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Extrusive Igneous activity that occurs at Earth’s surface.
F
Fault A break in a rock mass along which movement has occurred.
Felsic A term derived from feldspar and silica (quartz). It is a term used to describe
granitic igneous rocks.
Fissure A crack in rock along which there is a distinct separation.
Fissure eruption An eruption in which lava is extruded from narrow fractures or cracks
in the crust.
Flood The overflow of a stream channel that occurs when discharge exceeds the
channel’s capacity. The most common and destructive geologic hazard.
Flow A type of movement common to mass-wasting processes in which water-saturated
material moves downslope as a viscous fluid.
Fossil The remains or traces of organisms preserved from the geologic past.
Fracture (rock) Any break or rupture in rock along which no appreciable movement
has taken place.
Frost wedging The mechanical breakup of rock caused by the expansion of freezing
water in cracks and crevices.
G
Geology The science that examines Earth, its form and composition, and the changes it
has undergone and is undergoing.
Geosphere The solid Earth; one of Earth’s four basic spheres.
Glacier A thick mass of ice originating on land from the compaction and
recrystallization of snow. The ice shows evidence of past or present flow.
Glassy texture A term used to describe the texture of certain igneous rocks, such as
obsidian, that contain no crystals.
Granitic Igneous rocks composed mainly of light-colored silicates (quartz and feldspar)
are said to have this composition.
Groundwater Water in the zone of saturation.
H
Hardness A mineral’s resistance to scratching and abrasion.
Hydrologic cycle The unending circulation of Earth’s water supply. The cycle is
powered by energy from the Sun and is characterized by continuous exchanges of water
among the oceans, the atmosphere, and the continents.
Hydrosphere The water portion of our planet; one of the traditional subdivisions of
Earth’s physical environment.
Hydrothermal metamorphism Chemical alterations that occur as hot, ion-rich water
circulates through fractures in rock.
I
Ice sheet A very large, thick mass of glacial ice flowing outward in all directions from
one or more accumulation centers.
Igneous rock A rock formed by the crystallization of molten magma.
Inclusion A piece of one rock unit contained within another; inclusions are used in
relative dating. The rock mass adjacent to the one containing the inclusion must have
been there first in order to provide the fragment.
Infiltration The movement of surface water into rock or soil through crack and pore
spaces.
Inner core The solid, innermost layer of Earth, about 1216 km in radius.
Intensity (earthquake) A measure of the degree of earthquake shaking at a given locale
based on the amount of damage.
Interface A common boundary where different parts of a system interact.
Intermediate Compositional category for igneous rocks found near the middle of
Bowen’s reaction series, mainly amphibole and the intermediate plagioclase feldspars.

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