Over 500 million people throughout the world live in areas prone to volcanic eruptions. In the U.S., there are 169 active volcanoes accounting for 11 percent of the world’s total. Most of these volcanoes are in Alaska, Hawaii, and along the west coasts of California, Oregon, and Washington. Since 1980, there have been 45 eruptions at 33 U.S. volcanoes.
Volcano eruptions occur when magma (melted portions of the Earth’s mantle) is expelled through an opening in the Earth’s surface. Because magma is less dense than surrounding rocks, it rises and collects in magma chambers. When pressure builds from dissolved gasses or new magma enters an already filled chamber, the material is pushed upward, forcing an eruption.
Types of Eruptions
There are two general types of volcanic eruptions: effusive and explosive. Lava flows from Hawaii’s Kilauea and Muana Loa volcanoes are examples of effusive eruptions. Magma in effusive eruptions has low viscosity, which allows gas to escape easily as it rises to the surface. Effusive eruptions usually result in gently flowing lava flows, spatter cones, and lava fountains.
Mount Saint Helens
Mount Saint Helens in Washington is a well-known example of an explosive eruption. Explosive eruptions are caused by the tremendous pressure of trapped gasses in the magma. These eruptions produce high-speed avalanches of hot ash and gas (pyroclastic flows), giant mudflows (lahars), and a cloud of volcanic ash and rock fragments (tephra).
Measuring an explosion
Chris Newhall of the United States Geological Survey and Stephen Self at the University of Hawaii created the Volcanic Explosivity Index (VEI) in 1982. The VEI measures the amount of ejected volcanic material, how high it’s thrown into the atmosphere, and the length of an eruption. The VEI ranks volcanic explosions from 1 (gentle) to 8 (mega-colossal). Mount St. Helens, for example, rated a VEI of 5.