Andesite – an intermediate, aphanitic igneous rock that is the extrusive equivalent of diorite. It is usually medium gray. The rock is named after the Andes Mountains, where volcanic eruptions have produced lavas with this composition in great abundance.
Some andesites resemble rhyolite, so their identification must be finalized by microscopic examination to verify the abundance of plagioclase feldspar and ferro-magnesian mineral crystals.
Andesite is a volcanic rock with a so-called average composition. It is more felsic than basalt and more mafic than dacite. The main minerals are plagioclase and hornblende. Andesite is a common rock of subduction zone volcanoes. It is generally dark gray and may contain phenocrysts of plagioclase (usually andesine), biotite, and hornblende. Andesite is an extrusive equivalent of diorite.
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 is the next most abundant lava type after basalt and occurs most frequently along the convergent plate margins in island arcs and along continental margins. It is not found along oceanic ridges, and it is rare in oceanic islands or other intraplate settings related to mantle plumes.
Many of the deposits are not normal lava flows but rather flow breccias, mudflows, tuffs, and other fragmental rocks; the peperino near Rome and the trass of the Eifel district in Germany are examples. 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. The volcanoes Montagne Pelée, the Soufrière of St. Vincent, Krakatoa, Bandai-san, Popocatépetl, Fuji, Ngauruhoe, Shasta, Hood, and Adams have emitted great quantities of andesitic rock.
References: R.M.Busch, Encyclopaedia Britannica,Sandatlas