Illustrated Volcano Glossary
- VolcanologyAndesite 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 commonly erupts from stratovolcanoes as thick lava flows, some reaching several km in length. Andesite magma can also generate strong explosive eruptions to form pyroclastic flows and surges and enormous eruption columns. Andesites erupt at temperatures between 900 and 1100° C.
Andesite can be considered as the extrusive equivalent to plutonic diorite. Andesites are characteristic of subduction tectonic environments in active oceanic margins, such as the western coast of South America. The name andesite is derived from the Andes mountain range.
Andesite is a characteristic rock on the volcano peninsula Methana and on Nisyros island. Most lava domes on Methana are composed of andesitic rocks. Interesting is the phenomenon of magma-mixing that is thought to be a driving force in many explosive eruptions where the relatively cool andesitic lava is involved: basaltic intrusions in such a magma-chamber heat up the magma and chemical reactions activate such a magma. The result may be a lava dome like Merapi (Indonesia) or Montserrat.