With roughly 4.5 meters of snow falling on Blue Glacier annually, one would expect to see snow everywhere. But there is hardly any snow visible in most of the photographs in this book—the white stuff is almost exclusively firn.

Firn is old snow. More specifically, it is snow that has survived one or more summer melt seasons. The aging process is a little more complicated, however. Snow lands on a glacier in the beautiful and delicate shapes we mimic with paper and scissors. Gradually the individual crystals lose their pointed shape and become blunted, round grains of ice. The sharp points of a flake are dulled by the bumps and bruises it receives during landing, the subsequent redistribution by wind, and the compaction from an increasing blanket of snow. Water molecules also evaporate easily from the sharp ends of the flake and then condense back to ice in the center leading to a more spherical and granular shape.

Though it takes only a season for firn to form, the complete transition to ice may take many more. The physical blunting and melting processes continue, making the grains denser and larger until they freeze together along with small air bubbles. The whole process can take as long as a thousand years or as short as a single season. A plentiful supply of melt water, rain, and freezing temperatures contribute to rapid densification of snow on Blue Glacier.



intro | terminology | accumulation | firn | blue | ablation | water | equilibrium | massbalance | movement | crevasse | structure | algae | moraine | debris | erosion
pdf version | glacier glossary | bibliography | about blue ice

Benjamin Drummond 2002