F

fan

A fan-shaped sedimentary deposit that forms where rapidly flowing water enters a relatively open, flat area. As water slows down, it deposits sediment and gradually builds a fan. See alluvial fan.

fault

A fracture in the Earth along which one side has moved in relative to the other. Sudden movements on faults cause earthquakes.

fault scarp

A steep slope or cliff formed when movement along a fault exposes the fault surface.

feldspar

Family of silicate minerals containing varying amounts of potassium, sodium and calcium along with aluminum, silicon and oxygen. Potassium feldspars contain considerable potassium. Plagioclase feldspars contain considerable sodium and calcium. Feldspar crystals are stubby prisms, generally white, gray, or pink.

felsic

A term used to describe light-colored igneous rocks with an abundance of light-colored minerals, especially feldspars and quartz.

firn

Partially compacted snow that survives the summer melting season.

firn limit

A term used by glaciologists (scientists who study glaciers) for the boundary where the amount of snow loss from melting and evaporation equals the amount of snow accumulation from snowfall (also called the annual snowline).

fission tracks

Microscopic tunnels made in crystals by escaping nuclear particles emitted by radioactive elements. Most commonly studied are fission tracks in zircon crystals made by the radioactive decay of uranium, present as an impurity.

fissure

Elongate, narrow fractures.

flaggy

Term used to describe sedimentary or metamorphic rocks that tend to split into layers that are 1-10 cm thick.

flood

A lake, stream, or other body of water that flows over its natural confining boundaries. During a flood, water flows out over land not normally covered with water.

flood plain

A relatively flat surface next to a stream. During floods, when the stream overflows its banks, water flows over the flood plain. Streams construct flood plains that accommodate their maximum flood capacity.

flowstone

A general term for a type of cave decoration or speleothem that encrusts floors or walls of caves.

fluvial

Term used to describe river or stream-related features or processes. Fluvial deposits are sediments deposited by the flowing water of a stream.

focus

The location where an earthquake begins. Rock ruptures at this spot, then seismic waves radiate outward in all directions.

foliation

Aligned layers of minerals characteristic of some metamorphic rocks. Foliation forms in metamorphic rocks when pressure squeezes flat or elongates minerals so that they become aligned. These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied.

formation

A rock formation is a body of rock of considerable extent with distinctive characteristics that allow geologists to map, describe, and name it.

fossil

Mineralized remains or traces of organisms.

fossil fuel

General term for any hydrocarbon used as fuel, including coal, oil, natural gas, and oil shale.

fracture

Any break in rock along which no significant movement has occurred.

freeze-thaw cycle

In colder temperate regions, water trapped in fractures and between grains of rocks repeatedly freezes, then thaws during the winter months. In some areas this occurs on a daily basis as water freezes at night, then melts in warmer daytime temperatures. Only in the coldest regions does water remain frozen throughout the winter.

frost wedging

A process that mechanically breaks apart rock caused by expansion of water as it freezes in cracks and crevices.

fumarole

A volcanic vent that emits hydrogen sulfide or other gases.