Composite volcanoes, also known as stratovolcanoes, typically have a conical shape with steep sides and a central vent leading up to a crater at the summit. Stratovolcanoes have a symmetrical appearance because alternating flows of lava and ash build up the sides layer by layer.
Stratovolcanoes rise as high as 8,000 feet, nearly seven times as high as cinder cone volcanoes. Beneath each towering landmass, a large magma reservoir fills up with heat, dissolving stored gas and melting rock. As pressure builds inside the reservoir, magma and gas explode through tunnel-like vents leading out to the surface.
The intensity of volcanic eruptions is influenced by the fluidity of pent-up magma and gas in the reservoir. High-pressure stratovolcano eruptions are violent enough to blow out the sides of the slope, making them the most severe of all eruptions. Lava also flows out of existing fissures along the cone. Over time, the lava solidifies in the cracks, refortifying the volcano's structure.
Mount St. Helens, a 8,636-foot stratovolcano in Washington, caused severe economic devastation amounting to millions of dollars. The National Geophysical Data Center details how a May 1980 eruption destroyed 229 square miles of surrounding timber and blew off 396 miles of the volcano's upper structure.