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Geology glossary

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acceleration

Earthquakes
Generally, acceleration is increase of speed or velocity. During earthquakes, the shaking of ground is a swinging motion involving both acceleration and deceleration. Its maximum value is the largest increase in velocity recorded at a particular point. It largely influences how damaging an earthquake is in this area.

accretionary lapilli

Volcanology
Accretionary lapilli in an ash deposit on Santorini.
Accretionary lapilli in an ash deposit on Santorini.
Small spherical balls of volcanic ash
Accretionary lapilli are small spherical balls of volcanic ash that form from a wet nucleus falling through a volcanic ash cloud. They can flatten on hitting the ground or may roll on loose ash and grow like a snowball. -> See whole entry

aftershock

Earthquakes
Aftershocks following the M6.5 quake in Idaho on 31 Mar 2020
Aftershocks following the M6.5 quake in Idaho on 31 Mar 2020
Aftershocks are smaller quakes that follow a usually larger earthquake in the same rupture zone. The can occur up to weeks, months or even years after, depending on the size of the main shock.
Aftershocks are smaller than the mainshock and occur within 1-2 rupture lengths distance from the mainshock.

In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue. Large earthquakes can be followed by thousands of aftershocks.

The larger aftershocks following an earthquake can be particularly dangerous if many structures have been ... -> See whole entry

agate

Minerals
Agate from Milos island, Greece
Agate from Milos island, Greece
Agate from Milos
Agate from Milos
Agate is a microcrystalline variety of quartz (silica), chiefly chalcedony, characterised by its fineness of grain and brightness of color.
Agate is a microcrystalline variety of quartz (silica), chiefly chalcedony, characterised by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rocks but can be common in certain metamorphic rocks.

Colorful agates and other chalcedonies were obtained over 3,000 years ago from the Achates River, now ca... -> See whole entry

andesite

Volcanology
Typical andesite from the Methana peninsula (Greece)
Typical andesite from the Methana peninsula (Greece)
Andesite is a gray to black volcanic rock with between about 52 and 63 weight percent silica (SiO2). Andesites are typical for lava domes and stratovolcanoes.
Andesite is an igneous, volcanic rock, of intermediate composition, containing between about 52 and 63 weight % silica (SiO2).

Andesites contain crystals composed primarily of plagioclase feldspar and one or more of the minerals pyroxene (clinopyroxene and orthopyroxene) and lesser amounts of hornblende. At the lower end of the silica range, andesite lava may also contain olivine. Andesite magma ... -> See whole entry

ash

Volcanology: volcanic ash
Ash plume from an explosive eruption at Etna volcano (Italy)
Ash plume from an explosive eruption at Etna volcano (Italy)
Snow and ash from Hekla volcano's eruption in 2000
Snow and ash from Hekla volcano's eruption in 2000
Volcanic ash is the term for all fine-grained volcanic products (smaller than 2 mm), normally magma or older rock fragmented during explosive eruptions.
Volcanic ash has nothing to do with fire, but is a mere definition of grain-size. Ash can range in size from sandy to extremely fine; any fragment ejected by a volcano less than 2 mm in diameter is called ash. It may consists of freshly ejected lava (usually turned into a glass shard because of rapid cooling), older fragmented rock, or small crystals.

Ash is produced by explosive activity when ex... -> See whole entry

ashfall

Volcanology: volcanic ash fall
Ash plume and ash fall from Etna during the eruption in 2002
Ash plume and ash fall from Etna during the eruption in 2002
Effects of ash fall in a village during the 2002 eruption at Etna
Effects of ash fall in a village during the 2002 eruption at Etna
Volcanic ash fall is the deposition of ash from the air from eruption plumes emitted during explosive volcanic eruptions.

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asthenosphere

Geology
The uppermost viscous layer of the mantle where pressure and temperature conditions can allow partial melting of the mantle rocks, which forms magmas.
From the Greek "asthenos"=weak and "sphera"=sphere, the asthenosphere is the uppermost layer of the plastic, highly viscous mantle beneath the brittle crust. It extends to depths of 100-200 km.

Because the pressure in this zone is still comparably low while temperature is high, parts of the mantle rocks can be in a molten state and form magmas. This happens especially in areas where pressure is ... -> See whole entry

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