Our Mt St Helens Visit
The University of Washington completed their system of seismometers at Mt St Helens, and all along the Cascade volcanos, to measure earthquake activity. The system went online March 1, 1980.
Mt St Helens Eruption Sequence
On March 20, 1980 a 4.0 magnitude earthquake was measured directly beneath Mt. St. Helens.
Three days later another 4.0 was measured, followed by another of the same magnitude that evening. Then over the next few days 4.0 quakes were occurring at a rate of 3 per hour. Several overhead flights were reporting cracks in some of the glaciers near the summit.
In the early afternoon on March 27, while the mountain was shrouded in clouds, a loud explosion was reported coming from the direction of Mt St Helens. As the weather cleared a new crater was reported with a diameter of about 200 feet. Thus began a series of steam eruptions that continued into April and early May. The steam eruptions were as a result of ground water heating above the rapidly rising plug that was being formed in the central conduit of the volcano.
Two large east to west fractures were also observed on the north side of the volcano during the April to May period and a large dome was being formed between the two parallel fractures. The dome was observed expanding at an incredible rate of 6 feet per day, right up until the time of the big explosive eruption. By May 14, the steam eruptions had stopped.
On May 18, 1980 a 5.0 earthquake started a rapid series of events. The entire north slope above the bulge failed and slid down toward Spirit Lake, a beautiful pristine mountain lake. The sudden release of pressure caused by the massive landslide, resulted in a flash to steam of the ground water which initiated the huge lateral blast that followed.
Before the landslide could reach Spirit Lake, the blast would overtake it and go on to devastate a fan-shaped area almost 20 miles wide and 12 miles long. Within the first few miles all the old growth trees were laid down like matchsticks all pointing north. Further out the trees were singed from the 500 to 600 degree temperatures, and died standing. Every living thing died instantly in the fan shaped zone.
The sliding avalanche filled Spirit Lake rising the floor of the lake over 200 feet and doubled the lakes shoreline. It then entered the North Fork of the Toutle River and flowed for 15 miles (one of the longest avalanches ever recorded) to the west filling the valley with rock-filled, gooey deposits.
Within minutes a massive plume of rock, dust, and ash had risen from the crater to a height of 30 miles into the stratosphere. Ash fell for days, covering everything.
This massive natural disaster produced billions in property damage, destroyed billions of board feet of timber, and claimed 57 human lives, as well as the massive loss of wildlife.
The Mt. St. Helens National Volcanic Monument is a well thought out undertaking. The educational literature, well done talks, and incredible viewing areas, plus the amazing natural scenery make the area a well worth while stop. We entered the area from the east road. Visit the Mt St Helens National Volcanic Monument (website)