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F


fall
The fastest form of mass movement, occurring when rock or sediment breaks off from a steep or vertical slope and descends at a rate of 9.8 meters per second. A fall can be extremely dangerous.
fault
A fracture dividing a rock into two sections that have visibly moved relative to each other.
fault block
A section of rock separated from other rock by one or more faults.
fault-block mountain
A mountain containing tall horsts interspersed with much lower grabens and bounded on at least one side by a high-angle normal fault.
fault-zone metamorphism
The metamorphism that acts on rocks grinding past one another along a fault and is caused by directed pressure and frictional heat.
firn
Firmly packed snow that has survived a summer melting season. Firn has a density of about 0.4 gram per cubic centimeter. Ultimately, firn turns into glacial ice.
fission tracks
Marks left in the latticework of a mineral crystal by subatomic particles released during the fission of a radioactive atom trapped inside the crystal.
fjord
A deep, steep-walled, U-shaped valley formed by erosion by a glacier and submerged with seawater.
floodplain
The flat land that surrounds a stream and becomes submerged when the stream overflows its banks.
flood tide
A tide that raises the water surface of an ocean and moves the shoreline farther inland.
flow
A relatively rapid mass-movement process that involves a mixture of rock, soil, vegetation, and water moving downslope as a viscous fluid. Within a flow (such as a mudflow), each particle, regardless of its size, moves independently.
fluorescence
Emission of visible light by a substance, such as a mineral, that is currently exposed to ultraviolet light and absorbs radiation from it. The light appears in the form of glowing, distinctive colors. The emission ends when the exposure to ultraviolet light ends.
focus (plural foci)
The precise point within the Earth's crust or mantle where rocks begin to rupture or move in an earthquake.
fold
A bend that develops in an initially horizontal layer of rock, usually caused by plastic deformation. Folds occur most frequently in sedimentary rocks.
foliation
The arrangement of a set of minerals in parallel, sheet-like layers that lie perpendicular to the flattened plane of a rock. Occurs in metamorphic rocks on which directed pressure has been exerted.
forearc basin
A depression in the sea floor located between an accretionary wedge and a volcanic arc in a subduction zone, and lined with trapped sediment. See also backarc basin.
foreshock
A minor, barely detectable earthquake, generally preceding a full-scale earthquake with approximately the same focus. Major quakes may follow a cluster of foreshocks by as little as a few seconds or as much as several weeks.
foreshore
The portion of a beach that lies nearest to the sea, extending from the low-tide line to the high-tide line.
fossil fuel
A nonrenewable energy source, such as oil, gas, or coal, that derives from the organic remains of past life. Fossil fuels consist primarily of hydrocarbons.
fractional crystallization
The process by which a magma produces crystals that then separate from the original magma, so that the chemical composition of the magma changes with each generation of crystals, producing igneous rocks of different compositions. The silica content of the magma becomes proportionately higher after each crystallization.
fracture
(n) A crack or break in a rock. (v) To break in random places instead of cleaving. Said of minerals.
fringing reef
A reef that forms against or near an island or continental coast and grows seaward, sloping sharply toward the sea floor. Fringing reefs usually range from 0.5 to 1.0 or more kilometers in width.
frost wedging
A form of mechanical weathering caused by the freezing of water that has entered a pore or crack in a rock. The water expands as it freezes, widening the cracks or pores and often loosening or dislodging rock fragments. As the ice forms, it attracts more water, increasing the effects of frost wedging.

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