Formation of Mountains:
Since the world’s major mountain systems belong to the class of fold mountains, their process of formation is of great importance. While discussing the formation of fold mountains, the following points may be noted.
(i). The great mountain ranges are build of thick sequences of sedimentary rocks, which are usually highly metamorphosed in the deeper levels.
(ii). The sedimentary strata forming mountain ranges are usually folded, faulted, and intruded by igneous bodies (resulting in igneous rocks).
(iii). The mountain ranges occur in relatively narrow belts of great length on the earth’s surface.
It is generally believed that the mountains are originated from geosynclines. A ”geosyncline” may be defined as a long, relatively narrow mobile trough in which vast quantities of sediments, having a thickness of the order of 10 kilometers or more, are deposited. A geosyncline is formed at the margin of a continental mass and its bottom sinks gradually with the deposition of sediment. The width of a geosyncline varies from a few tens of kilometers to hundreds of kilometers and its length is usually more than 1000 kilometers. A geosyncline can be subdivided into two parts: (i) miogeosyncline, and (ii) eugeosyncline. An inner belt closer to the continental platform is called miogeeosyncline. it contains lesser thickness of sediments. The ”eugeosyncline” on the other hand, is the outer belt, which lie closer to an ocean basin. It contains a greater thickness of sediments.
After the accumulation of great thickness of sediments, horizontal forces from the seaward side of the geosyncline begin to compress the sediments. As a result, the sediments are folded and deformed, and a mountain system with a complex structure is formed. In this process of mountain building, which involves thickening and shortening of the earth’s crust, much of the sediments is also pushed deeper into the earth. Roots of mountains are formed in this way. The deeply buried sediments melt and produce magma. This magma moves upward and intrudes on the overlying sediments. The batholiths thus formed metamorphose the sediments intruded. In this way, a complex mountain system containing folded and faulted sedimentary rocks surrounding a core of igneous intrusions and metamorphic rocks are formed.
Although the geosynclinal concept of mountain building has many merits, it fails to answer the two vital questions: (i) what produced the subsidence in the geosyncline and (ii) why did the force come from the sea to squeeze the sediments. The mechanism of mountain building has nicely been explained by the ”plate tectonics theory”.
Role of tectonic movement in mountain building:
The earth’s crust is cracked into plates, and these plates are drifting either apart, or toward one another, or transversely. At some regions of tectonic movement, the earth’s crust is spreading, while at others it is squeezing. This movement is called ”plate tectonic movement”. According to the plate tectonic theory, fold mountains are formed by the movement and collisions of large plates that make up the earth’s crust. These plates are usaully very large in size and carry whole continents. The process of formation of fold mountains may be summarized as follow;
(1). A plate with a continent at the leading edge converge toward another plate carrying a continent but having an oceanic crust at the leading edges.
(2). In the early stage, plate convergence causes subduction of the oceanic crust. Then collision of continents takes place because the continental crust being buoyant can not be subducted.
(3) The collision of continents leads to the deformation and squeezing of the geosynclinal sediments which produce a mountain range and thickened continental crust. Subduction and partial melting of the oceanic crust initiate igneous activity. Numerous batholiths, which are intruded, metamorphose the adjacent sediments.
(4). Since the continents can not undergo subduction, the plate motion may stop altogether. Then the plate may fracture and a new subduction zone may be started at another place.
(5). An extinct subduction zone may now appear as a mountain belt within a continent. the Himalayas and the Ural Mountains are examples of such extinct subduction zones.