Mount St. Helens' dome still growing a year after its birth

It's been a busy year for Mount St. Helens.

After snapping back to life with a series of earthquakes that started last September, the volcano has been a veritable lava assembly line, churning out a dump-truck load every second. The new dome inside the crater is growing up to 16 feet a day, shoving aside a 700-foot-thick glacier as if it were papier-mâché.

And there's no end in sight, say U.S. Geological Survey (USGS) scientists.

"There's no reason to think it couldn't go on for another year, or for decades," said volcanologist Dan Dzurisin.

Scientists have been busy, too.

Since the eruption began, monitoring St. Helens has been an all-consuming job for experts at the USGS Cascades Volcano Observatory in Vancouver, Wash., said Cynthia Gardner, scientist in charge. Other projects have been shelved and $1 million devoted to the effort, not counting salaries.

That work is paying off with a richer picture of the region's most volatile volcano.

"It's been an exciting time, and we've learned a lot," Dzurisin said.

By analyzing the million small earthquakes that have rattled the volcano over the past year, scientists are beginning to construct a picture of the magma chamber 2.5 miles below the surface where the lava originates. Chemical analysis of the new dome suggests it takes about a week for molten rock to travel from the chamber to the surface.

For most of that trip, the rock remains fluid, Dzurisin said. About a half mile before emerging, it hardens up. As it thrusts out of the ground, the new rock forms whaleback ridges that climb, then crumble.

At its highest elevation, the top of the new dome stood 7,765 feet above sea level. It's lower now, but the volume has grown to 76 million cubic yards — 25 times the size of Safeco Field.

The magma itself is coming primarily from a pool that's been around since the volcano's spectacular eruption in 1980 and the series of smaller lava flows that followed, said geologist John Pallister. That's good news, he said, because it means the likelihood of another massive explosion is slight.

But a group of experts who examined the new lava rocks this summer found a bit of fresh magma mixed in, which may indicate the eruption is starting to tap into a deeper reservoir. If fresher, more explosive magma starts to move through the system, the chance of a big blow will increase, Pallister said.

The earthquake analysis also has yielded the first tangible evidence of a crack in the Earth's crust that extends for miles beneath the volcano. Scientists have long known it must be there — a weak spot in the crust that provides a pathway for molten rock, Dzurisin said.

"The volcano is there because magma has been rising through that zone of weakness for tens of thousands of years."

Scientists have been surprised at the eruption's persistence. Usually, such prolific lava outflows come in fits and starts. After the 1980 explosion left a hollowed crater, the volcano rebuilt a dome with multiple eruptions over the course of six years. In a single year, the current eruption has produced a dome nearly as big.

The current eruption came on with very little warning, which also was unexpected, Dzurisin said. The first earthquake swarm hit on Sept. 23, 2004. The first steam explosion was Oct. 1, and by Oct. 11 lava pushed through to the surface.

Could future, more destructive eruptions occur with even less notice?

"We're confident the volcano would give us some warning of a major change," Dzurisin said. "It could be as short as a day. It would more likely be a week or months."

On clear, fall days, St. Helens is likely to put on a nice show for visitors, with a steam plume rising high into the sky. From Johnston Ridge Observatory, about five miles north of the crater, the view is spectacular, Gardner said.

"Go to the volcano," she advised. "It's a once-in-a-lifetime opportunity."

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com