Mount St. Helens has erupted numerous times in its 300,000-year history, most notably on May 18, 1980. While much of its early history remains a mystery, geologic studies have concluded that Mount St. Helens is the most active volcano in the Pacific Northwest.
In determining the activity of Mount St. Helens, geologists study ash layers left behind by eruptions throughout its history. The resulting volcanic deposits are then sorted by relative age and broken down into stages marked by periods of dormancy. While the 1980 eruption was devastating, the largest eruption of Mount St. Helens was around the year 1850 B.C., which was four times as powerful as the eruption in 1980.