Saturday, 12 July 2014 10:53




Mountains make up a large proportion of the earth’s surface. Based on their mode of formation, four main types of mountains can be distinguished.

1. Fold mountains: These mountains are by far the most widespread and the most important. They are caused by large-scale earth movements, when stresses are set up in the earth’s crust. Such stresses may be due to the increased load of the overlying rocks, flow movements in the mantle, magmatic intrusions into the crust, or the expansion or contraction of same part of the earth. When such stresses are initiated, the rocks are subjected to compressive forces that produce wrinkling or folding along the lines of weakness.

Folding effectively shortens the earth’s crust, creating from the original level surface a series of ‘waves’ of rocks. The crests or upfolded waves are called anticlines and the troughs or downfolds are synclines. Due to the complexity of the compressional forces in the great fold mountains of the world (Himalayas, Rockies, Andes and Alps), the folds have developed much more complicated forms.

Since the rock strata have been elevated to great heights, fold mountains may be called mountains of elevation. The fold mountains are also closely associated with volcanic activity; they contain many active volcanoes, especially in the Circum-Pacific fold mountain system. They also contain rich mineral resources such as tin, copper, gold and petroleum.

2. Block mountains: When the earth’s crust bends folding occurs, but when it cracks, faulting takes place. Faulting may be caused by tension or compression, forces which lengthen or shorten the earth’s crust, causing a section of it to subside or to rise above the surrounding level.

Earth movement generates tensional forces that tend to pull the crust apart, and faults are developed. If the block enclosed by the faults remains as it is or rises, and the land on either side subsides, the upstanding block becomes the horst or block mountain. The faulted edges are very steep, with scarp slopes and the summit it almost level, e.g. the HunsruckMountains, the Vosges and Black Forest of the Rhineland.

Tension may also cause the central portion to be let down between two adjacent fault blocks forming a graben or rift valley, which will have steep walls. The East African Rift Valley system is 3,000 miles long, stretching from East Africa through the Red Sea to Syria.

Compressional forces set up by earth movements may produce a thrust or reverse fault and shorten the curst. A block may be raised or lowered in relation to surrounding areas. In general large scale block mountains and rift valley are due to tension rather than compression. The faults may occur in series and be further complicated by tilting and other irregularities. Denudation through the ages modifies faulted landforms.

3. Volcanic mountains: These are, in fact, volcanoes which are built up from material ejected from fissures in the earth’s crust. The materials include molten lava, volcanic bombs, cinders, ashes, dust and liquid mud. They fall around the vent in successive layers, building up a characteristic volcanic cone. Volcanic mountains are often called mountains of accumulation. They are common in the Circum-Pacific belt and include such volcanic peaks as Mt. Fuji (Japan), Mt. Mayon (Philippines), Mt. Merapi (Sumatra), and Mt. Cotopaxi (Ecuador).

4. Residual mountains: These are mountains evolved by denudation. Where the general level of the land has been lowered by the agents of denudation some very resistant areas may remain forming residual mountains, e.g. Mt. Aravalli and Mt. Rajmahal in India.