain, resulting when
natural levees are present.
Backshore The inner portion of
the shore, lying landward of the
high-tide shoreline. It is usually dry,
being affected by waves only during
storms.
Bajada An apron of sediment
along a mountain front created by
the coalescence of alluvial fans.
Banded iron formations Finely
layered iron- and silica-rich (chert)
layers deposited mainly during the
Precambrian.
Bar Common term for sand and
gravel deposits in a stream channel.
Barchan dune A solitary sand
dune shaped like a crescent with its
tips pointed downwind.
Barchanoid dunes Dunes
forming scalloped rows of sand
oriented at right angles to the wind.
This form is intermediate between
isolated barchans and extensive
waves of transverse dunes.
Barrier island A low, elongated
ridge of sand that parallels the coast.
Basal slip A mechanism of glacial
movement in which the ice mass
slides over the surface below.
Basalt An aphanitic igneous rock
of mafic composition.
Basaltic Term used to describe
igneous rocks that contain abundant
dark (ferromagnesian) minerals and
about 50 percent silica.
Base level The level below which
a stream cannot erode.
Basin A circular downfolded
structure.
Batholith A large mass of igneous
rock that formed when magma was
emplaced at depth, crystallized, and
was subsequently exposed by
erosion.
Baymouth bar A sandbar that
completely crosses a bay, sealing it
off from the main body of water.
Beach An accumulation of
sediment found along the landward
margin of the ocean or a lake.
Beach drift The transport of
sediment in a zigzag pattern along a
beach. It is caused by the uprush of
Glossary