In terms of origin and form, mountains can be divided into four main types: 1. Fold Mountain 2. Block Mountain 3. Mountain of Accumulation 4. Residual Mountain.
Type # 1. Fold Mountain:
A fold has two parts. The up-fold or bulged part is called anticline and the down-fold or the depressed part is called syncline.
The crust of the earth is made up of different strata of rocks. As a result of movements inside the earth, these rock strata at times are forced up or down and thus folds are created. Such folds can be said to be caused by the influence of internal forces.
On the other hand, the upper parts of the earth’s crust are subjected to erosion due to the influence of external forces. These eroded materials are deposited in low lying areas. When these deposits accumulate in the low lying areas for thousands of years, the weights on the bed increase. The bed of the depressed areas may thus subside under the weight and its two rims may come closer.
In the course of time, partly under the pressure from the materials accumulated above and partly due to pressure from the sides of the depression, the deposits turn into stratified sedimentary rocks. At the same time, these rocks become folded and Fold Mountains come into being. These fold mountains contain many fossils of marine origin. This proves that fold mountains have their origin in the depressed parts of the oceans.
Most of the lofty mountain ranges on the earth’s surface are Fold Mountains. The Himalayas, the Alps of Europe, the Rockies of North America and the Andes of South America are all examples of Fold Mountains. In the geological sense of time, these are rather new fold mountains. The Urals, Appalachia and the Tien Shan are some examples of ancient fold mountains. The Aravalli mountain of India is supposed to be the oldest fold mountain in the world.
Type # 2. Block Mountain:
As a result of violent movements inside the earth caused by the influence of internal forces, the earth’s crust at times cracks. The cracks are referred to as faults. If a number of parallel faults develop in an area, then the parts between two faults may rise up or subside. The subsided parts are known as rift valleys and up-thrown parts are known as block mountains and horsts.
The Black Forests of Germany, the Vosges of France and the Satpura of India are some notable examples of block mountains. The valleys of Jordan in West Asia, the Rhine in Europe and Narmada in India are examples of rift valleys.
Type # 3. Mountain of Accumulation:
Fluctuation of pressure and temperature often renders the materials deep inside the earth to be in liquid state. The materials then begin to increase in volume and try to escape to the surface. When the pressure from the molten materials becomes very high, the weaker spots in the rock strata crack and the materials come out to the surface through these cracks in the form of lava, ash and smoke.
These materials start accumulating in the form of cones, cool off gradually and become mountains. At times again, molten and gaseous material cannot come out to the surface. These then accumulate below the surface and begin to cool off slowly. In such a case, these materials start rising up due to upward pressure and eventually take the form of a mountain. This is known as Mountain of Accumulation.
Type # 4. Residual Mountain:
Since the rocks on the surface of the earth are of different types, their rate of erosion varies. Some rocks are eroded quickly and some slowly. The rocks are reduced by weathering and erosion. The softer rocks in the higher regions of earth’s surface are rapidly eroded away, but the harder rocks are left out.
The harder rocks stand out as highlands or hills. These are called Residual Mountains. The mountain ranges of Norway, Sweden, the King of Africa, the Sierra of Spain, the Eastern Ghats, Western Ghats, Pareshnath and Aravallies of India are some well-known examples of Residual Mountains.