Image: St. Helens dome
Mount St. Helens' volcanic dome comes to a steamy point, as seen Sept. 25 by a remotely operated camera on southeast Crater Rim.
updated 9/29/2006 10:54:36 PM ET 2006-09-30T02:54:36

Two years after Mount St. Helens began its low-key eruption, a process that has extruded tons of rock into the crater left by the volcano's deadly 1980 blast, scientists say the mountain seems to be slowing down.

But they're making no predictions about when the activity will end.

"Volcanoes throw you a lot of curve balls. I've been humbled enough not to call the pitch till it's over the plate," said Cynthia Gardner, scientist in charge at the Cascades Volcano Observatory, a U.S. Geological Survey facility about 50 miles (80 kilometers) from Vancouver, Wash., and 150 miles (240 kilometers) south of Seattle.

The southwest Washington mountain is going through another "dome-building" phase within its crater.

The volcano's May 18, 1980, eruption killed 57 people, sent superheated mud down the Toutle River Valley, flattened forests for miles and spewed ash across the state and, eventually, around the globe. It also reduced the 9,677-foot mountain to 8,363 feet (from 2,950 to 2,549 meters), and replaced its symmetrical, snow-covered cone with a gaping crater.

The blast was followed by a period of dome building that ended in 1986.

Extrusion rate slowing down
The current flow of magma began in October 2004 after weeks of low-level seismic activity. For the first year it averaged more than 3 cubic yards, about two big pickup truck loads, per second, Gardner said. The extruded rock has been piling up brittle new structures, which then collapse onto the crater floor and form the foundation for the next gush of lava.

Since October 2005, the extrusion rate has slowed to less than 1.3 cubic yards per second, Gardner said. Since April, "we're in the half-cubic-yard range," she said.

But confirming a trend "takes a long time, because a lot of these are very small changes," she said.

Seismic readings, which had ranged above magnitude 3, have also dropped over the past year, Gardner said. The rate of deformation — the swelling or shrinking of the volcano's flanks — also has slowed, but in very small amounts, she said.

"We're not seeing anything ... that tells us we're in store for a change in eruptive style in the near future," she said. "Right now it looks like we'll be continuing with fairly benign rock extrusion and rockfalls that can send ash over the crater rim."

There's no telling when, or if, the activity could ratchet up again. "A lull's OK, as long as it's not another 18 years. I don't have that much time left in my career," Gardner said.

18 years of silence
The current eruptive phase followed 18 years of silence.

A drumfire of seismic rumblings began on Sept. 23, 2004. A plume of ash and steam on Oct. 1, 2004, confirmed that an eruption was under way. Magma appeared in the crater 10 days later.

The Johnston Ridge Observatory about five miles from the peak was closed Oct, 2, 2004, due to safety concerns. It reopened the following May 6.

This summer, a hiking trail to the edge of the crater was reopened by the U.S. Forest Service.

"This is a great opportunity. I think people need to understand that this is so rare, that you can actually go up and see something like this — even for scientists," Gardner said.

Mount St. Helens, the youngest and most active of the Cascade Range volcanos, has a history of leveling and rebuilding itself. Scientists say the mountain that stood before 1980 was just 4,000 years old — the blink of an eye in geologic time.

There had been eruptions in the St. Helens area for hundreds of thousands of years, but for centuries they only produced small lava domes.

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