Mount St. Helens On March 20, 1980, Mount St. Helens experienced a magnitude 4.2 earthquake and, on March 27, steam venting started. By the end of April, the north side of the mountain had started to bulge. On May 18, a second earthquake, of magnitude 5.1, triggered a massive collapse of the north face of the mountain. It was the largest known debris avalanche in recorded history. The magma in St. Helens burst forth into a large-scale pyroclastic flow that flattened vegetation and buildings over 230 square miles (600 km2). More than 1.5 million metric tons of sulfur dioxide was released into the atmosphere. On the Volcanic Explosivity Index scale, the eruption was rated a five, and categorized as a Plinian eruption.