A 30-Year Memory: A Different Volcano

May 18, 1980 - 8:32 am
Mt. St. Helens Eruption

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 volcano that created a huge bulge and a fracture system on Mount St. Helens' north slope. An earthquake at 8:32:17 a.m. on Sunday, 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 a hot mix of lava and pulverized older rock toward Spirit Lake so quickly that it overtook the avalanching north face.

--Wikipedia