2 spots left on Iceland Geo Tour 16-30 July 2022

Illustrated Volcano Glossary

Updated: Jul 4, 2022 01:27 GMT - Refresh

accretionary lapilli

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. Read all


An adjective describing the chemical composition of magmas (or lavas) having relatively high content of silica (SiO2). It has little to do with the use of the term in chemistry where it is used to describe low pH values found in acids.
In petrology and volcanology, magmatic rocks (or the magmas they derived from) are often classified by their bulk silica (SiO2) content, which can typically range from around 40-75 weight %, an results in the 4 main types of magma by composition. These are, in order of increasing silica content: basaltic (45-53%), andesitic (53-62%), dacitic (62-69%), and rhyolitic (69-77%).
Rocks with high sili... Read all

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Typical andesite from the Methana peninsula (Greece)
Typical andesite from the Methana peninsula (Greece)
Andesite is a gray to black volcanic rock with intermediate silica content, typical for lava domes and stratovolcanoes in subduction zones.
Andesite is an igneous volcanic rock containing between about 52 and 63 weight % silica (SiO2). Andesite is the extrusive equivalent to plutonic diorite - if a dioritic magma cools beneath the surface as an intrusion, it will be diorite, if the same magma erupts during a volcanic eruption, the rock formed is andesite.

Where does andesite usually occur?

Andesite and diorite are among the m... Read all

Related keywords (2):

basalt - diorite


Volcanology: volcanic ash
Ash plume from an explosive eruption at Etna volcano (Italy)
Ash plume from an explosive eruption at Etna volcano (Italy)
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... Read all

Related keywords (4):

ashfall - explosive - pyroclastic flow - VAAC


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
Volcanic ash fall is the deposition of ash from the air from eruption plumes emitted during explosive volcanic eruptions.

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