On this date, 35 years ago, Mount St. Helens in Washington state erupted, killing 57 people.

Here are a few things you may not have known about the eruption.

One: There was plenty of warning that the mountain was going to erupt.

On March 18, a 4.2 magnitude earthquake was centered beneath the northern flank of the mountain followed by more than 150 small quakes in the next two days.

By May, the north face of the mountain had swelled some 500 feet.

Two: On May 18th at 8:32 a.m. Pacific Time, a 5.1-magnitude earthquake caused the north face of the mountain to give way, which in turn triggered pyroclastic flows and a 15-mile high ash column.

The ash turned day into night in parts of eastern Washington state, depositing 5 inches of ash in Yakima and closing Interstate 90 between Seattle and Spokane for 10 days.

The ash would eventually cover parts of 11 states, as far east as Minnesota and Oklahoma.

Our question: What was the only other major volcanic eruption in the contiguous 48 United States in the 20th century?