THE VOLCANIC EQUIVALENT of diorite, andesite is fine grained or porphyritic, and primarily consists of the plagioclase feldspar minerals andesine and oligoclase, plus one or more of the dark, ferromagnesian minerals, such as pyroxene or biotite. Unlike rhyolites, andesites do not contain quartz. Amygdaloidal andesite occurs when the voids left by gas bubbles in the solidifying magma are later filled in, often with zeolite minerals. Porphyritic andesite occurs when larger phenocrysts of feldspar and pyroxene form in a fine-grained matrix. Andesite erupts from explosive volcanoes and is commonly found interbedded with volcanic ash and tuff. The steep-sided volcanoes of the Pacific Rim, the Caribbean, and the Mediterranean are composed in large part of andesites.
- Rock type Intermediate, volcanic, igneous
- Major minerals Plagioclase feldspars
- Minor minerals Pyroxene, amphibole, biotite
- Color Light to dark gray, reddish-pink
- Texture Fine, porphyritic
Andesitic volcanoes form on continental or ocean crusts above subduction zones where one oceanic plate is sinking beneath another. Ancient andesites can therefore be used to map ancient subduction zones. The Soufrière Hills in Montserrat, West Indies, Krakatau in Indonesia, Popocatépetl in Mexico, and Mounts Shasta, Hood, and Adams in the USA have all expelled large quantities of andesitic rock.
Color Shades of grey, purplish, brown, green or almost black. Grain size Fine; less commonly partly glassy. Texture Frequently...
THE VOLCANIC EQUIVALENT of syenite, trachyte is commonly porphyritic. Some porphyritic trachytes contain phenocrysts of the mineral...