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Definition: andesite from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Volcanic igneous rock, intermediate in silica content between rhyolite and basalt. It is characterized by a large quantity of feldspar minerals, giving it a light colour. Andesite erupts from volcanoes at destructive plate margins (where one plate of the Earth's surface moves beneath another; see plate tectonics), including the Andes, from which it gets its name.


Summary Article: Andesite
from Rock and Gem

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.

  • Properties
  • Rock type Intermediate, volcanic, igneous
  • Major minerals Plagioclase feldspars
  • Minor minerals Pyroxene, amphibole, biotite
  • Color Light to dark gray, reddish-pink
  • Texture Fine, porphyritic

Andes

Andesite is named for the Andes Mountains of South America, a subduction zone, and therefore a characteristic locality.

Fine-grained

This fine-grained andesite specimen contains small phenocrysts of light plagioclase feldspar.

Volcanoes

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.

Mount Fuji

Mount Fuji, near Tokyo, Japan, is a classic andesitic volcano.

© 2008 Dorling Kindersley Limited

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