Mt st Helen's eruption By NicolasOrtiz125 Mar 16, 1980 First sign of activity Mar 24, 1980 20 earthquakes in an hour were recorded at the mountain. Mar 27, 1980 Its first eruption in over 100 years. Mar 28, 1980 Twelve more eruptions Mar 30, 1980 Ninety-three explosions in one day Apr 1, 1980 Plumes of steam and ash reached 20,000 feet Apr 3, 1980 The crater grew to about 1,300 ft in diameter