Mount St. Helens' Decade of Volcanic Lessons

By , Special to The Christian Science Monitor

THE 1980 eruption of Mount St. Helens provided scientists with a multitude of clues about how volcanoes work and a close-up look at the workings underneath the Earth's surface. Because of its proximity to the Portland, Ore., and Seattle metropolitan areas and to universities, the mountain has been the most studied volcano in the world.

Discoveries made from tests at Mount St. Helens have changed volcanology - the study of volcanoes - says Steven Brantley, a geologist at the Cascades Volcano Observatory in Vancouver, Wash.

``[Mount] St. Helens was a very important event,'' Mr. Brantley says. ``The eruption was so dramatic, within a matter of seconds it changed the landscape. We have had a rare opportunity to study that.''

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The 40,000-year-old Mount St. Helens is one of the youngest mountains in the Cascades. Mountains in that range belong to the same ``family'' that volcanoes across two hemispheres belong to - what is called the ``Pacific Ring of Fire.'' The ring includes dozens of volcanoes encircling the Pacific Ocean stretching from the US to South America, the South Pacific, and up along the Asian coastline.

All of these volcanoes are reacting to the same events below the Pacific Ocean - a grinding between ocean and continental plates.

Those forces were the cause of the 1980 eruption. Since then, scientists have examined the landscape around Mount St. Helens to retrace what happened.

``There were no big discoveries, but a series of small observations and findings,'' says Steve Malone, a University of Washington geophysicist. Nevertheless, several aspects of the Mount St. Helens eruption surprised scientists.

Among the biggest surprises was the observation of the eruption coming out of the side of the mountain instead of the top. The phenomenon, called ``collapsing volcanoes,'' occurred after rising magma from below the earth rose and battered away at the insides of Mount St. Helens.

The drastic change in air pressure between the inside and outside created a vacuum, and the magma and ash exploded out of the mountain's side at more than 100 miles per hour.

Scientists have since identified dozens of collapsing volcanoes around the world, thanks to the trail of mud, ash, and pumice Mount St. Helens left.

``We've realized that they are more common than previously thought,'' Brantley says.

Scientists also gained new insight into the formation of mudflows, and the destructive force they possess. Study of the landslide deposits has in turn provided a basis for interpreting landslide deposits around the world, Brantley says.

Maybe most significant was the scientists' opportunity to refine the way they measured earthquakes. Progress in that area in turn helped improve the ability to forecast eruptions. Scientists know that a volcano eruption is preceded by earth tremors. Since its major eruption, Mount St. Helens has registered 17 smaller eruptions. Scientists have accurately predicted each of them anywhere from weeks to hours in advance.

``We've come to learn to recognize different kinds of signals,'' Mr. Malone says.

And yet Malone, Brantley, and others quickly concede they cannot predict an eruption like the one on May 18, 1980. The landslide triggered by the earthquake prevented scientists from identifying the usual seismic buildup preceding a blast, Malone says.

``There's no way we could have seen it coming,'' he adds.

Scientists say that as the one of the youngest volcanoes in the Cascade Range, Mount St. Helens is most likely to erupt before others.

``All volcanoes tend to exhibit their own personality,'' Brantley says. ``Unfortunately, the lessons learned from one volcano are not necessarily applicable to another.''

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