Some of the most familiar stratovolcanos in the world are Mount Fuji, Mount Vesuvius, Mount Krakatoa, Mount Tambora and Mount Pinatubo. On the North American continent there are several stratovolcanos, Mount Shasta, Mount St. Helens, Mount Hood and Mount Rainier. Each of these volcanoes is a breathtaking, huge cone-shaped mountain. But what are the distinct characteristics of a stratovolcano?
Stratovolcanoes are sometimes called composite volcanoes because they are made up of layers of different materials. During eruptions of stratovolcanoes alternating layers volcanic ash, tephra, cinders, blocks, and bombs are blown out of a vent or a cluster of vents and deposited around a central crater usually found at the summit. These layers accumulate and build a mountain that can be as high as 8,000 feet from the base of the mountain to the top. Another component in the formation of a composite volcano is a layer of lava that flows from cracks in the crater wall or it may ooze from fissures or crevices on the sides of the mountain. Inside the stratovolcano there are conduits which allow magma to rise from deep in the earth’s crust to the surface which identify the volcano as a stratovolcano.
Stratovolcanoes are usually found along subduction zones or along the edges of two tectonic plates. When these plates meet, one edge will push over the top of the other. As the edge of one plate is pushed below the edge of the upper plate, a hot spot is created which is hot enough to melt the minerals and rocks which then become magma. As the magma continues to heat up it rises to the surface. As the magma rises to the surface, gases build up causing pressure beneath the surface. This pressure will become so great that it bursts through the surface of the earth in violent explosions called eruptions that are the calling card of a stratovolcano. The explosion will send rock, steam and ash thousands of feet into the atmosphere.
Another characteristic of a stratovolcano is that lava flowing from the volcano is very viscous or thick and does not flow as fast as thinner lava. It therefore does not spread as far before it cools and hardens. This tends to accumulate on the slopes of the cone which builds up the mountain even more. The viscosity of lava varies according to the amount of silica that it contains; lower silica content lava will flow faster than lava that has a higher concentration of silica.
Stratovolcanoes have killed more than 300,000 people since 1600 A.D. The violent eruptions usually are not the direct cause of the death of people, but the effects of a major eruption from a stratovolcano can be devastating. A pyroclastic flow, which is a mixture of hot volcanic debris, ash and gas, traveling at 93 miles per hour killed approximately 30,000 people when Mount Pelée erupted in the Caribbean in 1902. Another deadly effect is the mudslide that usually accompanies the eruption of a stratovolcano. A mudflow can be as thick as wet cement and quickly buries anything or anyone in its way. The city of Armero in Colombia was buried by a mudflow when Nevado del Ruiz Volcano erupted in 1985, killing nearly 23,000 people.
Although it is amazing to watch a stratovolcano eruption from a safe distance or on television, they are some of the most violent and deadly natural disasters to befall mankind.