POST-ERUPTION GLACIER DEVELOPMENT WITHIN THE CRATER OF MOUNT ST. HELENS, WASHINGTON, U.S.A

The cataclysmic eruption of Mount St. Helens on May 18, 1980 resulted in the formation of a large, north-facing amphitheater, with a steep headwall rising 600 m above the crater floor. The elevation of the highest point along the crater rim is about 2500 m. In this deeply shaded niche a new glacier has formed. Tongues containing crevasses extend from the main ice mass along the amphitheater backwall around both the east and west sides of the lava dome that occupies the center of the crater floor. Initially development of the glacier was delayed as the crater floor remained warm for several years after the eruption. Aerial photos of September 1996 reveal that the southwest portion of the amphitheater harbored a small glacier containing several crevasses and a bergschrund-like feature at the head of the ice body. The extent of the glacier in 1996 is hard to distinguish because of a light snow cover but probably was about 300 X 300 m. By September 2000 the debris-laden glacier had grown to about 1 km² in area, with a maximum thickness of about 200 m, and it contained an estimated 80,000,000 m³ (± 10,000,000 m³) of ice and debris. These estimates were calculated by subtracting the digital elevation model of the 1980 post-eruption topography from the 2000 photogrammetrically derived glacier surface. Approximately one-third of the volume of this glacier is thought to consist of rock debris derived mainly from rock avalanches from the surrounding amphitheater walls. As the 1980 eruption provided conditions for birth of a new glacier within the crater, it completely destroyed the Loowit, Leschi, and Wishbone Glaciers. In addition, large areas of the headward regions of the Shoestring, Ape, Nelson, and Forsyth Glaciers were lost (Brugman and Post, 1981). Not surprisingly these glaciers suffered severe shinkage in the intervening years. By September 2001 the Dryer, Shoestring, Nelson, and Forsyth Glaciers no longer existed, while the Toutle, Tallus, Swift, and Ape Glaciers have shrunk significantly from their pre-eruption size.