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Out of the Inferno:Volcanoes
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The Volcanoes of North America
Hell's Crust: Our Everchanging Planet
The Restless Planet: Earthquakes
Out of the Inferno: Volcanoes
Waves of Destruction: Tsunami
Link to movie: Mudflows
Link to movie: Mudflows

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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).
 

Design element
Paricutin volcano

 Paricutin volcano in Mexico.

Design element

Back in 1980, a string of three large quakes -- each around magnitude 6 -- rocked the caldera. Since then, swarms of small, imperceptible quakes have regularly lit up seismographs. Those quakes, some researchers suspect, mark the reemergence of volcanic activity in the caldera. There have been other indications too. "The center part of the volcano has been coming up -- doming --- at rates that vary from less than an inch to six inches a year," says Reid. "Different people have different interpretations, but I think that most people would agree that that means there is magma moving beneath the surface." Back in 1990, another dramatic sign of resurgent volcanic activity -- vast tree kills from huge amounts of carbon dioxide gas seeping out of the soil -- was first noted at Mammoth Mountain, near the ski resort town of Mammoth Lakes, on the southwestern edge of the caldera.

By some estimates, a magma body may be located about seven miles beneath the caldera. Reid's own work suggests that it could be large. She's precisely dated zircon crystals embedded in lava flows from two small eruptions, 115,000 and 625 years ago. Although Reid thought that the crystals would be about as old as the lava flows, they turned out to be much older -- around 230,000 years old. That means, Reid thinks, that the zircons crystallized in the same magma body over two hundred thousand years ago. The magma then stayed molten until at least 625 years ago. "To keep that magma hot for so long," Reid says, "you'd need to have a pretty big magma chamber below the surface, with perhaps two hundred cubic kilometers of material." Mount St. Helens, for comparison, released only one to two cubic kilometers of volcanic material when it erupted in 1980.

Article: Mountains of Fire | Sidebar One: Volcanoes of North America | Sidebar Two: Montserrat | Sidebar Three: Other Planets | ANIMATION
Hell's Crust: Our Everchanging Planet  |  The Restless Planet: Earthquakes 
Out of the Inferno: Volcanoes  | 
Waves of Destruction: Tsunamis
 

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