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

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