St. Helens' new dome growing rapidly

By Sandi Doughton
Seattle Times staff reporter

WILLIE SCOTT / USGS The new lava dome at Mount St. Helens and the uplifted area beneath it now encompass 70 acres and stand 750 feet high. That's about as big as Seattle Center and as tall as the 55-story Washington Mutual Tower in downtown Seattle.

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In 1980, many Northwesterners watched Mount St. Helens blow its top.

Now, they may have the rare opportunity to see it rebuilt during their lifetime.

If lava continues to pour out at the current rate, geologists say the new dome will be tall enough to be visible from Portland in less than two years. And in little more than a decade, the volcano would be back to the size it was before the cataclysmic blast that turned a snow-covered peak to a blackened shell and shaved 1,313 feet off its peak.

While it's unlikely that today's furious growth will continue that long, it's possible the volcano will erupt sporadically over the next 20 to 50 years or more, regaining its former height gradually, said volcanologist Willie Scott of the U.S. Geological Survey's Cascades Volcano Observatory.

One volcano in Guatemala has been steadily erupting for 80 years.

"Dome building can go on for weeks, months or decades," Scott said.

Since lava first broke the surface in early October, Mount St. Helens has been pumping it out at a prodigious rate: up to a dump truck load per second. The new lava dome and the uplifted area beneath it now encompass 70 acres and stand 750 feet high. That's about as big as Seattle Center and as tall as the 55-story Washington Mutual Tower in downtown Seattle.

As the new dome gets taller, the dangers increase, Scott added.

Unstable slopes can collapse, creating avalanches of hot rock and gas and triggering mud flows. And the bulk of the dome could put pressure on underground magma, raising the risk of explosive eruptions.

"This is a rather unpredictable volcano," he said. "It's very important we monitor the situation closely."
 
The eruption pattern has been significantly different than in the 1980s, the last time the volcano built a lava dome. Then, the molten rock pushed up in short bursts, followed by weeks or months of quiet. This time, the lava just keeps coming, Scott said.

Scientists initially used a bucket suspended from a helicopter to collect samples of the freshly minted rock.

Early this month, they made their first quick forays onto the new dome itself, using hammers to whack off chunks so hot they were difficult to hold.

"You can actually put your hand on some of the youngest rock on Earth up there," Scott said.

Analysis of crystals and minerals in the new rocks suggests that magma is flowing from a chamber four to five miles below the surface, making the trip in a week or less.

That explains why earthquake activity has been reduced to a slow rumble of tiny temblors, coming at a rate of one every minute, said USGS seismologist Seth Moran. The magma seems to be moving smoothly, meeting few obstacles that would lead to major rattling.

But if something interferes with that flow, then pressure could start to build in the system and possibly trigger an explosive eruption. The type of lava coming out of the volcano now closely resembles that produced in 1980, Scott pointed out.

"It's the kind that could drive the large explosive eruptions that St. Helens is well-known for in the past," Scott said.

And the mountain also has a long history of abrupt course changes, from liquid lava flows to sedate dome-building eruptions to spectacular explosions.

"Of the Cascade volcanoes, it certainly has the largest repertoire of things it will do," he said.

The growing dome itself poses several new risks.

"Lava domes are notoriously unstable," Scott said. The larger the dome, the more likely chunks will break off. The resulting avalanches of hot rock could travel several miles from the crater.

More significantly, they would melt the snow and ice in the dome as the weather cools, leading to mud flows, or lahars. The largest of these flows might extend up to 25 miles to a sediment dam on the North Fork of the Toutle River.

If the new lava dome gets significantly larger, it also could pose another hazard because of its mass and the pressure it exerts on underground magma. A partial collapse would release that pressure, allowing magma to rush to the surface and explode.

How well scientists are able to predict what the volcano will do depends on the number and type of instruments available to measure earthquakes and ground motion, said USGS technologist Rick LaHusen.

Over the past month, he and his colleagues designed and deployed seven of what they call spider packs: compact instrument packages painted bright orange and mounted on three sturdy legs.

The color allows the instruments to show up in aerial photographs. The legs provide stability in rugged terrain, and the instruments supply 24-hour readings on ground motion.

The data help to fill in the gaps in monitoring, especially when increasingly bad weather prevents scientists from going out in the field to make first-hand measurements, LaHusen said.

"With these, we can get an idea of what's going on even in the middle of a stormy night."

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