Effects of lava-dome growth on the crater glacier of Mount St. Helens, Washington

The process of lava-dome emplacement through a glacier
was observed for the first time as the 2004-6 eruption of
Mount St. Helens proceeded. 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 of extraordinary magnitude as compared to normal temperate alpine
glaciers. Unlike such glaciers, the Mount St. Helens 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
anomaly 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. Mechanical consideration of
the glacier-squeeze process also leads to an estimate for the
driving pressure applied by the growing lava dome.