Continental drift theory UPSC

The possibility of continental drift began when scholars observed symmetry on both sides of the Atlantic Ocean. Many scientists thought that the two Americas (North America and South America), Europe, and Africa, were once joined together due to matching the shoreline of Africa and South America facing each other.

Dutch map maker, Abraham Ortelius proposed first such similarities in about 1956. Later Antonio Pellegrini drew a map showing the three continents ( Americas, Europe, and Africa) together raising the possibilities of continental drift.

But it was German meteorologist, Alfred Wegener who put comprehensive arguments in form of "Continental drift theory " in 1912. 

Continental drift theory was given by Alfred Wegener in 1912 to explain the present arrangement of continents and oceans.

Continental drift theory of Wegener:

According to Wegener, all the continents formed from a single continental landmass, and all the oceans formed from a single mega ocean. The mega ocean surrounds the single continental landmass.  He named the supercontinent as PANGAEA which means all earth. The supercontinent is surrounded by the Mega Ocean called PANTHALASSA, which means all water.

Around 200 million years ago, the supercontinent began to splits. Pangaea first breaks into two large continental landmasses namely Laurasia (on the north side) and Gondwanaland (on the southern side). Laurasia and Gondwana's land continued to split many smaller continents that exist today.

From Laurasia the following continents were formed:

• North America
• Europe
• Asia except for the Indian subcontinent.

From Gondwanaland the following continents were formed:

• South America
• Africa
• Indian subcontinent
• Australia
• Antarctica 

Force for drifting;
Wegener suggested two forces for the drifting of the continent that is namely Polar-fleeing force and tidal force.

The polar-fleeing force is related to the rotation of the earth. As per Wegener, the earth has bulged at the equator due to the earth's rotation. Due to the earth's bulge at the equator and the earth's rotation, polar fleeing force is generated that pushes the continental to move towards the poles.

The tidal force is the second force as suggested by Wegener for the drifting of the continent. Tidal forces are generated due to the attraction of the moon and the sun that developed tides in oceanic water.

Wegener believed that these two forces polar-fleeing force and tidal forces when applied to million years, drift the continents.

However, most scholars criticized continental drift theory on these forces as these forces are inadequate for the drifting of such large land masses.

How mountain building is associated with continental drift theory?

As per the Wegener hypothesis, theory assumed that the sial layer is floating on the Sima layer, at someplace Sima had a barrier against the movement direction of the continent, and fold mountains are formed along with the barrier; rocky, Andes, Himalayan mountains are formed along with continental margins and near its barrier.

Evidence in support of continental drift:

The following evidence supports continental drift:

• The matching of continents ( jig-saw-fit)
• Rocks of the same age across the oceans
• Tillite deposits
• Placer deposits
• Distribution of fossils

The Matching of continents( Jig-saw-fit):

The coastlines of Africa and South America bear a remarkable and unmistakable resemblance to each other. Similarities are also seen in the coastlines of North America and Europe. So, this matching and fitting prove that Africa and South America were one in the past and later separated.

Rocks of the same age across the oceans;

By radiometric dating, the rock age of the coast of Brazil coincides with that of western Africa. The oldest marine sedimentary shorelines of South America and Africa are of the Jurassic age. This means that before that time (Jurassic era) the sea did not exist and the continents of South America and Africa were close.

Tillite;

Tillite is a sedimentary rock, formed by the deposition of glaciers. The Gondwana series of sediments found in India are also found in six different landmasses of the southern hemisphere namely Africa, the Falklands Islands, Madagascar, Antarctica, and Australia. These tracts also have a similar history as the similarity in the sediments of the Gondwana series is evident. Glacial tillite provides evidence of palaeoclimate and continental drift.

Placer deposits:

On the Ghana coast, rich placer deposits of gold are found but sources rock of gold is not present on the Ghana coast. But, sources of gold rocks do present on Brazil's coast. That means the gold deposits of Ghana are derived from the Brazil plateau. That means, in the past Africa and South America were the same land masses.

Distribution of Fossils:

Lemur is found in India, Madagascar, and Africa. That means these land masses were attached in the past.

Mesosaurus was a small reptile that was found in shallow saline water. Skelton of these reptiles are found in two locations; one on the coast of South Africa and the coast of Brazil, but both are located about 4,800 km away and there is an ocean between them.

The following are some criticism of continental drift theory:

• The tidal force and polar fleeing forces are not really so strong to drift such large land masses.
• The theory is also not able to explain the reason for moving a particular continent in a particular direction. It is not able to explain the why Indian subcontinent moved towards the northern direction and the Australian continent moved southern direction whereas both were part of Gondwana land.
• Continental drift theory is not able to explain the formation of mountains.
• Wegener in his theory gave contradictory statements, as Wegener assumed that sial is floating on sima without any barrier but Wegener also explained the mountains are built due to the barrier of sima against the movement.
• Continental drift theory is also not able to explain the causes of different natural events like earthquakes and volcanoes.
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