The permanency of position has been challenged by major theories such as continental drift theory, sea floor spreading theory and other theories of landform development. These theories help us understand the dynamic nature of Earth's surface and explain various geological phenomena observed across the globe.
INITIAL OBSERVATIONS
Distribution of Landmass and Water Bodies
There is more concentration of landmass in the northern hemisphere (approximately 75% of landmass) while there is more concentration of water in the southern hemisphere.
Shape of Continents
Except for Australia and Antarctica, most of the continents are roughly triangular in shape. Similarly, the majority of oceans are also triangular in shape.
Antipodal Arrangement
The triangles are arranged in such a manner that for the continents the base is to the north and apex is to the south, while for the oceans the base is to the south and apex is to the north. This is known as Antipodal Arrangement of the continents and water bodies.
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| Note:- Map Not to scale, For educational purposes only |
INTRODUCTION
The antipodal arrangement, paleoclimatic evidences, and changes in relief features have been explained with numerous theories, out of which the most significant one is the concept of Continental Drift.
Though the concept was initially given by Antonio Snider (1858) and F.G. Taylor in 1908, the most comprehensive theory was given by Alfred Wegener in 1912.
BASIC PREMISE OF THE THEORY
1. The theory aims to study past climatic changes that occurred due to the drifting of the continents.
2. It assumed that SIAL floats on SIMA without any resistance.
3. According to Wegener, there are two forces which cause the drift:
These forces cause the westward movement of continents.
These lead to the movement of continents northward, i.e., toward the equator.
4. Pattern Of Drift
According to Wegener, the drift started in the Carboniferous period due to the breaking of a major landmass known as Pangaea, which was surrounded by a super ocean known as Panthalassa.
THEORY
There was a supercontinent known as Pangaea which broke into two parts:
- Laurasia/Angaraland in the north
- Gondwanaland in the south during the Carboniferous period
This phase of breaking up of Pangaea is known as "Opening of Tethys".
The continents further broke and drifted apart. For example:
The westward drifting of northern South America caused the formation of the Atlantic Ocean (about 50 million years ago).
The northward drifting of Peninsular India and Africa resulted in the formation of the Indian Ocean.
The Gondwana landmass further broke into:
- South America
- Africa
- Peninsular India
- Madagascar
- Australia
- Antarctica
Australia, India and Africa continued moving at different rates, with Australia occupying its present position in the last 300 years.
Wandering of Poles
As the continents moved, the position of the equator and the poles also shifted, which Wegener called "Wandering of Poles".
• Once upon a time in the Silurian age (350 million years ago), the equator passed through Norway.
• During the Carboniferous period, the equator passed through South America, North Atlantic Ocean, and Eurasia.
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| Note:- Map Not to scale, For educational purposes only |
Thus the drifting of the continents ultimately gave rise to the present day arrangement of the continents.
According to Wegener, the edges of westward-moving continents were crumpled to give rise to fold mountains such as the Andes and Rockies due to the resistance of SIMA.
Similarly, island arcs of Indonesia, Malaysia, and Japan were formed because of the differential rate of continental drift.
CRITICISM OF WEGENER'S THEORY
1. Contradiction in Assumptions
Initially, Wegener assumed that SIAL floats on SIMA without resistance. But later, his theory states that mountains are formed due to resistance of SIAL over SIMA. Thus, there are contradictions in his theory.
2. Inadequate Forces Applied
Tidal force and force of buoyancy cannot properly explain the drift. Tidal force is highly inadequate for westward drifting of the continents, and similarly, the force of buoyancy and gravity is also very weak.
3. Unexplained Direction and Timing
Wegener could not properly explain the direction and time of drifting of the continents.
4. Trigger Mechanism Not Explained
Wegener could not explain why drifting started only in the Carboniferous period. In other words, he could not explain the trigger mechanism that caused the breaking of Pangaea in the Carboniferous period.
5. Limited Evidence
Even the evidences such as jigsaw fit of the continents or the distribution of fossil fuels have some limitations. Therefore, Wegener's theory suffered more criticism than praise.
POSITIVES/EVIDENCES
1. Jigsaw Fit of Continents
The evidences have proved that continents on either side of the Atlantic Ocean exactly fit each other, which means that these continents were once together.
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| Note:- Map Not to scale, For educational purposes only |
Ahmad and Krishna have proved that Southern India and Western Australia were once united to form a single landmass.
2. Geological Evidences
Presence of the same rocks on either side of the Atlantic provides strong evidence for continental drift.
The mountains of Appalachian (North America) are similar to the mountains of Ireland (England)and Mountains of Scandinavia (Norway).
3. Distribution of Fossils
The rocks of South America, Africa, Peninsular India, Australia, and Antarctica have the fossils of Glossopteris (a fern). This is the biggest evidence which shows that continents were once united in the form of Gondwana landmass.
There are numerous zoological evidences, including the behavior of Lemmings (rat-like animals), which have the tendency to migrate westward whenever there is an increase in their population.
CONCLUSION
Significance of Continental Drift Theory
Thus, the drifting of the continents ultimately gave rise to the present-day arrangement of the continents. Though Wegener's theory had many flaws and faced significant criticism, the basic idea that continents continuously changed their position led to serious thought and discussion about the dynamic nature of Earth's surface.
This eventually led to further studies in Sea Floor Spreading, which ultimately resulted in the development of the grand theory known as Plate Tectonic Theory.
Questions Asked in UPSC
Question 1: What do you understand by the theory of continental drift? Discuss the prominent evidence in its support. (UPSC Mains 2013)
1. The belt of ancient rocks from Brazil coast matches with those from Western Africa.
2. The gold deposits of Ghana are derived from the Brazil plateau when the two continents lay side by side.
3. The Gondwana system of sediments from India is known to have its counterparts in six different landmasses of the Southern Hemisphere.
Select the correct answer using the code given below:
a) 1 and 3 only
b) 1 and 2 only
c) 1, 2 and 3
d) 2 and 3 only
Ans:- C
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