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.
—Credit: USGS/L.C. Reed photograph/NSIDC
The Life of a Glacier
Most of the world's glaciers are found near the Poles, but glaciers exist on all of the world's continents, even Africa. Australia doesn't have any glaciers; however, it is considered part of Oceania, which includes several Pacific island chains and the large islands of Papua New Guinea and New Zealand. Both of these islands have glaciers.
Glaciers require very specific climatic conditions. Most are found in regions of high snowfall in winter and cool temperatures in summer. These conditions ensure that the snow that accumulates in the winter isn't lost by melt, evaporation, or calving during the summer. Such conditions typically prevail in polar and high alpine regions. Two main types of glaciers are valley glaciers and continental glaciers known as ice sheets.
The amount of precipitation whether in the form of snowfall, freezing rain, avalanches, or wind-drifted snow is important to glacier survival. In areas such as Antarctica, where the low temperatures are ideal for glacier growth, very low annual precipitation causes the glaciers to grow very slowly.
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.
Under the pressure of its own weight and the forces of gravity, a glacier will begin to move, or flow, outwards and downwards. Valley glaciers flow down valleys, and continental glacier ice sheets flow outward in all directions from a central point.
Glaciers move by internal deformation and/or by sliding at the base. Internal deformation occurs when the weight and mass of a glacier causes it to spread out due to gravity.
Glaciers carved many of the mountains and landforms seen in present-day Glacier National Park. A small glacier remained (in the uppermost cirque bowl-shaped hollow) in this photograph from 1925.
—Credit: Army Air Corps/Captain A. W. Stevens photograph/NSIDC
Sliding occurs when the glacier slides on a thin layer of water at the bottom of the glacier. This water may come from glacial melting due to the pressure of the overlying ice or from water that has worked its way through cracks in the glacier. Glaciers can also readily slide on a soft sediment bed that has some water in it. This is known as basal slip and may account for most of the movement of thin, cold glaciers on steep slopes or only 10 to 20 percent of the movement of warm, thick glaciers lying on gentle slopes.
When a glacier moves rapidly, internal stresses build up in the ice which cannot be relieved by deformation alone, and cracks called crevasses form at the surface of the glacier.
As large glaciers retreat, the underlying ground surface is typically scoured of most materials, leaving only scars on the underlying surface.
Glacier retreat, melt, and ablation, result from increasing temperature, evaporation, and wind scouring. Ablation is a natural and seasonal part of glacier life. As long as snow accumulation equals or is greater than melt and ablation, glacier health is maintained.
In the northern half of North America, glacial remnants from the last ice age may be reincarnated as vegetated hillsides. Views from an airplane window over the midwestern states and provinces reveal lines of eskers and herds of drumlins dotting the landscape.
Throughout advance and retreat, glacial debris sediment known as till is jostled in all directions. Till is thrust forward with the glacier, brushed aside as the glacier pushes past less mobile objects, such as a mountainside, or drawn along on the glacier's journey.
Over the past 60 to 100 years, glaciers worldwide have tended toward retreat. Alpine glaciers, which are typically smaller and less stable to begin with, seem particularly susceptible to glacial retreat. Whether this is due to a predictable climate trend or because of increased human impacts on global climate remains to be determined.