Glacier - Motion

Motion

Glaciers move, or flow, downhill due to the internal deformation of ice and gravity. Ice behaves like an easily breaking solid until its thickness exceeds about 50 m (160 ft). The pressure on ice deeper than that depth causes plastic flow. At the molecular level, ice consists of stacked layers of molecules with relatively weak bonds between the layers. When the stress of the layer above exceeds the inter-layer binding strength, it moves faster than the layer below.

Another type of movement is through basal sliding. In this process, the glacier slides over the terrain on which it sits, lubricated by the presence of liquid water. As the pressure increases toward the base of the glacier, the melting point of water decreases, and the ice melts. Friction between ice and rock and geothermal heat from the Earth's interior also contribute to melting. This type of movement is dominant in temperate, or warm-based glaciers. The geothermal heat flux becomes more important the thicker a glacier becomes.

The rate of movement is dependent on the underlying slope, amongst many other factors.

Glacier speed is not constant across the glacier. The top half of the glacier moves faster than the bottom, presumably because of friction. The sides also flow slower than the middle, also because of friction. The middle of the glacier at the equilibrium line flows fastest, the head and terminus flow slower. Above the equilibrium line the glacier grows faster than it melts; this is called the accumulation zone. Below the equilibrium line, the glacier melts faster than it grows; this is called the ablation zone.