Classification of Mountains
Mountains- An uplifted portion of the earth's surface is called a hill or a mountain.
Difference between Hills and Mountains
When the summit or top rises to more than 900 m above the base, then it is termed as Mountain while those with less than this elevation are called Hills.
Classification of Mountains
On the basis of their origin or mode of formation, the mountains are classified as
- Structural or Tectonic
- Residual or Dissected And
- Structural Mountains- These mountain systems are hundreds of kilometres wide and thousands of kilometres long. Many of them lie near or parallel to continental coastlines. All great mountain systems of the earth are of this type. Both the fold and the block mountains are included in this type.
Fold mountains are formed due to the folding of the crustal rocks by the compressive forces which in turn are generated by endogenetic forces.
- Folded mountains which have originated before tertiary period are called old fold mountains. - Caledonian, Hercynian, Vindhyachal and Aravalis. These are also called relict fold mountain because of denudation
- Some new fold mountains are the Alps in Europe, the Rockies of North America, the Andes of South America, the Himalayas of Asia and Atlas of North Africa. These young fold mountains are still rising under the influence of the earth's tectonic forces. They are known for a variety of rock structures, deep gorges and the high pyramidal peaks.
- The granitic core of such mountains is surrounded by metamorphic rocks, merging with sedimentary layers along the margins.
- The phenomenon of folding and faulting is most complex in the central areas of these mountains.
- The Urals, the Appalachians, the Tien Shan . and the Nan Shan were formed during an earlier mountain-building period.
- The highlands of Scotland and Norway and the Sayan and Stanovoy mountains in Russia are of still earlier period.
- These mountains are formed when great blocks of the earth's crust may be raised or lowered during the late stages of mountain-building
- During the uplift of structural mountains, sometimes magma flows upward into the crust.
- On its cooling and hardening beneath the surface, it contracts and the overlying rock may crack into large blocks moving up or down.
- An intense folding of rocks is generally followed by faulting of strata due to the horizontal force of tension.
- 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
- An old fold mountain may also be left as block mountains due to continuous denudation. These mountains have flat tops, steep fault scarps and the subsided portions between parallel fault are flat-bottomed.
- The Vosges in France, Black Forest mountains in Germany and the Salt Range in Pakistan are cited as typical examples of block mountains. Sierra Nevada of California (USA); Wasatch range in the Utah province are also examples of Block mountains.
- River Rhine in Europe flows through a rift valley.
- The Great Rift Valley of the world runs for about 6,000 kilometres from East Africa to Syria through the Red Sea.
- As these are formed by the accumulation of volcanic material, they are also known as mountains of accumulation.
- The matter is thrown out and deposited around the crater to form a mountain. If the lava is thin and basic in its composition, it spreads a long distance forming a flatter cone of gentler slope and of low elevation. If it is thick and of acid composition, a small volcanic cone sharply pointing out is the result.
- Sometimes lava is thrown out along with ash and cinders. Such a volcanic cone is termed as ash and cinder cone.
- Mount Mauna Loa in Hawai islands is an example of the former type.
- Fuji Yoma of Japan and Mt Popa in Central Myanmar are examples of the latter one.
Residual or Dissected Mountains
- They owe their present form due to erosion by different agencies.
- That is why they are also known as relict mountains or mountains of circumdenudation.
- They have been worn down from previously existing elevated regions.
- Hills like the Nilgiris, the Parasnath, the Girnar and Rajmahal in India are examples of this type.
- But Nilgiris got their present height as a result of subsequent uplift.
- All mountains of the Peninsula with the exception of the Aravallis are relict mountains