Main Page | See live article | Alphabetical index Andesite is an igneous, volcanic (extrusive) rock, of intermediate composition, with aphanitic to porphyritic texture. 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 phenocrystal. Example: olivinic andesite, if olivine is the principal accessory mineral. Andesite can be considered as the extrusive equivalent to the plutonic diorite rock. As diorites, they are characteristic of subduction tectonic environments in active oceanic margins, such as the western coast of South America. Due to this, the name andesite is derived from the Andes mountain range. See also: igneous rocks, list of rocks, diorite This article is from Wikipedia. All text is available under the terms of the GNU Free Documentation License.