When Washington state’s Mount St. Helens erupted in 1980, the volcano’s northern flank gave way, sending about 2.5 cubic kilometers of material down its slopes and into nearby valleys in what was the largest debris avalanche in recorded history. But roughly 22 million years earlier, one of the largest-known volcanic landslides the world has ever seen occurred in southwestern Utah, according to a study in Geology. That one, estimate the study’s authors, released between 1,700 and 2,000 cubic kilometers of ash, tuff and sandstone — nearly 1,000 times as much as Mount St. Helens — over a 3,400-square-kilometer area.
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