Set off by a 5.1-magnitude earthquake, Mount St. Helens´s violent eruption on May 18, 1980, launched landslides, lava and smoke. The landslide left an opening through which 540 million tons of bottled-up ash exploded across 22,000 square miles east of the volcano. Flowing lava quickly followed, destroying almost all the wildlife in the 230-square-mile forest surrounding the 8,300-plus-foot summit. Typical of a stratovolcano, Mount St. Helens´s thick magma walls trap expanding volcanic gases within the central vent, creating enormous internal pressure. When the stiff magma is disrupted by seismic activity or can no longer contain the gases, the volcano can suddenly explode. Despite recent rumblings, Mount St. Helens hasn´t celebrated the 25th anniversary of its famed eruption with a reprise of the powerful fireworks-yet.