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facies	(1) Sedimentary: a group of rocks and primary structures indicative of a given depositional environment; (2) Metamorphic: a set of metamorphic mineral assemblages formed under a given range of pressures and temperatures.
fault	A fracture on which one body of rock slides past another.
fault breccia	Fragmented rock in which angular fragments were formed by brittle fault movement.
fault creep	Gradual movement along a fault that occurs in the absence of an earthquake.
fault gouge	Pulverized rock consisting of fine powder that lies along fault surfaces; gouge forms by crushing and grinding.
fault scarp	A small step on the ground surface where one side of a fault has moved vertically with respect to the other.
fault system	A grouping of numerous related faults.
fault trace (or line)	The intersection between a fault and the ground surface.
fault-block mountains	An outdated term for a narrow, elongate range of mountains that develops in a continental-rift setting as normal faulting drops down blocks of crust, or tilts blocks.
faulting	Slip events along a fault.
felsic	An adjective used in reference to igneous rocks that are rich in elements forming feldspar and quartz.
Ferrel cells	The name given to the middle-latitude convection cells in the atmosphere.
fetch	The distance across a body of water along which a wind blows to build waves.
fine-grained	A textural term for rock consisting of many fine grains or clasts.
firn	Compacted granular ice (derived from snow) that forms where snow is deeply buried; if buried more deeply, firn turns into glacial ice.
fission track	A line of damage formed in the crystal lattice of a mineral by the impact of an atomic particle ejected during the decay of a radioactive isotope.
fissure	A conduit in a magma chamber in the shape of a long crack through which magma rises and erupts at the surface.
fjord	A deep, glacially carved, U-shaped valley flooded by rising sea level.
flank eruption	An eruption that occurs when a secondary chimney, or fissure, breaks through the flank of a volcano.
flash flood	A flood that occurs during unusually intense rainfall or as the result of a dam collapse, during which the floodwaters rise very fast.
flexing	The process of folding in which a succession of rock layers bends and slip occurs between the layers.
flocculation	The clumping together of clay suspended in river water into bunches that are large enough to settle out.
flood	An event during which the volume of water in a stream becomes so great that it covers areas outside the stream’s normal channel.
flood basalt	Vast sheets of basalt that spread from a volcanic vent over an extensive surface of land; they may form where a rift develops above a continental hot spot, and where lava is particularly hot and has low viscosity.
flood stage	The stage when water reaches the top of a stream channel.
flood tide	The rising tide.
floodplain	The flat land on either side of a stream that becomes covered with water during a flood.
floodplain flood	A flood during which a floodplain is submerged.
floodway	A mapped region likely to be flooded, in which people avoid constructing buildings.
flow fold	A fold that forms when the rock is so soft that it behaves like weak plastic.
flowstone	A sheet of limestone that forms along the wall of a cave when groundwater flows along the surface of the wall.
fluvial deposit	Sediment deposited in a stream channel, along a stream bank, or on a floodplain.
flux	Flow.
focus	The location where a fault slips during an earthquake (hypocenter).
fog	A cloud that forms at ground level.
fold	A bend or wrinkle of rock layers or foliation; folds form as a consequence of ductile deformation.
fold axis	An imaginary line that, when moved parallel to itself, can trace out the shape of a folded surface.
fold-thrust belt	An assemblage of folds and related thrust faults that develop above a detachment fault.
foliation	Layering formed as a consequence of the alignment of mineral grains, or of compositional banding in a metamorphic rock.
foraminifera	Microscopic plankton with calcitic shells, components of some limestones.
foreland sedimentary basin	A basin located under the plains adjacent to a mountain front, which develops as the weight of the mountains pushes the crust down, creating a depression that traps sediment.
foreshocks	The series of smaller earthquakes that precede a major earthquake.
foreshore zone	The zone of beach regularly covered and uncovered by rising and falling tides.
fossil	The remnant, or trace, of an ancient living organism that has been preserved in rock or sediment.
fossil assemblage	A group of fossil species found in a specific sequence of sedimentary rock.
fossil correlation	A determination of the stratigraphic relation between two sedimentary rock units, reached by studying fossils.
fossil fuel	An energy resource such as oil or coal that comes from organisms that lived long ago, and thus stores solar energy that reached the Earth then.
fossiliferous limestone	Limestone consisting of abundant fossil shells and shell fragments.
fractional crystallization	The process by which a magma becomes progressively more silicic as it cools, because early-formed crystals settle out.
fracture zone	A narrow band of vertical fractures in the ocean floor; fracture zones lie roughly at right angles to a mid-ocean ridge, and the actively slipping part of a fracture zone is a transform fault.
fresh rock	Rock whose mineral grains have their original composition and shape.
friction	Resistance to sliding on a surface.
fringing reef	A coral reef that forms directly along the coast.
front	The boundary between two air masses.
frost wedging	The process in which water trapped in a joint freezes, forces the joint open, and may cause the joint to grow.
fuel rod	A metal tube that holds the nuclear fuel in a nuclear reactor.
Fujita scale	A scale that distinguishes among tornadoes on the basis of wind speed, path dimensions, and possible damage.