| The reawakening of Mount St. Helens offered an unusual opportunity to document the rarely observed, short-lived phenomena that occur during explosive eruptions. An early explosive phase of activity was defined by the magmatic eruptions of May 18, May 25, June 12, July 22, August 7, and October 16-18, 1980. Of these, the eruptions of July 22 and August 7 were, at least with regard to flowage phenomena, the most closely observed and photographically best documented. This report presents some observations and photographs of eruption phenomena that occurred on July 22 and August 7, and suggests mechanisms to explain them. The emphasis is on the pulsating nature of the eruptions, the timing of pyroclastic density flow formation relative to vertical-eruption column development, and the generation and motion of pyroclastic flows and surges. Also considered are the emplacement temperatures of pyroclastic-flow deposits, and phenomena probably related to the displacement of air by an advancing pyroclastic density flow. -- Hoblitt, 1986 |
