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GLOSSARY
A
Aa A type of lava flow that has a jagged, blocky surface.
Abrasion The grinding and scraping of a rock surface by the friction and impact of rock
particles carried by water, wind, or ice.
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.
Angular unconformity An unconformity in which the older strata dip at an angle
different from that of the younger beds.
Anticline A fold in sedimentary strata that resembles an arch.
Asthenosphere A subdivision of the mantle situated below the lithosphere. This zone of
weak material exists below a depth of about 100 km 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.
B
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.
Batholith A large mass of igneous rock that formed when magma was emplaced at
depth, crystallized, and was subsequently exposed by erosion.
Bedding plane A nearly flat surface separating two beds of sedimentary rock. Each
bedding plane marks the end of one deposit and the beginning of another having
different characteristics.
Bedrock A general term for the rock that underlies soil or other unconsolidated surface
materials.
Breccia A sedimentary rock composed of angular fragments that were lithified.
C
Caldera A large depression typically caused by collapse of the summit area of a
volcano following a violent eruption.
Cap rock A necessary part of an oil trap, the cap rock is impermeable and hence keeps
upwardly mobile oil and gas from escaping at the surface.
Capacity The total amount of sediment a stream is able to transport.
Carrier beds are rock layers that are very permeable and transmit fluids.
Cavern A naturally formed underground chamber or series of chambers most
commonly produced by solution activity in limestone.
Chemical weathering The processes by which the internal structure of a mineral is
altered by the removal and/or addition of elements.
Cinder cone A rather small volcano built primarily of pyroclastics ejected from a single
vent.
Cleavage The tendency of a mineral to break along planes of weak bonding.
Compaction A type of lithification in which the weight of overlying material
compresses more deeply buried sediment. It is most important in fine-grained
sedimentary rocks such as shale.
Composite cone A volcano composed of both lava flows and pyroclastic material.
Conduit A pipelike opening through which magma moves toward Earth’s surface. It
terminates at a surface opening called a vent.
Continental slope The steep gradient that leads to the deep ocean floor and marks the
seaward edge of the continental shelf.

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