hen large areas of the surface of the globe are covered with ice sheets (large continental glaciers). Short ice ages, known as glaciations, are when glaciers are near their maximum extent.
             In polar and high-altitude alpine regions, glaciers generally accumulate more snow in the winter than they lose in the summer from melting, ablation (the loss of ice from a glacier caused by melting or vaporization), or calving (the process by which ice breaks off a glacier's end). If the accumulated snow survives one melt season, it is considered to be firn, snow that survives a year or more. Firn usually has a density greater than 550 kg/m3 (35 lb/ft3). The snow and firn are compressed by the overlying snow, and the buried layers slowly grow together to form a thickened mass of ice.
             The pressure created from the overlying snow compacts the underlying layers, and the snow grains become larger ice crystals randomly located in connected air spaces. These ice crystals can eventually grow to become several centimeters in diameter. Glacier ice crystals can grow to be as large as baseballs.
             As compression continues and the ice crystals grow, the air spaces in the layers decrease, becoming small and isolated. This dense glacial ice usually looks somewhat bluer than the less dense glacial ice before compre
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