A glacier forms when snow accumulates over time, turns to ice, and begins to flow outwards and downwards under the pressure of its own weight.

In polar and high-altitude alpine regions, glaciers generally accumulate more snow in the winter than they lose in the summer from melting, ablation, or calving. If the accumulated snow survives one melt season, it is considered to be firn. 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 oriented in connected air spaces. These ice crystals can eventually grow to become several centimeters in diameter.

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 blue.

Photo, top: Chickamin Glacier, bounded by mountains on both sides, flows past a cabin in this photograph taken in 1941. Chickamin Glacier is located in the coastal mountains shared by southeast Alaska and British Columbia, Canada.
(Source: USGS/L.C. Reed photograph at the World Data Center for Glaciology, Boulder )

The glacier story continues as the glacier, now grown, begins Moving Forward.

Photo, bottom: Leconte Glacier as seen in 1929. (Source: Photographer unknown. 1929. Leconte Glacier: From the Glacier Photograph Collection. Boulder, CO: National Snow and Ice Data Center. Digital Media.)