Warm ice has a temperature of around 0 degrees throughout its depth. This means that it is able to realise large amount of melt water, especially in high temperatures during the summer months. Cold ice is constantly below 0 degrees and therefore there is virtually no melt water. This is very relevant because it is the presence of melt water that enables a glacier to move.
Sometimes the ice surges and moves one hundred feet in a day, other times it moves only an inch or two a day so you can hardly notice the movement. No matter how slowly it is moving, when the ice field begins moving it is officially a "glacier." As the ice moves over the ground it starts to bend and crack because it is so big and it is moving over land that is not flat. The rocks also help crack the ice.
Even though glaciers are called "rivers of ice" they do not exactly move like a river. A real river is made of flowing water, but a glacier is made of brittle ice. Scientists have studied glaciers for a long time because they were curious about how they moved.
After a Swiss scientist built a hut on a glacier, he was quite surprised to find that it had moved more than one hundred yards downhill when he returned three years later to do some more research.
Using modern equipment, scientists have recently found that glaciers move in TWO ways. Glaciers slide on water that has melted. They slide along the ground on a thin layer of melt water under the ice. This is known as basal sliding. Basal sliding, the second way ice moves, can happen several different ways. True basal sliding means that the base of the ice sheet is near the pressure melting point, and that some water is present. The pressure melting point is reached because high pressure actually reduces the temperature at which ice will melt. Ice at base of a 2200 meter (1.36 miles) thick ice sheet, will melt at -1.6°C (29.12°F), rather than at 0°C (32°F). The thicker the ice, the lower the temperature at which it will melt, and the higher the chance that some water will be available at the glacier base to enhance movement. Large parts of the West Antarctic Ice Sheet are at the basal melting point, so there may be large areas under the ice sheet where a thin water layer exists.
Water reduces friction, and allows the ice to move faster. A thin layer of water may be present at the glacier base because the ice is at the pressure melting point. Or, the water may come from rain water or surface melt water that has worked through the cracks in the ice. Or, it may originate from melting upstream in the glacier. This water then flows toward the terminus (nose) of the glacier.
The other way is called "creep." The glacier is so extremely heavy that the crystals of ice make layers of ice one layer on top of another layer of ice. Then the different layers of ice start creeping and moving over each other.
Ice deforms under its own weight because of gravity. The deformation actually is because of the sum of tiny movements on the faces of the ice crystals making up the glacier. The thicker the ice, the faster the flow because of internal deformation. The warmer the ice, the faster the movement (hot honey will flow faster than cold honey!). The higher the pressure, the faster the movement.
Movement of a glacier by creep is very slow, and is on the order of tens of meters per year. Much of the movement of the interior of the Antarctic ice sheet is by internal deformation.
Movement by basal sliding is ten time faster than movement by creep. Basal sliding is extremely important in how much a glacier erodes the landscape, and the features that are created by the ice.
The above are the processes by which glaciers move but the reasons behind why they move are simple geographical and physical reasons. 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 ablation 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 down slope, 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.
