Layers of snow form glaciers. And, of course, Alaska is famous for its snowfall! It is not uncommon for some areas to receive twenty, thirty, even 100 feet of snow in a single winter. Each new snowfall bears weight down on the previous layer, compacting it into ice. In many places, the snow is so deep that it never totally melts during warmer months. This is especially true in the mountain ranges where glaciers are present. More snow blankets the layers remaining from previous seasons, and the cycle repeats itself. It goes without saying that this process did not occur overnight. It took Mother Nature thousands of years to construct these giant ice structures. The glaciers found in Alaska today are a result of the Neoglacial Period of the Late Cenozoic Ice Age, developing to their maximum about 3000 years ago during the Little Ice Age.
Glaciers are in constant motion, and move because of their mass and gravity. The rate at which they move depends on many factors, such as the amount of ice and water that make up the glacier, the amount of debris it carries, the slope of the ice surface and terrain over which it travels, the temperature, and friction encountered.
The term "receding" confuses many people. Can a glacier actually retreat up a mountain? The answer is yes. However, the ice mass is not really shifting into reverse and intentionally changing its direction of travel. A glacier is considered receding if the ice at its edge is melting faster than the glacier moves forward. A good example of this is Mendenhall Glacier located near Juneau. Mendenhall advances about two feet per day; however, because it descends into a warmer, maritime climate, its melting rate is faster than its forward movement. Therefore, Mendenhall is considered a receding glacier. An advancing glacier is the opposite. Its forward movement is faster than the rate at which the edge melts. And a stationary glacier's rate of melting and movement are equal. Advancing and stationary glaciers are typically found in the higher elevations where temperatures do not adversely affect the melting rate.
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