Introduction to Continents and Ocean Basins
Continents and ocean basins being fundamental relief features of the globe are considered as ‘relief features of the first order’. It is, therefore, desirable to inquire into their mode of possible origin and evolution. Different views, concepts, hypotheses and theories regarding the origin of the continents and ocean basins have been put forth by the scientists from time to time.
Before examining these views about their origin, we should know the characteristic features of the distributional patterns and arrangement of the continents and ocean basins as seen at present. About 70.8 per cent of the total surface area of the globe is represented by the oceans whereas remaining 29.2 per cent is represented by the continents.
Even the distribution of different continents and oceans in both the hemispheres is not uniform.
Distributional Patterns of the Continents and Ocean Basins:
The following characteristic features of the distributional pattern of the continents and ocean basins may be highlighted:
(1) There is overwhelming dominance of land areas in the northern hemisphere. More than 75 per cent of the total land area of the globe is situated to the north of the equator (i.e., in the northern hemisphere). Contrary to this water bodies dominate in the southern hemisphere.
If we divide the globe in two such hemispheres where the north pole stands located in the English Channel and the south pole near New Zealand, then the northern hemisphere would be ‘land hemisphere’ while the southern hemisphere as ‘water hemisphere’. Thus, the land hemisphere would represent 83 per cent of the total land area of the globe while the water hemisphere would carry 90.6 per cent of the total oceanic areas of the globe.
(2) Continents are arranged in roughly triangular shape. Most of the continents have their bases (of triangle) in the north while their apices are pointed towards south. If we take North and South Americas together, they represent equilateral triangles, the base of which would be along the arctic sea while the apex would be represented by Cape Horn. If we take these two continents separately, again they form two separate triangles.
Similarly, Eurasia also assumes the form of a triangle the base of which is along the arctic sea while its apex is near East Indies. The base of African triangle is towards north while its apex is the Cape of Good Hope. Australia and Antarctica are the exceptions to this rule.
(3) Roughly, the oceans are also triangular in shape. Contrary to the continents the bases of oceans are in the south while their apices are in the north. The base of the Atlantic Ocean extends between Cape Horn and Cape of Good Hope while its apex is located to the east of Greenland. The base of the Indian Ocean is in the south but its two apices are located in the Bay of Bengal and Arabian Sea. The apex of the Pacific Ocean is near Aleutian Islands while its base lies in the south.
(4) The North Pole is surrounded by oceanic water while south pole is surrounded by land area (of the Antarctic continent).
(5) There is antipodal arrangement (situation) of the continents and oceans. Only 44.6 per cent oceans are situated opposite to oceans and 1.4 per cent of the total land area of the globe is opposite to land area. More than 95 per cent of the total land area is situated diametrically opposite to water bodies.
There are only two cases of exceptions to this general rule i.e.:
(i) Patagonia is situated diametrically opposite to a part of north China, and
(ii) New Zealand is situated opposite to Portugal and Spain (the Iberian Peninsula).
(6) The great Pacific Ocean basin occupies almost one-third of the entire surface area of the globe.
The validity and authenticity of any hypothesis or theory dealing with the origin and evolution of the continents and the ocean basins would be determined in the light of aforesaid characteristics of the distributional pattern of the continents and ocean basins.
The presence of the great Pacific Ocean basin and island arcs and festoons of the Pacific Ocean is teething problem before scientists who venture in the precarious field of the postulation of the relevant theory of the origin of the continents and ocean basins.
Keeping the above facts in mind Lowthian Green postulated his ‘Tetrahedral Hypothesis’ to explain the intricate problems of the origin of the continents and oceans and characteristic features of their distributional pattern.
Besides, Lord Kelvin, Sollas, Love etc. have also attempted to explain the origin of the continents and ocean basins but their views are not discussed here because they are based on discarded and obsolete arguments and assumptions.
In fact, all the previous hypotheses and theories dealing with the origin of the continents and ocean basins have faded away after the postulation of plate tectonic theory. We will examine here only the concepts of Lowthian Green, F.B. Taylor, A.G. Wegener and of course plate tectonic theory.
Origin of Continents and Ocean Basins:
A few scientists have attempted to solve the problems of the origin of the continents and ocean basins on the basis of fundamental principles of geometry. The pantagonal dodecahedral hypothesis (dodeca is a Greek word which means twelve) of Elie de Beaumont is considered to be the first attempt in this field but the tetrahedral hypothesis of Lowthian Green is most significant of all the hypotheses based on geometrical principles.
‘An attractive hypothesis which has enjoyed a considerable vogue was initiated by Lowthian Green in 1875’. His hypothesis is based on the characteristics of a tetrahedron which is a solid body having four equal plane surfaces, each of which is an equilateral triangle (fig. 5.1).
Lowthian Green postulated his hypothesis after considering the characteristics of the distributional pattern of land and water over the globe.
Barring a few drawbacks and defects the tetrahedral hypothesis successfully explains the following characteristics of the continents and ocean basins:
(1) Dominance of land areas in the northern hemisphere and water areas in the southern hemisphere;
(2) Triangular shape of the continents and oceans;
(3) Situation of continuous ring of land around north polar sea and location of South Pole in land area (Antarctica) surrounded by water from all sides;
(4) Antipodal arrangement of the continents and oceans;
(5) Largest extent of the Pacific Ocean covering one third area of the globe and
(6) Location of chain of folded mountains around the Pacific Ocean.
The hypothesis of Lowthian Green propounded in the year 1875 is based on the common characteristics of a tetrahedron.
He based his hypothesis on the following two basic principles of geometry:
(1) A sphere is that body which contains the largest volume with respect to its surface area;
(2) ‘A tetrahedron is that body which contains the least volume with respect to its surface area’.
After many experiments Lowthian Green opined that a sphere if subjected to uniform pressure on all its sides would be transformed into the shape of a tetrahedron. He applied this principle in the case of the earth. According to him when the earth was originated it was in the form of a sphere. In the beginning the earth was very hot but it gradually began to cool down due to loss of heat.
First, the outer part of the earth cooled down and thus was formed the crust but inner part of the earth continued to cool down. Consequently, the inner part of the earth was subjected to more contraction due to continued cooling and thus there was marked reduction in the volume of the inner part of the earth. Since the upper part, the crust, was already cooled and solidified and hence it could not be subjected to further contraction.
This resulted into possible gap between the upper and inner parts of the earth. Consequently, the upper part collapsed on the inner part and ultimately the earth began to assume the shape of a tetrahedron. Lowthian Green has further maintained that the earth has not been as yet changed into a complete tetrahedron rather as it is being cooled; it is proceeding towards attaining the true shape of a tetrahedron.
He has further opined that the earth cannot be in the shape of a real tetrahedron because of its structural variations and thus it is natural that there may be some deviations from a true tetrahedron.
In a tetrahedron a plane face remains always opposite to an apex or coign. The apex or coign is more sharpened in the case of a real tetrahedron. In the case of the earth oceans represent the plane faces of the tetrahedron and land masses represent the apices or coigns but in the case of the earth the coigns are not much sharpened, rather they are flat and convex. According to Lowthian Green oceans were created on the plane faces of the terrestrial tetrahedron whereas the coigns became continental masses.
Four oceans (e.g., the Pacific Ocean, the Atlantic Ocean, the Indian Ocean and the Arctic ocean) were created on the four plane faces of the terrestrial tetrahedron. These plane faces could retain water because of the fact that these were lower than the level of the apices or coigns of the terrestrial tetrahedron.
Continents were formed along the apices or coigns of the tetrahedron. This fact may also be proved on the basis of an experiment. If we submerge a tetrahedron in a hemisphere of water, the flat surfaces of the tetrahedron would retain water while the edges or apices or coigns will project above the water.
Lowthian Green claimed to see a tetrahedral arrangement in the distribution of the continents and oceans in such a way that the earth was linked to a tetrahedron having four flat faces and standing on one point (fig. 5.2). The upper flat face represents the Arctic Ocean while the remaining three faces represent the Pacific Ocean, the Atlantic Ocean and the Indian Ocean.
Similarly, three vertical meridional edges represent North and South America, Europe and Africa and Asia while the lower point is represented by Antarctica. Thus, the presence of water around North Pole and the location of South Pole in land area (Antarctic continent) are very well explained on the basis of tetrahedral hypothesis.
Three coigns out of four coigns of four equilateral triangles are located in the northern hemisphere. Only the fourth coign is located in the southern hemisphere. These three coigns present the oldest rigid masses around which the present continents have grown. These three ancient shields are the Laurentian or Canadian shield, Baltic shield and Siberian shield.
The fourth coign or the pivot of the tetrahedron represents the Antarctic shield. The present continents have grown out of these four ancient shields represented by four coigns of the tetrahedron. All the continents developed along the edges of the tetrahedron taper southward and thus triangular shape of the continents is proved. The location of the oceans along four plane faces and the continents along the edges or coigns of the plane faces of the tetrahedron proves antipodal position of land and water.
Though Gregory accepted the tetrahedral hypothesis of Lowthian Green but he suggested certain modifications. According to Gregory due to shrinkage of the earth because of contraction on cooling ‘the portion of the vertical tetrahedral edges should be fairly constant, but three edges around the polar depression might develop sometimes in the northern and at others in the southern hemisphere’.
Though the tetrahedral hypothesis throws light on the problems of the continents and ocean basins and to major extent it successfully explains the characteristic features of the distributional pattern of the present- day continents and ocean basins but because of certain basic defects and errors the hypothesis is not acceptable to the modern scientific community.
It is argued that the balance of the earth in the form of a tetrahedron while rotating on an apex cannot be maintained. Secondly, the earth is rotating so rapidly on its axis that the spherical earth cannot be converted into a tetrahedron while contracting on cooling. Thirdly, this hypothesis believes more or less in the permanency of continents and ocean basins while the plate tectonic theory has validated the concept of continental drift.