The eruption of Mount St. Helens on May 18, 1980 triggered a huge landslide that caused a powerful laterally directed blast. A pyroclastic flow of rock fragments, highly charged gas, and super heated steam devastated almost 150,000 acres of private, state, and federal forests. In 1982 Congress established the 110,000 acre Mount St. Helens National Volcanic Monument, where no salvage and no tree planting were permitted.