How Volcanoes Work

 Image Described in Caption
Mt. St. Helens Pyroclastic Flow -- Subsequent, smaller eruptions followed the climactic eruption of Mt. St. Helens on May 18, 1980. The eruption shown here occurred on July 22, 1980. The pyroclastic flow shown in the foreground is advancing down the north flank of the volcano. Whereas some pyroclastic flows are generated by collapse of growing lava domes, the pyroclastic flow shown here was generated by partial collapse of the vertical eruption column. The vertical eruptive column was a sustained event that increased in intensity over a period of many seconds until pumice fragments bean to fall toward the crater flow, thus generating a fast-moving pyroclastic flow. Courtesy of J.W. Vallance, USGS.