Glaciers have two main sections: the accumulation area and the ablation area. The accumulation area is where temperatures are cold and snow collects, adding mass to the glacier. The ablation area is where temperatures are warmer, so some of the glacier melts. The ablation area could also be the point where the glacier meets the ocean. As the glacier extends onto the water, the ice floats, creating an ice shelf. Tidal forces flex the ice shelf up and down until it finally gives way. When huge ice chunks fall off of a glacier into the ocean, its called calving. The resulting floating ice chunks are known as icebergs.
The boundary between the ablation and accumulation areas shifts seasonally. In the spring and summer, there's more melting (ablation) going on, so the ablation area is larger. In winter, the accumulation area grows. The average balance between areas determines the stability of the glacier. A glacier with a much larger average accumulation area is growing, while one with a larger ablation area is a glacier that's shrinking and could eventually disappear. When the two areas are roughly equal, it's considered a stable glacier. Climate change can affect glacier stability over a long term. Recent trends suggest that many of the world's glaciers are shrinking at alarming rates [source: University of Zurich].
The front of a glacier is known as the terminus. If it's a stable glacier, the terminus will always be in the same place. The glacier is still moving, but an equal amount of ice is added to and melted away from the glacier each year.
In addition to crevasses, the thermal and dynamic forces that work on a glacier create several other interesting features.
- Moulins are vertical tubes that carry meltwater down through the glacier.
- Seracs are jagged columns or blocks of ice that form when softer ice falls away from pockets of dense ice, or when multiple crevasses intersect. They are dangerously prone to collapse.
- Ogives are wavelike structures that form at the base of an icefall (a place where the glacier moves over a cliff).
There are two main types of glaciers: alpine glaciers and ice sheets. There are only a few true ice sheets, but they're incredibly huge. One covers Antarctica, and another covers Greenland and a large area of the Arctic Ocean [source: Gallant]. Ice sheets move primarily by spreading, and may actually be made up of several smaller glaciers that form a conglomerate.
Alpine glaciers form at high elevations (not just the Alps) and "flow" down the mountain, usually through a glacial valley. Their movement is caused by basal slip.
Next, let's find out how glaciers have actually changed the shape of the planet.