Near-surface Faceted Crystals and Avalanche Dynamics in High-elevation, Tropical Mountains of Bolivia

The importance of near-surface faceted crystals in forming weak layers associated with snow avalanches has recently received greater attention. However, there is still much to be learned concerning the formation and growth of these crystal types, their geographical extent, and related avalanche activity. Here we report on a spatially extensive avalanche cycle that occurred during late September 1999 at high-elevations in the Bolivian Andes, claiming two lives. Climbers released one slide at about 5,300 m in the Cordillera Apolobamba (on El Presidente), and four days later snow scientists servicing a high-elevation meteorological site triggered another at 6,300 m near the summit of Illimani (Cordillera Real). Both slab avalanches followed lateral fracture propagation through 25-50 cm of relatively new snow; deeper pockets existed due to wind redistribution. Analysis of a snowpit on Illimani, from a nearby and safe location, showed that the avalanche ran on a thick layer of near-surface faceted crystals overlying the austral winter dry-season snow surface. Average size of the crystals was 5-7 mm, and individual crystals exceeded 10 mm in diameter. We evaluate local and regional meteorological information in an effort to understand what caused the growth of these large crystals and the resultant snowpack instability. Insights are offered regarding the avalanche hazard due to near-surface faceted crystal growth in high-elevation areas of the Tropics, where avalanches are not generally recognized as a significant hazard during the climbing season.