Geology of the Ice Age National Scenic Trail

Science Briefs

Science Brief 1: Why Glaciers? Why Not? Glaciers can exist only when and where more snow falls in the winter than melts in the summer . When snow that has fallen one winter is covered by new snow in the following winter, it compacts under the added weight and slowly changes to ice. As more weight from more seasons of snow is added, the snow particles compact and freeze together to form ice crystals. Then the ice crystals merge and eventually become solid ice with only a few air bubbles remaining. The process could take as few as 5 years under warm conditions with lots of snow or more than 100 years where it is very cold. The part of the glacier where more snow accumulates in the winter than melts in the summer is called the accumulation zone. Figure 9 shows a longitudinal profile of a hypothetical glacier. Although the cross section used is that of a valley glacier, the same concepts of ice movement apply to a large continental ice sheet. The glacier is divided into two zones: the accumulation zone mentioned above, where more snow falls in the winter than can melt in the summer, and the ablation zone, where more snow melts in the summer than falls in the winter. In accumulation zones the glacier gains in mass, and in the ablation zone a glacier loses mass over the year. The boundary between these two zones is called the equilibrium line. Figure 9 shows that unless there were some way to transfer ice from the accumulation zone to the ablation zone, the glacier would thicken in its upper part and disappear in its lower part. In fact, a glacier transfers its mass in three ways: ice moves from the accumulation zone to the ablation zone by bed deformation , by sliding at the glacial bed, and, if the ice is greater than about 100 feet thick, by internal flow (SB 2). The ice always moves from the accumulation zone to the ablation zone; that is, it always moves down the ice-surface slope. The glacier loses ice (ablates) through gradual melting or sudden release of icebergs. 24 Science Briefs “Ice advance” and “ice retreat” refer to changes in the position of the ice edge, or to what is often called the ice margin. Glacier ice always flows down the ice-surface slope toward the edge of the glacier, and changes in the position of the ice margin are due to changes in the balance between the amount of ice coming in from the accumulation zone and the amount of ice being lost by ablation. The glacier is analogous to the balance in your bank account. If you take out more money for your mortgage and groceries than you deposit, your bank account will shrink. Similarly, if the volume of ice lost is more than the volume of ice gained, the glacier shrinks and the ice margin retreats. With a somewhat delayed response, a glacier retreats if snowfall decreases (less accumulation) or summer temperatures increase (more ablation). Likewise, a glacier advances if snowfall increases (more accumulation) or the summer temperatures decrease (less ablation). Science Brief 2: How Do Glaciers Move? If glacier ice did not move, snow in the accumulation zone would pile thicker and thicker, stopping only when it touched the clouds that fed it with snow. As we know, this does not Science Briefs 25 Figure 9. Important parts and processes of a glacier. Several terms are discussed in the science briefs that follow. (Drafted by Mary Diman.) happen. Even though ice sheets can be over 2 miles thick in their central areas, there is a limit to how thick they can become. As more snow piles on top, gravity forces the ice beneath to move outward from the area of accumulation. How does the ice move? We think of ice as a brittle solid. If we drop an ice cube on the floor it shatters. In fact, ice in the top 100 feet of a glacier is brittle. But ice under thicknesses of accumulated snow and ice greater than about 100 feet behaves differently. This ice deforms, much like the way cold molasses or very thick pancake batter flows slowly, but the processes involve different mechanisms. The ice crystals actually melt at their edges, where the pressure is high, and refreeze in areas where the pressure is lower. Slippage within individual...