Anchorage Daily News

Update: The Alaska Volcano Observatory reported Thursday morning that seismic activity at Mount Redoubt gradually calmed overnight. The alert level remains at "watch/orange."

The frequency and intensity of tremors increased markedly late Tuesday night, a possible sign of magma moving in, Alaska Volcano Observatory geologist Allison Payne said. The volcano was experiencing near-continuous small earthquakes near its summit, and their intensity nearly doubled.

Though there was a slight decline in seismic activity Wednesday afternoon, activity was still elevated and an explosive eruption was looking likely in coming days or weeks, Payne said. The only question was when.

"It's kind of like the Nenana Ice Classic here," said Chris Waythomas, an AVO hydrologist. "Who's got the best estimate?"

During the burst Tuesday night, intense steam emissions, rising to about 18,000 feet, were briefly joined with "vigorous" ash emissions that capped off at about 12,000 feet, scientists said. Small amounts of ash were being detected on satellites overhead.

On Wednesday morning, a couple of secondary steam plumes were spotted in the drainage below the dome.

"It's a more active volcano than it was a few weeks ago," said Tina Neal, an AVO geologist.

Hot lava blocks have been shedding from the volcano's new lava dome, estimated to be about 850 million cubic feet to 1 billion cubic feet in volume -- about 10 times the volume of the Great Pyramid of Giza.

Payne said the dome has been slowly growing but that it is unstable. Pressure building underneath, either from viscous magma pushing up or a blocked vent, could make it explode.

"It's just really fresh, young lava that's very unstable," Payne said. "It's come out from the interior of the volcano where it's very hot, out to the fresh surface and the air, so it's thermally fractured, we would say. It's just all broken up from the change in pressure and temperature."

Ash drift models on Wednesday were suggesting that ash would blow in a southerly direction, away from population centers.

Redoubt began erupting in late March and last blew its top April 4, sending an ash plume more than 8 miles into the air. It also sent a massive mudflow, or lahar, on an hour-and-a-half run down the Drift River valley to Cook Inlet. The flow, about 30 feet deep, extended across the entire valley, topping trees and carrying with it boulders of glacier ice estimated at up to 50 tons, Waythomas said.

Lahars are caused when hot gas and rock melt snow and ice. Despite two previous lahars in this eruption, Waythomas said he thinks there's still plenty of ice -- perhaps 20 or 30 percent of what was there to begin with -- to rush the valley in another explosion. Water could also be stored in the crater, which could contribute to a torrent, he said.

"I don't think we're out of the woods yet with the lahar hazard," Waythomas said. "I think we'll see another lahar, and it could be as big or possibly bigger than any of the ones we've seen so far."

The biggest threat is to the Drift River oil terminal, which sits in a valley about 22 miles downriver from Redoubt. About 6.2 million gallons of crude were in two active tanks when the volcano first exploded March 22, but that has since been reduced to about 842,000 gallons, according to the U.S. Coast Guard.

Scientists planned an observation mission to Redoubt today. They intend to watch it for some time, regardless of whether it blows -- although most thought it would in coming days or weeks, Payne said.

"A lot of people do think that it looks like we're getting closer," she said. "But really, there's still a lot of unknowns out there. Some people think that this lava could just continue to extrude slowly, you know, this lava dome could just continue to grow gently, without eruptive events. So it's anybody's guess, I suppose."

Find James Halpin online at adn.com/contact/jhalpin or call him at 257-4589.