Volcanoes and Earthquakes

When did Mount St. Helens erupt?

Mount St. Helens, located in southwestern Washington state in the Cascades mountain range, erupted on May 18, 1980. Sixty-one people died as a result of the eruption. This was the first known eruption in the 48 contiguous United States to claim a human life. Geologists call Mount St. Helens a composite volcano (a steep-sided, often symmetrical cone constructed of alternating layers of lava flows, ash, and other volcanic debris). Composite volcanoes tend to erupt explosively. Mount St. Helens and the other active volcanoes in the Cascade Mountains are a part of the “Ring of Fire,” the Pacific zone having frequent and destructive volcanic activity. Mt. St. Helens erupted again in October 2004. A steam plume billowed 10,000 feet (3,048 meters) into the air. The eruption continued for 2.5 years building a new lava dome. The new lava dome measured about 125 million cubic yards (95.6 million cubic meters), or a volume equal to nearly 200 large sports stadiums. The amount of lava that erupted would have been enough to pave seven highway lanes, 3 feet (0.9 meter) thick, from New York City to Portland, Oregon. There was no loss of life during this eruptive period.

Volcanoes have not only been active in Washington, but also in three other U.S. states: California, Alaska, and Hawaii. Lassen Peak is one of several volcanoes in the Cascade Range. It last erupted in 1921. Mount Katmai in Alaska had an eruption in 1912 in which the flood of hot ash formed the Valley of Ten Thousand Smokes 15 miles (24 kilometers) away. And Hawaii has its famed Mauna Loa, which is the world’s largest active volcano, being 60 miles (97 kilometers) in width at its base.


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