Mt. St. Helens, Washington : initial eruptive events and morphologic changes

International Symposium on the Activity of Oceanic Volcanoes. Ponta Delgada, 4-9 August 1980.After a week of increasing seismic activity, on March 27, 1980, Mt. St. Helens erupted ash and steam to a height of some 2000 m above the new summit crater. This eruption broke a 125-year period of dormancy for this volcano, which had become a popular scenic attraction and center d outdoor recreational activity in the south western corner of the state of Washington. The U.S. Geological Survey hastily called in experts from Denver, Menlo Park, and the Hawaiian volcano observatory, and geologists and volcanologists converged on the mountain from all over the world. Toward the middle d April the frequency of eruptions declined, and local newspapers and television stations confidently reported that the eruptions were over, and that scientists were packing up and leaving. However, the U.S. Geological Survey continued to monitor a bulge on the north flank of the volcano, which grew outward at up to 2 m per day. Small phreatic eruptions took place again from May 7 to May 14, while growth of the bulge continued at a nearly constant rate. On the morning of May 18, a major eruption destroyed the summit and north flank of the mountain. In intensity and volume of material involved, this eruption appears to be equivalent to the 79 a.d. eruption of Vesuvius that destroyed Pompeii. Although the remote location of Mt. St. Helens prevented major loss d life, about 60 people are known dead or remain unaccounted for as a result of this eruption. This note summarizes observations d eruptive activity and morphologic changes in the volcano up to the eruption of May 18. Reference data include both aerial and ground based still photography and sketches, and 16 mm mode film taken from a U.S. Geological Survey observation camp about 15 km northwest d the mountain. Additional background data and details of eruptions and compositions of eruptive products through, August 1980, are given by Hoblitt et al., 1980.