Plate Tectonics and Continental Displacement | Continents | Geography

On the basis of the evidence of palaeormagnetism and sea floor spreading it has been now validated that the continents and ocean basins have never been sta­tionary or permanent at their places rather these have always been mobile throughout the geological history of the earth and they are still moving ill relation to each other. The scientists have discovered ample evidences to demonstrate the opening and closing of ocean ba­sins.

For example, the Mediterranean Sea is the re­sidual of once very vast ocean (Tethys Sea) and the Pacific Ocean is continuously contracting because of gradual subduction of American plate along its ridge.

On the other hand the Atlantic Ocean is continuously expanding for the last 200 million years. Red Sea has started to open (to expand). It may be mentioned that continental masses come closer to each other when the oceans begin to close while continents are displaced away when the oceans begin to open (expand).

Though the sequence of events of continental displacement based on the evidences of palaeomagnetism and sea floor spreading is available only for the last 200 million years but on the basis of general mechanism of plate tectonics and the evidence from the continents the sequence of earlier events may be reconstructed. Valentine and Moors (1970) and Hallam (1972) have attempted to reconstruct the chronological sequence of the continents and ocean basins from the beginning to the present time.

About 700 million years ago all the landmasses were united together in the form of one single giant landmass known as Pangaea I. About 600-500 million years before present first Pangaea was broken because of thermal convective currents coming from within the earth, most probably from the mantle and different landmasses drifted apart.

These landmasses were again united together due to plate motions in one land mass known as Pangaea II about 300-200 million years before present. According to A. Hallam Second Pangea began to break during early Jurassic period and N.W. Africa broke away from N. America and drifted away.

The zone of sea floor spreading continued to extend towards north and south. The separation of South America and Africa was accomplished during middle cretaceous period, and North America and Europe began to move away from each other (Fig.5.13).

The opening of North Atlantic was accom­plished in many phases. After the separation of North America from Africa, Europe and Greenland broke away from Labrador during late cretaceous period (about 80 million years before present) and thus Labrador Sea was formed.

This newly formed sea contin­ued to remain for some time as northern extension of the Atlantic Ocean. Rockall plateau was separated from Greenland during Tertiary period (about 60 mil­lion years before present). Labrador Sea and North Atlantic continued to expand between Europe and Greenland upto middle miocene period because the European and American plates continued to move eastward and westward respectively. The spreading of Labrador Sea stopped by middle miocene period (about 47 million years before present) but North Atlantic continued to expand.

Indian Ocean did not exist before cretaceous period. Indian plate began to move towards Asiatic plate through ‘Tethys Sea’ and Australian-Antarctic plates after breaking away from African plate began to move southward during cretaceous period. Dan Mackenzie and John Sclater have presented the chronological se­quence of the evolution of Indian Ocean on the basis of the study of magnetic anomalies.

According to them Indian plate began to move northward at the rate of 18 cm per year during early tertiary period but the move­ment stopped during eocene period. The same time Antarctica broke away from Australia. Thus, the Pacific Ocean began to shrink in size because of expansion of the Atlantic and Indian Oceans. Fig. 5.14 depicts the chronological events of the Atlantic Ocean during past 700 million years.

The Atlantic Ocean began to open about 700 million years before present because of break­ing of First Pangaea when the American and Africa- European plates began to move in divergent directions and thus the Atlantic continued to expand till 400 million years before present when the Atlantic again began to close.

Because of the closing of the Atlantic Ocean Applachian mountains of North America were formed. The Atlantic Ocean again began to open up about 150 million years before present when Second Pangaea was broken into several landmasses and it still continues to expand because of the movement of Ameri­can and European plates in opposite directions.

It may be pointed out that the Atlantic Ocean is continuously expanding for the past 200 million years but the Pacific Ocean is contracting in size because of westward move­ment of the Pacific Ocean. Fig. 5.14 depicts the probable situation of the continents and ocean basins during 50 million years hence.

The following examples demonstrate the trends and patterns of continental displacement, sea-floor spreading and contraction in the size of the oceans:

Red Sea and the Gulf of Aden:

Red Sea is an example of axial trough which is located between Africa and Arabian peninsula (fig. 5.15). The surveyed magnetic anomalies in this area show, as observed by A.W. Girdler, the pattern of stripes and these are similar to the magnetic anomalies of the ocean basins. F.J. Vine calculated the rate of the spreading of Red Sea on the basis of the data of magnetic anomalies in the year 1966.

According to him the Red Sea is spread­ing at the rate of one centimeter per year (total spread­ing 2 cm/year) since the past 3-4 million years. Alen and Morelli calculated the spreading rate in 1969 as 1.1 cm/year (total spreading 2.2 cm/year). Similarly, the rate of spreading of the Gulf of Aden has been calcu­lated on the basis of stripped magnetic anomalies as 0.9 to 1.1 cm/year (total spreading 1.8 to 2.2 cm/year).

The Red Sea and the Gulf of Aden are located at the junction of three plates viz. Nubian plate, Somali plate and Arabian plate. Nubian and Somali plates are sepa­rated by Ethiopian fault. Fig. 5.16 denotes the location of Red Sea, Gulf of Aden, Arabian, Nubian and Somali plates and the pole of rotation.

Gulf of California:

The Pacific Ocean is a wan­ing ocean because it is continuously being contracted in its size because of gradual encroachment of west­ward moving American plates. It is believed that like mid-Atlantic ridge there might have been a mid-oceanic ridge in the Pacific Ocean but it has now been remarkably deformed due to plate movement. The magnetic survey of the Gulf of California revealed the presence of stripped magnetic anomaly.

This situation validates two facts viz.:

(i) East Pacific Rise (ridge) is also located in the Gulf of California and there has been continuous spreading of the gulf along the ridge since the past four million years and

(ii) Baja, the Californian peninsula, was previously united with the mainland of North America but later on it broke away from the continent due to spreading of sea floor.

Evaluation:

It is commonly agreed by the majority of the scientists that plate tectonics has validated the concept of continental drift; rather continental drift has now become a reality. The only point of argument and question is related to the competent force responsible for the drifting of the continents. Most of the scientists still rely on the thermal convective currents coming from the mantle as the probable adequate force to move the plates (continents) in different directions.