On May 18, 1980, a major
volcanic eruption occurred at Mount St. Helens, a volcano located in the
state of Washington, United States. The eruption (a
VEI 5 event) was the only significant one to occur in the
contiguous 48 U.S. states since the 1915 eruption of
Lassen Peak in California. 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 the mountain's north slope.