1980 eruption of Mount St. Helens
Build-up to the eruption
Shocks of magnitude 3.2 or greater occurred at a slightly increasing rate during April and May, with five earthquakes of magnitude 4 or above per day in early April, and eight per day the week before May 18. Initially, no direct sign of eruption was seen, but small earthquake-induced avalanches of snow and ice were reported from aerial observations.
Build-up to the eruption
!Mount St. Helens seen from Monitor Ridge. This image shows the cone of devastation, the huge crater open to the north, the post eruption lava dome inside, and [Crater Glacier surrounding the lava dome. The small photo on the left was taken from Spirit Lake before the eruption, and the small photo on the right was taken after the eruption from roughly the same place. Spirit Lake can also be seen in the larger image, as well as Mount Rainier and Mount Adams).](//upload.wikimedia.org/wikipedia/commons/thumb/a/ab/StHelensfromMonitorRidgefeather.jpg/250px-StHelensfromMonitorRidgefeather.jpg)
Build-up to the eruption
At 12:36 pm on March 27, phreatic eruptions (explosions of steam caused by magma suddenly heating groundwater) ejected and smashed rock from within the old summit crater, excavating a new crater 250 feet (75 m) wide, and sending an ash column about 7,000 feet (2.1 km) into the air. By this date, a 16,000-foot-long (4.9 km) eastward-trending fracture system had also developed across the summit area. This was followed by more earthquake swarms and a series of steam explosions that sent ash 10,000 to 11,000 feet (3,000 to 3,400 m) above their vent. Most of this ash fell between 3 and 12 mi (5 and 19 km) from its vent, but some was carried 150 mi (240 km) south to Bend, Oregon, or 285 mi (460 km) east to Spokane, Washington.
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