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Geology glossary

Updated: Feb 1, 2023 23:24 GMT -

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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.
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A caldera is a large, usually circular depression at the summit of a volcano formed when magma is withdrawn or erupted from a shallow underground magma reservoir.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,... Read all

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.... Read all

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... Read all

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... Read all

dacite

Volcanology
A dacitic lava-dome at Nisyros Island in Greece
A dacitic lava-dome at Nisyros Island in Greece
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... Read all

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. Read all

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.
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effusive

Volcanology: effusive (volcanic) eruption
Lava flow at Mt Etna volcano - a typical effusive eruption
Lava flow at Mt Etna volcano - a typical effusive eruption
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. Read all

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