Site of the Mount St Helens Volcanic eruption, as it appeared in 1989. 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 volcanic eruption to occur in the contiguous 48 U.S. states since the 1915 eruption of Lassen Peak in California.[1] However, it has often been declared as the most disastrous volcanic eruption in United States history.