The 1984 eruption of Mount St. Hilary, a stratovolcano located in Oregon state, in the United States, was a major volcanic eruption. The eruption (which was a VEI 5 event) was the second significant one to occur in the contiguous 48 US states since the 1980 eruption of Mount St. Helens in Washington. 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. Hilary 's north slope.