<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 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> 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 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>
when did mount saint helens blow its top
when did mount saint helens blow its top
May 18 , 1980
May 18 , 1980