On the basis of their stage of development, size, shape and the relationship between the supply and flow areas, three types of glaciers have been distinguished. These are as:-
1. Mountain or Valley glacier;
2. Piedmont glacier;
3. Continental ice-sheets;
1. Valley Glaciers
These are also known as Mountain or Alpine- type of glaciers. They are confined to the pre-existing valleys in mountain areas and are fed by snow-fields which lie further up, above the snow-line. The ice flows down the mountain valleys with steep slopes to heights determined by the rate of supply from above and the rate of melting as it reaches warmer levels. They frequently descend below the regional snow-line.
The valley glaciers are characterized by a distinctly expressed supply area i.e. the snow-field where the snow is converted into firn and then into ice and a drainage area i.e. the area over which the glacier- ice moves and flows.
While the supply area is situated above the snowline the drainage area is made up of mountain valleys situated below the snow-line.
According to their characteristic features several kinds of valley glaciers have been distinguished, such as:-
(i) Simple glaciers, which are isolated glaciers consisting of single flow without any tributaries.
(ii) Complex or Polysynthetic glaciers, consist of a number of coalescing glaciers, the pattern of which resembles that of a river with tributaries.
Apart from the above, the types of glaciers included in the category of valley glaciers are as follows:
These glaciers originate in deep arm-chair shaped hollows situated at the valley heads. They are often at the snow-line and with hardly any flow.
These are valley glaciers which have become so thick that they spill over dividing ridges and join with other glaciers in adjoining valley. These are also known as avlanches or Hanging Glaciers.
In the case of valley glaciers the topography controls the motion and the movement is in one direction only clown the valley slope mainly due to gravity and the glaciers move several metres a day.
Valley glaciers are common in the young fold mountain areas such as the Alps, Himalayas, Tienshan, Pamirs and Caucasus. The Hubbard glacier in Alaska is the longest valley glacier in the world with a length of about 130 kms. Most of the Himalayan glaciers are small. The Gangotri glacier is about 24 kms. in length and the Siachan glacier is about 72 kms long.
2. Peidmont Glacier
These are also known as Intermediate type of glaciers. They are intermediate in form as well as origin between the valley glacier and the continental ice-sheets. Sometimes in colder climates valley glaciers may extend over a low-land and spreads out horizontally.
Several glaciers thus unite at the base of a mountain range forming an extensive and comparatively thick sheet of ice covering the low-lying ground. Such an ice-sheet is called a piedmont glacier. The Malaspina glacier of Alaska is the best known example of piedmont glaciers.
These glaciers are much larger in dimension than the valley glaciers. Their rate of movement is quite slow.
In contrast to the formation of ice-sheet at the foot-hill region of a mountain range the intermediate type of glaciers also include the formation of ice-caps on the flattened surfaces of the summits of ancient mountains covering them for hundreds of square kilometres.
These are also called plateau glaciers. Like continental ice-sheets they lie as a continuous mass covering a huge area on the plateau and in moving from the centre to the margins, these glaciers emerge through river-valleys and resemble mountain glaciers.
Thus plateau glaciers combine the characteristics of both continental ice-sheets and valley glaciers. Such glaciers are common in Scandinavia (Norway) and are therefore sometimes referred to as Scandinavian-glaciers.
3. Continental Glaciers or Ice-Sheets
These are the largest forms of accumulation of ice and they cover vast areas of the landmass including even the cliffs of mountains. At present they occur mostly in Antarctica and Greenland.
These glaciers are of enormous size and immensely thick. The thickness of the ice-sheet may reach even thousands of metres, as such all irregularities of relief are hidden by it. The topography has little or no control over the movement of such ice- sheets.
The surface of the ice-sheets has a plain-convex shape which resembles a shield. Their shape is not controlled by the bottom relief. Unlike the valley glaciers, they do not have distinctly separate supply and drainage area.
The movement of ice is radial. The movement of the ice is in many directions from points of high pressure within the ice-sheet towards the margin. The movement is very slow which takes place at the bottom, while the top of the ice-sheet remains almost stationary.
At the margins, the thickness of theice-sheet diminshes and mountain peaks and individual cliffs project through the ice, which are called nunataks by the Eskimoes.
Glaciers of the continental type are formed in polar regions and are located almost at sea-level.
At present, the Antarctica ice-sheet is the biggest continental type of glacier. In Greenland the ice-sheet covers almost the whole of the continent. The Antarctic ice-sheet covers 13 million square kilometres.
The complete ice-sheet does not reach the sea. The ice which enters the sea tends to float upon the water and its marginal part is buoyed up. It is thus easily broken by the waves giving rise to separated ice-mass known as Icebergs. These icebergs are large pieces of ice floating on the sea.
An important glacial feature of Antarctica is the presence of huge plates of floating glacial ice, known as ice-shelves. As the ice-sheets are not confined by valley walls the ice over-flows the coasts and presses out to sea as colossal floating ice-barriers (e.g. the Great Ross Barrier) from which the great tabular iceberg: of the Antarctica seas break off.