Glaciers form for a rather simple reason. Snow that does not melt or evaporate during the year is carried over to the next winter. If snow continues to accumulate year after year, eventually consolidating and beginning a slow downhill movement, a glacier is formed.
Within the old snow (the firn, or névé), the metamorphic conversion of snow crystals into grains of ice has been completed. Now the grains of ice are changed into glacier ice in a process called firnification. Firn turns into glacier ice when the air spaces between the grains become sealed off from each other so that the mass becomes airtight.
Part of the glacier ice is formed by refreezing of percolating meltwater each spring when the lower snow layers are still at temperatures below freezing. This refrozen meltwater forms ice layers within the firn. Therefore, by the time compaction and metamorphism have prepared an entire area of firn for conversion to glacier ice, it may already contain irregular bodies of ice.
Once glacier ice has formed, metamorphism does not cease. Through crystallographic changes, some of the ice grains packed in the glacier continue to grow at the expense of their neighbors, and the average size of the ice crystals increases with age. Large glaciers, in which the ice takes centuries to reach the terminus, may produce crystals more than 1 foot in diameter, gigantic specimens grown from minute snow particles.
In our imagination, we can follow the birth of a simple valley-type alpine glacier. Picture a mountain in the Northern Hemisphere that has no glaciers. Now suppose climatic changes that cause snow to persist from year to year in a sheltered spot on a northern exposure.
From the first, snow starts to How toward the valley in the very slow motion called creep. New layers are added each year, the patch of firn snow grows deeper and bigger, and the amount of snow in motion increases. The creeping snow dislodges soil and rock, while the melting, refreezing, and How of water around and under the snow patch add to impact on the surroundings. This small-scale process of erosion eventually leads to formation of a hollow where the winter snows are deposited in deeper drifts. The snow gets to be 100 feet deep or so. The lower layers have nearly turned to glacier ice, while the increasing pressure of the many upper layers of firn causes the flow to accelerate. A glacier is born.
With continued nourishment from heavy winter snows, the glacier flows toward the valley as a stream of ice. At some point in its descent, the glacier reaches an elevation low enough and warm enough that no more new snow accumulates. The glacier ice begins to melt. Eventually the glacier reaches the point, even lower and warmer, at which all ice carried down from above melts each year. This is the lower limit of the glacier.
Glaciers vary from stagnant masses with little motion to vigorously flowing rivers of ice that transport large masses each year from higher to lower elevations. Glaciers in relatively temperate climates flow both by internal deformation and by sliding on their beds. Differences in speed within the glacier are somewhat like that in a river, fastest at the center and surface and slower at the sides and bottom where bedrock creates a drag. Small polar glaciers present a striking difference in appearance from their temperate cousins, for they are frozen to their beds and can flow only by internal deformation. The polar glaciers look much like flowing molasses, while temperate glaciers are rivers of broken ice.
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