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Any member of a large family of rocks that occur in most of the world's volcanic areas, mainly as surface deposits and to a lesser extent as dikes and small plugs. The Andes, where the name was first applied, and most of the cordillera (parallel mountain chains) of Central and North America consist largely of andesites. They also occur in abundance in volcanoes along practically the entire margin of the Pacific basin. Andesites are most often porphyritic (having distinct crystals in a fine-grained base) rocks.
This entry comes from Encyclopædia Britannica Concise. For the full entry on andesite, visit Britannica.com.