Tuesday, May 17, 2005

Is Mount St.Helen about to celebrate its anniversary with a volcano eruption?

No one knows whether Mount St. Helens will put on a 25th anniversary show, but the region’s most active volcano isn’t likely to stop spitting and sputtering anytime soon.

The 40,000-year-old volcano returned to the limelight in late September when a swarm of earthquakes signaled it was about to awaken after 18 years of sleep.

On Oct. 1, steam and ash shot out of a vent, the first of five outbursts over four days. Two more explosive blasts took place in January and March. Both heaved large rocks across the volcano’s crater. The March event sent an eye-catching cloud up to 36,000 feet.

Lava began to ooze to the volcano’s surface Oct. 11, creating inside the crater a mound that hasn’t stopped growing. That’s no surprise to scientists at the Cascades Volcano Observatory in Vancouver, Wash., the home of a U.S. Geological Survey team that tracks the mountain’s behavior.

St. Helens could keep pumping masses of hot rock into its crater for years, perhaps decades, said John Pallister, an observatory geologist.

“We really have to wait and see where Mother Nature goes,” said Cynthia Gardner, also a geologist and the scientist in charge of the observatory.

What Gardner described as a “steady state eruption” has over the past seven months created a second lava dome inside the crater, near the one that formed in the six years after the 1980 eruption.

If the new dome continues to grow, it could reshape the top of the mountain, now capped by a huge, horseshoe-shaped crater.

Pallister compared the ongoing eruption to what has happened on the Caribbean island of Montserrat, where an eruption began in 1995 and lasted more than eight years.

On St. Helens, the pace of dome-building now is less than half of what it was last fall, but lava still is pushing its way to the surface. By February, the volume of the new dome had grown so large it was half the volume of the original.

A hazard warning issued last fall remains in effect, in part because scientists can’t predict the volcano’s steam-and-ash eruptions. The primary risk is to aircraft. An ash explosion could disable jet engines. Otherwise, a volcanic outburst is unlikely to cause harm except inside the crater and areas north of it.

A laboratory analysis of the lava shows it is the same type of igneous rock – known as dacite – that spit out of St. Helens in 1980, Pallister said. But it’s not as explosive.

“We’re dealing with largely leftover magma that lost much of its gas in 1980,” he said.

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