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In this article we will discuss about:- 1. Objectives of Thermal Contraction Theory 2. Mechanism of the Thermal Contraction Theory 3. Evaluation.
Objectives of Thermal Contraction Theory:
Jeffreys, a strong exponent of contraction theory, postulated his ‘thermal contraction theory’ to explain the origin and evolution of major reliefs of the earth’s surface (continents, ocean basins, mountains, island arcs and festoons) but his major objective was to explain the origin and distributional patterns of mountain systems of the globe.
Jeffreys was a contractionist. His theory was based on mathematical reasoning. He postulated his contraction theory because he could not find any strong reasons in the continental drift theory which advocated horizontal movement of the continents due to tidal force of the sun and the moon and the gravitational force as envisaged by A.G. Wegener.
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Jeffreys used the force of contraction resulting partly from cooling of the earth due to loss of heat through radiation from the earth’s surface and partly from the decrease of the speed of the earth’s rotation.
In fact, the forces invoked by Jeffreys are divided into two groups:
(1) Force coming through the cooling of the earth. The earth, after being formed, started cooling due to loss of heat through radiation. This process resulted in the gradual decrease of the size of the earth due to contraction on cooling. The resultant contraction provided adequate force (as believed by Jeffreys) to form various relief features including mountains.
(2) Force coming through the decrease in the speed of earth’s rotation. About 1600 million years ago the earth completed its one rotation in about 0.84 hour whereas it presently completes one rotation in about 24 hours. The decrease in the rotational speed caused contraction in the equatorial circumference of the earth. It may he concluded that the force of contraction was derived through the contraction of the earth due to (i) cooling of the earth and (ii) due to decrease in the speed of earth’s rotation.
Mechanism of the Thermal Contraction Theory:
Jeffrey’s theory is based essentially on the history of the contraction of the earth. According to Jeffreys the earth began to shrink because of contraction caused by gradual cooling of the earth due to loss of heat through radiation from the very beginning of its origin. He has mathematically calculated the extent of contraction on cooling.
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A decrease of temperature upto 400°C in the 400 km thick outer shell of the earth would cause shortening of the diameter of the earth by 20 km and the circumference by 130 km due to cooling and resultant contraction. He calculated the maximum shortening of the crust due to contraction to be 200 kilometres and the reduction in surface area upto 5 × 106 cm2.
According to Jeffreys the earth is composed of several concentric shells (layers). The cooling and resultant contraction take place layer after layer but the cooling is effective upto the depth of only 700 km from the earth’s surface. ‘The region of the earth from the centre to somewhere about 700 kilometres from the surface may have undergone no appreciable change of temperature, and consequently no marked change in volume’. Within the zone of 700 km from the earth’s surface every upper layer has cooled earlier and more than the layer immediately below the upper layer.
Thus, each upper layer contracted more [Hah the layer just below it. Further, each upper layer continued to cool unless obstructed by the immediate lower layer. The outer layer began to cool first due to loss of heat through radiation. It may be pointed out that there is a limit of cooling beyond which no further cooling is possible.
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After maximum cooling and resultant contraction of the upper layer lower layer just lying below the upper layer begins to cool and contract, with the result already cooled and contracted upper layer becomes too large to fit in with the still cooling and contracting lower layer. The core of the earth is not affected by cooling because of exceptionally high temperature prevailing there.
Thus, the core obstructs the contraction of the layer lying above it. The cooling and contracting layer lying below the already cooled and contracted upper layer becomes too short to fit in with the core of the earth. There is such a layer between the upper and lower layer where contraction is such that the intermediate layer can fit in with the lower layer. This layer is called level of no strain.
The layer lying over the level of no strain is too big to fit with the lower layer and hence the upper layer has to collapse on the lower layer so that it can fit with the lower layer. This process (collapse of upper layer on lower layer) results in the decrease in the radius of the earth which causes horizontal compressive stress which leads to buckling and folding of the rocks of upper layer. Thus, the mountains are formed. The lower layer below the level of no strain is too short to fit with the core of the earth and hence the lower layer has to stretch horizontally.
This process implies a lateral spreading and thinning out of the materials of the lower layer below the level of no strain. The spreading and thinning of the lower layer introduces a state of stress which causes fractures and fissures resulting into breaking of rocks. This mechanism allows further collapse of the already cooled outer layer and thus already formed mountains are subjected to further rise in height.
Jeffreys has also explained various aspects of mountain building e.g., period of mountain building, zones of mountain building, direction of mountains etc.
Period of mountain building:
According to Jeffreys the process of aforesaid mechanism of mountain building is not always active throughout the geological periods rather is confined to certain periods only. There is continuous accumulation of compressive and tensile forces resulting from contraction of the earth due to cooling and this process continues until the accumulated forces exceed the rock strength.
When this state (when accumulated compressive and tensile forces exceed the rock strength) is reached, folding and faulting are introduced and the process of mountain building sets in and this process continues till the compressive and tensile forces are strong and active. When these forces become weak, mountain building stops and the period of quiescence sets in.
Again the process of accumulation of compressive and tensile forces starts and the next process of mountain building begins when these forces again become strong enough to fold the crustal rocks. Thus, two periods of mountain building are separated by a long period of quiescence.
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Zones of mountain building:
According to Jeffreys mountain building depends upon the nature and strength of rocks. The areas having soft and elastic rocks are most affected by the process of mountain building as the rocks are easily folded by compressive forces caused by contraction but the regions having hard and less elastic rocks are affected by tensile forces and thus several faults and fractures are formed because such rocks are easily broken into blocks. It is, thus, apparent that mountain building is localized in certain zones of the globe.
Direction of force:
According to Jeffreys not all the areas below the earth surface are equally affected by the mechanism of cooling and contraction. The cooling process was more active below the oceanic crust than the continental crust because of dissimilar structure of these two zones.
Thus, the rocks below the oceanic crust experienced more cooling and contraction than the rocks below the continental crust. Thus, the force of contraction is directed from oceanic crust towards the continental crust. This mechanism results in the formation of mountains along the continental margins parallel to the oceans. Rockies and Andes are the examples of such situation.
Direction of mountains:
According to Jeffreys the compressive force generated by contraction of the earth due to cooling was directed from oceanic areas towards the continental areas almost at right angle and thus the mountain ranges were formed parallel to the oceanic areas.
The layout and direction of the Rockies and Andes mountains are very well explained on the basis of this theory because these mountains run north to south along the western margins of North and South America respectively and are parallel to the Pacific Ocean but the west-east extent of the Alps and the Himalayas cannot be explained on the basis of this theory.
Evaluation of the Thermal Contraction Theory:
Though Jeffreys has attempted to explain the origin and evolution of surface features of the earth and has presented several evidences in support of his thermal contraction theory but his theory has been severely criticised and attacked on the following grounds:
(1) The force of contraction resulting from the cooling of the earth is not sufficient enough to account for the origin and evolution of major surface reliefs of the globe. A. Holmes has remarked that ‘the calculated reduction of area (by Jeffreys) is seriously in deficit of the amount to explain mountain building.’
(2) The concept of cooling of the earth in the system of concentric shells (layers) is erroneous and is not acceptable.
(3) The impact of decrease in the speed of rotation of the earth on mountain building is doubtful. J.A. Steers (1932) has aptly remarked, ‘It may, in fact, be safely concluded that whatever effects the changing speed of rotation in geological times may have had, it was totally inadequate to influence mountain building in any marked way.’
(4) It is improper to believe that contraction would have been so immense about 200 million years ago so that it might have formed such gigantic mountains of Tertiary period as the Rockies, the Andes, the Alps, the Himalayas etc.
(5) As per thermal contraction theory of Jeffreys the continents and oceans should have been uniformly distributed as the earth was contracted from all sides but presently there is uneven distribution of continents and oceans.
(6) According to this theory the situation of mountains should always be parallel to the oceans. The arrangement of the Rockies and Andes is justified on the basis of this theory but the arrangement of European Alpine mountains and the Himalayas cannot be explained.
(7) If we believe in the competence of the force of contraction to form mountains it cannot produce great ranges of mountains as they are found at present over the globe but it would produce a larger number of small puckers or minor folds.
(8) According to this theory there should not be any definitive distributional pattern of mountains as they may be formed everywhere because all parts of earth’s crust experienced contraction but contrary to this mountains are found in certain patterns e.g., along the margins of the consents extending either north- southward or west-eastward.