Mount St. Helens and other Northwest volcanoes are all part of the Ring of Fire, an arc encompassing more than 75 percent of the world's active and dormant volcanoes. It stretches from New Zealand, along the eastern edge of Asia, north across the Aleutian Islands of Alaska, and south along the coast of North and South America.
In the Pacific Northwest, the Ring of Fire runs between Northern California and British Columbia.
The Ring of Fire is located at the borders of the Pacific Plate and other tectonic plates. Plates are like giant rafts of the earth's surface which often slide next to, collide with, and are forced underneath other plates.
Around the Ring of Fire, the Pacific Plate is colliding with and sliding underneath other plates. This process is known as subduction and the volcanically and seismically active area nearby is known as a subduction zone. A tremendous amount of energy is created by these plates and they easily melt rock into magma, which rises to the surface as lava and forms volcanoes.
Both Mount St. Helens and Mount Rainier are composite volcanoes, or stratovolcanoes. They are typically steep-sided, symmetrical cones of large dimension built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise as much as 8,000 feet above their bases.
Other stratovolcanoes are Mount Hood in Oregon, Mount Shasta in California, Mount Fuji in Japan and Mount Cotopaxi in Ecuador.
Mount St. Helens is the youngest stratovolcano in the Cascades and the most active. Geologists have identified at least 35 layers of tephra erupted by the volcano in the past 3,500 years.
The essential feature of a stratovolcano is a conduit system through which magma from a reservoir deep in the Earth's crust rises to the surface. The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash, etc., are added to its slopes.
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. All but basalt commonly generate highly explosive eruptions. A stratovolcano typically consists of many separate vents, some of which may have erupted cinder cones and domes on the volcano's flanks.
Source: U.S. Geological Survey