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Geology glossary

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bentonite

Minerals
Bentonite from the Greek island of Milos
Bentonite from the Greek island of Milos
Bentonite is an absorbent aluminium phyllosilicate generally impure clay consisting mostly of montmorillonite.
Bentonite is an absorbent aluminium phyllosilicate generally impure clay consisting mostly of montmorillonite. There are a few types of bentonites and their names depend on the dominant elements, such as K, Na, Ca, and Al. As noted in several places in the geologic literature, there are some nomenclatorial problems with the classification of bentonite clays. Bentonite usually forms from weathering... -> See whole entry

block

Volcanology: (volcanic) block
Large ballistically ejected block on Santorini (Greece)
Large ballistically ejected block on Santorini (Greece)
Volcanic blocks are solidified rock fragments greater than 64 mm in diameter. Blocks commonly are ejected during explosive eruptions and consist of older pieces of the volcano's edifice, e.g. parts of the conduit, lava domes or older lava flows.
During violent eruptions, blocks of up to several meter size can be thrown to several km distance. For example, during the Minoan eruption (ca. 1613 BC) of the Santorini volcano in Greece, meter-sized blocks were thrown to up to 7 km horizontal distance and impacted violently into the ground, some of them destroying houses of ancient settlements. The time some of these blocks spent on their trajec... -> See whole entry

bomb

Volcanology: volcanic bomb
Large bomb ejected from Etna volcano's SE crater.
Large bomb ejected from Etna volcano's SE crater.
Ejected fragments of fresh magma larger than 64 mm in diameter, often shaped aerodynamically during their flight.
Volcanic bombs are lava fragments larger than 64 mm in diameter that were ejected while still viscous and partially molten. Many bombs acquire rounded aerodynamic shapes during their travel through the air. Volcanic bombs include breadcrust bombs, ribbon bombs, spindle bombs (with twisted ends), spheroidal bombs, and "cow-dung" bombs. -> See whole entry

breadcrust bomb

Volcanology
Breadcrust bomb, ca. 50 cm long, from Lokon volcano (N-Sulawesi, Indonesia)
Breadcrust bomb, ca. 50 cm long, from Lokon volcano (N-Sulawesi, Indonesia)
Perfectly shaped breadcrust bomb from Nea Kameni, Santorini (Greece)
Perfectly shaped breadcrust bomb from Nea Kameni, Santorini (Greece)
Volcanic bomb with a cracked surface, similar to bread, caused by the slow expansion of the interior gas bubbles while cooling.
A breadcrust bomb is a volcanic bomb with a cracked and checkered surface, sometimes resembling the surface of a loaf of bread. The cracks develop when the outer surface of a partially molten lava fragment cools to form a brittle surface and then subsequently cracks as the hot interior expands due to the continued growth of gas bubbles. -> See whole entry

calcite

Minerals
Large crystal of Calcite on display at the National Museum of Natural History in Washington, DC. With its special property of showing double refraction, it is also known as Icelandic Spar.
Large crystal of Calcite on display at the National Museum of Natural History in Washington, DC. With its special property of showing double refraction, it is also known as Icelandic Spar.
Calcite is a very common carbonate mineral and the most stable form of calcium carbonate (CaCO3).
Namecalcite
Category
FormulaCaCO3
Crystal systemtrigonal
Colortypically white to colorless; sometimes also gray, yellow, brown, orange, red, green or even blue
Streakwhite
Lustervitreous to pearly
Crystal habitcrystalline, granular, stalactitic, concretionary, massive, rhombohedral
Mohs hardness3 (definition mineral)
Specific weight2.7
Usesproduction of lime (industrial scale), acid neutralization, abrasive, soil conditioner, high-grade optical instruments
Other
Other polymorphs of calcium carbonate are the minerals aragonite and vaterite. Aragonite will change to calcite at 380–470 °C, and vaterite is even less stable. -> See whole entry

caldera

Volcanology
View of the 13x8km large caldera of Santorini, Greece, formed during several Plinian eruptions including the Minoan eruption around 1613 BC.
View of the 13x8km large caldera of Santorini, Greece, formed during several Plinian eruptions including the Minoan eruption around 1613 BC.
Öraefajökull volcano, SE-Iceland. The summit caldera of this large, explosive volcano that erupted last in 1728, is occupied by a glacier, which is part of the Vatnajökull ice cap that covers most of SE Iceland.
Öraefajökull volcano, SE-Iceland. The summit caldera of this large, explosive volcano that erupted last in 1728, is occupied by a glacier, which is part of the Vatnajökull ice cap that covers most of SE Iceland.
Large crater, usually several kilometers across, formed by the collapse of the roof of a magma chamber emptied by large explosive eruptions.
A caldera is a large, usually circular depression at the summit of a volcano formed when magma is withdrawn or erupted from a shallow underground magma reservoir. The removal of large volumes of magma may result in loss of structural support for the overlying rock, thereby leading to collapse of the ground and formation of a large depression. Calderas are different from craters, which are smaller,... -> See whole entry

crater

Volcanology
Marum's crater on Ambrym volcano, containing a small lava lake.
Marum's crater on Ambrym volcano, containing a small lava lake.
A depression often on the top or the flanks of a volcano usually created by explosions of lava from the vent.

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caldera

crust

Geology
The outermost layer of the Earth, ranging from about 5 to 65 km in thickness worldwide, composed of plutonic, metamorphic, sedimentary and volcanic rocks.
There are two main types of crust: oceanic crust, which is thin (5-10 km) and consists primarily of basalt, diabase, and gabbros. It is formed continuously at oceanic spreading centers.

Continental crust is typically from 30-50 km thick and consists of intrusive rocks (e.g. granites) and metamorphic (e.g. shists, gneiss) rocks at depth and sedimentary and volcanic rocks often forming its surface.... -> See whole entry

crust

Earthquakes
Continental and oceanic crust (image: USGS)
Continental and oceanic crust (image: USGS)
The earth's crust is the outermost major layer of the earth, which forms its varied surface both above and under water that we live on. Its thickness ranges from about 10 to 65 km worldwide. The uppermost 15-35 km of crust is brittle enough to produce earthquakes.
The crust is composed by magmatic, sedimentary and metamorphic rocks. There are two major types of crust: oceanic and continental crust.

Oceanic crust is usually less than 10 km thick and primarily composed of volcanic rocks rich in iron and magnesium (called: mafic rocks). Oceanic crust is much thinner and heavier than continental crust, having a mean density of approx. 3.0 grams per cub... -> See whole entry

crystal

Geology
Quartz crystal from the Greek volcano island Milos
Quartz crystal from the Greek volcano island Milos
In chemistry, mineralogy, and materials science, a crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions.
In chemistry, mineralogy, and materials science, a crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions.

The word crystal originates from the Greek word κρύσταλλος (krystallos) meaning clear ice, as it was thought to be an especially sol... -> See whole entry

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