Looking back at the Mount St. Helens eruption

Forty years ago, on May 18, 1980, a volcano in Washington state erupted, killing 57 and triggering a debris avalanche that destroyed hundreds of square miles of forest. Correspondent Luke Burbank talks with Steve Olson, author of “Eruption: The Untold Story of Mount St. Helens,” about the largest volcanic event in U.S. history.