Glacier as geological agent

Glacier

The glacier is the large slow moving masses of the land ice formed by the crystallisation of the snow. More than 50% of the world’s land area is occupied by the flowing ice. In simple terms, a glacier is defined as the large natural accumulation of the ice with the downward or outward movement from the snowfield under the influence of the gravity.

Formation of the glacier

The glacier is formed from the accumulation of snow and then involves the subsequent transformation. For the formation of the glacier, a low mean annual temperature falling as snow, a favourable landscape, and the existence if the shallow slopes and depressions protected from the sun and the wind are necessary.

The areas where the snowfall is heavy and the average annual mean temperature is sufficiently low, the snow remains throughout the year. In the frigid zones even the land, which lies only slightly above sea level may be covered by the snow and ice. The lower limit of the perpetual snow is known as the snowline. The height of the snowline varies with the latitude of the place and is modified to some extent by the local climatic conditions and the topographic pattern. The Himalayas snowline is about 5100m.

With the falling of the temperature below 0 ^oC, some atmospheric moisture is precipitated in the form of the hexagonal ice crystals commonly known as the snowflakes, which is highly porous having specific gravity 0.05. With the successive falling causes the compaction of the lower layers. In the summer, the snowflakes melt at day and freezes at night. The melted water seeps deeper into the loose snow and melts the snow crystals further down. When the melted snow refreezes around the snow crystals, they assume the shape of the grain which forms ice granules. When the snow becomes granular, its porosity decreases and the specific gravity increases and the granular mass is firm. This process repeats and the firn becomes more compact, the individual crystalline aggregate converts into frozen mass forming which is known as glacier ice. The glacier ice is transparent with specific gravity 0.8 and impermeable.

Glacier movement

Physically the glacier appears to be the stationary mass of the ice but the glaciers like rivers move much more slowly. The rate of the forward progress ranges from less than an inch to as much as 100 ft. per day. When the sufficient amount of the ice is accumulated, the pressure is exerted on the ice in the lower layers and it requires plastic properties which enable the ice mass to move outward or down the hill and the glacier comes into the existence. The thickness of the ice is more in the central than in the periphery which causes the more pressure at the central part and the movement of the glacier is more at the central parts than in the periphery.

The various numbers of factors control the rate of movement of the glacier which is as follows:

• Thickness of the glacier
• Gradient of slope it covers
• Temperature of ice
• Rate of evaporation
• The intensity of retarding friction along the slope.

Types of the glacier

The glacier is usually divided into:

1. Mountain or valley glacier
2. Piedmont glacier
3. Continental glacier or ice sheet glacier

1. Mountain or Valley glacier

The valley glacier is also known as alpine or mountain glacier. These glaciers originate at the snowfields at the end of the mountain valleys. Their downward movement follows old stream-cut valley. The valley glaciers are characterised by the distinctly supply area and the drainage area.

2. Piedmont glacier

Sometimes two or more valley glaciers emerge from the adjacent mountain valleys on the plains below. The lower end of such glacier unites to form a broadly rounded mass of the ice is called piedmont glacier. These are intermediate in form as well as origin between the valley glacier and continental glacier. These glaciers are larger in the dimensions but the rate of the movement is quite slow.

3. Continental glacier

The broad ice that covers large land areas are known as ice thick, this glacier spread outward from the center of the landmass until they cover much of the continent's highland lowlands. The movement is very slow which takes place at the bottom but the top of the ice sheet remains stationary.

Surface feature of the glacier

Glaciers usually have rough and uneven glacier due to the presence of the gaping fissures known as crevasses. The movement through the mountain valley is the major irregularities of the earth’s surface, which gives the differential movement within the glacier mass resulting crevasses.

Geological action of valley glacier

Erosion

The glacier as a geological agent consists erosion, transportation and the deposition. The glacier erosion is accomplished by following features:

1. Plucking

Plucking involves the lifting out of the bedrocks by moving the ice, lose with the freezing and thawing of water in fracture beneath the ice. It involves two processes, frost wedging and quarrying. The frost wedging causes shattering of the rocks while the quarrying causes lifting of the shattered rocks.

1. Rasping

Rasping is carried out by the abrasion. The plucked blocks and the other rock debris scratch and polish the bedrock over which the glacier passes.

• Avalanching

It is the process of the mass wasting.

Transportation

The glacier carries great quantities of the earth’s material and some of the rock fragments in which most of them are transported on the top of the glacier and constitutes the super-glacial load. The material frozen at the interior is known as the glacial load, while the sub-glacial consists of the rocks and soil carried at the bottom of the ice mass. The sub-glacial load is responsible for much of the abrasive action of the glacier.

Deposition

When the ice begins to melt, the transportation capacity of the glacier slows down resulting in the deposition of the materials carried by the glacier itself. The deposition mainly takes place in the downstream parts of the glacier. The rocks and the soil randomly intermingled without regard to the size, weight or composition as the glacial load and when the ice melts, it will drop the debris, forming the variety of the deposits, which are designated as glacial drifts. There are two types of glacial deposits:

• Unstratified deposits

The unstratified deposits are also called till laid down by the ice deposits of till are composed of the rocks of varying size. Boulder clay is the most extensive of the drift deposits consisting of the large flat masses of all the sizes. Deposits of the till from topographic features are known as moraine ridges of mounds or boulder, gravels, sand and clay deposit by the glacier.

• Stratified deposits

The stratified deposits are also known as the glaciofluvial deposits. These are the deposits formed by streams of glacial melt water called outwash. They are sorted and stratified. The melted water deposits coarse material near the end of the end of the glacier and the finer material are carried further away. The important features are outwash plain, valley trains, eskers kames, kame terraces, kettles.

References:

Keller, E.A. Environmental Geology. Columbus, Ohio: Charles E. Miller Publishing Company, Bell and Howell Company, 1985.

Mahapatra, G.B. Textbook of Physical Geology. Shahadra,Delhi-110032: CBS Publishers and Distributers Pvt.Ltd., 1992.