The Mount Adams ice-and-rock avalanche of August 31, 1997, was noteworthy in at least three respects. (1) Although the avalanche was not large by worldwide geological standards, it was probably the largest on a Cascades volcano (other than Mount St. Helens) since the Little Tahoma avalanche at Mount Rainier in 1963. The Mount Adams avalanche contained about 5 million cubic meters of material, more than 90% of which appeared to consist of snow and ice. (2) The avalanche involved wholesale removal of the upper reaches of a steep glacier and part of the underlying hydrothermally altered rock. The same area has repeatedly spawned similar avalanches. (3) Although the avalanche contained much snow and ice mixed with clay-rich rock debris, an insignificant fraction of the avalanche mobilized into debris flows.
BACKGROUND
The 1997 avalanche originated in the aptly named Avalanche Glacier cirque, which heads at about 12,000 feet elevation on the southwest flank of Mount Adams. The same cirque produced a similar although smaller ice-and-rock avalanche in July, 1983. In 1921 the cirque produced a larger avalanche, which involved about 4 million cubic meters of rock debris (estimated from the deposit geometry) and an unknown amount of snow and ice. About 200 years ago a larger event from the same source area produced the Salt Creek lahar, estimated to have a volume of 15 million cubic meters. About 6000 years ago the Trout lake lahar (volume ~66 million cubic meters) issued from the same general source area, including the cirque of the adjacent White Salmon Glacier. This lahar inundated the Trout Lake lowland, which today is occupied by many people.
Rock exposed in the cirque headwalls above and adjacent to the Avalanche and White Salmon glaciers appears intensively hydrothermally altered and very weak. However, no known measurements of the physical properties of this rock exist. Hildreth and Fierstein (Bull. Volcanology, 1997) estimate that 2-3 cubic kilometers of intensively altered rock exists in Adams' summit block, but chemical analyses of the Mount Adams summit andesite reported by Hildreth and Fierstein reflect the state of the parent rock and not the altered products.
1997 AVALANCHE: NARRATIVE OF EVENTS
Eyewitness accounts and seismic evidence indicate that the 1997 avalanche was actually a series of at least two and perhaps more events spaced over the course of about 1 1/4 days. Eyewitness accounts come from Charles Hyman, a recreational backpacker who was camped at about 6000 ft. elevation near Salt Creek the nights of August 29 and 30. The largest and culminating avalanche occurred at about 6:35 AM PDT on Sunday, August 31, and stopped about 400 m short of Charles Hyman's tent. The account below is synthesized from accounts of Hyman and from photographs taken at various times by Hyman and by Darryl Lloyd, a resident of Glenwood near Mount Adams.
Avalanche activity began sometime Saturday, August 30. Weak seismic signals recorded near Mount St. Helens indicate that the first pulse of activity occurred shortly after midnight. This timing is corroborated in part by Hyman photographs. (Both August 29 and August 30 were characterized by clear skies in the Mount Adams area.) A Hyman photo taken of the southwest face of Mount Adams on August 29 at 8:00 PM shows no evidence of avalanching, whereas a photo taken in nearly the same spot around 8:00 PM August 30 shows a substantial avalanche scar below about 11,600' elevation in the accumulation zone of the Avalanche Glacier and fresh, white avalanche debris at about 7800' elevation along the crest of the neoglacial moraines that enclose the glacier's terminus. However, Hyman neither heard nor observed this avalanche -- perhaps because it occurred while he was asleep or perhaps because he was hiking to the north when it occurred. Anecdotal accounts by various Trout Lake residents place the timing of this avalanche from 10:00 AM to 2:00 PM on August 30, but these same anecdotal accounts maintain that this Saturday avalanche was "the" avalanche, and the accounts may reflect times when residents first noticed a scar on the mountain, rather than the time of avalanche occurrence. (Note that the southwest face of Mt. Adams, where the avalanche occurred, is deeply shadowed in early morning sun.) In any event, there seems to be no compelling evidence to discredit the seismic inference that the first phase of avalanching occurred very early on August 30.
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