Earth & Space Science: Session 6

A Closer Look: Glaciers

What are glaciers?

Glaciers are huge sheets of moving ice that erode the land as they move. Due to their sheer mass, glaciers flow like very slow rivers. Glaciers store about 75% of the world's freshwater. Some glaciers are as small as football fields, while others grow to be over one hundred kilometers long (about 100,000 football fields, or 62.2 miles).

How do they form?

Glaciers begin to form when snow accumulates and remains in an area year-round. If temperatures do not rise enough to completely melt the snow, snow continues to accumulate. Each year, new layers of snow bury and compress the previous layers. The weight of the overlying snow puts enough pressure on the bottommost snow layers to compress them into large, thickened ice masses. This compression forces the snow to re-crystallize. During recrystallization, the crystals interlock to create ice that essentially behaves like rock. Once a mass of compressed ice reaches a critical thickness, it becomes so heavy that it begins to move.

How do glaciers move?

  • Internal deformation: Ice buckles under its own weight because of gravity. As this happens, the mass of ice crystals in the glacier slowly changes shape without completely breaking or melting, causing it to flow downward or outward. The thicker and warmer the ice is, the faster the flow. Movement of a glacier by internal deformation is very slow, on the order of tens of meters per year.
  • Basal sliding: Basal sliding occurs when water is present under the glacier. Water is formed toward the bottom of the glacier because of the pressure of the overlying ice. This melt water reduces friction and allows the ice to move more quickly. Water may also come from surface water that has worked its way through the cracks in the ice. Or, it may originate from melting upstream in the glacier.
  • Deforming substrate: Water is not the only material that can cause sliding. The rock debris under the ice sheet can also increase movement at the base of a glacier. If the glacier is sitting on a soft or loose bed of sediment that contains water, the sediment can move and carry the ice sheet with it.

What are some glacial features?

The glacier terminus is the leading edge of a glacier. The accumulation zone is the area where snowfall is added (usually near the top of the glacier). The ablation zone is the area of the glacier where glacial ice is being lost through evaporation, melting, or calving (see below). When a glacier flows rapidly over uneven land, the top of the glacier bends and breaks, creating huge cracks called crevasses. Internal stresses build up in the ice that cannot be relieved by internal deformation alone, and crevasses form at the surface of the glacier. Glaciers leave grooves, gouges, and scratches on the landscape called glacial striations that record their passage.

What are the different kinds of glaciers?

  • Ice sheets, ice caps, and ice fields are all masses of glacial ice of various sizes that spread laterally across the landscape and vary in size. Ice caps are smaller than ice sheets. Ice fields are similar to ice caps, but are typically smaller, and are therefore more influenced by the underlying topography.
  • Ice shelves occur when ice sheets extend over water. They are large, flat-topped sheets of ice that are attached to land along one side and float in an ocean or lake on the other side.
  • alpine glacier
    Glacial melt from an alpine glacier.
    Mountain (Alpine) glaciers develop in high mountainous regions, often flowing out of ice fields. The topographical features of the surrounding mountains define their shape and control the direction of their flow. Mountain glaciers include cirque glaciers, named for the bowl-shaped depressions they occupy; piedmont glaciers, which occur when steep valley glaciers spill out into relatively flat plains; and valley glaciers that flow within the walls of a mountain or river valley. Valley glaciers may be very long, often flowing far enough to reach sea level.
  • Tidewater glaciers are valley glaciers that flow far enough to reach out into the sea. Tidewater glaciers “calve,” which is when portions of a glacier break off into the sea, becoming small icebergs.
  • Hanging glaciers cling to steep mountain slopes and terminate at or near the top of a cliff; they are wider than they are long. Hanging glaciers are common in the Alps, where they often cause avalanches due to the steep slopes with which they are associated.
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