andezit

Andezit is an extrusive igneous rock of intermediate silica content, chemically and mineralogically between basalt and rhyolite. The term andezit is the Turkish spelling for andesite, and in geology the rock is commonly referred to simply as andesite in English-language literature. Andesite forms from magmas produced in subduction-zone environments and is a hallmark of volcanic arcs around the world.

Mineralogy: the essential minerals are plagioclase feldspar and a ferromagnesian mineral such as pyroxene or hornblende; quartz may be present in quartzandesite varieties. Accessory minerals can include magnetite, ilmenite, biotite, and zircon.

Texture: andezit rocks range from fine-grained to porphyritic. Aphanitic varieties cool rapidly at the surface, producing a uniform texture, while porphyritic andesite contains phenocrysts—larger crystals of plagioclase and pyroxene or hornblende—embedded in a finer-grained groundmass.

Formation and distribution: andesitic magmas are commonly produced by the partial melting of subducted oceanic crust and/or the fractional crystallization of higher-magnesium basaltic magmas. They are characteristic of convergent plate boundaries and calc-alkaline magmatism. Global occurrences include the Andes, the Cascades, the Japanese archipelago, New Zealand's arcs, and many other Pacific Rim volcanic regions.

Uses and significance: andezit is widely used as a construction rock in some regions, and provides important clues about crustal processes at subduction zones and the evolution of volcanic arcs.