F

FAULT: A planar or gently curved fracture in the Earth's crust across which there has been relative displacement of the two blocks of rock parallel to the fracture.

FAULT-BLOCK MOUNTAINS: A mountain or range formed when the crust is broken into blocks of different elevations by normal faulting.

FAULTING: The processes by which crustal forces cause a rock formation to break and slip along a fault.

FELSIC: An adjective used to describe a light-colored igneous rock that is poor in iron and magnesium and contains abundant feldspars and quartz.

FERRIC IRON: Iron with a 13 charge (Fe³¹).

FERROUS IRON: Iron with a 12 charge (Fe ²¹). FIRN: Old, dense, compacted snow.

FISSURE ERUPTION: A volcanic eruption emanating from an elongate fissure rather than a central vent.

FJORD: A former glacial valley with steep walls and a U-shaped profile, now occupied by the sea.

FLOOD BASALT: A basltic plateau extending many kilometers in flat, layered flows originating from fissure eruptions.

FLOODPLAIN: A level plain of stratified, unconsolidated sediment on either side of a stream, submerged during floods and built up by silt and sand carried out of the main channel.

FLOOD TIDE: The part of the tide cycle during which the water is rising or leveling off at high water.

FLUX: The flow of Earth's chemicals from one reservoir to another. (See also Geochemical cycle.)

FOCUS (EARTHQUAKE): The point along a fault at which the rupture occurs. Also called the "hypocenter." (See also Epicenter.)

FOLD: A bent or warped stratum or sequence of strata that was originally horizontal, or nearly so, and was subsequently deformed.

FOLD AXIS: Within each stratum involved in a fold, the axis connecting all the points in the center of the fold, from which both limbs bend.

FOLD BELT: Synonym for Orogenic belt.

FOLDING: The processes by which crustal forces deform an area of crust so that layers of rock are pushed into folds.

FOLIATION: A set of flat or wavy planes in a metamorphic rock, produced by structural deformation.

FORAMINIFER: A group of single-celled organisms whose secretions and calcite shells account for most of the ocean's carbonate sediments.

FORAMINIFERAL OOZE: A calcareous pelagic sediment composed of the shells of dead foraminifera.

FORESET BED: One of the inclined beds found in cross-bedding; also an inclined bed deposited on the outer front of a delta.

FORESHORE: The marine zone between the upper limit of wave wash at high tide and the low-tide mark.

FORMATION: The basic unit for the naming of rocks in stratigraphy; a set of rocks that are or once were horizontally continuous, that share some distinctive feature of lithology, and that are large enough to be mapped.

FOSSIL FUEL: A general term for combustible geologic deposits of hydrocarbons of biologic origin, including coal, oil, natural gas, oil shales, and tar sands.

FRACTIONAL CRYSTALLIZATION: The separation of a cooling magma into components by the successive formation and removal of crystals at progressively lower temperatures.

FRACTURE: The irregular breaking of a crystal along a surface not parallel to a crystal face; serves to identify minerals.

FRINGING REEF: A coral reef that is directly attached to a landmass not composed of coral.

FUMAROLE: Small volcanic vent that emits gas and steam from which minerals precipitate onto surrounding surfaces.