Firn is dense old snow on its way to becoming solid glacier ice
Fresh snow is fluffy and full of air. As layers bury it, grains round, bond, and densify into firn — the halfway material between powder and blue glacier ice.
Over years to decades, remaining pores close and firn becomes impermeable ice that can flow. Accumulation zones are firn factories feeding the glacier below.
Snow graduates into ice. Firn shows the quiet metamorphism that turns mountain snowfall into the engines of ice sheets and alpine glaciers.
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