How Did Mt St Helens Erupt?

Currently, there are fifteen subduction-related volcanoes active along the Cascade chain. Before 1980, the last one to erupt was Mt. Lassen in California, in 1914. In the mid-1970's scientists were concerned that Mt. Baker, in northern Washington, might be the site of the next volcanic eruption, based on increased activity on the volcano. However, in 1978, Dwight Crandell and Don Millineaux of the USGS suggested that Mt. St. Helens was potentially the most likely volcano to erupt in the twentieth century; based their evidence on the relatively young age of the volcano, and its frequent historical eruptions (Explore). Since about 1400 A.D., eruptions have occurred at approximately one every 100 years. Before the 1980 eruption, it had been 130
St. Helens. The volcano was shrouded in clouds, however, a summit eruption was verified by a news team from the Vancouver Columbian. As they circled the summit in an airplane, they spotted a dense column of ash rising through the clouds to a height of about 2000 m. As the weather cleared later in the day, a new crater was visible, with a diameter of about 70 m, and snow on the summit area was covered by a thin layer of dark ash. The summit eruption on March 27 was typical of several small eruptions that would occur through April and early May. None of these eruptions were magmatic in character, but instead they were steam eruptions generated by the heating of groundwater above a rising plug of magma that had invaded the central conduit of the
Helens erupted. Shaken by an earthquake measuring 5.1 on the Richter scale, the north face of the tall symmetrical mountain collapsed in a massive rock debris avalanche. Quickly, the rock and ice slammed into Spirit Lake (on the northeast side of the mountain), crossed a ridge 1,300 feet high, and roared 14 miles down the Toutle River. The avalanche rapidly released the pressurized gases within the volcano. A tremendous lateral explosion ripped through the avalanche and developed into a “turbulent, stone-filled wind” that swept over ridges and toppled trees (Gifford Pinchot). Nearly 150 square miles of forest was decimated or left dead and