Mount Redoubt roared to life Sunday and Monday, blasting a column of ash and steam almost 12 miles above Cook Inlet.
The eruptions -- which started Sunday night, persisted through the early hours Monday, then struck again Monday evening -- canceled commercial airline flights and spurred Alaskans north of Anchorage to protect their cars and homes.
"That's a very high plume," Alaska Volcano Observatory geophysicist John Power said of the largest ash cloud, which rose 60,000 feet high.
"That's about as high as they go."
The latest explosion, which occurred at 7:41 p.m., prompted the National Weather Service to declare a new ash fall advisory for Susitna Valley and the Bristol Bay area to the west, which have since expired.
The AVO's alert level remained at red this morning, with no new explosions.
Earlier Monday, lower-level winds carried Redoubt's abrasive volcanic particles roughly due north over the Susitna Valley, and significant ash fall was reported in Skwentna, Willow, Trapper Creek and Talkeetna, according to the National Weather Service.
Traces of ash were also reported in communities as far-flung as Healy to the north and the village of Nikolai to the west.
Anchorage emerged unscathed, and that should remain true again today, if the volcano erupts once more, said weather service meteorologist Nate Hardin.
"The threat should be even smaller tomorrow," Hardin said Monday. "The (low-level) wind should carry the majority of the particles even further to the west and northwest than it did today."
Higher-elevation winds above 40,000 feet were beginning to veer east toward Anchorage, but the very lightweight particles that could attain that height aren't expected to fall locally, said NWS Anchorage office meteorologist-in-charge Bob Hopkins.
"Eight miles up -- that's going to stay up there," Hopkins said. "But that will affect aircraft at that altitude."
The eruptions west across Cook Inlet from Kenai apparently damaged the "RSO" seismometer on the south flank of the volcano, as well as the AVO webcam inside a hut six miles from the summit, AVO hydrologist Chris Waythomas said.
Two additional seismometers on Redoubt's north and east slopes were nonoperational early Monday, but that was due to a power outage on the Kenai Peninsula, he said.
By midmorning residents in Kenai began reporting a sulfur smell in the air, but no ash had fallen there, officials said.
Redoubt began erupting Sunday night, with the first explosion coming at 10:38 p.m., followed by another at 11:02 p.m., a third at 12:14 a.m. Monday and a fourth at 1:39 a.m., the AVO reported.
Ted Stevens Anchorage International Airport remained open, though some airlines canceled or diverted flights. Alaska Airlines reported canceling 19 flights in and out of Anchorage because of the ash, but other flights were operating.
Elmendorf Air Force Base reported that 60 planes, including fighter jets, cargo aircraft and a Boeing 747 commercial plane, were being sheltered. The base initially ordered only essential personnel to report for duty; that was later changed to a call for everyone to report to work.
Mount Redoubt, a 10,197-foot stratovolcano 100 miles southwest of Anchorage, last erupted during a fourth-month period in 1989-90. Its recent period of volcanic unrest began Jan. 25.
The Weather Service advised people in areas of ash fall to seal windows and doors, protect electronics and cover air intakes and open water supplies as well as minimize driving.
Cook Inlet Pipe Line Co., which operates the Drift River Oil Terminal on the western shore of Cook Inlet downriver from the volcano, said early Monday that it had begun shutting the facility down. Two of seven tanks at the facility are in use and each contains about 74,000 barrels of crude oil.
Previously, the company would not say how much oil was being stored there, citing security concerns.
About 10 Drift River terminal workers -- the entire crew -- were evacuated from the site Monday morning, as flooding from melting ice and snow reached the edge of the terminal, said Waythomas, the AVO hydrologist.
"We had confirmation that water is flowing over the helipad and it's on the runway," he said.
While the water was rising in some places, dikes constructed around the tank farm after the 1989 eruption appeared to be working, Waythomas said. The AVO staff was due to fly to Redoubt Monday afternoon to observe the area firsthand.
Preliminary data indicate the recent heavy snowfall is keeping destructive mud flows in the Drift River Basin to a minimum.
"These flows are really, really watery," Waythomas said. "They're not these big slurrylike mud flows ... They're more waterlike floods."
At an afternoon press conference, Powers, the AVO geophysicist, said more eruptions are possible today, and that Redoubt's previous history suggests the volcano could remain explosive for months.
"Those of us in Anchorage today are lucky," he said, "... that we're not out there sweeping up ash right now."
More than the inconvenience ash fall brings is the danger the highly abrasive particles pose to aircraft, he said.
"You can imagine flying an airplane into a sandblaster -- that's what it's like."
A key objective of the creation of the Alaska Volcano Observatory two decades ago was warning the aviation community about imminent volcanic eruptions.
For two hours prior to Redoubt's first explosion Sunday night, AVO scientists reported heightened seismicity at the volcano and issued warnings that it could escalate to an eruptive stage.
The volcano had been placed on aviation color code orange and alert level "watch" Saturday evening, and AVO staff changed its status to red Sunday night. It remained at red late Monday night.
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What happened when
Eruption began Sunday night The first explosive event occurred about 10:38 p.m.
Continued into Monday Explosions continued that morning, culminating with a 4:31 a.m. eruption that sent an ash cloud 60,000 feet high. Another eruption was reported at 7:41 p.m.
Ash moved north The ash cloud headed north into the Susitna Valley, avoiding the Kenai Peninsula, Anchorage and Wasilla. By afternoon, no additional ash clouds were being reported and the ash advisory for Su Valley was lifted.
Ash hits ground Up to a quarter-inch was reported in Skwentna and a dusting occurred in Trapper Creek, Talkeetna and Denali National Park.
Anchorage No ash fall, but flights in and out of Anchorage were disrupted.