Usually constructed over a period of tens to hundreds of thousands of years, stratovolcanoes may erupt a variety of magma types, including basalt, andesite, dacite, and rhyolite. A stratovolcano volcano consists of many separate vents (http://volcanoes.usgs.gov).

             Of all the mountains in the Cascade Range, Mount Saint Helens has been the most active. She has not gone more than 500 years without major volcanism and has dominant eruptions every century or two (Crandell et. al., 1987). Figure 1.0 shows the recent volcanic activity within the Cascade Range. Why is there so much volcanic activity around the Cascade Range? The answer to this is plate tectonics.

             Mount Saint Helens is a good example of a subduction volcano. It is still active as of its last eruption in 1981. In Mount Saint Helen"s case, it is also part of a volcanic arc, the cascade. This arc is created by the subduction of the San Juan plate under the North American plate. It creates a chain of mountains from Mount Garibaldi in British Columbia to Mount Shasta in California.

             Plate tectonics are the driving force for all volcanism. In the Pacific northwest, the Juan de Fuca plate is subducting beneath the North American plate. Due to the intense pressure, the edge of the subducting plate to melts into magma. Because of the magma"s low density and buoyancy, it makes it way to the surface through weak spots in the crust (Carson ,2000). When the magma reaches the top, it spews out onto the surface causing volcanism. .

             According to research scientists, Mount Saint Helens has been active for at least the past 18000 years. Looking at documentation from the 1840"s and 1850"s it is noted that Mount Saint Helens was quite active during this timeframe. In 1842, one of the first recorded eruptions of Mount Saint Helens was by a Methodist missionary in the Willamette Valley (Harris, 1988). According to J.L. Parrish, .

             "It was a silent awe inspiring spectacle.