LANDFORMS FORMED BY INTERNAL FORCES

MOUNTAINS According to the Penguin Dictionary, mountain is a portion of the land surface rising considerably above the surrounding country either as a single eminence or in a range or chain. Some authorities regard eminences above 600 m (2,000 feet) as mountains, those below being referred to as hills.
Mountains make up a large portion of the earth's surface. They are formed in the process of orogenesis. Based on their mode of formation, four main types of mountains can be distinguished.

1. Fold Mountains Fold mountains are formed under compression in which the sedimentary rock strata are squeezed into a succession of folds. Folds are the buckling of once horizontal rock strata. Horizontal compression results in the formation of upfolds called anticlines and downfolds called synclines.
Fold mountains are the most widespread. They are of two types: young fold mountains and old fold mountains. Young fold mountains have been formed relatively recently and are higher than the old fold mountains. They have pointed peaks and rugged features like steeper slopes and deeper valleys. Examples include the Andes, the Rockies, the Alps and the Himalayas. Old fold mountains have been formed long ago. They have rounded peaks and gentler slopes. The Appalachians, the Urals and the Aravalis are good examples of the old fold mountains.

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. The land between the two parallel faults either rises, forming block mountains or horsts, or subsides into a depression termed as a rift valley or graben.

The Arabian Peninsula, the Sinai Peninsula, the Vosgus (France), Black Forest mountains (Germany), and Salt Range in Pakistan are cited as typical examples of block mountains. In India, the Vindhyas and the Satpuras are examples of block mountains.

3. Volcanic Mountains Volcanic mountains are, in fact, volcanoes which are built up from material ejected from fissures in the earth's crust. These materials include molten lava, volcanic bombs, cinders, ashes, dust and liquid mud. Volcanic mountains are often called mountains of accumu­lation, as these are formed by the accumulation of volcanic material. They are common in the Circum-Pacific belt and include such volcanic peaks as Mt Fuji (Japan), Mt Mayon (the Philippine), Mt Merapi (Sumatra), Mt Agung (Indone­sia) and Mt Catopaxi (Ecuador).

4. Residual Mountains Residual mountains owe their present form due to erosion by different agencies. That is why they are also known as relict mountain or mountain of
circumdenudation. The residual mountains stand alone in the surrounding area reduced in height. Examples of residual mountains include Mt Manodnock (USA), and the Nilgiri,