As Davis suggested a glacial topography is a climatic accident that happens to normal cycle of erosion, i.e., climate gets very cold and the river freezes. Instead of rivers of water there are rivers of ice, called ‘Glaciers’ which operate as the main geomorphic agent
Long well and Flint have defined a glacier as follows:- A glacier is a body of ice. consisting mainly of recrystallized snow, flowing on a land surface.
[Introduction to Physical Geology Published by John Wiley & Sons, New York and London-1961]y
In simple terms a glacier may be defined as a large natural accumulation of ice with a downward or outward movement from the snow-field under the influence of gravity. They move down the preexisting valleys or radiate out in great lobes.
At present it is estimated that approximately 10 percent of the earth’s land surface and 7 per cent of the oceans are covered by glacier ice. The largest extent was attained during the Pleistocene or Great Ice Ages.
Formation of Glaciers
Glaciers originate in snow fields. Glaciers are formed through the accumulation of snow and its subsequent transformation. In areas where annual snow fall exceeds the amount of loss of snow due to evaporation and melting, snow starts accumulating. Snow accumulates when the air temperature is too low for it to melt. As we know, low temperature prevails in regions of high latitudes and at the mountain summits where there is not enough warmth in the summer time to melt the accumulated snow. Thus the snow mass grows from year to year.
Ice forms only where the snow cover is thick and continuous and is not melted during a summer thaw. A number of factors play important roles in the process of changing snow in to ice. These factors are:-
1. A low average annual temperature.
2. Continuous and abundant precipitation falling as snow.
3. The altitude in the mountainous regions and the latitude of a locality which effects the formation of glaciers to a major extent; since the climatic zones of the earth are related to the degree of latitudes.
4. Existence of a suitable landscape and relief forms.
With the falling of temperature below 0°C, some atmospheric moisture is precipitated in the form of hexagonal ice crystals commonly known as snow-flakes. Newly fallen snow is highly porous with a specific gravity of 0.05. Successive snow falls bring about the compaction of the lower layers and the increasing pressure causes a slight lowering of the melting point of snow.
Besides, in the summer the snow at the surface begins to thaw, the snowflakes melt and at night during the recurrence of frost they refreeze. A part of the melt water seeps deeper into the loose snow through the interstices and melts the snow-crystals further down.
When the melt water refreezes around the snow crystals they assume the shape of grains. Thus ice granules are formed. As the snow becomes granular its porosity diminishes and specific gravity increases.
This process is known as regelation. The granular masses of ice are called ne ‘ve’ in French and firn in German language. Firn is a dull white, impermeable and structure less form of ice.
This process, as described above, is repeated with each new snowfall. With further addition of snow and under the influence of pressure the fun becomes more compact and the separated crystalline aggregates merge into frozen masses forming what is known as glacier- ice.
The glacier-ice is transparent and has a bluish tint. It has a specific gravity of 0.8 and is impermeable. The glacier-ice constitutes the main body of glaciers. Thus Arthur Homes has defined glaciers as follows:
Glaciers are masses of ice which, under the influence of gravity, flow out from the snow fields where they originate.
[Principles of Physical Geology, Published by the English Language Book Society & Nelson 1975]
As we know, temperature drops by 0.5° to 0.6°C per each 100 metre rise in altitude; besides the temperature also decreases with an increase in the latitude i.e. from the equator to the poles.
In any region, the lowest limit of perpetual snow is known as the permanent snow line or commonly as Snow Line. Above the snow-line are the snow fields that persist throughout the summer season.
The height of the snow line varies with the latitude of the place and are modified to some extent by the local climatic conditions and topographic pattern. In the polar regions it lies at altitudes close to sea-level (Antarctica); in Norway and Alaska it is at an altitude of 1500 metres, in the Himalayas at 5100 metres in Assam- region and approximately 6000 metres in Kashmir region. It is important to note that permanent snow fields occur in almost all the continents except Australia.
An upper limit of the snowline has also been imagined on the basis of the fact that the moisture content of the atmosphere diminishes with the rise of altitude and that there may be conditions unfavourable for snow to form at some particular height.
This gives an impression that the peaks of the mountains would be free from snow even after their tops reach that height. The term ‘Hionosphere’ is used for the region between the Permanent Snow Line and the imaginary Upper Snow Line.