by Carolyn Gramling Thursday, January 5, 2012
After a series of five explosive eruptions from Sunday night through Monday morning, Alaska's Redoubt volcano quieted for about 15 hours Monday afternoon — long enough for scientists from the U.S. Geological Survey's Alaska Volcano Observatory to travel to the volcano to make observations and repair equipment (including the Redoubt webcam). A sixth explosive eruption followed Monday night at about 7:40 p.m.
Following the initial explosive onset, the volcano "entered a phase of dome-building," during which the magma rises to the surface, said research geophysicist Stephanie Prejean at a press conference today. Although the weather has prevented direct observation of the dome, "the seismicity indicates the dome is there," Prejean said. During Redoubt's 1989-1990 eruption, the volcano went through a series of about 20 similar dome-building periods. Typically, after building up for some time, the dome will become unstable and fail; in Redoubt's case, oozing lava that is "very thick, like toothpaste — more like a pile of rocks" rather than a very fluid flow, she said. Current seismic data indicate that this dome growth pattern will likely continue, she added. "It's a fairly new field of volcano seismology, to understand dome-building eruptions."
While the volcano was relatively quiet Monday, scientists observed the volcano's impact during a helicopter excursion over Cook Inlet and across the Drift Glacier Valley. Watery mudflows and lahars had completely inundated the glacier valley, research geologist Kristi Wallace said. The scientists observed hundreds of fallen trees in the valley, while others still standing were stripped of bark and limbs by the abrasive, muddy flows and suggesting a high water mark of about eight meters in the valley. A huge flow of muddy water was carving a deep channel in the glacier, producing some "really impressive waterfalls," Wallace said. Large blocks of ice were also observed in the mudflows throughout the valley, indicating that the glacier had been seriously disturbed, she said.
The scientists landed on a high ridge north of the volcano to repair two seismic stations and to collect volcanic ashfall for analysis. In addition to coarse ash particles up to about eight centimeters across, they saw pumice, a conclusive indicator of a magmatic eruption, Wallace said. Samples collected during Redoubt's steam explosion on March 15, by contrast, showed no indication that magma had yet reached the surface; the samples were just fragments of rock from beneath the glacier. Pumice, however, is indicative of new magma, she said.
Although two seismometers were repaired, the eruption claimed a third as well as a Redoubt webcam, which has since been repaired. AVO currently has eight seismometers taking measurements on the volcano, Prejean said. The mudflows also threatened the Drift River Oil Terminal, at the mouth of Cook Inlet, which is currently shut down and evacuated due to the threat. Although the mudflows inundated the nearby hangar and runways, the oil tanks are still intact, and no release of oil or fumes have been detected, said Sarah Francis of the U.S. Coast Guard at the press conference. Two of the terminal's seven tanks are operational, containing 74,000 barrels of crude oil each.
The AVO team will continue to be on watch around the clock to monitor the volcano, Prejean said — but providing early warnings of an impending eruption can be difficult for Redoubt, due to its relatively quiet seismicity, Prejean said. "Sometimes we see less than we hope to," she said. Although many volcanoes provide ample earthquake warning, much of the seismic activity observed at Redoubt prior to Friday indicated only shallow sources — such as hydrothermal fluids — rather than deep, magmatically induced earthquakes. But by Friday, the seismic activity had turned more ominous, prompting officials to raise the aviation color code — which warns pilots of potential ash hazards from a volcano — from yellow to orange, indicating an impending eruption. On Sunday, as the eruption began, the color was raised all the way to red.
Although Redoubt is not currently erupting, its seismic activity is still elevated, according to AVO. However, with no ash emissions since Monday night, the National Weather Service's ashfall advisory is no longer in effect. Meanwhile, AVO is gearing up for an observation and gas measurement flight to obtain more information about the volcano's status.
The first, at about 8:34 a.m., shot clouds of ash about 10,000 meters into the air, and the second, at about 9:24 a.m., sent clouds of ash nearly 22,000 meters into the air.
As expected, there was no short-term seismic warning before the latest eruptions, said research geophysicist Sarah Prejean of the Alaska Volcano Observatory at a press conference Thursday. "On Sunday, we saw seismic signs that something was pushing toward the surface, but at this point we have a wide open system," Prejean said. "We don’t expect to see warnings." These eruptions don't appear to be related to a collapsing dome, she said, but are probably from a deeper magma source.
Ash clouds from the eruptions are headed toward the Kenai Peninsula, but no ash is expected to land on Anchorage. The National Weather Service has ashfall advisories for the region.
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