Continental Drift Theory – Wegener

The Continental Drift Theory is considered as a significant theory that produced numerous compelling proofs to demonstrate the movement of the earth's crust, but it failed to define the mechanism behind the process. Until the early twentieth century, the continents were assumed to be fixed geographical masses for a long time. The theory of continental drift was proposed by Alfred Wegener, a geologist, in 1915.  This article will explain the concepts of continental drift theory- Wegener, which is an important part of the geography syllabus in the UPSC and Other State PSCs Exams and other competitive exams.

Continental Drift Theory

• The movement of continents across the ocean bed is known as continental drift. This drifting takes millions of years to complete this process.
• Continental drift theory was given by Alfred Wegener in 1915. This theory has explained the origin of continents and oceans basins. Though it was a Dutch cartographer, Abraham Ortelius, who was the first to work on the concept of symmetrical coasts on the Atlantic Ocean's sides.
• According to Wegener, all the continents formed a single continental mass which is called “Pangaea” which meant all earth. This supercontinent was surrounded by a mega ocean called “Panthalassa” meaning all water.
• Subsequently, Laurasia and Gondwanaland continued to break into various smaller continents that exist today. Thus, Wegener proposed that continents are floating and constantly drifting on the earth’s surface. His hypothesis later on became the basis of present day plate tectonic theory. 

Various Stages of Continental Drift Theory

• The first stage occurred during the Carboniferous period, when Pangea, a supercontinent was encircled by Panthalassa, a mega-ocean.
• In the second stages around 200 million years ago in the Jurassic period, the super continent, Pangaea, began to split. Pangaea first broke into too large continental masses as Laurasia and Gondwanaland forming the northern and southern components respectively.
• In the third stages, the Tethys Sea progressively filled the area between Laurasia and Gondwanaland during the Mesozoic epoch, and it gradually broadened.   
• In the fourth stage around 100 million years ago when North and South America drifted westward, resulting in the emergence of the Atlantic Ocean. The Rockies and Andes were formed by the westward drift of North and South America.
• The Orogenetic Stage in which mountain-building activity took place is the fifth stage.
• Note: The Himalayas and Alps were constructed by the folding of Tethys Sea deposits.

Forces responsible for Continental Drift

• There are two factors which are responsible for continental drift which are as follows:
• The continental drift was equatorward due to the combined action of gravitational forces, pole-fleeing force, and buoyancy force because the planet is not perfectly round and has a bulge at the equator.
• Note: The 'pole-fleeing force' is caused by an increase in centrifugal force from the poles towards the equator.
• The continental drift was westward due to tidal currents caused by the earth's rotation.
• However, these two factors were eventually discovered to be insufficient reasons for continent drifting, which is considered a criticism of Wegener's theory.

Evidence that supports the Continental Drift Theory

• Alfred Wegener provided some evidence to support his theory, which is mentioned below.
• The Matching of Continents (Jig-Saw-Fit): When facing one other, the shorelines of South America and Africa form a similarity. Similarly, when matched, Africa, Madagascar, and India's east coast all fit together.
• Rocks of Same Age Across the Oceans: Radiometric dating techniques have been used to correlate rock development across continents. It suggests that the 2,000 million-year-old strip of ancient rocks off the coast of Brazil corresponds to the mountain ranges of Western Africa. Similarities can also be found between the Caledonian and Appalachian mountains. It also implies that the early marine deposits along Africa's and South America's coastlines date from the Jurassic period, implying that the ocean did not exist before then.
• Tillite: Tillite is a type of sedimentary rock formed by glacier deposits. The Gondwana system of sediments from India has been discovered to have parallels in six different landmasses of the Southern Hemisphere, including Africa, the Falkland Islands, Madagascar, Antarctica, Australia, and India. It reveals that the antiquity of both landmasses were strikingly similar.
• Placer Deposits: Gold placer deposits can be discovered along the Ghana coast (West Africa). However, there is no source rock in the immediate vicinity. The fact that gold-bearing veins may be found in Brazil is incredible. When the two continents are laid side by side, it appears that Ghana's gold reserves are sourced from the Brazil plateau.
• Distribution of Fossils: Identical species and animal were found on both the side of marine barrier. For example, Mesosaurus, a freshwater crocodile-like reptile that lived between 286 and 258 million years ago, is only found in Southern Africa and Eastern South America.

Criticism of Continental Drift Theory

• Wegener discusses how buoyancy, tidal currents, and gravity all play a role. These, however, were insufficient to move continents.
• He favours westward or equatorial travel, but movements have occurred in all directions.
• Alfred Wegener was unable to explain the pre-carboniferous period of time. He didn't say why the drift started in the Mesozoic era and not earlier.
• Oceans were not taken into account in the theory.
• Oceanic ridges and Island arcs were not explained by the idea.
• Large-scale motions are thought to be impossible due to the rigidity of the Earth's crust. Wegener's theories failed to provide a convincing mechanism for supporting the displacement of bigger masses during long voyages.

Conclusion

• The Continental Drift Theory was rejected by the majority of scientists, and it was intensely disputed for decades following his death in 1930. The concept of conventional currents in the upper mantle was conceived in the 1920s. However, due to his early death, Alfred Wegener was unable to add the concept of conventional currents as the most compelling rationale for continent movement.
• Although the Continental Drift theory is no longer valid, the central premise of the theory was the driving force behind all subsequent modern hypotheses, such as plate tectonics and seafloor spreading.

MCQs

Question. Which of the following phenomena might have influenced the evolution of organism? (UPSC CSE- 2014)

1. Continental drift
2. Glacial cycle

Select the correct answer using the codes given below:

1. 1 only
2. 2 only
3. Both
4. Neither 1 nor 2

Answer: (c)

Explanation:

• The Continental drift must have moved some animals from one continent to others and few getting stuck in the same place, which must have influenced the evolution. Hence statement 1 is correct.
• Glacial cycles would affect evolution as many might not able to tolerate extreme cold and would have extincted. Hence statement 2 is correct. Therefore (c) is the correct option.

Question. Consider the following statement related to continental drift theory?

1. The Rockies and Andes were formed by the westward drift of North and South America.
2. The Himalayas and Alps were constructed by the folding of Tethys Sea deposits.

Select the correct answer using the codes given below:

1. 1 only
2. 2 only
3. Both
4. Neither 1 nor 2

Answer: (c)

Explanation:

• In the third stages, the Tethys Sea progressively filled the area between Laurasia and Gondwanaland during the Mesozoic epoch, and it gradually broadened. Himalayas and Alps were constructed by the folding of Tethys Sea deposits. Hence statement 2 is correct.
• In the fourth stage around 100 million years ago when North and South America drifted westward, resulting in the emergence of the Atlantic Ocean. The Rockies and Andes were formed by the westward drift of North and South America.
• The Orogenetic Stage in which mountain-building activity took place is the fifth stage. The Rockies and Andes were formed by the westward drift of North and South America. Hence statement 1 is correct. Therefore (c) is the correct option.