what happened after the mt st helens eruption
An earthquake which caused the entire weakened north face to slide away, creating the largest landslide recorded
1980 eruption of Mount St. Helens
An earthquake at 8:32:11 a.m. PDT (UTC−7) on Sunday, May 18, 1980, caused the entire weakened north face to slide away, creating the largest landslide recorded. 
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1980 eruption of Mount St. Helens [SEP] Introduction
Less severe outbursts continued into the next day, only to be followed by other large, but not as destructive, eruptions later that year. Thermal energy released during the eruption was equal to 26 megatons of TNT.