Emplacement of a Silicic Lava Dome Through the Crater Glacier of Mount St.
Helens, USA , 2004--2007
[Joseph S. Walder] (USGS, Cascades Volcano Observatory, 1300 SE Cardinal Ct.,
Bldg. 10, Suite 100, Vancouver, Washington 98683, USA; ph: 1-360-993-8948; fax:
360-993-8981; email: jswalder@usgs.gov); Richard G. LaHusen (USGS, Cascades
Volcano Observatory, 1300 SE Cardinal Ct., Bldg. 10, Suite 100, Vancouver,
Washington 98683, USA; ph: 1-360-993-8924; fax: 360-993-8981; email:
rlahusen@usgs.gov); James W. Vallance (USGS, Cascades Volcano Observatory,
1300 SE Cardinal Ct., Bldg. 10, Suite 100, Vancouver, Washington 98683, USA; ph:
1-360-993-8959; fax: 360-993-8981; email: vallance@usgs.gov); Steve P. Schilling
(USGS, Cascades Volcano Observatory, 1300 SE Cardinal Ct., Bldg. 10, Suite 100,
Vancouver, Washington 98683, USA; ph: 1-360-993-8939; fax: 360-993-8981; email:
sschilli@usgs.gov)
The process of lava-dome emplacement through a glacier was observed in detail for
the first time after Mount St. Helens reawakened in September 2004. The horseshoeshaped 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, and the termini have advanced by several hundred
meters. Photography, photogrammetry and geodetic measurements document glacier
deformation of an extreme variety, with strain rates of extraordinary magnitude as
compared to those of normal alpine glaciers. Mechanical consideration of the glaciersqueeze process leads to an estimate of about 1.3 MPa for the driving pressure applied
by the growing lava dome on the glacier.
All previous measurements on temperate glaciers (glaciers at the melting point) reveal
speed-up at the beginning of the ablation season and marked diurnal speed
fluctuations during the ablation season. These phenomena result from development of
a meltwater drainage system that conveys water along the glacier bed to the terminus.
The drainage system modulates the stress state at the glacier bed and thus the rate of
glacier sliding. The Mount St. Helens crater glacier is unique in manifesting neither
spring speed-up nor diurnal speed fluctuations. 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 drains into 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. The rubble was derived mostly from rock avalanches
shed from the crater walls before the glacier began to grow.
Volcano/glacier interactions are commonly thought to result in rapid meltwater
generation either as magma contacts the glacier bed or as lava or pyroclasts are
erupted onto the glacier surface. At Mount St. Helens, however, glacier melt
associated with dome emplacement has been minor, with no more than 10% of the
glacier mass being lost so far. The reason is two-fold: the solidified exterior of the
lava dome, and the talus aprons surrounding it, greatly reduce the rate of heat transfer
from the dome’s interior to the glacier; furthermore, hot material has been emplaced
on the glacier surface only rarely as rock avalanches have been shed from the dome.
INVITED ORAL
CORRESPONDING AUTHOR: JOSEPH S. WALDER