Geology glossary

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.

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... Read all

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. Read all

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.

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 (... Read all

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 ... Read all

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

Foreshocks are earthquakes of varying magnitudes and intensities which precede a larger and stronger earthquake. This larger and stronger earthquake is called the main shock. Earthquakes or a series of quakes in one area cannot often be immediately identified as a foreshock. By definition, a main shock should have occurred first before any foreshocks and aftershocks can be identified.
The ... Read all

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... Read all

Granite is the most well-known and one of the most common intrusive magmatic (plutonic) rock type. It is formed when an intrusion of viscous magma with high silica content (68-75 wt %) remains under the surface of the earth, where it cools and crystallizes slowly inside the crust.

Granite is the equivalent of its extrusive (volcanic) rock type rhyolite. It is granular and coarse-grained in texture. Its principal minerals are feldspars, quartz, and mica.
Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. Read all

Hauyn is a blue mineral, found f.e. in the pumice of the Laacher See eruption in Germany

Hauyne, haüyne or hauynite is a tectosilicate mineral with sulfate and chloride with formula: (Na,Ca)4-8Al6Si6(O,S)24(SO4,Cl)1-2. It is a feldspathoid and a member of the sodalite group. Hauyne crystallizes in the isometric system forming translucent, vitreous typically twinned crystals with highly variable color (blue, white, grey, yellow, green, pink). It has a Mohs hardness of 5 to 6 and a spec... Read all