Defence Exam Notes: Continental Drift theory

Upon careful observation of the coastlines of the Atlantic Ocean, one can easily say that the coastlines might have been connected at some point in time. It's not a big surprise that numerous researchers thought about this resemblance and about the two Americas, Europe and Africa, to be once combined.

Among the pioneers of such a proposal was Abraham Ortelius, a Dutch mapmaker, who initially proposed such a possibility in 1596. Another pioneer, Antonio Pellegrini drew a map which indicated the three landmasses together. Finally, it was Alfred Wegener—a German meteorologist who set forth an extensive theory as "the continental drift theory" in 1912.

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Continental Drift Theory

As proposed by Wegener, all the continents today formed a giant mainland mass and all the oceans formed one great ocean. All the continents combined were called PANGAEA, which translates to ‘all earth’ and the giant sea was called PANTHALASSA, which means ‘all water’.

He also proposed that around 200 million years ago, Pangaea, started dividing. It initially broke into two huge mainland masses, named Laurasia and Gondwanaland which formed the northern and southern parts respectively.

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The division of Laurasia and Gondwanaland continued further into different smaller landmasses that exist today. Evidence was offered in support of this theory. In this article, we will look at the evidence given by Alfred Wegener in support of his theory:

The matching of shorelines of continents or the Jig-Saw-Fit
- The shorelines of Africa and South America separated by the Atlantic Ocean have a strikingly matching fit.
Rocks of Same Age Across the Oceans
- Study of rock systems across different continents suggests that they have had the same phases of development and at the same time. This suggests that these rock systems developed together and were once a part of a greater system of rocks. This is possible only if the theory is true. For example, the belt of old rocks from 2,000 million years ago in Brazilian coast matches with those from western Africa. The ancient marine deposits along the coastline of South America and Africa are from the Jurassic age.
Tillite
- Tillite is a type of sedimentary rock formed out of the glacial deposition. The best example is the Gondwana rock system in India. It has six similar counterparts in six distinct landmasses of the Southern Hemisphere. At the bottom, the rock system has thick tillite formation indicating an extensive and long period of glaciation. Counterparts of Gondwana rock system can be found in Africa, Falkland Island, Madagascar, Antarctica and Australia. This indicates that these landmasses had a remarkably similar history.
Placer Deposits
- The occurrence of rich placer deposits of gold in the Ghana coast and the absolute absence of source rock in the region is an amazing fact. The gold-bearing veins are in Brazil and it is obvious that the gold deposits of Ghana are derived from the Brazil plateau when the two continents lay side by side.
Distribution of Fossils
- Similar species of plants and animals, adapted according to the same environment have been found across different continents. This created a problem in the documentation for a scientist named Lemur. According to his observations in India, Madagascar and Africa, he proposed the existence of a contiguous landmass ‘Lemuria’, which comprised these three landmasses. Species that were discovered by Lemur were Mesosaurus, a small reptile adapted for living in shallow saltwater. The skeletons of Mesosaurus were discovered only in two places: the Southern Cape region of South Africa and Traver formations of Brazil. The two regions are now separated by 4,800 km of sea.
Force for Drifting
- Wegener reasoned that the force responsible for this drifting of landmasses is pole-fleeing force and tidal force. He related the pole-fleeing force with the revolution of the earth. The earth is not a perfect sphere; it has a lump at the equator. This lump is attributed to the revolution of the earth. The second force that was proposed by Wegener, was the tidal force caused by the gravity of the moon and the sun. Wegener proposed that although these forces may not appear to be strong enough but would cause a major change when applied over millions of years. However, the majority of researchers do not believe that these forces are sufficient.

Image Source Brainkart.com

Criticism or Weakness:

It only talked about the movement of continents stating that the continent floated over the ocean floors like ship floats on the ocean and this is not true.
It does not talk about the movement of ocean floors.
Reasons for the drift are not right.
Wegner did not talk about the condition before the Pangea starting the explanation from only 250 million years before present.

Later, it was found that there were more such continents and their breaking. Wegner tried to explain every part despite not having suitable reasons.

Arthur Homes in 1930 discovered convection and convectional current in the mantle. The generation of this current due to radioactive elements causing thermal differences in the mantle portion. This convectional current was the cause of the movement of the earth crust. It was an explanation for the issue of drift-force, based on which contemporary scientists discarded the continental drift theory.

Plates of the Earth

The major plates are as follows:

Antarctica and the surrounding oceanic plate
North American plate (the western Atlantic floor is separated from the South American plate at a point near the Caribbean islands)
South American plate
Pacific plate
India-Australia-New Zealand plate
Africa with the eastern Atlantic floor plate
Eurasia and the adjacent oceanic plate.

Some important minor plates are listed below:

Cocos plate: lies between Central America and Pacific plate
Nazca plate: lies between South America and Pacific plate
Arabian plate: the majority of the Saudi Arabian landmass
Philippine plate: lies between the Asiatic and Pacific plate
Caroline plate: lies between the Philippine and Indian plate (North of New Guinea)
Fuji plate: North-east of Australia.