9 Summary table
10 See also
11 Notes
12 References
13 External links

Buildup to the eruption

Mount. St. Helens remained dormant from its last period of activity in the 1840s and 1850s until March 1980.[6] Several small earthquakes beginning as early as March 15, 1980, indicated that magma may have been moving below the volcano.[7] Then on March 18 at 3:45 p.m. Pacific Standard Time (all times will be in PST or PDT), a shallow Richter magnitude 4.2 earthquake (the initial reading was 4.1), centered below the volcano's north flank,[7] signaled the volcano's violent return from 123 years of hibernation.[8] A gradually building earthquake swarm saturated area seismographs and started to climax at about noon on March 25, reaching peak levels in the next two days, including an earthquake registering 5.1 on the Richter scale.[9] A total of 174 shocks of magnitude 2.6 or greater were recorded during those two days.[10]
Erupting conical volcano
USGS photo showing a pre-avalanche eruption on April 10.

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 8 per day the week before May 18.[8] Initially there was no direct sign of eruption, but small earthquake-induced avalanches of snow and ice were reported from aerial observations.

At 12:36 p.m. on March 27, at least one but possibly two nearly simultaneous phreatic eruptions (exploding groundwater-derived steam) ejected and smashed rock from within the old summit crater, excavating a new crater 250 feet (76 m) wide[8] and sending an ash column about 7,000 feet (2,100 m) into the air.[10] By this date, a 16,000-foot (4,900 m) long, east-trending fracture system had also developed across the summit area.[11] 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.[8] Most of this ash fell within 3 to 12 miles (5 to 20 km) from its vent, but some was carried as far as 150 miles (240 km) south to Bend, Oregon, and 285 miles (460 km) east to Spokane, Washington.[12]
Side of mountain
Photo showing the cryptodome on April 27.

A second, new crater and a blue flame was observed on March 29.[12][13] The flame was visibly emitted from both craters and was probably created by burning gases. Static electricity generated from ash clouds rolling down the volcano sent out lightning bolts that were up to two miles (3 km) long.[12] Ninety-three separate outbursts were reported on March 30,[12] and increasingly strong harmonic tremors were first detected on April 1, alarming geologists and prompting Governor Dixy Lee Ray to declare a state of emergency on April 3.[14] Governor Ray issued an executive order on April 30 creating a "red zone" around the volcano; anyone caught in this zone without a pass faced a $500 fine or six months in prison.[15] This excluded many cabin owners from visiting their property.




Tankless Hot Water Heater
smart car