Continental Drift Theory: Evidences and Criticism
To explain the present distribution of oceans and continents, various theories have been proposed. Continental Drift Theory is considered a pivotal theory which provided various conclusive proof to establish the movement of the earth crust but the theory failed to establish the mechanism behind the process.
People for a very long time, till the early 20th century, thought that the continents were fixed land masses. But it was in 1912, Alfred Wegener, a geologist came up with the theory of continental drift. He further expanded the idea in his book The Origin of Continents and Oceans which was published in 1915. Though Abraham Ortelius, a Dutch cartographer was the first one to work on ideas of symmetric coastlines on the sides of Atlantic ocean.
Continental Drift Theory
Continental drift means the movement of the continents across the ocean bed. This drifting happens very, very slowly, over hundreds of million years!
- According to Alfred Wegener, all the continents formed a single continental mass known as Pangea (Pan=all + Gea=earth).
- Pangea was surrounded by a mega-ocean, Panthalassa (Pan=all + Thalassa=ocean).
- Wegener further argued that about 225 million years ago, Pangea began to split. It first broke into two large continental masses- Laurasia (the northern component) and Gondwanaland (the southern component).
- The intertwining part between Laurasia and Gondwanaland was known as Tethys Sea, a shallow and meandering waterbody.
- Subsequently, Laurasia and Gondwanaland continued to break into smaller continents that we see today.
Figure showing different stages of Continental Drift
Stages of Continental Drift
- First stage - During the Carboniferous period in which a supercontinent Pangea was surrounded by mega-ocean, Panthalassa.
- In the second stage - Around 200 million years ago, Flight of continents took place, continents began to drift gradually and broke into pieces, Laurasia (Angaraland) and Gondwanaland. ( India was a part of Gondwanaland.)
- In the third stage - During the Mesozoic era, the space between Laurasia and Gondwanaland got filled with Tethys Sea and it gradually got widened.
- Fourth stage - around 100 million years ago-Westward drift of North America and South America led to the opening of Atlantic Ocean.
- Fifth stage is the Orogenetic Stage-in which mountain building activity took place. While Himalayas and Alps were formed with the folding of sediments of Tethys Sea, and westward drift of North and South America led to folded edges and formation of Rockies and Andes.
Forces responsible for Continental Drift
- Due to the cumulative effect of gravitational forces, 'pole-fleeing force' and force of buoyancy, the continental drift was equator wards. The 'pole-fleeing force' is due to increase in centrifugal force from poles towards the equator as the earth is not perfectly round but there is a bulge at the equator.
- Due to tidal currents resulting from rotation of the earth, the continental drift was westwards. Tidal currents are due to the attraction of moon.
However, later, these two forces were found to be insufficient reasons for drifting of the continents which is counted as the criticism of Wegener's theory.
Evidence in support of the Continental Drift Theory
To justify his theory, Alfred Wegener came up with some evidence which are listed below.
- Jig-Saw Fit evidence or the Matching of continents- the shorelines of South America and Africa when are facing each other shows a remarkable fit. Similarly, Africa, Madagascar and east coast of India fit into each other when matched. Figure showing jig-saw fitting of continents of Africa with South America
- Rocks of Same Age across the Oceans- The radiometric dating methods have correlated rock formation across different continents. It suggests that the belt of ancient rocks formed 2,000 million years ago from Brazil coast matches the mountain belt found in Western Africa. the Caledonian and Appalachian mountains also show similarity. It also suggests that the early marine deposits along the coastline of Africa and South America belong to the Jurassic age indicating that the ocean did not exist prior to that time. Figure showing matching mountain ranges across different continents
- Tillite evidence- Tillite refers to the sedimentary rock made out of deposits of glaciers. In revealing evidence, it has been found that the Gondwana system of sediments from India has its counterparts in six various landmasses of the Southern Hemisphere- Africa, Falkland Island, Madagascar, Antarctica and Australia and India. It demonstrates that these landmasses had remarkably same antiquities. Figure showing continuity of glaciers across different landmasses
- Placer deposit- In the Ghana coast (West Africa), rich placer deposits of gold are found. But there is a complete absence of source rock in the area. It is amazing to know that the gold-bearing veins are in Brazil. It suggests that the gold deposits of Ghana are derived from the Brazil plateau when the two continents lay side by side.
- Fossil evidence- The observations that Lemurs occur in Africa, Madagascar and India suggest that a contiguous landmass Lemuria existed connecting the three. Another amazing fact is that skeleton of Mesosaurus (a small reptile, adapted to shallow brackish water) are found only in South Africa and Iraver formations of Brazil. Figure showing fossils of similar species in different continents
Criticisms of the Continental Drift Theory
- There are flaws in jig-saw fitting, though it seems appropriate if 500-fathom Isobath is considered as an outline of the continents.
- Wegener talks about the role of forces like buoyancy, tidal currents and gravity. But these were not strong enough to drift continents.
- He advocates directional movement either westward or equatorward but movements have taken place in all directions.
- Alfred Wegener failed to explain the Pre-carboniferous history. He did not explain that why the drift began only in Mesozoic-era and not before.
- The theory did not take oceans into consideration.
- The theory did not explain the formation of oceanic ridges and Island arcs.
- Earth's crust is believed to be too rigid to permit large-scale motions. Wegener's ideas have not offered a suitable mechanism justifying the displacement of larger masses for long journeys.
Present status of the Continental Drift Theory
The Continental Drift Theory was not accepted by most of the scientific community and was hotly debated off and on for decades even after his death in 1930. In the 1920s, the concept of conventional currents in the upper mantle was developed. But Alfred Wegener was not able to incorporate the concept of conventional currents as the most justifying reason for the movement of continents due to his untimely death.
Although, the Continental Drift theory have become obsolete the main idea of the theory of drift of continent was the driving force behind all other modern theories including the theory of plate tectonics and seafloor spreading.