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Geology glossary

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fault

Earthquakes
Illustration of the main types of tectonic faults (source: USGS)
Illustration of the main types of tectonic faults (source: USGS)
A fault is a fracture in the rigid crust of the earth, along which the two blocks on either side have moved relative to each other and often may do so again in the future. When blocks move along a fault, tectonic earthquakes are generated.
Depending on the movement, there are strike-slip faults (side-wards movement), normal faults (caused by extension), and reverse or thrust faults (caused by compression). A reverse fault with a small dip angle is called a thrust fault. -> See whole entry

feldspar

Minerals
Compositional phase diagram of the different minerals that constitute the feldspar solid solution. Image credit: Muskid / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
Compositional phase diagram of the different minerals that constitute the feldspar solid solution. Image credit: Muskid / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
Feldspars are a group of closely related rock-forming silicate minerals that are by far the most common in the earth's crust, making up about 41% of its weight. Feldspar is the most common mineral formed during crystallization of magmas and almost always present in magmatic rocks. Chemically, the feldspar minerals primarily bind varying proportions of the most common light elements potassium (K), sodium (Na), calcium (Ca), and aluminum (Al) ions, into a crystal lattice of silica (Si) ions.
Namefeldspar
Category
FormulaKAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8
Crystal systemtriclinic
Colorpink, white, gray, brown, blue
Streakwhite
Lustervitreous
Crystal habit
Mohs hardness6.0–6.5
Specific weight2.55–2.76
UsesFeldspar is a common raw material used to produce glass and ceramics. To lesser extent, it is used in filters, paints, plastics, and rubber.
Other
Feldspars are divided into several groups, based on their chemical compositions of potassium (K), sodium (Na) and calcium (Ca). Thus, there are 3 end-members and numerous intermediate compositions:

- potassium: (K-spar) endmember KAlSi3O8

- albite: NaAlSi3O8

- anorthite: CaAl2Si2O8



Intermediates between K-feldspar and albite (... -> See whole entry

fissure vent

Volcanology
A fissure vent, also known as a volcanic fissure or simply fissure, is a linear volcanic vent through which lava erupts.
A fissure vent, also known as a volcanic fissure or simply fissure, is a linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is usually a few meters wide and may be many kilometers long. Fissure vents can cause large flood basalts and lava channels. This type of volcano is usually hard to recognize from the ground and from outer space because it has no ... -> See whole entry

focusSynonym of: hypocenter

Earthquakes
Epicenter and hypocenter of an earthquake (Source: USGS earthquake glossary)
Epicenter and hypocenter of an earthquake (Source: USGS earthquake glossary)

foreshock

Earthquakes
Foreshocks are smaller earthquakes that precede the largest earthquake in a series, which is termed the mainshock. Not all earthquakes arre preceded by foreshocks.

Related keywords (2):

aftershock - mainshock

fossile

Geology
Fossils are the mineralized or otherwise preserved remains or traces (such as footprints) of animals, plants, and other organisms.
Fossils (from Latin fossus, literally "having been dug up") are the mineralized or otherwise preserved remains or traces (such as footprints) of animals, plants, and other organisms. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous (fossil-containing) rock formations and sedimentary layers (strata) is known as the fossil record. The study of fossils a... -> See whole entry

 

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