Recent views on mountain building | Geomorphology | Principle Of Geography

 

Geomorphology

Principle Of Geography

Geography Complete Study Material

(Paper - I)

Recent view on mountain building

                       Mountains are significant relief features of the second order on the earth's surface. A mountain may have several forms viz. (i) mountain ridge, (ii) mountain range, (iii) mountain chain, (iv) mountain system, (v) mountain group and (vi) cordillera. A mountain ridge is a system of long, narrow and high hills. Generally, the slope of one side of a ridge is steep while the other side is of moderate slope but a ridge may also have symmetrical slopes on both the sides. A mountain range is a system of mountains and hills having several ridges, peaks and summits and valleys. In fact, a mountain range stretches in a linear manner. In other words, a mountain range represents a long but narrow strip of mountains and hills. All of the hills of a mountain range are of the same age but there are structural variations in different members of the range. A mountain chain consists of several parallel long and narrow mountains of different periods. Some times, the mountain ranges are separated by flat upland or plateaus. A mountain system consists of different mountain ranges of the same period. Different mountain ranges are separated by valleys. A mountain group consists of several unsystematic patterns of different mountain systems. Cordillera consists of several mountain groups and systems. In fact, cordillera is a community of mountains having different ridges, ranges, mountain chains and mountain system. The mountainous region of the western part of North America is the best example of a cordillera.

CLASSIFICATION OF MOUNTAINS

A mountain may have several forms. Important among them are:

• Mountain Ridge,
• Mountain Range,
• Mountain Chain,
• Mountain System,
• Mountain Group, and
• Cordillera.

Mountain Ridge: - It is a linear, steep-sided high hill, or spur. The slope of one side of a ridge is steep, while the other side is of the moderate slope. A ridge, however, may have symmetrical slopes on both sides. The Shimla Ridge is a good example of a mountain ridge.

Mountain Range:  - A mountain range is a linear system of mountains and hills having several ridges, peaks, summits, and valleys.

Mountain Chain: – A mountain chain consists of several parallel long and narrow mountains of different periods.

Mountain System: – A mountain system consists of different mountain ranges of the same period. In a mountain system, different mountain ranges are separated by valleys.

Mountain Group: – A mountain group consists of several unsystematic patterns of different mountain systems.

Cordillera: – It is a Spanish term referring to a system or major group of mountains. A cordillera consists of several mountain groups and systems. In other words, cordillera is a community of mountains having different ridges, ranges, mountain chains, and mountain systems. It usually refers to an orogenic belt at a continental scale, e.g., the Western Cordillera of the U.S.A., which includes all the ranges between the Pacific and the Great Plains.

PLATE TECTONICS AND MOUNTAIN BUILDING

• The theory of plate tectonic as outlined by. Hess and R. Dietz and postulated by W. J. Morgan is the most modern, most scientific, and most acceptable theory. It explains the origin of mountains in a scientific manner with the mechanism of plate movements. It recognizes three types of plate boundaries:
  • Divergent boundaries or junctures at the mid-oceanic ridges of rifts,
  • Shear boundaries or junctures where two plates pass across each other, and
  • Convergent boundaries or junctures where two plates collide against each other and one of the two plates is lost by subduction into the trenches.
• The divergent and the convergent boundaries are particularly important because oceanic ridges and rift valleys are formed on the divergent boundaries, and the folded mountain ranges are built on the convergent boundaries.
• The young folded mountains of the world-the Alps, the Himalayan mountain system as well as the Circum Pacific belt of mountains are located on convergent plate boundaries where there is a state of collision between two plates. It may therefore be stated as a general rule that where there is convergence or collision of two plates, mountains are formed as a result of compression in the earth’s crust. The Convergence of plates is possible under three different conditions:
• Collision between a continental and an oceanic plate or continent-ocean collision.
• Collision between two continental plates or continent-continent collision, and
• Collision between two oceanic plates or ocean-ocean collision.

CONTINENT-OCEAN COLLISION

• This is the most common type of collision, and the mountain ranges encircling the Pacific Ocean are all located at places where there is a collision of continental and oceanic plates. Its simplest and best example is found along the Pacific coast of South America. There exist a steady-state continuous collision between the oceanic and the continental plates, and the oceanic plate is thrust down under the continental plate in the trenches.
• As a result of the intense pressure, the deposition on the continental margin is compressed and folded. As the mobile core of the orogenic belt develops the deformation of the plate margins increases with increasing temperature and intense pressure. There is gravity sliding on account of uplift and thrusting on account of compression. The mobile core pushes the metamorphosed rocks towards the continent and the continental edge is uplifted to form mountains.
• According to the plate tectonic theory the Andes were formed during the early Mesozoic era. The subduction of the oceanic plate beneath the S. Amerian plate started about this time resulting in the deformation of the Palaeozoic marine sediments. After this, the South American plate started moving west which intensified the process of mountain formation in the mid-Mesozoic and early cretaceous times.
• As the oceanic plate descended further the pressure against the South American plate also increased and there was further intensification of these activities and the area of orogenesis also increased.

CONTINENT-CONTINENT COLLISION

• The Alpine Himalayan mountain system provides the best example of mountains formed as a result of this type of collision. In Mesozoic times India together with the southern continents formed part of Gondwana land, and there existed the Tethys sea between the main Asiatic landmass (Laurasia) and the Gondwana land. After the Mesozoic, Gondwanaland started breaking up, and India started moving to the north at the rate of 16 cm per year and joined the Asiatic landmass about 30 to 60 million years ago. As a result, the Tethys sea became narrower and ultimately closed.
• At about the same time Africa also started moving north and the portion of the Tethys sea between Europe and Africa became narrower, As a result of the collision of the Indian and the Asiatic landmasses the marine sediments and the crust located in between the two were folded and thrust and the Himalayan mountain chain came into existence from 2 to 30 million years ago.
• On account of the relative buoyancy of the continents, the upper rocks suffered more folding and thrusting. On the uplifted folded sediments are found deposits of flysh derived from the erosion of the former and of molasses in the coastal regions. Similarly, as a result of the collision of the African and European plates, the sediments along the continental margins have been folded and thrusted to form the Alps in Southern Europe and the Atlas Mountain in northwest Africa.

OCEAN-OCEAN COLLISION

• Where oceanic plates exist on both sides of a convergent plate boundary, the oceanic crust of one plate is subducted under the other plate in the trenches, and the resultant compression leads to the formation of island festoons and islands arcs. This type of mountain is especially found off the western coast of the Pacific Ocean and the northeastern coast of the Indian Ocean. Suess was the first to point out that this arc like island groups are tops of drowned young folded mountain ranges and are extensions of the mountain systems found on the continents.
• Between the continents and the island arcs are shallow seas which are called back-arc basins. The Sea of Japan is a good example of a back-arc basin. Towards the oceanic margin of each island arc is a deep oceanic trench. It appears as if the trenches owe their origin to the descent of the plate. Here the oceanic plate together with the deposited sediments descends under the adjacent oceanic plate and on account of compression on the continental margin of the trenches; there is the formation of metamorphic rocks.
• It may be pointed out that sometimes mountains may be formed where there is a collision of continent and islands arc. This kind of situation exists in New Guinea where the mountains of New Guinea have come into existence about 20 million years ago as a result of the convergence of the island arc lying to the north and the northern edge of Australia.