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Continental Drift.

If we see the earth today we see various land masses surrounded by the water bodies, these land masses are called Continents. These continents were once united together to form a single land mass. Continental Drift refers to the horizontal movement of the continents on a vast-scale.  Alfred Wegener in 1910 proposed the continental drift theory. He suggested that at the beginning of the Mesozoic era (200 million years ago) all the continents of the earth were united together to form a single large super-continent which he called “Pangaea”. The huge ocean which surrounded it was called “Panthalassa”. Panthalassa broke into Laurasia and Gondwanaland and Tethys sea emerged between them.

Wegener proposed that this vast continent began to break up into smaller continents at the beginning of the Mesozoic era which then drifted to their present positions just like pieces of wood floating on water. The continents were drifted westward and equator-ward under the “differential gravitational forces”.

 

 

The term  “Gondwanaland” was used for the southern huge landmass which included Africa, South Australia, Antarctica and India, and “Laurasia” for the northern landmass which included North America, Greenland and Eurasia. To support his theory Wegener gave many geological evidences, some are as follows:

1. Similarities in Shape of Coastlines:- The Atlantic coasts of South America and Africa have a roughly similar shape. They would fit in nicely if they are brought in contact with each other. However, the coast liner are not reliable geological feature because their shape changes with the rise and fall of the sea level relative to the land. The real edge of a continent occurs at the continental slope where sea bottom falls rapidly down to deep ocean floor. The mapping of the continental slopes of eastern South America and Africa has indicated that their contours match excellently. This strongly suggests that these two continents were once joined together.

 

2. Glossopteris Flora:- The occurrence of plant fossil Glossopteris of late Paleozoic age in the rocks in South America, Africa, India and Australia would be better explained with the assumption that these continents were once joined, although these localities are now widely separated from one another.

 

3. Glaciation:-  Unmistakable evidences of widespread glaciation towards the Paleozoic era found on the continents of the southern hemisphere support the idea of continental-drift. If South America, Africa, India and Australia were spread over the earth in the Paleozoic extensive glaciation would have had to prevail over almost the whole world. But no such evidence of Paleozoic glaciation is found in the northern hemisphere.

 

4. Similar Orogenic Belts :-  If the eastern cost of South America and the western coast of Africa are fitted together, the orogenic belts of the two continents which have the same range of age and similar structural trend, are found to align themselves across the join. For example, in Ghana near Accra (west Africa)  there is a clear boundary between 2000 million year old rocks and the Atlantic Ocean in the southwest direction. The same boundary occurs in Brazil (eastern South America) at Sao Luis. These data provide some of the best evidence supporting their original continuity.

 

5. Position of India:- In order to fit Australia, Antarctica and India together, the geology is used as a guide. The Paleozoic mountain belt in Antarctica and Australia indicate the way in which they were joined together. The same pattern of belts also continues into Africa and South America. Further, the edges of Antarctica and Australia at 1000 meters line also match nicely. However there is doubt about the proper position of INDIA. Ahmad, an Indian geologist, has suggested that close links exist between the geology of southeast India and northwest Australia. He reached to this conclusion when he studied the sedimentary basin s of Permian age in these two continents and found them very similar.

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