Andesite is the most common volcanic rock after basalt. It is porphyritic, that is, consists of coarse crystals (phenocrysts) embedded in a granular or glassy matrix (groundmass). Having a silica content of 57%, it is in the intermediate category (52–66% silica) of the silicic–mafic scale. The large volcanic mountain chains of North and South America, including the Andes (for which andesite is named), are composed largely of andesite. Indeed, andesite is common in all the mountain-building zones that rim the Pacific Ocean. The transition from the oceanic crust of the main basin of the Pacific to the andesitic rocks around its perimeter is termed the andesite line. The crust on the deep-sea side of the andesite line is a product of sea-floor spreading, and the andesitic mountains on the other side are a product of orogenic volcanism. The andesite line thus marks the geological border of the true Pacific basin.

The primary ingredient of most andesites is andesine, a feldspar of the plagioclase series. Smaller amounts of quartz or minerals rich in iron and magnesium such as olivine, pyroxene, biotite, or hornblende are also present. Andesites are ordered in three classes according to the identity of their non-feldspar components: from most silicic to most mafic, these are (1) quartz-bearing andesites, (2) pyroxene andesites, and (3) biotite and hornblende andesites. All are intermediate in composition between diorite (an intrusive igneous rock consisting mostly of plagioclase feldspar) and rhyolite, a volcanic rock having the same composition as granite (i.e., feldspar plus quartz). In other words, andesites are higher in feldspar than rhyolite but lower in feldspar than diorites.

Andesite's character usually results from the melting and assimilation of rock fragments by magma rising to the surface. Rocks nearer the surface tend to be higher in silicon, because silicon is less dense than iron and magnesium, components that increase at greater depths. An andesite can thus be viewed very roughly as a basalt contaminated with excess silicon (and perhaps other ingredients). Indeed, many olivine-bearing andesites are so close to basalt in appearance that they can only be distinguished on the strength of chemical analysis.

Andesites high in quartz—dacites—are sometimes classed as a separate group.

Mount St. Helens erupted explosively because its magma is andesitic, with increased amounts of trapped gas. AP/Wide World. Reproduced by permission.

See Also

Bowen's Reaction Series; Crust; Earth, Interior Structure; Minerals; Volcanic Eruptions