Monday, May 17, 2010

Andesite is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and granite. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals. Alkali feldspar may be present in minor amounts. The quartz-feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams. Relative alkali and silica contents are illustrated in TAS diagrams. Andesite can be considered as the extrusive equivalent of plutonic diorite. Andesites are characteristic of subduction zones, such as the western margin of South America. The name andesite is derived from the Andes mountain range.

Andesites can be found mainly as surface deposits and, to a lesser extent, as dikes and small plugs. Not only the Andes, where the name was first applied to a series of lavas, but most of the cordillera (parallel mountain chains) of Central and North America consist largely of andesites. The same rock type occurs in abundance in volcanoes along practically the entire margin of the Pacific Basin. Andesite most commonly is fine-grained, usually porphyritic. In composition, andesites correspond roughly to the intrusive igneous rock diorite and consist essentially of andesine. It forms at convergent plate margins and is thought to be the product of partial melts of the water-rich subducting oceanic crustal basalts or of the intervening wedge of lower crustal rocks above the subducting plate.