landforms created by glacier

THE WORK OF MOVING ICE :

SNOW – FIELDS :

·           Wherever the rate of snow melting or its evapouration is lower than the rate of snowfall in a year, snow accumulates into great mass of ice.

·           Permanently snow covered regions of this type are called snow - fields.

·           Snow - fields occur in polar regions and on high mountainous areas.

·           Snowfields are always found above the snow line.

·           Snow line is an imaginary line above which there is permanent snow.

·           The height of the snow - line is not uniform and is affected by latitude, amount of snowfall, direction of winds and slope.

GLACIER :

·           A moving mass of ice and snow is called a glacier.

·           In region experiencing snowfall, the snow keeps on accumulating in layers one above the other.

·           Its overlying pressure is applied to the underlying snow.

·           It is so great that snow in lower layers becomes granular, hard and compact.

·           The pressure also quickens the melting of some of the snow, which on refreezing starts turning into a granular ice.

·           Again it is the pressure of the overlying layers which makes this solid mass of ice mobile.

·           This great mass of ice moving more under its own weight is called a glacier.

·           Its velocity is very low and it moves from a few centimetres to a few metres in a day.

Types of Glaciers :

 (i) continental glaciers and

(ii) valley glaciers.

(i) Continental Glaciers :

·           A thick ice sheet covering vast area of land is called a continental glacier.

·           The thickness of ice in such regions goes upto thousands of metres.

·           Glaciers of this type build up at the centre and move outward in all directions.

·           Continental glaciers of today are found mainly in Antarctica and Greenland.

·           The precipitation in these regions occurs in the form of snow.

·           It gets accumulated year by year because of relatively slower rate of its melting.

(ii) Valley Glaciers :

·           When a mass of ice from the high mountainous regions starts moving down into the pre-existing valleys, it is called a valley glacier or a mountain glacier.

·           The shape of the valley glaciers depends on the valley it occupies.

·           Where the valley is broad, the glacier spreads outwards and where the valley is narrow, the glacier contracts.

·           The longest glacier in India is the Siachen Glacier in Karakoram range which is 72 kilometres long.

·           Gangotri Glacier in Uttarakhand is 25.5 kilometres long. There are many smaller glaciers in other parts of the Himalaya. Their length varies from 5 to 10 kilometres.

·           The two important rivers of India, the Ganga and Yamuna, origin nate from Gangotri and Yamunotri glaciers respectively.

LANDFORMS PRODUCED BY GLACIER :

(A) Erosional work of glacier :

·           As a glacier moves over the land, it drags rock fragments, gravel and sand along with it.

·           These rock fragments become efficient erosive tools. With their help glacier scrapes and scours the surface rocks with which it comes in contact.

·           This action of glacier leaves behind scratches and grooves on rocks.

The landforms created by glacial erosion are:

(i) Cirque (or Corrie), (firn) :

·           Snow collects at the upper end in a bowl shaped depression, is called cirque.

·           Layers of snow in the process of compaction and recrystallization are called firn.

·           Sometimes the deepest parts of these hollows are occupied by accumulated-water, to form Corrie Lake (or Tarn).

(ii) ‘U’ - shaped Valley :

·           The glacier does not carve a new valley like a river but deepens and widens a preexisting valley by smoothening away the irregularities.

·           In this process the glacier broadens the sides of the valley.

·           The shape of the valley formed in this manner resembles the letter ‘U’.

·           It is therefore called a ‘U’ - shaped valley.

·           Such a valley is relatively straight, has a flat floor and nearly vertical sides.

(iii) Hanging Valley :

·           Just like tributary streams of river, there are tributary glaciers also which join the main glacier after moving over their mountainous path.

·           These tributary glaciers like the main glaciers carve U - shaped valleys.

·           However, they have less volume of ice than the main glaciers and thus their rate of erosion is less rapid.

·           As a result their valleys are smaller and not as deep as that of the main glacier.

·           Due to this difference in deepening; the valley of the tributary glacier is left at a higher level than that of the main glacier.

·           The valley of the tributary glacier just looks like hanging downwards

at the point of its confluence with the main valley.

·           This type of a topographical feature is called a hanging valley.

·           This feature is visible when ice has melted in both the valleys.

·           When the ice in the hanging valley melts, a waterfall is formed at the point of confluence of this stream with the main river.

4.Eskers :

·           Glaciers can also contain sinuous flows of meltwater that occur in ice tunnels at the base of the ice.
·           The beds of these sub-surface glacial streams are composed of layers of sand and gravel.
·           When the ice melts from around the meltwater tunnels, the beds of sand and gravel are deposited on the Earth's surface as long twisting ridges known as eskers.

5.Drumlins :

·           feature of continental glaciation are hill shaped deposits of till known as drumlins .
·           A couple theories exist to explain their formation.
·           The most excepted theory suggests they form when saturated ground sediments oozes up into hollows at the base of an advancing glacier.
·           The sediment is then stretched out and molded into a streamline form as the ice moves forward.

6.Kettle hole :

·           When glaciers are rapidly retreating, numerous blocks of ice can become detached from the main body of the glacier.
·           If glacial drift is then placed around the ice, a depression on the surface called a kettle hole can be created when the ice melts .
·           Kettle holes are commonly found on moraine and outwash plain deposits.
7.Outwash plain :

·             Outwash deposits are formed when sand is eroded, transported, and deposited by meltwater streams from the glacier's snout and nearby till deposits to areas in front of the glacier. 
·           Outwash plain develops when there are a great number of meltwater streams depositing material ahead of the glacier.
8.Kame :

·           Glaciofluvial deposits are also associated with the melting of stagnant ice at the front of the glacier.
·           Where sediment rich water flows into a crevasse or depression in the ice, a conical-shaped pile of sand and gravel, known as a kame .

(B) Transportational work of Glacier :

·           Although the glacier moves very slowly, it drags with it large boulders and rock fragments.

·           Glacier gets this material from the mountain slopes, valley sides, valley bottom and from air.

·           This material is called the load of glacier.

(C) Depositional work of Glacier :

·           When the glacier melts or retreats, it deposits its load in different parts. 

·           The debris thus deposited are called moraines. Depending upon their location in the valley.

moraines are of four types :

(i) Terminal Moraine :

·           When the glacier melts, the debris are deposited at the end of the valley glacier in the form of a ridge.

·           Morainic material ranges from fine clay to large angular boulders.

(ii) Lateral moraine:

·           The moraine which is deposited on either side of a glacier is called lateral moraine.

(iii) Medial moraine:

·           When two glaciers join each other their lateral moraines also join.

·           Moraines thus formed on the confluence of two glaciers are called medial moraines.

(iv) Ground moraine:

·           It consists of deposits left behind in areas once covered by glaciers.