PREDICTIONS OF THE GROWTH AND STEADY-STATE FORM OF THE MOUNT ST. HELENS CRATER GLACIER USING A 2-D GLACIER MODEL

Since the 1980 eruption of Mount St. Helens, a glacier has been growing around the lava dome in the volcano's crater. Informally known as the Crater Glacier, the ice mass averages approximately 100 meters in thickness and is reportedly the fastest growing glacier in the continental United States. In 2004, the glacier contained about 120 million cubic meters of snow and ice, a volume roughly equivalent to that of all the glaciers that existed on the mountain prior to the 1980 eruption. The size that this glacier may eventually reach, as it grows toward a steady-state condition, is of considerable interest because the storage of water in the crater increases the risk of mudslides that could result from even minor eruptions - increased activity in the crater during 2004 lifted and cracked the glacier and pierced it in at least two places. To estimate how this glacier may develop in the future and what size it might eventually obtain, we are modeling glacier development in the crater using a general purpose 2-D glacier simulator designed to examine the climatic sensitivity of alpine glaciers. The simulator includes a detailed treatment of effects high-relief topography on net radiation, a physically-based treatment of the other components of the surface energy balance and a transient solution to a set of non-linear equations describing 2-D ice flow. Using this model, whose inputs are primarily a digital elevation model (DEM) and pseudo-vertical lapse rates and precipitation gradients, we calibrate to the observed Crater Glacier development by adjustments to uncertain energy balance parameters, like albedo. With the calibrated model, we predict what the glacier might look like in the future, and describe the primary uncertainties involved in predicting its evolution. Results should be useful as a means of improving risk analysis associated with the geologic hazards that the volcano represents.