A glacier is a mass of snow and ice that moves slowly over the land away from its place of accumulation. Avalanches are masses of ice formed on cirques that flows down hill with a great velocity.
Formation of Glaciers:
Glaciers originate from the snow field which may form on plains, plateaus or mountains. Three conditions are necessary for the formation of a snow field: first abundant snow fall; second the cold climate and third, low rate of summer melting and evaporation. In cold regions, precipitation takes place in form of snow fall or ice. Snow consists of delicate thin tabular hexagonal crystals in form of tiny flakes like foam or cotton.
When it falls on the ground, it is in form of granules and is called as neve. Successive falls press this granular ice into a flaky structure, air is squeezed out and the mass assume a granular structure. This form is known as firn.
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This change is brought about by partial melting and subsequent freezing known as regelation. Firn is intermediate between snow and compact ice. Gradual accummlation of snow, neve and compact ice give rise to snow field.
The lower or outer edge of a snow field above which all the snow does not melt during summer is known as snowline. Snow line occurs at any altitude at high latitudes and at high altitude at low latitudes.
Thus, snow line occurs at about 5,500 to 6,000 meters in the equatorial belt, around 4,000 meters in the Himalayas, while almost at sea level in Antarctica and the Arctic regions. On the surface of glacial ice, cracks are developed due to various reasons. These cracks are termed as crevasses. When the glacier bends over an inclined surface, the cracks parallel to the rocky wall are known as bergschrund. Creavasses may be longitudinal, transverse or marginal.
Movement of Glaciers:
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The lay man’s concept is that the glaciers are streams or rivers of ice. Actually, the repeated processes of melting and freezing with fracture, shear and gravity are responsible for the movement of glaciers. Presence of shear planes, thrusts and bands in the lower parts of the glaciers act as slippage planes.
The rate of motion varies from glacier to glacier. Long and large glaciers move faster than the smaller ones. Generally the larger glaciers move faster than smaller ones but during summer all the glaciers move faster in comparison to winter season. Even within the body of glacier there is differential velocity i.e. the top and central part of the glaciers move faster than that of the bottom and sides which are obstructed by friction of walls.
The movement varies from a fraction of a centimeter to nearly 30 meters per day. In the Himalayas, most of the glaciers move from a few centimeters to a meter per day. Fedechenkow glacier in the Pamir Himalayas moves more than 10 meters per day.