COMPOSITE VOLCANOES

Composite volcanoes are constructed from multiple eruptions, sometimes recurring over hundreds of thousands of years, sometimes over a few hundred. Andesite magma, the most common but not the only magma type forming composite cones, produces lava more brittle than basaltic lava because of its higher viscosity. Although andesitic composite cones are constructed dominantly of fragmental debris, some of the magma intrudes the cones as dike or sills. In this way, multiple intrusive events build a structural framework that knits together the voluminous accumulation of volcanic rubble, which can stand higher than cones composed solely of fragmental material. Composite cones can grow to such heights that their slopes become unstable and susceptible to collapse from the pull of gravity. Famous examples of composite cones are Mayon Volcano Philippines, Mount Fuji in Japan, and Mount Rainier, Washington, U.S.A. Some composite volcanoes attain two to three thousandmeters in height above their bases. Most composite volcanoes occur in chains and are separated by several tens of kilometers. There are numerous composite volcano chains on earth, notably around the Pacific rim, known as the "Rim of Fire".

St. Augustine volcano, Alaska. Composite cone. Photograph by Harry Glicken.

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