The 1980 eruption of Mount St. Helens was the most significant to occur in the contiguous 48 U.S. states in recorded history ( VEI = 5 , 0.3 cu mi , 1.2 km3 of material erupted ) , exceeding the destructive power and volume of material released by the 1915 eruption of California 's Lassen Peak . The eruption was preceded by a two-month series of earthquakes and steam-venting episodes caused by an injection of magma at shallow depth below the mountain that created a huge bulge and a fracture system on Mount St. Helens ' north slope . An earthquake at 8:32 am on May 18 , 1980 , caused the entire weakened north face to slide away , suddenly exposing the partly molten , gas - and steam-rich rock in the volcano to lower pressure . The rock responded by exploding into a very hot mix of pulverized lava and older rock that sped toward Spirit Lake so fast that it quickly passed the avalanching north face
1980 eruption of Mount St. Helens
1980 eruption of Mount St. Helens
Ring of Fire