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Geology glossary

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dacite

Volcanology
Dacite is an igneous, volcanic rock with a high iron content found at many lava-domes.
Dacite (pronounced /deɪsaɪt/) is an igneous, volcanic rock with a high iron content. It is intermediate in compositions between andesite and rhyolite, and, like andesite, it consists mostly of plagioclase feldspar with biotite, hornblende, and pyroxene (augite and/or enstatite). It has an aphanitic to porphyritic texture with quartz as rounded, corroded phenocrysts, or as an element of t... -> See whole entry

debris avalanche

Volcanology
A sudden collapse of volcanic material from an unstable side of a volcano. Debris avalanches are a particularly violent type of pyroclastic flows (in its broader meaning).

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pyroclastic flow

diorite

Volcanology
Diorite sample (image: Michael C. Rygel via Wikimedia Commons)
Diorite sample (image: Michael C. Rygel via Wikimedia Commons)
Diorite is an intrusive igneous rock of intermediate silica content (53-63%). It is the equivalent of its extrusive (volcanic) counterpart andesite.
Diorite is composed principally of the silicate minerals plagioclase feldspar (typically andesine), biotite, hornblende, and/or pyroxene. The chemical composition of diorite is intermediate between gabbro and granite. -> See whole entry

earthquake

Earthquakes
An earthquake (also called quake, tremor or temblor, rumbling etc) is the shaking of the surface of the earth resulting from a sudden release of energy in the Earth's interior, usually by sudden movements along fault lines. This movement releases energy that propagates as seismic waves.
The size of earthquakes is determined by the amount of energy released in this process. It can range in size between many orders of magnitude and is commonly given as a number on the logarithmic Richter scale. It ranges from so tiny that they are not felt and can be only detected by the most sensitive modern instruments (seismometers) to so strong that entire regions are shaking violently.

-> See whole entry

effusive

Volcanology: effusive (volcanic) eruption
An effusive vent at Mt Etna volcano
An effusive vent at Mt Etna volcano
A lava flow is the result of an effusive eruptions (here Mt. Etna in Nov 2006)
A lava flow is the result of an effusive eruptions (here Mt. Etna in Nov 2006)
Effusive means flowing out of lava as opposed to explosive eruptions.
If magma is sufficiently fluid and if it is not framented by expanding gasses when reaching the surface vent, it can erupt to form lava flows. This is called effusive eruption. The opposite of effusive is explosive, i.e. fragmentation of magma. -> See whole entry

epicenter

Earthquakes
The point on the Earth’s surface vertically above the point (focus or hypocenter) in the crust where an earthquake occurs, i.e. where the seismic rupture nucleates.

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hypocenter

explosive

Volcanology: explosive (volcanic) eruption
A small explosive eruption at Mt Etna volcano
A small explosive eruption at Mt Etna volcano
Explosion of magma at Krakatau at the beginning of a vulcanian eruption
Explosion of magma at Krakatau at the beginning of a vulcanian eruption
Explosive eruptions occur, when the erupting magma is ejected as fragments into the air, as opposed to effusive eruptions producing lava flows.
Explosive eruptions are called so, when the erupting magma is fragmented when exiting the conduit. The reason are expanding gasses from the magma itself or external water e.g. from an aquifer. The resulting fragments of the magma itself are called tephra, and consist in small and large pieces: Ash, lapilli and bombs are the typical products of explosive eruptions.

The reason for explosive eruptio... -> See whole entry

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

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