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Introduction to Glaciers

A glacier is a perennial mass of snow or ice that is large enough and heavy enough to flow, like a very thick fluid.

Birth of a Glacier

Glaciers form wherever more snow accumulates than is lost each year. As new snow accumulates, it buries and compresses the old snow. Under the weight of the overlying snow, the old snow is transformed from a fluffy mass of ice crystals into dense, hard ice. This process occurs on the upper part of a glacier, at higher altitudes, where more snow accumulates than is lost each year. This is called the “accumulation zone”, and is typically covered with snow year-round. The glacier is in constant motion, and the ice in the accumulation zone flows down to lower altitudes, which is called the “ablation zone”. The ablation zone is located in the lower part of the the glacier where more snow is lost than accumulates. In late summer, when the seasonal snow has melted, the bare ice of the ablation zone is exposed.

Cross section of an alpine glacier showing snow being converted into glacier ice (the left side of the figure) and the two major zones of a glacier’s surface. The red arrows show the direction and relative speed of different parts of the glacier. The longer the arrow, the faster ice is moving.

Why do Glaciers Move?

Once a mass of compressed ice reaches a critical thickness, around 18 meters thick, it becomes so heavy that it begins to deform and move. The sheer girth of the ice, combined with gravity's influence, causes glaciers to flow very slowly. Ice may flow down mountain valleys, fan across plains, or in some locations, spread out to the sea. Movement along the underside of a glacier is slower than movement at the top due to the friction created as it slides along the ground's surface.

Glaciers periodically retreat or advance, depending on the amount of snow accumulation or albation that occurs. This retreat or advance refers only to the position of the terminus, or snout, of the glacier. Even as it retreats, the glacier still deforms and moves downslope, like a conveyor belt. For most glaciers, retreating and advancing are very slow occurrences, noticeable only over a long time. However, when glaciers retreat rapidly, movement may be visible over a few months or years. For instance, massive glacier retreat has been recorded in Glacier Bay, Alaska. Other glaciers have been photographed at intervals showing dramatic recession.

Alternatively, glaciers may surge, racing forward several meters per day for weeks or even months. In 1986, the Hubbard Glacier in Alaska began to surge at the rate of 10 meters per day across the mouth of Russell Fiord. In only two months, the glacier had dammed water in the fiord and created a lake. This illustrates how quickly a surging glacier can change its surroundings.

The Anatomy of a Glacier

Essentially all glaciers have accumulation and ablation zones. The boundary between these zones, the equilibrium line, is the transition where accumulation equals ablation.

The accumulation zone has three major layers. The top layer is seasonal snow that thickens further up glacier. The next layer is the “firn”, or a transitional form between the snow and solid ice below. The bottom layer is ice. The ablation zone, on the other hand, is composed entirely of ice. During the winter, the ice of the ablation zone is covered with snow, which entirely melts away in summer.

In a cool year, the accumulation zone may cover the entire glacier, and in a warm year the ablation zone may cover the entire glacier. For larger glaciers in equilibrium with climate the accumulation zone typically covers about 65% of the glacier.

Several visible features are common to most glaciers. The awesome force of the glacier's own movement causes it to deform, often creating giant cracks in the ice called crevasses, which may make travel across a glacier treacherous. Underneath the glacier, where glacier ice meets the ground, large amounts of rock and soil are ground up by the tremendous weight of the glacier.

Another glacial feature are moraines, created when the glacier pushes or carries along this rocky debris as it moves. These long, dark bands of debris are visible on top and along the edges of glaciers. Medial moraines run down the middle of a glacier, lateral moraines along the sides, and terminal moraines are found at the terminus, or snout, of a glacier. Sometimes one glacier flows into another, also creating moraines. Because they represent evidence of glacial motion, crevasses and moraines are features that often help scientists decide whether a mass of ice is truly a glacier.

Types of Glaciers

A cirque glacier is a small glacier that occupies a bowl-shaped basin at the head of a mountain valley. Cirque glaciers are usually the remnants of much larger valley glaciers.

Valley glaciers are glaciers confined to a valley (usually a former stream valley). Valley glaciers are commonly found in Washington and Alaska.

The largest type of glacier is a continental ice sheet, that can cover thousands of square kilometers. Only two ice sheets of this type exist on the planet today, one on Greenland, the other in Antarctica. During the Pleistocene Ice Age, ice sheets covered nearly all of Canada reaching down into the northern contiguous United States.

Three principle types of glaciers

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