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Mt. St. Helens mudflows.

The Volcanoes of North America

by Kathy Svitil

Mount St. Helens is arguably North America's most famous volcano, but you don't have to live in southwestern Washington state to have an active volcano in your back yard. Popocatepetl, the Mexican volcano located just outside of Mexico City and featured in the "Out of the Inferno" program in the SAVAGE EARTH series, reawakened on March 5, 1996, and has been shaking and smoking ever since. The United States is home to 50 active volcanoes (defined as having erupted sometime in the last 200 years). A whopping 80 percent of those are located in Alaska's remote Aleutian Islands chain. The volcanic island chain, which stretches west from the mainland toward Kamchatka on the northwest Asian coast, is the result of the sinking of the Pacific plate beneath the North American plate. The chain has more than 40 active volcanoes, including Mount Spurr, Mount Redoubt, and Mount Augustine.

"We usually have one or two decent eruptions a year," says volcanologist Chris Nye of the Alaska Volcano Observatory, which monitors activity in the Aleutian chain. "On average, we have three or four or five days a year when the eruption columns reach up high enough into the atmosphere to interfere with air traffic." Air traffic may seem inconsequential in such a remote region, but the region is actually an important corridor for international air traffic. "Almost all of the air freight which moves between North America and Asia and Europe and Asia comes through Alaska to refuel. Sixty to eighty thousand wide-body aircraft fly over the Aleutian volcanoes every year."
 

In the Cascade Range in the Pacific Northwest, the home of Mount St. Helens, geophysicists are continuously monitoring a number of other volcanoes that have erupted within the past two centuries, including Mount Shasta, Mount Hood, Lassen Peak, and Mount Rainier. "There is a lot of concern about Mount Rainier because it is so close to Seattle and capable of damaging mudflows," says geologist Mary Reid of the University of California at Los Angeles.

Reid and others have focused their scientific sights on the Long Valley Caldera in northeastern California. The giant depression was formed 760,000 years ago in a massive volcanic explosion that blanketed the entire western United States in ash and volcanic rock. The volcano has been relatively quiet ever since, except for eruptions from some smaller volcanoes inside the caldera (crater).