Saturday, December 03, 2005

Mt. St.Helen's gentle eruption

The satellite trucks and news reporters have long gone. The crowds of tourists have thinned. No plumes of ash have risen above Mt. St. Helens for nine months.Daniel Dzurisin, a volcanologist at the U.S. Geological Survey's Cascades Volcano Observatory in Vancouver, Wash., said people often asked him when St. Helens would erupt again."When I tell them it's erupting today," Dzurisin said, "they're surprised."The mountain has a split personality.

The cataclysmic eruption on May 18, 1980, blew off the top 1,300 feet of the mountain, flattened 86,000 acres of forest and killed 57 people.The current eruption, now in its 15th month, is quiet, as volcanic eruptions go. It shows no signs of turning violent, and there isn't even lava.Instead, what is coming out of the ground is a tube of rock that, while still hot, solidified perhaps a mile underground and then was pushed upward.

The process is like holding a toothpaste tube vertically and squeezing the toothpaste out.Each second, about a cubic yard of new mountain--roughly a pickup truck's worth--is pushed to the surface, adding to a dome growing in the crater.In earlier months, the cylinder of new rock, which is about 200 yards in diameter, toppled to the side as it rose. Now, the new rock is buried beneath earlier material and just pushes up the entire hill."It's looking pretty impressive," said Jon Major, a hydrologist at the observatory.

"There's quite a pile of rock and rubble."For the scientists at the volcano observatory, the past year has been an unexpected bonanza, one that is giving them new insight into Mt. St. Helens, the youngest and most active of the volcanoes in the Cascade Range. It also may give clues about the 60 other volcanoes on the U.S. mainland that have erupted in the past 10,000 years and are thus presumed to have the potential to erupt again.Among all volcanoes in the United States, the long-term average is two eruptions a century.

"That doesn't sound like a lot," said John Ewert, a Geological Survey scientist. "But when you consider the size of the volcanoes and consider most of them are covered with snow and ice, it becomes a much more significant number."An eruption can melt the snow and ice, setting off avalanches and gargantuan flows of debris rolling down the mountain.

But for now, the Geological Survey has few instruments keeping watch over them.Many of the scientists observing Mt. St. Helens were there when it erupted in 1980 and continued to observe the mountain as about 20 smaller eruptions, some lasting only a few days, continued through 1986. Then the mountain fell quiet. Scientists did not expect another eruption in their lifetimes.Last September, a swarm of small earthquakes shook the volcano.

The first eruption of ash and steam rose upward a couple of weeks later.Mt. St. Helens tossed up a few more small clouds of steam and ash. The reporters went elsewhere, but scientists have a rare, close-up view with relatively little danger. Gases such as carbon dioxide and sulfur dioxide--the ingredients that make volcanoes explosively deadly--are largely missing this time."It's become an incredible scientific experiment," said geologist John Pallister.

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