when did mt st helens first erupt?
1980 eruption of Mount St. Helens
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 In late fall or early winter of 1842 , nearby settlers and missionaries witnessed the so @-@ called " Great Eruption " . This small @-@ volume outburst created large ash clouds , and mild explosions followed for 15 years . The eruptions of this period were likely phreatic ( steam explosions ) . Josiah Parrish in Champoeg , Oregon witnessed Mount St. Helens in eruption on November 22 , 1842 . Ash from this eruption may have reached The Dalles , Oregon , 48 miles ( 80 km ) southeast of the volcano . 

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 The 1980 eruption of Mount St. Helens in Washington , which ripped apart the volcano 's summit , was a Plinian eruption of Volcanic Explosivity Index ( VEI ) 5 . 

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 On March 20 , 1980 , Mount St. Helens experienced a magnitude 4 @.@ 2 earthquake ; and , on March 27 , steam venting started . By the end of April , the north side of the mountain had started to bulge . On May 18 , a second earthquake , of magnitude 5 @.@ 1 , triggered a massive collapse of the north face of the mountain . It was the largest known debris avalanche in recorded history . The magma in St. Helens burst forth into a large @-@ scale pyroclastic flow that flattened vegetation and buildings over 230 square miles ( 600 km2 ) . More than 1 @.@ 5 million metric tons of sulfur dioxide were released into the atmosphere . On the Volcanic Explosivity Index scale , the eruption was rated a five ( a Plinian eruption ) . 

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 Mount St. Helens , which exhibited phreatic activity just prior to its catastrophic 1980 eruption ( which was itself Plinian ) . 

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 The active Mount St. Helens was radically different from its dormant form , now featuring an enormous bulge and several craters . In the week preceding the eruption , cracks formed in the north sector of the volcano 's summit , indicating a movement of magma from the bulge and towards the caldera . 
