16.2 How Glaciers Work
There are two main types of glaciers. Continental glaciers cover vast areas of land in extreme polar regions, including Antarctica and Greenland (Figure 16.7). Alpine glaciers (a.k.a. valley glaciers) originate on mountains, mostly in temperate and polar regions (Figure 16.1), but even in tropical regions if the mountains are high enough.
Earth’s two great continental glaciers, on Antarctica and Greenland, comprise about 99% of all of the world’s glacial ice, and approximately 68% of all of Earth’s fresh water. As is evident from Figure 16.8, the Antarctic Ice Sheet is vastly bigger than the Greenland Ice Sheet; it contains about 17 times as much ice. If the entire Antarctic Ice Sheet were to melt, sea level would rise by about 80 m and most of Earth’s major cities would be submerged.
Continental glaciers do not flow “downhill” because the large areas that they cover are generally flat. Instead, ice flows from the region where it is thickest toward the edges where it is thinner, as shown in Figure 16.9. This means that in the central thickest parts, the ice flows almost vertically down toward the base, while in the peripheral parts, it flows out toward the margins. In continental glaciers like Antarctica and Greenland, the thickest parts (4,000 m and 3,000 m respectively) are the areas where the rate of snowfall and therefore of ice accumulation are highest.
The flow of alpine glaciers is primarily controlled by the slope of the land beneath the ice (Figure 16.10). In the zone of accumulation, the rate of snowfall is greater than the rate of melting. In other words, not all of the snow that falls each winter melts during the following summer, and the ice surface is always covered with snow. In the zone of ablation, more ice melts than accumulates as snow. The equilibrium line marks the boundary between the zones of accumulation (above) and ablation (below).
Above the equilibrium line of a glacier, not all of the winter snow melts in the following summer, so snow gradually accumulates. The snow layer from each year is covered and compacted by subsequent snow, and it is gradually compressed and turned into firn within which the snowflakes lose their delicate shapes and become granules. With more compression, the granules are pushed together and air is squeezed out. Eventually the granules are “welded” together to create glacial ice (Figure 16.11). Downward percolation of water from melting taking place at the surface contributes to the process of ice formation.
The equilibrium line of a glacier near Whistler, B.C., is shown in Figure 16.12. Below that line, in the zone of ablation, bare ice is exposed because last winter’s snow has all melted; above that line, the ice is still mostly covered with snow from last winter. The position of the equilibrium line changes from year to year as a function of the balance between snow accumulation in the winter and snowmelt during the summer. More winter snow and less summer melting obvio