©H.C. Tupper
photo of the southwest side of Mount Adams, with source of the recent avalanche identified. The avalanche sped downhill along the dark track visible below Avalanche Glacier in the lower center of the mountain.

A two-mile-long avalanche of ice and rocks large enough to rattle seismometers has reworked the southwest face of Mount Adams.

The volcano is usually very quiet, with few of the tremors that occur occasionally at other Cascade volcanoes such as Mount Hood. So the seismic signal from Mount Adams on Aug. 1 stood out to Cynthia Gardner of the Cascades Volcano Observatory in Vancouver, who first noticed it.

"It is a very large signal at a volcano that has a very quiet background," she told The Oregonian on Wednesday.

She passed on her observations to researchers at the Pacific Northwest Seismic Network at the University of Washington. There, seismologist Robert Norris recognized it as the likely signature of a major avalanche or rockfall, Norris said.

He relayed the information to Darryl Lloyd, a Hood River photographer and longtime authority on the Mount Adams region. Lloyd reported back that indeed a major avalanche beginning at Avalanche Glacier had tumbled about two miles down the mountain.

The Avalanche Glacier area of the mountain is aptly named, as it has released major avalanches in the past. A rock-and-ice avalanche from the same area in August 1997 ran about three miles down the mountain. It contained an estimated 5 million cubic meters of material and was the largest avalanche in the Cascades since one on Mount Rainier in 1963, according to the Cascades Volcano Observatory.

The avalanche this month was not that big, Lloyd said. It was more akin to a 1983 avalanche from the same area.

Norris said one line of thinking for how the avalanches occur is that ice and snow piling up on the mountain eventually overwhelms the strength of the loose volcanic rock below it and both come tumbling down. All three of the known large avalanches from this area on Mount Adams have occurred in a summer following a winter of especially heavy snow, Norris and Lloyd noted.

"That weak substrate can only support so much ice," he said.

In the past avalanches, a large part of Avalanche Glacier has tumbled down, Norris said. Lloyd said it's not clear how much of the glacier remains.

Such avalanches are a major force in reshaping Cascade volcanoes between eruptions, Gardner said.