After nearly two weeks of seismic calm, geologists at the Alaska Volcano Observatory on Tuesday downgraded the volcanic hazard color code for Mount Redoubt from orange to yellow and its alert status from "watch" to "advisory."
While the volcano remains restless with abnormally high gas emissions and melting of the summit glacier, the newly molten rock beneath it "does not show any signs of upward movement at this time," the agency reported.
"We stopped seeing volcanic tremor," said AVO geologist Kristi Wallace. "We still think magma was intruded (upward) to shallow depths, approximately three miles beneath the volcano, but that volume is quite small and perhaps not large enough yet to result in an eruption."
The 10,197-foot volcano 100 miles southwest of Anchorage last erupted over a four-month period from December 1989 through April 1990. Six weeks ago it began to shake and rumble again.
As earthquakes and volcanic tremor suddenly began to spike on Jan. 25, the AVO staff thought it might repeat the pattern of the '89 episode -- when Redoubt escalated from a state of calm to a full eruption in less than 24 hours.
But its tendency this time to hold back and simmer might be more comparable to its second-most recent eruption, which began in 1965 and didn't culminate until 1968.
"I could foresee that we could toggle back and forth between an orange and yellow color code for months," Wallace said. "(Or) things could ramp up again very quickly ... in the order of 24 hours."
Shifting Redoubt from an orange to yellow color code means geologists at the observatory will no longer be monitoring Redoubt every minute of the day, Wallace said. But the staff there will check hourly, and data collection from the volcano will continue.
It also means Redoubt now shares code-yellow status with two other Alaska volcanos -- Cleveland and Okmok in the Aleutian Islands. Cleveland and Okmok both erupted last summer, as did Kasatochi volcano in the western Aleutians.
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By GEORGE BRYSON
gbryson@adn.com