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In this article we will discuss about:- 1. Objectives of the Sliding Continent Theory 2. Axioms of the Sliding Continent Theory 3. Mechanism 4. Evaluation.
Objectives of the Sliding Continent Theory:
Daly postulated his theory of sliding continents in his book ‘Our Mobile Earth’ in the year 1926 to explain the origin and evolution of different relief features of the earth’s surface. Though Daly attempted to throw light on major reliefs of the globe but his main objective was to explain the causes and processes of mountain building. He attempted to explain salient aspects of folded mountains e.g., origin, successive upheavals, distributional patterns and orientation and extent.
The main force implied by Daly for the origin of the mountains has been the force of gravity. The whole theory of Daly is based on the nature and rate of downward slide of the continents fostered by gravitational force. ‘The key to the Daly’s views is the idea that there has been downhill sliding movement of continental masses. In other words, the controlling factor has been gravity’. Daly himself proclaimed that his theory based on gravitational force was competent to deal with all the problems of mountain building satisfactorily.
Axioms of the Sliding Continent Theory:
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Daly has assumed certain axioms (self-proved facts) in support of his theory. If we look into his theory it appears that ‘a major part of the theory is based on self-proved facts or axioms’. It may be pointed out that Daly did not elaborate his axioms. He admitted himself that ‘his theory can well explain the problems of orogenesis on the force of gravity alone.’
According to Daly a solid crust was formed just after the origin of the earth. He named this solid crust as primitive crust. In early times there existed a series of ancient rigid masses which were generally situated near the poles and around the equator. These rigid masses have been named by Daly as polar and equatorial domes.
Thus, there were three belts of rigid masses e.g.:
(i) North polar domes,
(ii) Equatorial domes, and
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(iii) South polar domes.
These three belts of rigid masses were separated by depressed regions which were called by Daly as mid latitude furrows and primeval Pacific Ocean. These depressed regions were in fact oceanic areas (or say geosynclines) the beds of which were formed of primitive crust which was formed with the origin of the earth.
The crust, according to Daly, composed of granites, was heavier than the rocks of substratum below the crust. The crust was composed of heavier granites while the substratum was formed of lighter glassy basalt. It may be pointed out that this view of Daly is isostatically totally wrong.
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He further assumed that the water bodies occupied about half of the globe and Tethys geosycline (northern mid-latitude furrow between north polar dome and equatorial dome) ‘was a marked feature throughout much of geological time’. Land masses (polar equatorial domes-rigid masses) projected above the water bodies and the polar and equatorial domes were sloping towards mid-latitude furrows (which were in fact geosynclinal tracts) and the Pacific Ocean.
Mechanism of the Sliding Continent Theory:
Daly has believed in the collapse of the primitive crust but has not elaborated the mechanism of collapse. It may be surmised that the primitive crust would have been probably bad conductor of heat and so the surface temperature would have fallen soon to that of the present time but the loss of heat from the interior into the exterior part continued and hence the interior part contracted away from the outer shell or crust.
Consequently, crust would have collapsed on the still contracting interior due to:
(i) the weight of the oceanic water,
(ii) the weight of geosynclinal sediments and
(iii) gravitational force of the centre of the earth.
It may be pointed out that the impact of gravitational force was more under the oceanic crust than the continental domes because the former was nearer to the earth’s centre.
It appears (though not described by Daly) that the mid-latitudinal furrows were formed as geosynclines due to collapse of outer crust on the contracting interior of the earth and due to the gravitational force coming from the centre of the earth.
The sediments derived through the erosion of polar and equatorial domes (more precisely continental domes) were deposited by the rivers into the mid- latitudinal furrows and the Pacific Ocean (geosynclines). Continuous sedimentation and weight of the oceanic waters exerted enormous pressure on the beds of oceans (geosynclines) with the result their beds were subjected to continuous subsidence.
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Thus, downward pressure on the oceanic (geosynclinal) beds due to continuous sedimentation and resultant subsidence of geosynclinal beds caused lateral pressure on the continental masses, with the result they were transformed into broad continental domes known as polar and equatorial domes.
As the oceanic beds were depressed downward due to gravitational force of the earth’s centre, and weight of oceanic water and geosynclinal sediments, the size of domes continued to increase. It may be pointed out that gradual increase in the lateral pressure resulting from continuous downward movement of geosynclinal beds was responsible for increase in the size and height of continental domes.
The sediments of the continental domes began to expand because of increase in the size and height of the domes and consequently sediments of the domes began to lose weight and became lighter in weight. In order to compensate the loss of weight of sediments of the continental domes there began underground flowage of dense materials from below the oceanic (geosynclinal beds) beds towards the continental domes.
Because of this process denser materials began to accumulate in the continental domes from below. Because of the repetition of the above processes the continental domes continued to grow in size and height, ‘probably not as rapidly in the centre as towards their peripheries’.
The increase in the size of domes caused pressure on the crust under the oceanic beds (geosynclinal beds). As the size of domes continued to expand, the resultant pressure on oceanic beds also continued to increase. When the tolerance limit of the oceanic crust to withstand the ever-increasing pressure was crossed, the oceanic beds began to rupture and break.
Thus, the support of the continental domes was removed due to rupture of the oceanic beds which introduced strong tensional movements due to which larger blocks of continental mass began to slide towards the geosynclines. The geosynclinal sediments were thus squeezed and folded due to compressive force coming from the sliding continental blocks, (fig. 11.10) giving birth to folded mountains.
According to Daly the broken continental blocks and parts of oceanic crust founder in the substratum because the density of outer crust is more than the substratum. On the other hand, the geosynclinal sediments do not founder in the substratum rather these float on the substratum because these are less dense than the substratum. Because of this fact geosynclinal sediments are more folded by the compressive forces generated by sliding continental blocks.
It is thus obvious that greater the amount of slipping of continental blocks, the more geosynclinal sediments are squeezed and more and greater folded mountains are formed. Daly has further pointed out that the foundered continental blocks in the substratum are melted due to high temperature and thus rise in the volume of molten continental blocks causes further rise in the mountains.
The sliding continental theory of Daly also well explains the distributional patterns of folded mountains e.g., north-south and west-east extents. According to Daly folded mountains are formed because of squeezing and folding of geosynclinal sediments by compressive forces caused by sliding of the continental blocks towards the geosynclines.
Thus, west-east extending mountains (e.g., Alpine chains and the Himalayas) were formed due to sliding of polar and equatorial domes towards mid-latitude furrow (Tethys geosyncline) and north-south extending mountains (e.g., Rockies and Andes) were formed due to sliding of continental masses towards Pacific Ocean. Similarly, the island arcs and festoons parallel to the Asiatic coast wore formed due lo sliding of Asiatic mass towards Pacific Ocean.
Evaluation of the Sliding Continent Theory:
Though the ‘sliding continent theory’ by Daly is based on well-known principle of gravitational force and tries to explain the problem of mountain building in a simple manner yet it does not present coherent account of the problem as the theory does not go into details and there is a wide gap between theoretical and practical aspects of the theory. Major part of the theory is based on self-proved facts (axioms).
The theory suffers from the following limitations:
(1) The sliding continent theory presents erroneous concepts about the structure of the interior of the earth. His concept, that the outer crust is denser than the substratum, is against the evidences of seismology because it is now proven fact that the density increases with increasing depth in the interior of the earth.
(2) Daly’s theory is based on several guesses and surmises. Why did the earth’s crust become asymmetrical? Why the continental domes were sloping towards mid-latitude furrows (geosynclines)? How was the Pacific Ocean formed? Daly does not offer any convincing explanation to these and many more questions.
(3) This theory presents erroneous views about geosynclines because these are generally considered as long, narrow and relatively shallow depressions of water but Daly’s geosynclines were in fact oceans (e.g., mid-latitude furrows and Pacific Ocean). If these are accepted as geosynclines, they would have never been filled with sediments and thus no mountains could have been formed.
(4) Daly has also presented confusing ideas and erroneous concepts about the mechanism of mountain building. In fact, this theory does not care for the extension and depth of oceans and amount of sediments deposited in them but expects mountains from every ocean (geosyncline).
(5) The theory provides wrong views about the mechanism and process of gravity. The theory does not throw light on the termination of pulling effects of gravity and the beginning of the rupture of the beds of the geosynclines. Thus, there is no coherence between different events related to mountain building-as envisaged by the sliding continent theory. In fact, the theory presents some piecemeal analysis of mountain building rather than a complete or perfect perspective.
(6) The theory to certain extent believes in such distribution of land and sea (midlatitude furrows and Pacific Ocean and polar and equatorial continental domes) as to suit its own purpose.
Wooldridge and Morgan have aptly remarked, ‘complete rejection of the idea may be premature, but it is a fair comment to say that the cause of primary “bulges” which start the slipping has in no sense been satisfactorily indicated.