After several months of seismic buildup, Mount St. Helens’ eruption on May 18, 1980 may have been among the most anticipated in modern history. But the size and force of the blast, the landslide and mudflow from superheated snow were far beyond expectations.
A magnitude 5.1 earthquake below the volcano triggered a blast that blew out the top 1,300 feet of the volcano, pushing some 3.7 billion cubic yards of debris to the north and west. Temperatures in the blast zone reached an estimated 600 degrees Fahrenheit. Trees were snapped off and laid down like dominoes. An avalanche of debris rushed down the North Toutle River, raising it as much as 600 feet in some spots, wiping out bridges and burying roads. Fifty-seven people died.
A cloud of ash rose an estimated 15 miles into the atmosphere in less than 15 minutes, and was pushed east-northeast by the winds. In two hours it reached Eastern Washington and North Idaho and by the next morning it had spread across Wyoming, Colorado and the Dakotas, the western edge of Minnesota and the northern edge of New Mexico.