Continental Drift Theory - UPSC Geography Notes

Continental Drift Theory

This theory was given by German Meteorologist named Alfred Wegener in 1912 and elaborated it in his book the 'Origin of continents and Oceans' in 1915. The theory states that the present continents were once joined together as a single super continent called Pangea, which later broke apart and drifted to their present positions. This theory was the first scientific attempt to explain the similarity of coastlines, rocks, fossils and geological structures found on different continents.

Concept of Continental Drift Theory

According to Wegener:-

• Around 250 million years ago, all land masses formed one giant supercontinent called Pangea.
• Pangea was surrounded by a super ocean called Panthalassa.
• A shallow sea was formed when Pangea was divided into two parts. The northern part of Pangea, situated north of the Tethys Sea, was called Laurasia, and the southern part was called Gondwanaland. The northern part is also known as Angaraland.
• Further rupturing of Laurasia formed North America and Eurasia.
• Further rupturing of Gondwanaland formed South America, Africa, Antarctica, India, Australia and Madagascar.
• Because of rupturing of the continents and their respective movement formed different oceans also like:-
  • Between North America and Eurasia intrusion of Panthalassa water formed North Atlantic and later on South Atlantic was formed between America and Africa.
  • Indian ocean was formed between Africa, Australia and Antarctica.
  • Arctic Ocean was formed when the Panthalassa water near North pole was surrounded by North America and Eurasia almost from all the sides.
  • Pacific ocean i.e. largest ocean on the planet is remnant of the old Panthalassa.
• Over geological time due to fragmentation of the land masses and their gradual shift has led to the present arrangement of the continents.
• Wegener believed that the continents are composed of lighter rocks i.e. silica and aluminium, floating over dense oceanic crust which is made up of silica and magnesium allowing them to move.

Evidence supporting this theory

1. Jigsaw fit of continents:- The coastlines of continents appear to fit together like pieces of a puzzle. e.g. the eastern coast of Brazil/South America matches closely with the western coast of South Africa. The best fit is obtained by matching continental shelves rather than present coastline. This suggested that these 2 continents were once joined.
2. Geological Evidence:- Rock formation and mountain belts of similar age and composition are found on opposite sides of the ocean e.g. the Pre-Cambrian rock formations occur in Africa and South America. These similarities indicate a common geological stream Similarly the Appalachian mountain of North America continue into Norway also a part of Caledonian mountain range. It suggests that these mountains were formed when North America and western Europe were together.
3. Fossil Evidence:- Identical fossils are found on continents now separated by oceans. e.g.
   1. Mesosaurus:- Fossil of Mesosaurus are found in South America. It was a fresh water reptile. It could not have crossed the Atlantic ocean.
   2. Lystrosaurus and Cynognathus:- Their fossils are found across multiple southern continents. Though these were land reptiles and unable to swim.
   3. Glossopteris:- It is fossil of a plant/fern found in India, Australia, Antarctica, Africa and South America. However, this plant grows in cold climatic conditions. Such fossil distribution strongly suggest that these continents were once connected.
4. Paleoclimatic Evidence:- Evidence of ancient climates also support continental drift. e.g. Permo-Carboniferous glacial deposits are found in India, South Africa, Australia, South America and Antarctica. These regions are now widely separated but once were part of Gondwanaland near the south pole. Other examples include the presence of vast coal deposits in Russia, Kazakhstan, US, UK (near Atlantic region) and in Antarctica. Coal forms in warm humid conditions indicating these areas were once located in different climatic zones.
5. Presence of Gold Placer Deposits near Guinea coast of Africa:- Many rivers carry gold deposits/minerals and deposit them into the sea but gold reserves are not found along with West African coast instead these are found in Brazil. If we put jigsaw fit evidence and the presence of gold placer deposits along with the fact that gold reserves are found in Brazil then it clearly becomes evident that these two continents were together in their geological past.

Forces Responsible for Continental Drift

Three types of forces were suggested by Wegener which were responsible for:-

1. Rupturing of Pangea
2. Movement of continents from pole to equator
3. Movement of continents to the west like North America and South America For rupturing of Pangea he made tussle between the gravitation and force of buoyancy.

• According to him since these two forces act in the opposite direction and gravity is more powerful force thus Pangea ruptured about 200 million years ago.
• Pole-fleeing Force:- It is caused by earth's rotation it is basically centrifugal force due to which continents were believed to move from the pole to equator like India, Africa, Eurasia, etc.
• Tidal Force:- It is generated by gravitational attraction of sun and moon which rise in the east and set down in west because of the earth rotates from west to east.

Limitations of Continental Drift Theory

1. Inadequate explanation of driving force:- The theory could not explain that
   1. Why Pangea ruptured:- According to Wegener, it was because of tussle between the gravity and force of buoyancy but scientists have proved that it was a wrong interpretation. Gravity keeps the things intact.
   2. Pole-fleeing force was held responsible for continental drift from poles toward equator:- Scientists have proved that not only this force is too weak to move the continents but also it should also move the mass i.e. both continent and ocean floor instead of continents only as suggested by Wegener.
   3. The tidal force of sun and moon:- Which were made responsible for the westward drift of the continent if taken seriously then this force should be 90,000 times greater than the present tidal forces of sun and moon and in such case it could have stopped the rotation of earth itself.
2. Apart from these limitations related to insufficiency of force he also lacked some conceptual clarity also e.g.
   1. According to him continents were drifting over the ocean floors this is conceptually wrong as plate tectonics has proved that it is lithospheric slabs that are drifting over the asthenosphere.
   2. Secondly, he tried to explain the formation of folded mountains and island arcs or archipelago. To explain these he said that when continents were drifting over ocean floor where frictional resistance along their leading margin that's why continents got folded and mountains were formed. Island arcs are those continental part found at trailing margin of the continents which could not match the rate of the drift of the moving continents e.g. Japan, Indonesia, Caribbean Island.

Conclusion

Alfred Wegener's continental drift theory was a landmark contribution to geology. Although it failed to explain the exact mechanism of continental movement, its extensive geological, fossil, and climatic evidence strongly suggested that the continents were once united as Pangea. Later discoveries in sea floor spreading and plate tectonics confirmed the essence of Wegener's idea, making the continental drift theory the foundation of modern geoscience and our understanding of the Earth's dynamic nature.