Click the photo to see the full-size version. In general, color is a good clue to the silica content of lavas, with basalt being dark and felsite being light. Although geologists would do a chemical analysis before identifying andesite in a published paper, in the field they readily call a gray or medium-red lava andesite. Andesite gets its name from the Andes mountains of South America, where arc volcanic rocks mix basaltic magma with granitic crustal rocks, yielding lavas with intermediate compositions. Andesite is less fluid than basalt and erupts with more violence, because its dissolved gases cannot escape as easily. Andesite is considered the extrusive equivalent of diorite.
See more andesites in the gallery of volcanic rocks.
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