USGS/Cascades Volcano Observatory, Vancouver, Washington
Mount Adams, Washington:
August 31, 1997 Debris Avalanche -
Information courtesy of:
Richard Iverson, Hydrologist, USGS/CVO,
September 12, 1997
Between about 10:00 AM and 2 PM (PDT), August 30, 1997
-- (Web note: date revised - see revised observations) -- ,
avalanche of snow, ice, and rock debris
descended the southwest flank of
Mount Adams, Washington. No known direct observations of the avalanche
exist. This synopsis of the event is preliminary and is based on one
hour of aerial reconnaissance on September 8 and one person-day of
on-the-ground field reconnaissance September 11, supplemented by reports
from local residents.
The avalanche headed at about 12,000 feet elevation in the cirque of the
Avalanche Glacier on Mount Adams' southwest flank.
It descended to about
6,300 feet elevation and traveled a maximum horizontal distance of about
3 miles. Most debris ponded behind glacial moraines at about 7400 feet
elevation, but perhaps one tenth of the debris overtopped moraines and
funneled down the valleys of Salt Creek and Cascade Creek.
No part of the avalanche reached the Round-the-Mountain hiking trail.
However, a small debris flow discharged from the toe of the avalanche
in one location, traveled down the east fork of Cascade Creek to about
6000 feet elevation, and caused minor damage to the Round-the-Mountain
trail. Streamflow observed below the avalanche on September 11 did not
Total volume of the avalanche is probably between 1 million and 10
million cubic meters. A more precise volume estimate will require more
detailed work than has been possible to date. Most of the avalanche
volume appears to consist of snow and glacial ice, although most of the
avalanche deposit is veneered by muddy rock debris. The deposit surface
is quite unstable -- much like a debris-covered glacier terminus -- and
shifting of debris and ice is common.
A continuing hazard exists in the immediate vicinity of the avalanche
deposit in the area bounded by Crofton Ridge to the southeast and
Stagman Ridge to the northwest. In this area minor shifting of
avalanche debris can be expected to occur, and sporadic rockfall will
likely continue from the avalanche source area and deposit. Hazards are
greatest along the channels of Salt Creek and Cascade Creek at
elevations above 6000 feet and are less at lower elevations and away
from stream channels.
No obvious trigger, such as an earthquake or volcanic activity,
precipitated the avalanche. Instead, the avalanche is likely the result
of long-term weakening of volcanic rocks and perhaps of a smaller
precursory avalanche that involved only snow and ice. The rock exposed
in the cirque of the Avalanche Glacier, where the avalanche originated,
appears intensively altered by hydrothermal activity.
Rock debris exposed on the avalanche deposit is dominated by
fine-grained, clay-rich material.
Few fragments of solid rock larger than 1 meter exist in the deposit.
Stratigraphic and cross-cutting relationships between deposited snow,
ice, and rock debris suggest that the first pulse of the avalanche may
have consisted mostly of snow and that a subsequent, larger pulse of
glacial ice followed by broken, clay-rich rock arrived later. More work
is needed to test this hypothesis.
The avalanche followed nearly the same path as a larger
rock avalanche that occurred in 1921
and of still larger prehistoric avalanches.
Recurring avalanches in this area result from the large volume of weak,
altered rock in the areas occupied by the Avalanche and White Salmon
Glaciers. Refer to
USGS Open-File Report 95-492, "Volcano Hazards in
the Mount Adams Region, Washington"
for more information on past
avalanche and volcanic activity at Mount Adams.
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