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. An earthquake at 8:32:17 a.m. PDT (UTC−7) on Sunday, May 18, 1980, caused the entire weakened north face to slide away creating the largest landslide ever recorded. This suddenly exposed the partly molten, gas- and steam-rich rock in the volcano to lower pressure.