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A fracture in the Earth’s crust in which the rock on one side of the fracture moves measurably in relation to the rock on the other side.

fault scarp

An escarpment directly beside a fault line, where the ground on one side of the fault has moved vertically with respect to the other side, creating step-like topography.


An extremely common group of rock-forming minerals found in igneous, metamorphic, and sedimentary rocks.
There are two groups of feldspar: alkali feldspar (which ranges from potassium-rich to sodium-rich) and plagioclase feldspar (which ranges from sodium-rich to calcium-rich). Potassium feldspars of the alkali group are commonly seen as pink crystals in igneous and metamorphic rocks, or pink grains in sedimentary rocks. Plagioclase feldspars are more abundant than the alkali feldspars, ranging in color from light to dark.
Feldspars are commercially used in ceramics and scouring powders.


Igneous rocks with high silica content and low iron and magnesium content. They are light in color and are typically found in continental crust.

filter feeder

An animal that feeds by passing water through a filtering structure that traps food. The water may then be expelled and the food digested. This strategy is employed by a wide range of animals today, from clams and krill to flamingos and whales.


Compacted glacial ice, formed by the weight of snow on top. Individual flakes break down by melting, refreezing, and bonding to the snow around them, eventually forming compacted grains.


A deep, narrow, glacially scoured valley that is flooded by ocean water.
See also: glacier

flank collapse

A dramatic mass wasting event that occurs when the flank of a shield volcano collapses under its own weight. Fractures formed by gravitational stress can slip rapidly, resulting in a massive collapse that shears off to create steep cliffs and huge debris fields.
See also: fracture, mass wasting, shield volcano


A hard, high-quality form of chert that occurs mainly as nodules and masses in sedimentary rock. Due to its strength and the fact that it splits into thin, sharp flakes, flint was often used to make tools during the Stone Age. Flint will also create sparks when struck against steel, and has been used to ignite gunpowder in more modern times.


The land around a river that is prone to flooding. This area can be grassy, but the sediments under the surface are usually deposits from previous floods.

fluorite, fluorspar

The mineral form of calcium fluoride (CaF2). Fluorite is used in a variety of commercial applications, including as lenses for microscopes, the production of some glass, and the chemical industry.
Fluorite lent its name to the phenomenon of fluorescence, which occurs in some fluorites due to impurities in the crystal.


See outwash plain

flux (mineral extraction)

A mineral added to the metals in a furnace to promote fusing or to prevent the formation of oxides.


The arrangement of the constituents of a rock in leaflike layers, as in schists. During metamorphism, the weight of overlying rock can cause minerals to realign perpendicularly to the direction of pressure, layering them in a sheet-like pattern.


A class of aquatic protists that possess a calcareous or siliceous exoskeleton. Foraminifera have an extensive fossil record.

foreland bulge

An area of uplift on the far side of an inland basin. Mountain building associated with plate convergence generally results in downwarping, that is, a basin associated with the load of mountains. Away from the area of maximum subsidence, the basin gradually shallows, followed by an area of uplift (the foreland bulge).


Preserved evidence of ancient life, including, for example, preserved skeletal or tissue material, molds or casts, and traces of behavior. Fossilization may alter biological material in a variety of ways, including permineralization, replacement, and compression.
Remains are often classified as fossils when they are older than 10,000 years, the traditional start of the Holocene (Recent) epoch. However, this date is only a practical guideline - scientists studying successions of plant or animal remains would not recognize any sudden change in the material at 10,000 years, and would typically refer to all material buried in sediments as fossil material.
The word fossil is derived from the Latin word fossilis, meaning “dug up.”

fossil fuels

Fuel for human use that is made from the remains of ancient biomass, referring to any hydrocarbon fuel source formed by natural processes from anaerobically decomposed organisms, primarily coal, petroleum, natural gas (methane), and peat. Fossil fuels are non-renewable, meaning that because they take thousands to millions of years to form, the rate of use is far greater than the rate of formation, and eventually we will run out.

fracture (mineral)

A physical property of minerals, formed when a mineral crystal breaks; also a crack in rocks, sometimes known as a joint. This process is separate from cleavage, which occurs when a mineral breaks in a characteristic way along a specific plane of weakness.

frost wedging

Physical weathering that occurs when water freezes and expands in cracks.


A material substance possessing internal potential energy that can be transferred to the surroundings for specific uses—included are petroleum, coal, and natural gas (the fossil fuels), and other materials, such as uranium, hydrogen, and biofuels.