On May 18, 1980, a 5.1 magnitude earthquake shook Mount St. Helens. The bulge and surrounding area slid away in a gigantic rockslide and debris avalanche, releasing pressure and triggering a major pumice and ash eruption of the volcano. Thirteen hundred feet of the peak collapsed. As a result, 24 square miles of valley were filled by a debris avalanche; 250 square miles of recreation, timber, and private lands were damaged by a lateral blast; and an estimated 200 million cubic yards of materials were deposited into the river channels.

Photo by: Austin Post/U.S. Geological Survey