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Debris Avalanche Hazards at Three Sisters

South, Middle, and North Sister, as well as Broken Top are high, steep-sided peaks that could produce debris avalanches.

Remnant of Villard Glacier is in cirque below. Thick summit lava flow makes up "Glisan Pinnacle," which overlies hundreds of thin rubbly lava flows that are intercalated with red-oxidized agglutinate and scoria falls exposed on 800-m- (2600-ft-) high face below. Flows that form ridge at upper right are thicker than those on most of edifice. Ash-rich phreatomagmatic fallout layers in lowest 150 m (500 m) are pervasively altered to yellow-orange clay-like material and riddled with dikes, many of which also cut through overlying stack of thin lava flows.

Avalanches of modest volume (less than about 10 million cubic meters – football stadium size) are the most probable and would affect areas primarily close to the volcanoes. Nevertheless, even modest-sized avalanches that contain sufficient water could transform into lahars that travel ten or more kilometers (six or more miles) down valleys. Similar to what occurred at Mount St. Helens in 1980, very large avalanches, those involving hundreds of millions of cubic meters of rock debris, would likely be preceded by pronounced changes in volcanic activity such as increased earthquake frequency, deformation of the volcano, and thus would give advanced warning.