Volcano collapse promoted by hydrothermal alteration and edifice shape, Mount Rainier, Washington

Catastrophic collapses of steep volcano flanks threaten many populated regi ons, and understanding factors that promote collapse could save lives and p roperty. Large collapses of hydrothermally altered parts of Mount Rainier h ave generated far-traveled debris flows; future flows would threaten densel y populated parts of the Puget Sound region. We evaluate edifice collapse h azards at Mount Rainier using a new three-dimensional slope stability metho d incorporating detailed geologic mapping and subsurface geophysical imagin g to determine distributions of strong (fresh) and weak (altered) rock. Qua ntitative three-dimensional slope stall calculations reveal that sizeable f lank collapse (>0.1 km(3)) is promoted by voluminous, weak, hydrothermally altered rock situated high on steep slopes. These conditions exist only on Mount Rainier's upper west slope, consistent with the Holocene debris-flow history. Widespread alteration on lower flanks or concealed in regions of g entle slope high on the edifice does not greatly facilitate collapse. Our q uantitative stability assessment method can also provide useful hazard pred ictions using reconnaissance geologic information and is a potentially rapi d and inexpensive new tool for aiding volcano hazard assessments.