USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Gigantic Debris Avalanche of Pleistocene Age from Ancestral Mount Shasta
Volcano, California, and Debris-Avalanche Hazard Zonation
--
Dwight R. Crandell, 1989,
Gigantic Debris Avalanche of Pleistocene Age from Ancestral Mount Shasta
Volcano, California, and Debris-Avalanche Hazard Zonation:
U.S. Geological Survey Bulletin 1861, 32p.
Abstract
The deposits of an exceptionally large debris avalanche extend from the base of
Mount Shasta volcano northward across the floor of Shasta Valley in northern
California. The debris-avalanche deposits underlie an area of about 675 square
kilometers, and their estimated volume is at least 45 cubic kilometers.
Radiometric limiting dates suggest that the debris avalanche occurred between
about 300,000 and 380,000 years ago. Hundreds of mounds, hills, and ridges
formed by the avalanche deposits are separated by flat areas that slope
generally northward at about 5 meters per kilometer. The hills and ridges are
formed by the block facies of the deposits, which includes masses of andesite
lava tens to hundreds of meters across as well as stratigraphic successions of
unconsolidated deposits of pyroclastic flows, lahars, air-fall tephra, and
alluvium, which were carried intact within the debris avalanche. The northern
terminus of the block facies is near Montague, at a distance of about 49
kilometers from the present summit of the volcano. The flat areas between hills
and ridges are underlain by the matrix facies, which is an unsorted and
unstratified mudflowlike deposit of sand, silt, clay, and rock fragments derived
chiefly from the volcano. Boulders of volcanic rock from Mount Shasta are
scattered along the west side of Shasta Valley and in the part of Shasta Valley
that lies north of Montague, at heights of as much as 100 meters above the
adjacent surface of the debris-avalanche deposits. The boulders represent a lag
that was formed after the main body of the avalanche came to rest, when much of
the still-fluid matrix facies drained away and flowed out of Shasta Valley down
the Shasta River valley and into the Klamath River.
The debris avalanche probably originated in a quick succession of huge
landslides of water-saturated rock on the northwest flank of ancestral Mount
Shasta, each of which cut progressively deeper into the volcano. Evidence is
lacking of contemporaneous volcanic activity, and the cause of the landslides is
not known.
The possible lengths of future volcanic debris avalanches from Mount Shasta, as
well as other volcanoes, with volumes of 1 cubic kilometer or more can be
estimated from the equation L=H/f,
where L is length, H is the altitude
difference between the summit of the volcano and points 20 kilometers downslope
from it, and the value of
f is 0.075. This value of f is based on the average
height-to-length ratios of 11 representative volcanic debris avalanches with
volumes of at least 1 cubic kilometer. For the purpose of hazard assessment, a
safety factor could be added to the calculated value that is commensurate with
local conditions of topography, land use, and population at risk.
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02/03/06, Lyn Topinka