Mount Redoubt released a plume of steam and ash that rose about three miles above sea level Sunday afternoon, and geologists at the Alaska Volcano Observatory immediately upgraded volcano's aviation color code status to orange and its alert status to "watch."
Ashfall from the explosion could be seen on the upper south flank of the volcano, but the National Weather Service had yet to detect any airborne ash that might create an aviation hazard, AVO chief scientist Tom Murray said.
It was the most explosive episode that's occurred at Redoubt since its current state of unrest began in late January, Murray said, but his staff was not yet describing the event as an actual eruption.
The ash didn't appear to be generated by new magma, Murray said. It may have been ash residue from previous eruptions that was sent airborne by the explosion of steam.
"It was visible from Kenai, and it put some ash up in the air, but it's not the thing ... that could have a big impact on all of Cook Inlet."
A team from the observatory flew to Mount Redoubt after the explosion was detected around 1 p.m. and observed steam still issuing from the volcano by mid-afternoon.
"It went on for about four hours, then things gradually declined," AVO geologist Chris Waythomas reported Sunday evening. "Now we're getting these discrete earthquakes with a little bit of tremor."
"We're continuing to watch closely," Murray said. "The situation could evolve quickly."
If weather permits, an AVO crew will return to Redoubt in a helicopter on Monday and examine the nature of the volcanic ash that settled on its slopes, Waythomas said.
Mount Redoubt is a 10,197-foot stratovolcano based on the west side of Cook Inlet, about 100 miles southwest of Anchorage.
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By GEORGE BRYSON