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.
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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