Glaciers are sometimes called ‘rivers of ice ’. However, this is misleading since glaciers do not normally form from rainfall, but by the transformation of snow to ice. To initiate a glacier, winter snowfall needs to be great enough for some of the snow to last throughout the following summer. This process is then repeated for several years. Finally, under the pressure of its own weight the snow turns into ice. If the ice is thick enough, it flows under the influence of gravity. This transformation of snow to ice is often a long and complex process, since both the nature of the transformation and the time involved depend on temperature and the depth of further, overlying snow. The transformation is most rapid in temperate regions, such as the Alps and the Western Cordillera of North America, where ice can form from snow within five to ten years. In contrast, the transformation in high polar latitudes or at high elevations may take hundreds of years.

From snowflake to glacier ice

Although snow crystals tend to have a hexagonal structure, with characteristic six-sided symmetry, snow falls in myriad forms. Snowflakes may come as delicate, feathery crystals a centimetre or so across, or as relatively hard grains that have the feel of sand. They have their most intricate and varied forms when they fall close to freezing point, and can form a very light snow layer 20 times less dense than water.