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Andesite

Very fine crystalline extrusive rock of volcanic origin composed largely of plagioclase feldspar (oligoclase or andesine) with smaller amounts of dark-colored mineral (hornblende, biotite, or pyroxene). The extrusive equivalent of diorite.

Any member of a large family of rocks that occur in most of the world's volcanic areas, mainly as surface deposits and to a lesser extent as dikes and small plugs. The Andes, where the name was first applied, and most of the cordillera (parallel mountain chains) of Central and North America consist largely of andesites. They also occur in abundance in volcanoes along practically the entire margin of the Pacific basin. Andesites are most often porphyritic (having distinct crystals in a fine-grained base) rocks.

Andesite forms from the eruption of intermediate-composition lava. Such eruptions often begin in an explosive manner, so deposits of layers of pyroclasts are common on and around the volcano. The explosive phase of the eruption is then followed by fluid flow of lava from the volcano which cools, hardens and protects the underlying layer of pyroclasts from erosion. The volcanos resulting from a series of such eruptions are layered (stratified), and so are referred to as strato- or composite volcanos.

Andesite most commonly is fine-grained, usually porphyritic. In composition, andesites correspond roughly to the intrusive igneous rock diorite and consist essentially of andesine (a plagioclase feldspar) and one or more ferromagnesian minerals, usually amphibole or biotite. The larger crystals of feldspar and ferromagnesian minerals are often visible to the naked eye; they lie in a finer groundmass, usually crystalline, but sometimes glassy. There are three subdivisions of this rock family: the quartz-bearing andesites, or dacites, sometimes considered to be a separate family; the hornblende- and biotite-andesites; and the pyroxene-andesites.

Andesite 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. While andesite is common in younger arc systems such as the Cascades, it is nearly absent in the older Sierra Nevadas, possibly a consequence of erosion.

Material Notes:

Igneous rock, with a fine texture and grain size < 1mm (extrusive). It is intermediate in the acidic-basic range, with a total silica content of 55-65%. Predominent minerals in this generally medium-dark colored rock are hornblende and plagioclase feldspar.

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