<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> <P> An eruption column rose 80,000 feet (24 km; 15 mi) into the atmosphere and deposited ash in 11 U.S. states . At the same time, snow, ice and several entire glaciers on the volcano melted, forming a series of large lahars (volcanic mudslides) that reached as far as the Columbia River, nearly 50 miles (80 km) to the southwest . Less severe outbursts continued into the next day, only to be followed by other large, but not as destructive, eruptions later that year . Thermal energy released during the eruption was equal to 26 megatons . </P>

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