EMPLACEMENT OF A SILICIC LAVA DOME THROUGH A CRATER GLACIER: MOUNT ST. HELENS, 2004-07

The process of lava-dome emplacement through a glacier was observed for the first time after Mount St. Helens reawakened in September 2004. The glacier that had grown in the crater since the cataclysmic 1980 eruption was split in two by the new lava dome. The two parts of the glacier were successively squeezed against the crater wall. Photography, photogrammetry and geodetic measurements document glacier deformation of an extreme variety, with strain rates 2 to 3 orders of magnitude greater than typically measured in temperate alpine glaciers. Little ice volume (no more than about 10% of the pre-eruptive total) has been lost because (i) conductive heat transfer from the dome to the glacier is inefficient, and (ii) the glacier surface has rarely been swept by hot debris. Unlike normal temperate glaciers, the crater glacier shows no evidence of either speed-up at the beginning of the ablation season or diurnal speed fluctuations during the ablation season. Thus there is evidently no slip of the glacier over its bed. The most reasonable explanation for this anomalous behavior is that meltwater penetrating the glacier is captured by a thick layer of coarse rubble at the bed and then enters the volcano's groundwater system rather than flowing through a drainage network along the bed.