The surfaces of glaciers are usually rough and uneven because of the presence of gaping fissures known as Crevasses which may be open and visible, but are often masked by snow. The surface part of the glacier is brittle.
Brittleness of the surface part makes it crack as it is subjected to tension, whereas the ice beneath behaves like a plastic substance and moves by slow flowage. Crevasses are seldom more than 30 metres deep and 7 metres wide.
The flow of ice at depth prevents the formation of crevasses at depths of 30 metres or so.
Movement through mountain valleys and major irregularities of the earth’s surface, gives rise to differential movement within the mass of glacier which results in the development of crevasses. However, the formation of crevasses is conditioned by a variety of factors as follows:
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(i) Relief of the subglacial bed;
(ii) Variations in the cross-section of the valley thri glacier moves,
(iii) Thickness of ice,
(iv) Rates of flow of glacial ice i.e. the differential mo middle and marginal part of the glacier, etc.
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On the basis of the mode of formation and nature of the cracks, crevasses are classified into three main types as:
– Transverse crevasses.
– Longitudinal crevasses.
– Marginal crevasses.
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Transverse Crevasses
When there is a slight change in the gradient of the valley or there is a marked steepening of its slope the glacier is subjected to a considerable tension which is relieved by the development of a series of cracks, transverse to the direction of flow, at the bends.
Since the glaciers move more rapidly in the middle than at the sides, these crevasses become curved with the convex side facing downward.
When the change of gradient is more pronounced the glacier is broken into a huge jagged mass of ice pinnacles known as Seracs. These are similar to waterfalls in a river. Accordingly they are also known as ice-falls. An abrupt steepening of the slope forms what is known as rock-step.
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Longitudinal Crevasses
When there is a sudden widening of the valley, the glacier expands sideways and assumes its shape. Such spreading out may develop cracks which are more or less parallel to the length of the glacier i.e. parallel to the direction of flow.
Marginal Crevasses
These crevasses are formed due to the differential rate of movement of glacier at the middle and the marginal parts (i.e. the valley sides). While the middle part of the glacier moves more rapidly, the marginal parts move quite slowly.
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Accordingly cracks are developed along the valley sides which are oblique to the course of the glacier and are pointing to up-hill direction. These crevasses are also known as lateral- crevasses.
A wide and very deep crevasse that opens near the top of the firn field of a cirque where the head of a glacier is pulled away is known as the Bergschrund (in German). Such crevasses usually open in summer.
As the glacier creeps down the slope to the foot-hill region where the gradient is gentle the cracks close up and the crevasses disappear. The presence of debris on the surface of the glacier tends to cause rapid melting and thus sometimes melt a hole in the ice which are known as dust-wells.
The dust-wells often unite forming a depression of the shape of a bath-tub, commonly called bagnoire. The melt water gathering into streams mostly fall into crevasses and by their melting and pot-hole action deep cauldrons are formed in the glacial ice, which are known as glacier mills or moulins. The water escapes to the front of the glacier through a tunnel.