An intermediate volcanic rock, andesite usually has 55 to 65 percent total silica content. Plagioclase feldspar (andesine or oligoclase) is the most significant constituent, along with pyroxene, amphibole, and biotite mica.

TEXTURE A fine-grained, often porphyritic rock. The phenocrysts set into the matrix are usually white tabular feldspar crystals or biotite, hornblende, or augite.

ORIGIN This rock forms as lava flows from andesitic volcanoes, which are second in abundance only to basaltic volcanoes. Andesitic volcanoes are often associated with sub duction zones, as in the Andean mountains of South America.

Group: IGNEOUS

Origin: Extrusive

Grain size: Fine

Crystal shape: Anhedral, Euhedral

Classification: Intermediate

Occurrence: Volcano

Color: Medium