<Tr> <Th> Impact </Th> <Td> Approximately 57 deaths, about $1.1 billion in property damage; caused a partial collapse of the volcano's flank, deposited ash in 11 U.S. states and five Canadian provinces </Td> </Tr> <Tr> <Td_colspan="2"> Map of eruption deposits </Td> </Tr> <P> On May 18, 1980, a major volcanic eruption occurred at Mount St. Helens, a volcano located in Skamania County, in the State of Washington . The eruption (a VEI 5 event) was the most significant volcanic eruption to occur in the contiguous 48 U.S. states since the much smaller 1915 eruption of Lassen Peak in California . It has often been declared as the most disastrous volcanic eruption in U.S. history . 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 large bulge and a fracture system on the mountain's north slope . </P> <P> 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 allowed the partly molten, high - pressure gas - and steam - rich rock in the volcano to suddenly explode northwards toward Spirit Lake in a hot mix of lava and pulverized older rock, overtaking the avalanching face . </P>

When was the last time mount saint helens exploded