| As Mount St. Helens illustrated so dramatically on May 18, 1980, debris avalanches from volcanoes pose significant hazards to people and property. Debris avalanches may occur without warning, move great distances at high speed, cover large areas, initiate later blasts, and, if they enter the sea, cause tsunamis. The May 18 eruption was the first time eye-witness accounts and photographs documented the emplacement of a large volcanic debris avalanche. The debris-avalanche deposit at Mount St. Helens has provided a basis for interpretation of similar deposits elsewhere and has led to the realization that large-scale gravitational slope failures of volcanoes are more common than previously thought. Since 1980, volcanic hazard assessments have included consideration of hazards posed by debris avalanches in addition to other, more common products of eruptions, such as pyroclastic flows, lahars, lava flows, and tephra. -- Brantley and Glicken, 1986 |
