What are the basic types of glaciers? Glaciers are divided into two basic types. A valley glacier is a river of ice that forms in the cold heights of mountain ranges and moves downslope through a valley. A continental glacier is a thick, slow-moving sheet of ice that covers a large part of a continent or other large landmass. Today, continental glaciers cover much of Greenland and Antarctica.
How do glaciers form? Glaciers form where climates are cold enough that snow, instead of melting completely in summer, is transformed into ice by recrystallization. As snow accumulates, either at the tops of valley glaciers or at the domed centers of continental glaciers, the ice thickens. Its thickness increases until it becomes so massive that gravity starts to pull it downhill.
How do glaciers shrink or grow? Glaciers lose ice by melting, sublimation, iceberg calving, and wind erosion. The glacial budget is the relationship between ablation (the amount of ice a glacier loses annually) and accumulation. If ablation is balanced by accumulation of new snow and ice in the glacier’s upper reaches, the size of the glacier remains constant. If ablation is greater than accumulation, the glacier shrinks; conversely, if accumulation exceeds ablation, the glacier grows.
How do glaciers move? Glaciers move by a combination of plastic flow and basal slip. Plastic flow dominates in very cold regions, where the glacier’s base is frozen to the ground. Basal slip is more important in warmer climates, where meltwater at the glacier’s base lubricates the ice.
How do glaciers shape the landscape? Glaciers erode bedrock by scraping, plucking, and grinding it into sizes ranging from boulders to fine rock flour. Valley glaciers erode cirques, horns, and arêtes at their heads; excavate U-shaped and hanging valleys; and create fjords by eroding their valleys below sea level at the coast. Glacial ice has both high competence and high capacity, which enable it to carry abundant sediment particles of all sizes. Glaciers transport huge quantities of sediments to the ice front, where melting releases them. The sediments may be deposited directly by the melting ice as till or picked up by meltwater streams and laid down as outwash. Moraines and drumlins are characteristic landforms deposited by ice. Eskers and kettles are formed by meltwater. Permafrost forms where summer temperatures never rise high enough to melt more than a thin surface layer of soil.
What does the geologic record tell us about past ice ages? Glacial drift of Pleistocene age is widespread over high-latitude regions that now enjoy temperate climates. This widespread drift is evidence that continental glaciers once expanded far beyond the polar regions. Studies of the geologic ages of glacial deposits on land and in marine sediments show that continental ice sheets advanced and retreated many times during the Pleistocene epoch. The most recent glacial advance, known as the Wisconsin glaciation, covered the northern parts of North America, Europe, and Asia with ice and exposed large areas of continental shelves. During interglacial intervals, sea level rose and submerged the shelves.