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