Andesite is an igneous, volcanic (extrusive) rock, of intermediate composition, with aphanitic to porphyritic texture. Its mineral assembly is usually quartz and plagioclase. Biotite, hornblende and pyroxene are common accessory minerals. Alkali feldspar is absent.

Classification of andesites may be refined according to the most abundant phenocryst. Example: olivinic andesite, if olivine is the principal accessory mineral.

Andesite can be considered as the extrusive equivalent to plutonic diorite. As diorites, they 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.

Andesitic magma in island arc regions (i.e. active oceanic margins) comes from wet melting of mantle wedge peridotite. When an oceanic plate is subducted, it contains a lot of water. This water is removed from the subducting slab because of the increasing pressure and temperature. Flow in the mantle wedge carries the water down to sites directly below the volcanoes, where the water enables the melting of the mantle peridotite. The initial melt is usually of basaltic composition. On its way to the surface, the melt stalls and cools, enabling the fractional crystallization of silica poor minerals, thus raising the silica content of the remaining melt and resulting in andesitic magma.

See also

• list of rocks

References

• Origins of the Continental Crust
• Island arc magmatism
• Experimental and Theoretical Constraints on Peridotite Partial Melting in the Mantle Wedge