Mount St. Helens' 1980 eruption was related to complex interactions between the Pacific Plate, the North American Plate and the much smaller Juan de Fuca Plate. It was the first major eruption to take place in the 48 contiguous U.S. states since Mount Lassen erupted in 1915.
Mount St. Helens is part of the Pacific Ring of Fire, a series of volcanoes encircling the Pacific Ocean. The Cascade Volcanic Arc segment of the Ring of Fire has over 160 active volcanoes and also includes Mount Ranier, considered one of the United States' most dangerous volcanoes. Mount St. Helens is still active as of 2014 and continues to be monitored by volcanologists.