Mount St. Helens
Mount St. Helens spews steam and gray ash from a small explosive eruption in its crater on Oct. 1, 2004. St. Helens is only one of several potentially dangerous volcanoes in the Cascade Range. REUTERS

Volcanic eruptions like the one from Hawaii's Kilauea volcano on March 5 offer spectacular views of lava flows and are a vivid reminder to islanders that the ground they live on can become hazardous very fast. For people on the mainland of the U.S., such hazards can seem far away.

But volcanoes are actually as great a danger - perhaps more so - in the continental U.S. Many remember the 1980 eruption of Mount St. Helens, which resulted in 57 deaths and the destruction of 200 homes. Before that, Lassen Peak in California erupted in 1915 and created a Devastated Area that is still sparsely forested and dropped ash as far away as Winnemucca, Nev., some 200 miles away.

According to the U.S. Geological Survey, one of the most dangerous volcanoes in the U.S. is Mount Rainier. Rainier has not erupted since the 1820s, but it is listed as a volcano with a good chance of erupting within the next 10 years. If it were to erupt with the same power as Mount St. Helens did in 1980, it would be much worse because of the larger populations that live in the area. The city of Portland, Ore., is 50 miles from Mount Hood, another active peak that has been relatively quiet. But Mount Hood's last eruption was just before the Lewis and Clark expedition arrived in 1805, and there has been minor activity since then, most recently in the early part of the last century.

But while the volcanoes in the lower 48 states pose dangers, at the same time there has been a marked increase in the sophistication of the instruments used to monitor them. Charles Mandeville, associate program coordinator of the Volcanic Hazards Program at the USGS, notes that when Mount St. Helens erupted, there was no GPS, for example, and satellite photos were harder to come by. There's really been a revolution in the kinds of instruments we use, he said.

GPS stations can monitor small movements of the land underneath them, tilt meters can tell when the slope of the side of a mountain alters, and satellites can take photos and beam radar at the mountains and accurately measure the changes in shape. All these are important to seeing the swelling of a volcano that often occurs just before an eruption. Other instruments can measure the gases that a volcano gives off just before an explosion and tell scientists when one might occur.

This doesn't allow for absolute predictions. Shan de Silva, volcanologist and professor of Geosciences at Oregon State University, notes that there is a big difference between likely to erupt and will erupt and it is sometimes difficult to tell just how close a volcano is to blowing its top.

But that makes the Cascade Range, where much of the current active volcanism in the lower 48 states is, all the more important to monitor. Both de Silva and Mandeville say that more monitoring equipment is needed to make sure that there is some advance warning.

What makes the Cascade volcanoes so important is that while they do not erupt nearly as often as their counterparts in Hawaii, when they do it is far more explosive. There are currently a dozen that are considered active.

This is because of the type of volcano that dominates the region. The Cascade volcanoes are stratovolcanoes. Stratovolcanoes are formed from successive layers of material that comes out of the ground, forming the classic conical shape. They release lava that contains higher amounts of silica, which makes it thicker and more like a paste. In Hawaii, the landscape is dominated by shield volcanoes (such as Hawaii's Mauna Loa). A Hawaiian volcano tends to have lava flows that are faster, more like liquid water.

Thicker magma doesn't let gas escape as easily. When it rises to the surface it is under tremendous pressure, and when it breaks through during an eruption the gas escapes. The process is similar to what happens when you shake a bottle of carbonated water and take the cap off. Stratovolcanoes are also more likely to produce pyroclastic flows, which is a blast of hot gases carrying ash that travel at up to hundreds of miles an hour at temperatures up to 1,000 degrees Celsius.

On top of that, some of the volcanoes in the Cascades - notably Rainier and Hood - are covered by glaciers. When they melt, they can add water to a pyroclastic flow, creating what is called a lahar, which is essentially a high-speed, very hot mudslide that is as dense as concrete. Such a flow engulfed the town of Armero, in Colombia, in 1985, killing 20,000 people.

The ice also can increase the explosive power of an eruption. When the bottom of the glacier melts during an eruption, the water runs into the crater and vaporizes. That means that even a relatively small eruption can loft ash and even large pieces of rock high into the air. You can have a small eruption which doesn't ordinarily pose any danger, but with ice it is multiplied by orders of magnitude, de Silva said.

A number of the more dangerous volcanoes in the region are monitored, Mandeville noted. But many are not. And more data is needed to provide accurate forecasts, because much of the forecast is based on what the volcano did in the past, and what its current levels of activity are. But unless volcanologists have a good picture of what is normal for a given mountain it is hard to make accurate estimates of when a volcano might erupt.

Mount St. Helens last erupted in 2004, and continued to shake and belch steam and ash for another four years. The eruption sent ash plumes into the air several times, some of which were visible all the way from Seattle. That eruption was small enough that it caused few problems. But de Silva notes that it isn't a matter of if one of the volcanoes in the area will erupt again, but when, and people need to prepare for that eventuality. All the big ones in the area have gone off at least once in recorded history, he said. And a hundred years is a short time to a geologist.

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