Andesite and dacite
Nomenclature and compositional varieties
The name andesite was originally coined by Leopold von Buch in 1826 for porphyritic volcanic rocks of the Andes. The rocks were said to consist mainly of “albite” and horn-blende. The name was not generally used, however, until a more refined definition was introduced by J. Roth who, in 1861, applied it to porphyritic Cenozoic rocks consisting chiefly of plagioclase and either hornblende or augite. The term was subsequently broadened to include subalkaline (hypersthene-normative) rocks of any age in which plagioclase has an average composition more sodic than labradorite. Modern usage follows the definition adopted by the IUGS Subcommission on the Systematics of Igneous Rocks (Streckeisen, 1978), which places less emphasis on the composition of plagioclase than on silica content and color index (volumetric proportion of dark minerals). The latter has been set at 35% or less in the mode or molecular norm (40% in the CIPW norm). Many recent...
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- Andesite line; Calc-alkaline rocks; Circum-Pacific plutonic and volcanic activity; Diorue and tonalite; Explosive volcanic eruptions—classification; Granodiorite; Igneous rocks—classification and nomenclature; Igneous rocks—tectonic setting; Intermediate igneous rocks; Island arcs—petrology; Ultramafic lavas; Volcanic rocks.