when did mt. st. helens erupt last
acquired October 28, 2008 download large image (809 KB, JPEG, 1440x960) When Mount St. Helens erupted on May 18, 1980, the north face of the mountain collapsed, and a massive avalanche of rock, mud, and volcanic debris thundered down the mountain. The description of the event on the Mount St. Helens National Monument Website reads, âNearly 230 square miles of forest [were] blown down or buried beneath volcanic deposits.