Mt.Saint Helen's eruption was a devastating event the blow was so big that it killed everything within a 100 miles, and covered Washington in ash. Where it happened was in Seattle Washington the explosion was in May 18, 1980. The ash cloud was so big that the ash was all the way past central Colorado before the beautiful mountain exploded into ash. The eruption of Mt St Helen's in 1980 was a result of major earth movements and massively affected the earth and the people in the area.
What caused it? Magma caused this deadly eruption magma started to come into the mountains around late winter and early spring of 1980 by may 18 the bulge had reached its instability point. Something else caused this eruption, two tectonic plates were moving away from each other. That is how Mount St Helen's erupted.
Effect on earth The earth was affected by the series of small earthquakes that came before the deadly eruption. there was also a big earthquake that tore the north side of the mountain off it was the biggest landslide recorded on earth that was how big it was.
Affect on people The affect on people's lives were hard to get through when the mountain erupted Seattle Washington was covered in ash which caused people to wear masks so they would not breath any ash to their lungs. The eruption was so big it killed everything within 100 miles on the mountain.
That was all about the Mount Saint Helen's eruption of May 18, 1980. Also, Mt St Helen is a beautiful place for sightseeing.