A characteristic feature of glaciers is their ability to flow. When sufficient ice gets piled up, it exerts substantial pressure on the ice in the lower layers and it acquires plastic properties which enable the ice-mass to move outward or downhill and thus an active glacier comes into being.
When there is thick accumulation of ice on a slope the glacier under the influence of gravity begin to flow slowly down a valley until it reaches a point where the rate of melting exactly balances the increment of ice.
A glacier may extend far below the snow-line. When the flow of ice exceeds the amount lost by melting the glacier advances, and if the melting is in excess, the termination of the glacier recedes up the valley and the process is called the retreat of a glacier.
On level surface the movement of ice is not ordinarily noticed. In such cases, the thickness of the snow in the snow-fields increase indefinitely and the flow is along the radii from the centre to the periphery.
As we know, the accumulation of snow is more in the central part of the glacier and there is a decrease in the thickness of icye near the marginal parts. Accordingly the pressure on the lower-layers of ice in the central parts is more where they acquire plasticity and start moving along the radii towards the periphery.
The melting of glacier becomes more towards the peripheral region and the glacier thins out closer to the margin. Thus differential pressure on the glacier plays an important role in its movement on a level surface.
Glacier motion is very slow and its velocity (i.e. the rate of movement) varies from a few centimeters per day to a few metres per day. The rate of movement depends on a number of factors, such as:
(i) Thickness of the glacier;
(ii) Gradient of the slope which it covers;
(iii) Temperature of the ice;
(iv) Rate of evaporation and melting;
(v) The intensity of retarding friction along the slope, etc.
Apart from the above, it is also important to know that the movement of the glaciers is more in the central parts than at the sides. At the sides frictional drag retards the rate of movement and therefore the lateral parts move slowly in comparison to the middle parts. Some mountain glaciers move very rapidly and are called Surging Glaciers.
The rate of movement of the Himalayan glaciers varies from 2 to 4 metres per day. The glaciers of the Alps move at the rate of 0.1 -0.4 metres per day. Some of the glaciers of Greenland move at a rate of about 20 metres a day.