- Glaciers flow under the force of gravity as snow accumulates on the upper parts of the glacier and wants moves down slope.

- The snow compresses to become ice and flows through the glacier into the ablation zone where it is lost.

- If the accumulation equals the ablation than the glacier is said to be in equilibrium and its position will not change. This does not mean that the ice will not flow!

http://users.aber.ac.uk/tmh/glacier2.htm

Accumulation Zone
The area where inputs occur into a glacier system. This usually occurs near the top of the glacier or ice sheet and such inputs to the system include snowfall, wind blown snow, rain and avalanches.

Ablation Zone
The region in which more mass is lost than gained in a glacier system. This usually occurs at the end and sides of the glacier. Forms of losses include wind ablation, avalanching, iceberg calving and melting.

Glaciers flow through three different mechanisms: (1) by internal deformation; (2) by basal sliding; and (3) by subglacial deformation.

Glacier Flow Mechanisms

Internal Deformation
Ice deforms under its own weight due to gravity and the movements of tiny ice crystals. Thicker and warmer ice deforms more rapidly although the overall movement is very slow, only around tens of meters a year. There are two main processes of internal deformation; creep, which forms fold structures, and faulting, which occurs when ice cannot creep fast enough and forms superficial tensional fractures.

Basal Sliding
Enhanced Basal Creep - Stress concentrations around the upstream side of an obstacle result in locally high strain rates which causes ice to accelerate around the obstacle. The basal ice continually modifies its shape to allow a continued sliding. This process works best when the obstacle is over 1m in size.
Regelation - The process allows glacier ice to slide over rough beds by melting and refreezing on the downglacier side. It occurs as the most resistance to glacier movement is provided by the upstream side of obstacles. This results in locally high pressures and the consequent encouragement of ice melting immediately upglacier of the obstacle. The resulting meltwater migrates to the lower pressure area downstream where it refreezes. This process is most effective when objects are less than 1m in size.

Subglacial deformation
Sediment has a lower yield strength than rock and ice and so it is deformable. As the sediment deforms, it moves the ice sheet with it.

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