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March 15-19
A number of very small earthquakes are recorded, but are not recognized as immediate precursors to possible volcanic activity. -
March 20
A magnitude 4.1 earthquake, unlike any that had been previously detected in the area, occurred just northwest of the summit of Mount St. Helens. Seismologists were uncertain as to whether or not these first earthquakes were related to volcanic activity. They decided to deploy additional seismometers in order to better monitor future activity. -
March 27 to April 18
Earthquakes and steam-driven explosions occur off and on during this period. -
April 29
State officials asked the governor to close a large area around the volcano. The plan called for a Red Zone (no public access) and a Blue Zone (restricted access). Emergency services officials are frustrated because the public appeared to remain unaware of the da -
May 7-13
Small explosions of steam and ash are emitted from the volcano. Intermittent earthquakes up to magnitude 4.9. -
May 17
Law enforcement officials escorted about 50 carloads of property owners into the Red Zone to retrieve possessions. -
May 18
Here is the sequence of events that occurred during the May 18 eruption of Mount St. Helens: * A 5.1-magnitude earthquake * The bulge and surrounding area on the north face of the volcano slid away, resulting in a huge avalanche of rocks, mud, and debris the filled 24 square miles of valley * The resulting release of pressure from within the volcano released a plume of ash and pumice * Mount St. Helens was reduced by over 1,300 feet in height * Volcanic ash fell as far as 93