PORTLAND, Ore. (KOIN) — The signs had been apparent since the early spring of 1980. Mount St. Helens, the most active volcano in the Cascade Range, awakened on March 16, 1980 with a series of small earthquakes.
Hundreds more earthquakes followed and on March 27, steam explosions blasted a crater through the volcano’s summit ice cap. By May 17, more than 10,000 earthquakes shook the volcano and the north flank.
Then it happened.
At 8:32 a.m., Sunday, May 18, 1980, a magnitude 5.1 earthquake hit. Within 20 seconds, the volcano’s bulge and summit slid away in a huge landslide — the largest on Earth in recorded history. The landslide depressurized the volcano’s magma system, triggering powerful explosions that ripped through the sliding debris.
Scientists estimate that the eruption reached its peak between 3:00 and 5:00 p.m. Over the course of the day, prevailing winds blew 520 million tons of ash eastward across the United States and caused complete darkness in Spokane, Washington, 250 miles from the volcano.