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Restless volcano signals eruptions to come, seismologists say

George Bryson

On the surface, all was relatively calm at Redoubt volcano Tuesday in the wake of six hugely explosive eruptions the previous day and night.

Earthquakes and volcanic tremor had fallen off sharply. A stately, white plume of ash-free steam twisted leisurely into the sky. Flood waters from super-heated rock and gas that raced down the icy mountain only hours earlier had finally settled down.

But scientists at the Alaska Volcano Observatory back in Anchorage are not fooled. They're almost certain that another big eruption at Redoubt is on its way -- within days -- and AVO geophysicist Stephanie Prejean can explain why.

One reason is its history. When the volcano about 100 miles southwest of Anchorage last erupted over a four-month span nearly 20 years ago -- from December 1989 to April 1990 -- it fell into a steady, telltale pattern that volcanologists call "dome-building."

Unlike volcanoes in Hawaii that release liquid-like lava that slides down the side of the mountain, most volcanoes in Alaska contain a more viscous form of magma that barely oozes out the top before it hardens into a stopper-like cap. (Molten rock inside a volcano is called "magma;" molten rock outside a volcano is called "lava.")

Two things usually happen next. One: Pressure from stoppered-up heat and gas inside the volcano builds, testing the dome. Two: The brittle dome collapses -- then gets blown sky-high by the venting volcano.

"That's what we saw yesterday (when Redoubt erupted for the sixth time in less than 24 hours), and it's likely continuing," Prejean said of Monday's last eruption. "The seismicity last night and this morning is consistent with this continual dome growth at the volcano."

As a geologist whose specialty is seismology, Prejean pays close attention to the "sounds" volcanoes make. Monday night, about an hour before Redoubt erupted, seismometers positioned downslope from Redoubt's summit began to transmit back to Anchorage the very rhythmic drumbeat that dome-building volcanos make. What Prejean describes as "a 'push ... push ... push.' "

Now she and other AVO seismologists were hearing a muted echo of that signal again on Tuesday.

As it so happened Monday night -- just before the drumbeat began -- AVO geologist Kristi Wallace and two colleagues were still standing near the side of the volcano.

There on an AVO mission, they'd been analyzing the damage, measuring the ash fall, sampling the rock, repairing battered seismometers and webcams. When they were done, they departed in their chartered helicopter on a return flight to Kenai a little after 6 p.m. -- a mere hour and a half before Redoubt erupted again.

What they observed during that first up-close, post-eruption glimpse of the volcano -- detailed by Wallace Tuesday at an AVO press conference -- sounded almost cinematic.

A new Niagara-like waterfall was cascading into a yawning sinkhole in the Drift Glacier. Large blocks of glacial ice the size of small cars, carried away by powerful mud flows, were stuck high and dry in stalled-out mud.

The flood that earlier roared unseen down the Drift River basin left a high-water mark 20 to 25 feet higher than normal. In the valley were "hundreds" of mature but toppled trees. Others had been stripped of their bark by the sand-blasting effect of the ash-laden floodwater.

Though the flood had clearly subsided, there was still "a huge flow of muddy water cascading down the middle of the Drift Glacier cutting a deep channel," Wallace said. Perhaps most impressive of all, however, was how much of the glacier was still intact and waiting to be turned into more floodwater with the next hot eruption. Both Wallace and Prejean think a lot of it may have melted away Monday night.

That's because volcanoes that build lava domes -- then blast them into the air -- also generate something that geologists call "pyroclastic flows," a powerful mixture of white-hot gas and rock that rains back down on the mountain and runs down the side at high speed.

"Think of rock debris that is about 600 degrees centigrade avalanching down the side of the mountain," says AVO geophysicist John Power. "It's very fast and very deadly."

During the '89-'90 Redoubt eruption, pyroclastic flows triggered by all the dome building totally devoured the Drift Glacier, generating, fast, super-heated mud flows called "lahars" that melted more ice and snow. The flood that ensued, AVO geologists say, temporarily turned the normally mild Drift River into the most powerful river in America.

But the dome-collapse explosions that accompanied Redoubt's last eruptive phase 20 years ago didn't just happen once or twice. There were more than 20 explosive events during that four-month span, Prejean said. Now she won't be surprised if Redoubt erupts again this week. And maybe again and again.

Find George Bryson online at adn.com/contact/gbryson or call 257-4318.


By GEORGE BRYSON
gbryson@adn.com