1. Geosynclines
The ‘Geosynclines’ are major structural and sedimentational units of the crust of the earth. They are elongated trough-like depressions. submerged beneath the sea water. They are considered to be the future sites of mountain building activity and of fold mountains.
These basins become filled with very great thickness of sediments and alongwith the accumulation of pile of sediments, there occurs progressive subsidence of the basin floor.
Types of geosyncline
There are seven types of geosynclines as follows :
(i) Ortho-geosyncline. These are elongated basins which become filled with very great thickness of sediments, which is subsequently deformed to form a fold-mountain chain.
(ii) Eugeosyncline. In these geosynclines, the pile of sediments are found with an abundance of volcanic rocks ; they are formed at some distance from the shield areas, i.e., Kratons.
(iii) Miogeosyncline. These are formed adjacent to the Kraton, -where there is a thinner development of sediments which lack ‘volcanic rocks.
(iv) Taphrogeosyncline. This is an elongated depression, formed because of faulting. These are also known as ‘graben’ or ‘rift-valley’.
(y) Parageosyacline. This is the geosyncline which lies within the Kraton.
(vi) Zeugo-geosyncline, These are parageosyncline with marti nal uplifts.
(vii) Auto-geosynclize. It is a para-geosyncline without matginal uplifts.
Most orogenic-belts arise on the sites of geosynclines and the resulting mountains therefore consist of sediments and volcanic rocks deformed and metamorphosed to a greater or lesser extent according to their position and depth in the oropenic belt.
The fact that the sedimentary units are thicker along the mountain belts than outside indicates that most sediments now exposed in mountain chain were deposited in geosynclines. True geosynclinal accumulations presently lie off the eastern and gulf-coasts of America under the continental shelves and continental slopes.
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2. Mountains
Mountains are isolated or interlinked masses of land elevated appreciably above the average altitude of their ‘surroundings and are characterised by the presence of pointed or ridge-like tops known as ‘peaks’.
Terminologies associated with mountains
(i) The smallest unit of an elelvated land-mass is known as a ‘hillock or mound’.
(ii) Larger elevated land-masses are know as ‘hills or mountains’, But mountains are usually 1,000 metres high or above, and ‘those which are having less heights are the ‘hills’.
(iii) A series of inter-connected mountains constitute a range’,
(iv) Anumber of genetically related ranges, together form a system’.
(v) A few systems combine to form a ‘Chain’.
(iv) Several inter-related chains form a ‘Cordillera’.
Types of mountains
On the basis of their mode of origin, ‘MOuntains can be grouped into three categories as follows :
(a) Mountains of accumulation.
(b) Relict or residual mountains.
(c) Tectonic mountains.
Mountains of accumulation are due to accumulation of volcanic materials like lava and other pyroclastic materials, into heaps Or cones, which produce volcanic mountains. Sand-dunes formed duc to accumulation of piles of sands belong to this category.
Relict mountains are because of differential erosion, which is noticeable in regions which are composed of rocks of various Strength and durability.
Tectonic mountains are formed because of the roles played by the Diastrophic forces, and are of three types as :
(i) Fold mountains. The bending of rock-strata due to comPressional forces acting tangentially or horizontally towards a Common point or plane from opposite directions is known as folding; Which results in fold mountains.
(ii) Fault mountains. These are also known as Block mountains, which are because of faulting. Horst or a block mountain is an uplifted landmass located between two adjacent faults. A graben or rift valley is the block lowered between two adjacent faults.
(iii) Dome mountains. Igneous instrusions, sometimes, make room for themselves by lifting up the overlying layers of the country rocks. Thus a dome shaped structure is formed, which are sometimes large enough to be described as dome mountains e.g., laccolith.
Causes of mountain building
So many hypothesis have been put forwarded to explain the origin of mountains. Some of the important hypothesis may be described as follows :
(a) Geosynclinal hypothesis
(b) Theory of isostasy.
(c) Contraction hypothesis.
(d) Convection current hypothesis.
(e) Continental drift hypothesis.
(f) Theory of plate tectonics.
(a) Geosynclinal hypothesis. Some hold that, under the load of sediments the floor of the subsiding geosynclinal basin is likely to be broken and thus the sediments would meet the internal heat and expands in volume. Asa result of which the upper sedimentary layer would be uplifted. But this process does not account for the compressional forces.
Others believe that the geosynclinal basin on receiving the lodes will sink down and in this process, the two sides of the shallow trough will be brought nearer. This will generate compressional forces and may account for the formation of fold mountains.
(b) Theory of isostasy. Isostatic adjustment is believed to play an important role in mountain building, but this process only accounts for. vertical upliftment and not the compressional forces.
(c) Contraction hypothesis. It states that earth through the radiation of heat is cooling and as a result the earth will shrink ard wrinkles will be. developed on its surface, just as a mango if left exposed to heat will shrink and develop: wrinkles upon its surface. But it does not explain the occurrence of fold mountains along 4 few belts only, besides the fact that radioactive substances produced heat on disintegration has not been taken into account.
(d) Convection current hypothesis. It is postulated by Holmes and Griggs. It is believed that the earth has a thin solid crust and a metallic core, while the intervening layer is made up of molten silicates. Since silicates are bad conductors of heat particularly in the molten state, it transmits heat by convectional process.
In this process, where currents are flowing horizontally under the surface of the crust, they exert a powerful drag on the crust and throw it into tension where they diverge and into compression, where they converge. Thus we should expect orogenic belts to be formed where two approaching currents turned down.
(c) Continental drift hypothesis. According to this hypothesis, the sialic blocks during their course of movement through the sima are affected by the resistance of sima. This resistance crumple and. throws the frontal moving parts of the continent. / According to. Wagener, the Rockies and Andies are formed in this way, by the Westward drifting of the continents. The Himalayas and the Alpine mountain belts are thought to bave been formed by the equatorial movement of India and Africa.
(f) Theory of plate tectonics. It explains that the top crust of the earth is a mosaic of several rigid segments called plates, which include not only the solid upper crust but also part of the denser mantle below. They float on the plastic upper mantle known as ‘asthenosphere’
Plates may diverge, converge or move in parallels. Plates are said to diverge when two adjacent plates move apart and hot magma comes up through the crack and solidifies, accounting for the formation of mid oceanic ridges. Plates are said to converge when they come together and collide. When two continental plates converge, the lateral pressure exerted by them crumples and compresses them into folds. When this crumpling and folding continues high mountains form.
Accordingly, the Himalayas were formed because of the northward movement of the Indian plate against the Chinese plate.
All the aforesaid hypothesis explain the origin of mountains some or other ways.