Near-surface faceted crystals, avalanches and climate in high-elevation, tropical mountains of Bolivia

The importance of near-surface faceted crystals in forming weak layers asso ciated with snow avalanches has recently received greater attention. Howeve r, there is still much to be learned concerning the formation and growth of these crystal types, their geographical extent, and related avalanche acti vity. Here we report on two avalanches that occurred during late September 1999 at high-elevations in the Bolivian Andes, Climbers released one slide at about 5200 m in the Cordillera Apolobamba (on El Presidente), which clai med two lives. Four days later and 200 km to the southeast, snow scientists servicing a high-elevation meteorological site triggered another at 6300 m , near the summit of Illimani (Cordillera Real). Both slab avalanches fract ured through 25-50 cm. of relatively new snow, with deeper pockets of wind redistributed snow. Snowpit analyses on Illimani showed the avalanche ran o n a thick layer of near-surface faceted crystals overlying the austral wint er dry-season snow surface. Average crystal size was 5-7 mm, and individual crystals exceeded 10 mm in diameter. We evaluate local and regional meteor ological information in an effort to understand what caused the growth of t hese large crystals and the resultant snowpack instability. Dust and chemis try profiles indicate that the crystal growth occurred through the austral winter, prior to a snowfall event in the days prior to the avalanche. Tempe rature profiles measured just above and just below the snow surface indicat e that bi-directional, large gradients of temperature and vapor pressure ex ist through the dry winter season, Insights are offered regarding the avala nche hazard due to near-surface faceted crystal growth in high-elevation ar eas of the Tropics, where avalanches are not generally recognized as a sign ificant hazard during the climbing season. (C) 2001 Elsevier Science B.V. A ll rights reserved.