There was a lot of excitement in Mt. Saint Helens, Washington. Geologists were monitoring the Mount Saint Helens volcano. Seismicity began several days before March 20, 1980, when an earthquake of 4.2 centered under the volcano commanded wide attention. The first of a series of small phreatic explosions occurred on March 27, accompanying the opening of a crater northward at the summit of the cone. Strong seismicity continued, at times bursts of deep volcanic tremor was felt statewide in early April but died away without returning. By mid-April a bulge was obvious on the north flank of the volcano. Seismicity continued into May, with fewer but larger earthquakes, and phreatic activity was intermittent.5
An earthquake registering 5.1 on the Richter scale occurred on May 18, 1980 at 8:32 A.M. This struck beneath the Mt. Saint Helens volcano. Thirteen minutes later Mt. Saint Helens erupted. Within seconds of the earthquake, the volcano's bulging north flank slid away in the largest landslide recorded in history.6
The landslide removed confining pressure on the cryptodome and its surrounding hydrothermal system. Hot gas was rapidly released from the cryptodome7 triggering a destructive, lethal lateral blast of hot gas, steam, and rock debris that swept across the landscape at fast as 1,100 kilometers per hour. Temperatures within the blast reached as high as 300 degrees Celsius. Snow and ice on the volcano melted, forming torrents of water and rock debris that swept river valleys leading from the volcano. Within minutes, a massive plume of ash thrust 10 kilometers into the sky, where the prevailing wind carried about 490 tons of ash across 57,000 square miles of the western United States.8
The eruption of May 18, 1980, was caused by a sudden collapse and sliding of the north slope of the volcano. This released the pressure on...