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 . More than 1.5 million metric tons of sulfur dioxide were released into the atmosphere. On the Volcanic Explosivity Index scale, the eruption was rated a 5, and categorized as a Plinian eruption.