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Geological Terms Beginning With "F"



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Facies:

The characteristics of a rock mass that reflect its depositional environment. These characteristics enable the rock mass to be distinguished from rocks deposited in adjacent environments.

Farm-Out Arrangement:

A contractual agreement in which a mineral rights owner or lessee assigns a working interest to another party who will become responsible for specific exploration, development or production activities.

Fault:

A fracture or fracture zone in rock along which movement has occurred.

Fault-Block Mountain:

A linear mountain that is bounded on both sides by normal faults.

Faunal Succession:

A principle of relative dating that is based upon the observed sequence of organisms in the rock record. The relative age of two rock units can frequently be determined by matching the fossils found in those rocks to their positions in the rock record.

Felsic:

A term used to describe an igneous rock that has a large percentage of light-colored minerals such as quartz, feldspar, and muscovite. Also used in reference to the magmas from which these rocks crystallize. Felsic rocks are generally rich in silicon and aluminum and contain only small amounts of magnesium and iron. Granite and rhyolite are examples of felsic rocks. (See mafic to contrast.)

Fjord:

A deep, narrow, steep-walled, U-shaped valley that was carved by a glacier and is now occupied by the sea.

Flood:

An overflow of water onto lands that are normally above local water levels. Can be caused by stream discharge exceeding the capacity of the stream channel, storm winds and reduced pressure drawing water from a lake or ocean onto the coastline, dam failure, lake level increase, local drainage problems or other reasons.

Flood Basalt:

A sequence of parallel to subparallel basalt flows that were formed during a geologically brief interval of time and which covered an extensive geographic area. Thought to have formed from simultaneous or successive fissure eruptions.

Flood Plain:

An area of alluvium-covered, relatively level land along the banks of a stream that is covered with water when the stream leaves its channel during a time of high flow.

Flood Stage:

A water height that is reached when the discharge of a stream exceeds the capacity of the channel.

Flood Tide:

A tidal current that generally moves landward and occurs during the part of the tide cycle when sea level is rising. (See neap tide for contrast.)

Flowing Well:

A well that taps an aquifer that is under enough pressure to force water to the surface. Caused when the aquifer has a recharge area at a higher elevation.

Fluid Inclusion:

A small amount of fluid (liquid and/or gas) trapped within a rock and which is thought to represent the fluid from which the rock crystallized.

Focus:

A point beneath Earth's surface where the vibrations of an earthquake are thought to have originated. Also known as a hypocenter.

Fold:

A bend or flexure in a rock unit or series of rock units that has been caused by crustal movements.

Foliation:

The planar or layered characteristics of metamorphic rocks that are evidence of the pressures and/or temperatures to which the rock was exposed. These can be structural such as cleavage, textural such as mineral grain flattening or elongation, or compositional such as mineral segregation banding.

Foraminifer:

A group of single-celled organisms, mostly marine, that produce a calcium carbonate shell. Their shells can make up a significant portion of the carbonate sediment in some areas.

Foraminiferal Ooze:

A calcareous sea-floor sediment composed of foraminifer shells.

Foreign Operations:

Activities located outside of the United States, its offshore territorial waters, commonwealth territories, and protectorates.

Forset Beds:

The distinctly dipping sediment layers deposited on the front of a prograding delta or on the lee side of a sand dune.

Formation:

A laterally continuous rock unit with a distinctive set of characteristics that make it possible to recognize and map from one outcrop or well to another. The basic rock unit of stratigraphy.

Fossil:

Remains, imprints or traces of an ancient organism that have been preserved in the rock record. Bones, shells, casts, tracks and excrement can all become fossils.

Fossil fuel:

A carbon-rich rock material or fluid, of organic origin that can be produced and burned as a fuel. Coal, oil and natural gas are examples of fossil fuels.

Fumarole:

A vent that emits hot gases, usually associated with past or current magmatic activity below.

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