Faceting around ice crusts
Weak faceted crystals are often observed near hard crust layers
in the snowpack. The combination of weak facets adjacent to a
hard ice crust often creates dangerous, persistent, and tricky
avalanche conditions. During the 2001/2002 winter a faceted snow
and crust combination formed in the mountains of northern Utah.
This layering was responsible for numerous large avalanches and
contributed to three avalanche fatalities. A recent article
in The Avalanche Review summarizes this cycle.
Two processes that can create or contribute to this dangerous
layering scenario are radiation recrystallization and melt-layer
recrystallization (the latter is sometimes called wet layer recrystallization).
These near-surface faceting processes are described in this
article, as well as Bruce Tremper's widely available book,
Staying Alive in Avalanche Terrain. In addition to these
processes, it is believed that the denser and more conductive
ice adjacent to the less conductive snow might concentrate temperature
gradients next to the crust (Adams and Brown, 1988; 1990). This
phenomenon could result in the enhanced faceted growth near the
ice crust long after near-surface recrystallization has diminished.
This layering may also allow faceted growth to continue in a thermal
environment that is generally conducive to rounding.
Ethan Greene (Colorado State University), working with Kelly
Elder of the USDA-Forest Service Rocky Mountain Research Station,
Montana State University's Ed Adams, the Swiss Federal Institute
for Snow and Avalanche Research's Martin Schneebeli, and the National
Avalanche Center, is conducting laboratory work to carefully analyze
the metamorphic processes around buried ice lenses. Ethan sucessfully
competed for an National Science Foundation Dissertation Improvement
Grant that helped to support this research. He did the bult of
his experiments during the 2004-05 winter, then cast his samples
and shipped them over to Switzerland packed in dry ice! Then he
went to Switzerland to analyze the data with help from Martin
Schneebeli. This research will form the basis of his PhD dissertation.
Ethan Greene, working in Montana State University's
cold laboratory, conducts initial experiements on faceting around
Adams, E.E., and R.L. Brown. 1988. On the effect of strong density
layering on metamorphism of seasonal snowcover. Proceedings of
the 1988 International Snow Science Workshop, Whistler, British
Adams, E.E., and R.L. Brown. 1990. A mixture theory for evaluation
heat and mass transport processes in nonhomogeneous snow. Continuum
Mechanics and Thermodynamics 2, 31-63.