Glacial Movement
Glacier movement is not constant over time, or even through the glacier itself.
Factors Affecting Rate of Movement:
Thickness: a glacier moves because pressure generated by its own weight causes it to deform and/or slide, consequently, thick glaciers often flow faster than thinner glaciers. This also explains why high velocity glaciers tend to occur in areas of high snowfall.
Temperature: in general, temperate glaciers flow at greater velocities than polar glaciers. This is because temperate glacial ice is warmer and is therefore able to deform more easily and, further, the presence of meltwater at their base promotes basal sliding.
Bedrock: more rapid movement occurs over easily deformable rock, such as clay or shale, because bedrock deformation increases the rate of basal sliding. Greater velocities also occur over impermeable rock surfaces
Types of flow:
Basal slippage- Lower ice melts allowing glacier to move as one body (temperate glaciers)
Deformation- Individual ice crystals moving within the glacier. Ice at the surface and in the middle of glacier move faster due to less friction (temperate and polar glaciers).
Compressional flow- Reduction in gradient leads to glacier de-acceleration so ice thickens.
Extensional flow- Gradient becomes steeper so glacier accelerates, and ice thins.
Rotational flow- Ice in cirque pivots around a point which deepens corrie floor.
Regelation creep- Due to pressure melting point (melting point drops below 0 degrees), water melts, flows to an area of less pressure and re-freezes.
Glacial surge- Excessive build up of meltwater leads to ice moving forward rapidly.
Factors Affecting Rate of Movement:
Thickness: a glacier moves because pressure generated by its own weight causes it to deform and/or slide, consequently, thick glaciers often flow faster than thinner glaciers. This also explains why high velocity glaciers tend to occur in areas of high snowfall.
Temperature: in general, temperate glaciers flow at greater velocities than polar glaciers. This is because temperate glacial ice is warmer and is therefore able to deform more easily and, further, the presence of meltwater at their base promotes basal sliding.
Bedrock: more rapid movement occurs over easily deformable rock, such as clay or shale, because bedrock deformation increases the rate of basal sliding. Greater velocities also occur over impermeable rock surfaces as compared to permeable since the percolation of meltwater into permeable rock masses will reduce lubrication at the ice/rock interface and retard flow.
Gradient: glaciers with steep surface gradient flow faster than those with gentler gradients, since the former are affected by greater gravitational forces.
Altitude: affects precipitation and temperature; the greater the precipitation and lower the temperature, the greater the chance of a positive mass balance.
Friction: bedrock and valley walls create friction that must be overcome before a glacier can move. Glaciers flow faster towards their centre.
Glacier Mass: the heavier the ice the more force is needed to overcome increased friction
Bedrock Permeability: temperate glaciers move faster over impermeable surfaces as basal meltwater is retained – aiding slippage.
Meltwater: this lubricates the base of the glacier, thus reducing friction.