Landforms of Second Order (Mountains, Plateaus and Plains)
Major landforms which are found on continents and ocean floors are known as landforms of second order. Mountains, plateaus and plains are the major landforms found on the continents. Continental shelf, continental slope, deep sea plains etc. are formed on the sea floor. These features have already been discussed.
A mountain is that part of land which is at least 900 metres above sea level and its slope makes an angle of 25° and 35° with the horizontal plain. Following are the main types of mountains.
Fold mountains have been formed due to folding of sedimentary rock strata. It is believed that there were narrow, elongated shallow seas or lake basins in which huge amount of sediment was deposited. These low-lying narrow and elongated areas are termed as geosynclines. The sedimentary deposits on the bed of geosynclines suffered compression which was cause d by the tectonic forces working inside the earth. The compression led to folding of sedimentary rock strata and gave birth to mountains. Thus, “out of geosynclines, have come the mountains”. The sedimentary material today marked on the tops of fold mountains or marine fossils found inside such rock-strata point out the uplift of the deposition filling up the geosyncline in the distant past.
The lateral tectonic forces causing folds can either work from both the sides of the geosyncline or one edge may be stable and the other is moving which may squeeze the sediment. The Himalayas, the Alps, the Rookies, the Andes are important examples of fold mountains.
These mountains were formed due to faults caused by tensional forces. An intense folding of rocks is generally followed by faulting of strata due to 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 garben. The Vosges in France and Black Forest mountains in Germany are cited as typical examples of black mountains.
These mountains are formed due to accumulation of thick lava as a result of volcanic eruption in the surrounding areas. Fujiyama of Japan, Mt. Popa of Myanmar and Mt. Mauna Loa of Hawaii are some of the outstanding examples of volcanic mountains.
Residual or Dissected Mountains
They owe their present form due to erosion by different agencies such as river, wind, glacier, etc. 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 upli. Almost all the hills of Peninsula are India are examples of residual mountains. Some authorities treat Aravalis also as a relict mountain but opinions differ on this issue.
A plateau is an elevated area as compared to its surrounding areas. In contrast to mountains, it has a large area on its top and has at or undulating surface. Its sides are marked by a steep cli except in a plateau which is surrounded by high mountains. It is, therefore, often termed as ‘table land’. Depending on their location, plateaus are of the following three types.
The plateaus which are partly or fully enclosed by mountains are known as intermontane plateaus. The highest and extensive plateaus of the world, such as Tibet, Bolivia and Mexico are of this category.
These plateaus are surrounded by sea or plains form all the sides. They are the result of continental uplift and are quite vast. The plateau s of Brazil, South Africa, West Australia, South India, Greenland, Antarctic and Arabian Peninsula are important examples of continental plateaus. Their heights vary from 600-1,500 metres.