DESCRIPTION:
Mount Shasta's Debris Avalanche Deposit

Shasta82_hummocks_09-22-82_med.jpg
USGS Photograph taken September 22, 1982, by Harry Glicken.
[medium size]

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.

[Map,26K,InlineGIF]
Inferred minimum extent of Mount Shasta's debris-avalanche deposits in Shasta Valley.
-- Modified from: Crandell, 1989, USGS Bulletin 1861

Debris Avalanches

Debris avalanches are flowing or sliding, incoherent and chaotic, wet or dry mixtures of soil and rock debris that move away from their source at high speed. Volcanic-debris avalanches occur occasionally at large central-vent volcanoes and are among the most hazardous of volcanic events. Such avalances form when part of a volcanic edifice fails catastrophically and moves downslope away from the volcano. Disruption of a volcanic cone may be the result of intrusion of magma and earthquake shaking, as at Mount St. Helens in 1980 (Voigt and others, 1981), or of a volcanic blast as apparently occurred at Bezymianny in Kamchatka, U.S.S.R., in 1956 (Gorshkov, 1959; Bogoyavlenskaya and others, 1985). Steep-sided volcanic cones may also fail due to the influence of gravity after gradual weakening by hydrothermal alteration.

Debris avalanches produce thick, hummocky deposits that can extend as far as several tens of kilometers from a volcano and cover an area of a few tens of square kilometers, as occurred at Mount St. Helens in 1980, to a few hundred square kilometers, as occurred at Mount Shasta between 300,000 and 360,000 yr ago (Crandell and others, 1984). Debris avalanches are most likely at steep-sided volcanoes and, thus, are a potential hazard primarily at Mount Shasta, Lassen Peak, and possibly Mammoth Mountain in Long Valley.

The avalanche from Mount Shasta flowed northwesterly a distance of about 45 km and the deposit covers an area of at least 450 sq.km. -- [webnote: use updated figures from Crandell, 1989, USGS Bulletin 1861, above] -- A late-Pleistocene avalanche at Mammoth Mountain was considerably smaller and traveled less than 6 km from its source (Lipshie, 1976). About 300 yr ago, three rockfall-debris avalanches occurred from domes at the Chaos Crags eruptive center near Lassen Peak (Crandell and others, 1974). The Chaos Crags avalanches traveled as far as 4.3 km from their source areas.

Debris avalanches destroy everything in their paths by impact or burial beneath tens of meters of debris. Because debris avalanches occur with little warning and can travel at high speeds, areas that might be affected should be evacuated before such avalanches occur. Therefore, local government officials might decide to evacuate some areas in advance of threatened eruption.

For more than a century, hundreds of mounds, hills, and ridges of volcanic rocks in Shasta valley, north-central California, had puzzled geologists. In a guidebook of the Western United States, geologist John Diller wrote in 1915 that the hummocks were "in part at least, the products of minor and local eruptions that broke through the Cretaceous beds, each vent contributing its little pile of material." Subsequent interpretations suggest that the hillocks were moraine deposits left by a glacier, or small bodies of magma that had risen from a sill intruded beneath Shasta valley, or erosional remnants of old volcanic rocks of the Western Cascades. Comparison with the Mount St. Helens debris-avalanche deposit, however, clearly establishes that the hills are part of a giant debris-avalanche deposit derived from a volcano ancestral to Mount Shasta, according to a recent report by U.S. Geological Survey geologists Rocky Crandell, C. Dan Miller, Harry Glicken, Christopher Newhall, and Robert Christiansen.

In addition to similarities in surface morphology, the internal structure of both deposits is similar -- the Mount Shasta deposit also consists of block facies and matrix facies. The block facies form hummocks and include individual debris-avalanche blocks ranging in size from tens to hundreds of meters in maximum dimension. Such blocks consist of a single rock type of layers of airfall tephra, pyroclastic flows, lahars, and pieces of soil. These blocks formed part of the former volcanic cone. The debris-avalanche blocks are generally not as shattered as in the Mount St. Helens deposit. Surrounding these debris-avalanche blocks, sometimes forming dikes that penetrate narrow cracks and joints in the blocks, is a mixture of clasts that range in size from microns to meters in diameter -- the matrix facies. Most of the largest clasts in the matrix facies are andesitic rocks, similar to rocks that make up the present Mount Shasta volcano. Other clasts include sedimentary and metamorphic rocks, and rocks derived from alluvial and lake sediments incorporated by the avalanche as it traversed the floor of Shasta Valley. Fossils of lacustrine organisms are also found in the matrix facies at several localities.

The Mount Shasta debris-avalanche deposit covers and area of at least 450 square kilometers with about 26 cubic kilometers of debris -- [webnote: use updated figures from Crandell, 1989, USGS Bulletin 1861, above] -- roughly 10 times the volume of the 1980 Mount St. Helens avalanche deposit. Radiometric ages of rocks in the deposit and of a post-avalanche basalt flow indicate that the avalanche occurred between about 300,000 and 360,000 years ago.

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