GROWTH AND FLOW OF A NEW GLACIER IN MT. ST. HELENS CRATER

The well shaded north-facing crater at Mt. St. Helens, which formed during the eruption of 1980, has accumulated snow steadily since 1982-83, forming the newest and fastest growing glacier in Washington. Between 1982 and 1988, roughly 3.33 million m³/yr of snow fell in the crater, and by late 1998, the glacier had a total volume of ~71,000,000 m³ of ice. In summer, 2001, the glacier was clearly visible to the west of the lava dome. In this study, we assess the mass balance and flow dynamics of the glacier in order to predict its steady-state size under the current climate.

The most recent topography available (1998) indicates that the glacier has two lobes separated by an ice divide oriented N-S on the south side of the lava dome. Lobe A flows around the dome to the east and Lobe B flows around the dome to the west. The total areas of Lobes A and B as of 1998 are ~0.5 km² and 0.9 km², respectively. By assuming that the equilibrium-line altitude is approximated by the ice-firn boundary on Lobe B, and assuming an accumulation-area ratio of 0.65, we estimate that the ablation zone of each lobe at equilibrium will be ~0.20 km² and 0.35 km², respectively. Projecting these areas north of the lava dome roughly perpendicular to the modern slope suggests that, at equilibrium, the ablation zones will merge on the north side of the dome. However, this growth may be moderated once the ice reaches beyond the protective shading of the crater walls.

We use the equation t_b=rgH sin a to estimate the basal shear stress (t_b) of the two lobes in the central accumulation zone as of 1998. To do this, we combine estimates of glacier thickness (H), a modified ice density (r) to account for extra mass contributed to the ice by rockfall, and a surface slope (a) for each glacier. Based on historical observations, we estimate the thickness values for Lobes A and B as ~70 m and 80 m, respectively. We estimate that the glaciers contain approximately 30% dacitic rock (density of 2.7 g/cm³) from the collapse of the surrounding crater walls, which results in an average density of 1.5 g/cm³ for the glacier. The surface slopes of Lobes A and B along their centerlines are ~7.35° and ~5.57°, respectively. Combining these values indicates that the basal shear stress for Lobe A is ~125 kPa and for Lobe B is ~110 kPa, both within, but at the higher end, of values for normal glaciers.