Stratovolcanoes, also known as composite cones, are the most dangerous type of volcano because of their explosive nature. Most are located around the Pacific plate in a zone called the "ring of fire." This zone includes the volcanic mountain ranges of North and South America, such as the Andes and the Cascades, as well as those of Hawaii and Japan.
The magma of stratovolcanoes has an andesitic composition. This type, also called an intermediate composition, has at least 25 percent dark silicate minerals and a high percentage of plagioclase feldspar, an aluminum silicate mineral. This magma is rich in gas. As the gases are released from the magma, they tend to build up inside the central vent and underground. Eventually, the high pressure and gases will cause an explosion, in which ash, debris and magma will be spewed from the central vent to Earth's surface.
The classic structure of a stratovolcano is a large, symmetrical cone with a broad base composed of lava and pyroclastic materials. Pyroclastics are volcanic rocks ejected during an explosive eruption; they include ash, bombs and blocks. Mount Fujiyama in Japan and Mount Mayon in the Philippines are classic examples of stratovolcanoes.
The eruption of a stratovolcano begins when a central vent emits pyroclastic material and lava. This will usually happen as an explosion of gases, unlike a Hawaiian volcano, in which lavas seem to flow like thick water. Rather, the silica-rich magma is a slow, viscous fluid that only travels short distances and may ooze from fissures in the base of the cone. This may occur simultaneously or alternately of explosive events. The cylindrical cone will typically display intermingled layers of lava and pyroclastics.
When an explosion occurs, rock and debris are ejected into the air. The coarser materials will fall almost immediately and contribute to the base of the composite cone. Finer materials, like ash, are spread out over great distances and can remain in the atmosphere for months, even years. Additionally, the seismic movement caused by the eruption can also generate tsunamis if the volcano is located near a body of water.
Excessively large eruptions can cause summit depressions -- horseshoe-shaped holes where the top of the composite cone has partially collapsed. Additionally, once the eruption is over, the andesitic lava may still be visible in the base of the cone. Finer material, like ash and rock debris, can become saturated with water, either from snowmelt on top of the volcano or by heavy rainfalls, and create huge landslides called lahars. If the layers of ash and debris are thick, these lahars can be crushing, wiping out everything in their path, as happened with the eruption of Mount St. Helens on May 18, 1980.
Mount St. Helens -- A Case Study
Mount St. Helens in southwestern Washington state erupted on May 18, 1980. The eruption devastated the region. The initial blast exploded the northern flank, literally lowering the summit of the mountain by 400 meters. The heat melted the snow at the cap, creating lahars that flattened the forested mountainside. Approximately one cubic kilometer of ash and debris was ejected from the mouth into the stratosphere, where it was carried as far away as Oklahoma and Minnesota. In total, the eruption claimed 59 lives. Some were close enough to be thrown by the blast or entrapped in the mudflows, while others suffocated from the ash and gas cloud that spread over the area. The volcano erupted again beginning Oct. 16, 1980.