Forty years ago, after two months of earthquakes and small explosions, Mount St. Helens cataclysmically erupted. A high-speed blast leveled millions of trees and ripped soil from bedrock. The eruption fed a towering plume of ash for more than nine hours, and winds carried the ash hundreds of miles away. Lahars (volcanic mudflows) carried large boulders and logs, which destroyed forests, bridges, roads and buildings.