ORIGIN OF CRUST
Several theories have been put forward regarding the origin of the continents and oceans. Of these theories only the Plate Tectonics Theories have received wide acceptance and rest are generally carry historical and academic value. Kelvin was of the view that higher and lower parts of the earth’s surface were determined quite early in the history of the earth in the process of contraction of rocks. The low and high density rocks solidified to form higher and lower longitudinal columns of earth surface; for example, north and south Americas are low density rocks aligned along Atlantic Ocean of high density rocks followed by Europe and Africa again of low density rocks. But, Kelvin’s hypothesis does not stand the test of science. Since the rotation of the earth has not changed since its origin, Kelvin’s logic would place columns of high and low density rocks on latitudinal axis and not on the longitudinal as these are place and as Kelvin suggested.
Several theories have been put forward regarding the origin of the continents and oceans. Of these theories only the Plate Tectonics Theories have received wide acceptance and rest are generally carry historical and academic value. Kelvin was of the view that higher and lower parts of the earth’s surface were determined quite early in the history of the earth in the process of contraction of rocks. The low and high density rocks solidified to form higher and lower longitudinal columns of earth surface; for example, north and south Americas are low density rocks aligned along Atlantic Ocean of high density rocks followed by Europe and Africa again of low density rocks. But, Kelvin’s hypothesis does not stand the test of science. Since the rotation of the earth has not changed since its origin, Kelvin’s logic would place columns of high and low density rocks on latitudinal axis and not on the longitudinal as these are place and as Kelvin suggested.
Sollas suggested that the distribution of elevations and declivities is related to the original atmosphere so that in areas of high atmospheric pressure the molten material was pushed downward.
Thus Sollas presumed that the original atmospheric pressure belts were distributed longitudinally. But this is not corroborated by geological evidence. The pressure belts have wandered in geological past and that there are evidence showing that the axis of earth deviated in past by 11 to 30 degrees but there no research till date has suggested that earth’s axis deflected by 90 degree at any point of time in the past. Put simply, the rotational axis of the earth has never been horizontal, the essential pre-condition for Sollas’ model to be true. Even if his principles are applied on the earth, the arrangement of the continents and oceans would be along latitudes in relation to pressure belts, with low pressure regions comprising continents and high pressure belts oceans. But the earth’s orientation is markedly different
There are many other view. The supporters of the Planetesimal Hypothesis attribute the crustal distribution to unequal collection of planetesimals during the formation and solidification of earth.
According to Lapworth, the continents and oceans have been formed through of large scale folding on the surface of the earth so that the anticli8nes represent the land and synclines constitutes the sea. However, it does not appear logical to invoke large scale folding merely on the ground of contraction and cooling of the earth. In 1907 Love gave a mathematical grab to the hypothesis of Lapworth and tried to prove that since the gravitational centre and geometrical centre of the earth are not the same, there will be a series of harmonic deformation in the shape of the earth resulting in upwarpings and depressions and thus in the formulation of the continents and oceans.
In the beginning of the 20th century, a new hypothesis was proposed by Jeans later supported by Sollas as well. According to this view, after the earth had given birth to the moon , the earth became pear-shaped in the process of attaining rotational stability. On this pear shaped earth, the neck would form the oceans and the edges would constitute higher land. Jenas postulated that these two hemispheres in the process fo cooling would press against each other due to mutual attraction, resulting in equatorial buldge and the formation of higher land.
Sollas tried to show that it was possible to explain the present distribution of land and sea or water the the help of Jeans’s hypothesis but the idea was not accepted. In this connection, it may be pointed out that recently, Osmond Fisher of Australia has suggested that the depression which was left on the earth after the separation of the moon, gave way to the pacific ocean in due course. This hypothesis has the merit of explaining the principal asymmetry on the earth’s surface and also the difference between the Atlantic and Pacific coasts.
In 1875, Lothian Green proposed his tetrahedral hypothesis. He pointed out that the distribution of land and water shows a tetrahedral arrangement. Green stated that the earth was an inverted tetrahedral which stood on its top. In such a position the four oceans would occupy the four flat faces of the tetrahedron and the upper horizontal face would be occupied by the artic ocean. ON the edges of the upper horizontal face would be found the landmasses which would form a ring around the arctic ocean. On the three vertical edges would be the continents extending from north to south are north and south Americas, Europe and Africa and Asia and Australia respectively. At the bottom would lie the south pole around which would extend landmass of Antarctica. He pointed out that the shape of the earth could not an exact geometrical tetrahedron and its edges which constituted the continents would be convex and not sharp and angular.
In recent years, J W Gregory has tried to present the above theory in a more detailed and systematic manner with reference to the characteristics of the distribution of land and water. But this hypothesis has now been rejected for not standing the test of mathematical principles. According to the laws of mathematics, a big body like the earth can not mountain its tetrahedral shape in gravitational equilibrium.
The continental drift theory of Wegener also tried to explain the present status of the crust but failed to provide scientific reasoning. The major breakthrough was achieved by Harry Hess (1956-60) when he presented Sea Floor Spreading Theory. This theory provided basic, relevant and scientific information regarding evolution of oceanic crust. Thereafter came the Plate Tectonics Theories, mainly by Wilson and Morgan. Now, it was viewed that the initial cooling of the earth solidified the upper layer of the earth body. This solidified upper layer is called the crust. Solidification or the formation of the crust was not a homogenous or regular phenomenon rather it was in the form of crustal blocks, which are known as plates.
But these plates are not irregularly spaced, they are arranged in a way to that two plates have a common boundary known as Rim. This boundary is a space through which internal energy of the earth escapes. The plate margins are defined as tectonic zones because the margins are subjected to folding, fracturing, squeezing, volcanism and passage of seismic waves. The characteristics are related to the functional behaviour of the energy flow as explained under the convectional current theory of Arthur Holmes.
All modern views suggest that boundary planes are characterized by upwelling and sinking of energy matter, that is, magma. Whenever there is upwelling there will be volcanism i.e. it brings crustal accretion- formation of basaltic crust, as explained by Harry Hess. All mid-oceanic ridges are the centres of crust formation. Divergin trend of energy cycle takes crustal columns apart and the vacuum is occupied by the new basaltic rocks or magma forming crust.
But whatever crust is added at or near the mid-oceanic ridge is compensated in the subduction zone where crustal plates are destroyed. Subduction plate margins are found at the place of convergence of two plates generally of varying densities. At the site of collision the low density rocks will go upward and high density rocks downward. The upward column is subjected to up-folding and sinking column undergoes melting in the Benioff Zone. Granitic crust develops on the continents by means of volcanism caused by subduction zone or Benioff zone effect.
Crust formation is compensated by the process of weathering and erosion. There is a rock cycle and whatever continental crust is formed is consumed by the rock cycle.
Crustal evolution is thus a dynamic phenomenon and modern geophysical and geomorphological evidence favour this modern model of origin of the crust. In spite of having gained wide acknowledgement this modern theory of evolution of crust based on sea floor spreading and plate tectonics theory is not beyond the scope of criticism. According to this model there must be equal, parallel and continuous subduction column to compensate the sea floor spreading or crustal accretion but we don’t find equal and continuous subduction column. Secondly, the rate of upliftment or crustal formation exceeds the rate of crustal destruction. However, this model is the best one at our disposal for understanding geomorphological phenomena.
