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In this article we will discuss about:- 1. Objective of Plate Tectonic Theory 2. Base of the Plate Tectonic Theory 3. Mechanism 4. Evaluation.
Objective of Plate Tectonic Theory:
Plate tectonic theory is a comprehensive theory which offers explanations for various relief features and tectonic events viz. mountain building, folding and faulting, continental drift, vulcanicity, seismic events (earthquakes) etc. The theory belongs to a host of scientists of different disciplines.
Plate tectonic theory is, in fact, the outcome of combined efforts of many scientists of different countries working together and separately. The theory came into light in the 1960s. It envisages the formation of mountains due to collision of plate boundaries.
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The orogenetic force to form mountains is provided by the compressive forces caused by the collision of two convergent plates along the destructive plate boundaries. Thermal convective currents originating in the mantle have been accepted as the competent force for the movement of plates.
The plates move in different directions relative to each other under the impact of thermal convective currents. Plate movements take place in accordance with the Euler’s geometrical theorum which envisages the movement of plates in the form of simple rotation along a pole of rotation (see fig. 5.10).
Base of the Plate Tectonic Theory:
The rigid lithospheric slabs or rigid and solid land masses having a thickness of about 100km composed of earth’s curst and some portion of upper mantle are technically called ‘plates’. The term ‘plate’ was first used by Canadian geologist J.T. Wilson in 1965. The whole mechanism of the evolution, nature and motion and resultant reactions of plates is called ‘plate tectonics’.
Plate tectonic theory, a great scientific achievement of the decade of 1960s, is based on two major scientific evidences e.g.:
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(i) Evidence of palaeomagnetism, and
(ii) Evidences of sea-floor spreading.
Six major and 20 minor plates have been identified so far (e.g. Eurasian plate, Indian-Australian plate, American plate, Pacific pate, African plate and Antarctic plate).
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McKenzie and Parker discussed in detail the mechanism of plate motions on the basis of Euler’s geometrical theorum in 1967. Hary Hess (1960) elaborated the mechanism of plate movement on the basis of the evidences of sea-floor spreading. W.J. Morgan and Le Pichon elaborated the various aspects of plate tectonics in 1968.
Three types of plate boundaries (see fig. 5.7) have been identified e.g.:
(i) Constructive plate boundaries,
(ii) Destructive plate boundaries, and
(iii) Conservative plate boundaries.
(i) Constructive plate boundaries also called as ‘divergent plate boundaries’ or ‘accreting plate boundaries’ represent zones of divergence along the mid- oceanic ridges and are characterized by continuous addition (accretion) of materials as there is constant upwelling of molten materials (basaltic lavas) from below the mid-oceanic ridges.
These basaltic lavas are cooled and solidified and are added to the trailing margins of the divergent plates and thus new oceanic crust is continuously formed. In fact, oceanic plates split apart along the mid-oceanic ridges and move in opposite direction (see fig. 5.7) and thus transform faults are formed.
(ii) Destructive plate boundaries also known as ‘consuming plate boundaries’ or ‘convergent plate boundaries’ are those where two plates collide against each other and the leading edge of one plate having relatively lighter material overrides the other plate and the overriden plate boundary of relatively denser material is sub-ducted or thrust into the upper mantle and thus a part of the crust is lost in the mantle (see fig. 5.8). This mechanism results in constant loss of crustal materials.
(iii) Conservative plate boundaries also known as ‘shear plate boundaries’ are those where two plates slip past each other without any collision along the transform fault and thus crust is neither created nor destroyed.
Mechanism of the Plate Tectonic Theory:
According to plate tectonic theory mountains are formed due to collision of two convergent plates. Mountains are always formed along the destructive plate boundaries. It is obvious that the process of mountain building is associated with destructive plate boundaries of two convergent plates.
The plate tectonic theory envisages the formation of mountains due to compression of sediments caused by the collision of two convergent plate boundaries. Two plates moving together under the impact of thermal convective currents collide against each other and the plate boundary having relatively denser materials is sub-ducted under the other plate boundary of relatively lighter materials.
This subduction zone is also called Benioff zone. The subduction of plate boundary causes lateral compressive force which ultimately squeezes and folds the sediments and materials of the margins of the plates and thus mountains are formed. The subducted part of the plate after reaching a depth of 100km or more in the mantle is liquefied and thus expands in volume because of conversion of the portion of plate into magma. This expansion of molten materials causes further rise in the mountains.
The convergence and consequent collision of plate boundaries occurs in three situations viz.:
(i) Collision of two oceanic plates,
(ii) Collision of oceanic-continental plates and
(iii) Collision of two continental plates .
(i) Convergence of two oceanic plates:
The collision of two oceanic plates and subduction of the boundary of the plate of relatively denser materials results in the formation of the fold mountain ranges of island arcs and festoons, for example, island arcs and festoons formed by Japanese islands, Phillippines etc. around the western margin of the Pecific Ocean off the east coast of Asia.
The fold mountain ranges of island arcs and festoons ‘form where a section of the ocean floor is subducted in the ocean basin away from a continent i.e. where ocean floor crust is on either side of the convergent plate boundary’.
The best example of the formation of mountains due to collision of two oceanic plates is the situation of Japanese island arc. Mountains of Japan range in height from 3000m to 4000m AMSL.
It may be pointed out that all the mountains of Japan are of volcanic origin. Though Japanese mountains exhibit a number of characteristic features of folded mountains but they can no longer be regarded as Fold Mountains like the Alps and the Himalayas. Honshu Island represents the most characteristic example of the situation of the convergence of two oceanic plates.
Honshu is bordered by Japan Trench in the east and Japan Sea in the west. The western part of the island is more frequented by volcanic activities than the eastern part. The island is characterized by two belts of metamorphic rocks on either side. It is believed that the Japan Trench was formed due to the subduction of Pacific Oceanic plate under the oceanic crust to the east of Japan.
According to plate tectonic theory the subducted portion of plate after reaching a depth of 100km or more starts melting due to high temperature prevailing in the upper mantle. The magma, thus formed, ascends and appears as volcanic eruption about 200km away from the oceanic trench. Since Japan is very close to the Japan Trench and hence western part of Japan is more frequented by volcanic activities.
This process is still continuing as the Pacific plate is being continuously subducted under the oceanic crust along the Japan Trench. The eruption of volcano in the month of June, 1991 in Japan after a dormant period of about 200 years and the eruption of Mt Pinatubo on June 9, 1991 in Manila, Phillippines, validate the authenticity of this theory of plate tectonics. The volcanic eruptions caused by subduction of oceanic plates under the oceanic crust off the Japanese coast resulted into continuous accumulation of volcanic rocks and consequent increase in the height of island arc and thus the formation of volcanic mountains could be possible.
(ii) Convergence of continental and oceanic plates:
The collision of continental and oceanic convergent plates results in the formation of cordillera type of folded mountains, e.g., the western cordillera of North America (including the Rockies). When one continental and the other oceanic plates collide due to their convergence along subduction or Benioff zone, the oceanic plate boundary being heavier due to comparatively denser materials is subducted below the continental plate boundary.
The sediments deposited on the continental margins are squeezed and folded due to compressive forces caused by the subduction of oceanic plate (see fig. 5.8). The Rockies and the Andes mountains were formed due to subduction of the Pecific ocean plate under the American continental plate.
(iii) Convergence of two continental plates:
When two convergent plates composed of continental crusts collide against each other, the continental plate having relatively denser materials is subducted under the other continental plate having comparatively lighter materials than the former. The resultant lateral compression squeezes and folds the sediments deposited on either side of continental plate margins and the sediments of the geosynclines lying between two convergent continental plates and thus forms gigantic folded mountains e.g. the Alps and the Himalayas.
The origin of the Alpine mountains of Europe and Asia is well explained on the basis of this mechanism (collision of two convergent continental plate boundaries) of plate tectonics. There existed a long Tethys geosyncline between Eurasian plate in the north and African-Indian plate in the south during Mesozoic Era.
The geosynclinal sediments of Tethys sea were squeezed and folded into Alpine-Himalayan mountain chains due to lateral compressive forces caused by the convergence and collision of Eurasian and African-Indian continental plates during Cenozoic Era. It may be pointed out that the formation of Alpine- Himalayan mountain chains could be possible due to continued collision of continental plates and consequent orogenesis along several subduction zones for long period of time.
About 70-65 million years ago there was an extensive geosyncline, known as Tethys geosyncline, in the place of the Himalayas. Tethys geosyncline was bordered by Asiatic plate in the north and Indian plate in the south. Tethys geosyncilne began to contract in size due to movement of Indian and Asiatic plates together. About 60-30 million years ago the Indian plate came very close to Asiatic plate. The Indian plate began to actively subduct under the Asiatic plate.
The convergence and collision of Asiatic and Indian plates and consequent subduction of Indian plate under the former caused lateral compression due to which the sediments of Tethys geosyncline were squeezed and folded into three parallel chains of the Himalayas about 30-20 million years ago. It has been estimated that the crust has been shortened by 500km between Asiatic and Indian plates due to convergence of two plates and subduction of Indian plate.
Alpine mountains of Europe were formed due to convergence and collision of European and African plates. Since the collision of these two continental plates was very complex and hence the structure of the European Alpine mountains is also very complex. The African plate is still moving northward and is being subducted under European plate to the south of Aegean arc. Similarly, Indian plate is also being continuously subducted under Asiatic plate.
Evaluation of the Plate Tectonic Theory:
The overwhelming majority of the scientists all over the world is of the view that plate tectonic theory has almost solved the problems of the origin of continents and ocean basins and of mountain building. In fact, the continental drift has now become a reality on
the basis of evidences of palaeomagnetism and sea-floor spreading. Plate tectonic theory also satisfactorily explains the cyclic pattern of mountain building.
It may be pointed out that 4 major periods of mountain building have been identified e.g.:
(i) Pre- Cambrian Orogeny,
(ii) Caledonian Orogeny,
(iii) Hercynian Orogeny and
(iv) Tertiary Orogeny.
It may be mentioned that plates are always in motion due to which sometimes all the land masses unite together to form Pangaea and again break up and move away relative to each other and new distributional pattern of continents and ocean basins is evolved. The past history of the earth upto 200 million years has been reconstructed on the basis of evidences of palaeomagnetism.
About 200 million years before present all the continents were united together in the form of Pangaea II (a super continent). It is believed that before the situation of Pangaea II, the continents were separated from each other. These continents might have moved relative to each other in such a way that they might have united together to form a super continent.
It is believed that the continents moved together due to plate motions and were united together in the form of Pangaea I during Pre-Cambrian period, about 700 million years ago. About 600-500 million years ago Pangaea I was disrupted. About 460 million years ago the Atlantic Ocean began to close down due to convergence of American and Eurasian plates and the Caledonian mountains were formed.
About 300 million years ago the Atlantic Ocean was completely closed and the orogenesis of the Applachian Mountains was completed during Permian period. At the same time Hercynian mountains of Europe were formed (see figure 5.12).
About 200 million years ago all the continents were again united to form Pangaea II. About 150 million years ago Pangaea was again disrupted and the Atlantic Ocean was reopened. The Alpine mountains were formed clue to plate movements during Tertiary period.
The only point of argument and question is related to the competent force responsible for the movement of plates and drifting of continents. Most of the scientists still rely on the thermal convective currents originating from the mantle as the probable adequate force to move the plates (continents) in different directions relative to each other.