Definition of andesite

A fine-grained, dark-colored intermediate volcanic

rock, andesite typically has phenocrysts of zoned sodic

plagioclase, and biotite, hornblende, or pyroxene. It has

56–63 percent silica, although basaltic andesites with silica

contents down to 52 percent have a composition that is transitional

with basalts. Andesite is the extrusive equivalent of

diorite and is characteristic of volcanic belts formed above

subduction zones that dip under continents. The name was

coined by Buch (1826) for rocks in the Andes Mountains of

South America.

Andesite is generally associated with continental margin

or Andean-type magmatic arcs built on continental crust

above subduction zones. Their composition is thought to

reflect a combination of processes from the melting of the

mantle wedge above the subducting plate, plus some contamination

of the magmas by partial melting of the continental

crust beneath the arc.

The average composition of the continental crust is

approximately andesitic to dacitic. Many models for the

formation and growth of continents therefore invoke the

formation of andesitic to dacitic magmas at convergent

margins, with the andesitic arcs colliding to form larger

continental masses. This is known as the andesite model of

continental growth.

See also CONVERGENT PLATE MARGIN PROCESSES; PLATE

TECTONICS; VOLCANO.

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