Gigantic debris avalanche of Pleistocene age from ancestral Mount Shasta Volcano, California, and debris-avalanche hazard zonation

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 km*, and their estimated volume is at least 45 km3. Radiometric limiting dates suggest that the debris avalanche occurred between about 300,000 and 380,000 yr ago. Hundreds of mounds, hills, and ridges formed by the avalanche deposits are separated by flat areas that slope generally northward at about 5 m/km. 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 km 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 m 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 progress contemp ively deeper into the volcano. Evidence is lacking of oraneous volcanic activity, and the cause of the landslides is not known. Manuscript approved for publication June 6, 1988. The possible lengths of future volcanic debris avalanches from Mount Shasta, as well as other volcanoes, with volumes of 1 km3 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 km downslope from it, and the value off is 0.075. This value off is based on the average height-to-length ratios of 11 representative volcanic debris avalanches with volumes of at least 1 km3. 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.