Sudden collapse

William E. Scott

Lee Siebert's article "Blown Away" [10/05] details how volcanoes can collapse. Last year, when Mount St. Helens reawakened, my colleagues and I at the U.S. Geological Survey (USGS) were asked whether the mountain's southern flank was unstable. We concluded that the volcano had been so eviscerated by the avalanche in 1980 that another large collapse was unlikely until the mountain rebuilt itself. Data showed that the current eruption was having little effect on the southern flank, except during a brief time when a new lava dome first reached the crater wall. The lava pushed the flank outward a few centimeters--nowhere near the 1.5 meters that the northern flank deformed each day before the 1980 collapse.

William E. Scott

US. Geological Survey

David A. Johnston Cascades

Volcano Observatory

Vancouver, Washington

Lee Siebert mentions the controversial models of giant tsunamis that a collapse of island volcanoes might generate. We have calibrated our models with data from recent tsunamis caused by smaller, Mount St. Helens-size island-arc volcanoes. We were able to model the main features of an 1888 tsunami, caused by the collapse of the Ritter Island volcano, northeast of New Guinea, by assuming that a landslide traveled underwater at around ninety miles an hour.

Since then, our group has made sonar surveys and seafloor photographs of deposits made by the 1888 landshde, which show that the landshde left tracks similar to the ones left by the Mount St. Helens landshde at Johnston Ridge. Computer simulations of the 1888 landslide also show it must have been moving about ninety miles an hour to leave the traces it left. The agreement between landslide deposit data and tsunami data give us confidence that scaled-up models may predict what could happen when the flank of a volcano the size of La Palma, in the Canary Islands, heads for the ocean floor.

Steven Ward

Simon Day

University of California

at Santa Cruz

Santa Cruz, California

Lee Siebert replies: Public reaction to the current eruption of Mount St. Helens was indeed colored by the catastrophic events of 1980. Data obtained by USGS scientists were important in demonstrating that the present eruption is not likely to lead to a 1980-scale landslide and ensuing lateral blast. Unfortunately, the tide "Blown Away" which the editors of Natural History gave to my article, reinforces the incorrect perception that volcanic explosions caused the destruction of the summit and the northern flank of Mount St. Helens. The dominant process was actually massive landsliding.

The new study of the 1888 collapse contributes to the understanding of the impacts that the collapses of much larger oceanic volcanoes could cause.

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