The landslide and eruption of Mount St. Helens on May 18, 1980, demonstrated how a volcano can collapse catastrophically, fill an adjacent river valley with rocks and debris, and generate a destructive lahar. With this unforgettable example in mind, scientists from around the world have taken a new look at the geologic record of other volcanoes in hopes of finding similar landslide deposits. They've found several hundred deposits, some more than 10 times larger than the 1980 landslide at Mount St. Helens!

It is clear from these new discoveries that volcano landslides occur far more frequently than had been recognized by scientists before the 1980 eruption of Mount St. Helens. More than 20 landslides >0.1 km³ have occurred in the past 500 years, most of them associated with either magmatic or phreatic explosive eruptions. Landslides are now widely considered as an important volcano hazard that must be taken into account when a hazard assessment is prepared.

Discovery of so many landslide deposits was made possible because of three characteristics that geologists can easily recognize in the field:

• hummocks or mounds that form the surface of many landslide deposits
• shattered but otherwise well-preserved volcanic rock layers within the deposit
• lahar deposits rich in hydrothermally-altered silt and clay particles and volcanic rocks downstream from a volcano

Landslide hummocks