Continental Drift Theory by Alfred Wegener

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Continental drift theory by Alfred Wegener (1912) is regarded as the source of most ideas which lead to formation of plate tectonic theory. Wegener was a climatologist. He found fossils of those animals in a climatic region where they should not be. For instance, if you find fossils of penguins (which live in cold climate) in a hot climate region, you may conclude that the climate of that region has changed. However, Wegener had two hypothesis to explain such occurrences i.e. either the climatic zones have shifted from one continent to other continent or the continents has drifted from one climatic zone to another. He argued that climatic zones can not wander to such a large extent because the tilt of axis of the earth remains between 21°-24°, therefore, it is the continents which have drifted from colder to warmer zones and vice-versa. Wegener chose the continental drift theory because he observed that the continents can fit with each other lie a jigsaw puzzle.

Continental Drift Theory in Simple Words

  • Basic Assumption: He assumed that continents are landmasses which float freely on liquid layer called Mantle without any friction like a boat on water.
  • Basically, this theory suggests that there was a single large continent called Pangaea which formed during 299-273 Million year ago (MYA).
  • A single large ocean, Panthalasa, surrounded the Pangaea. Pangaea was located at the south pole.
  • The Pangaea floated on the top of earth’s liquid layer called Mantle without any resistance.
  • Around 200 MYA, a long narrow ocean, called Tethys, opened in the middle of Pangaea and divided it into two halves. The southern part is called Gondwanaland and norther part Laurasia.
  • With time, the Gondwanaland and Laurasia further broke in to smaller pieces called Continents and drifted apart from each other to form the present world.

    Fig. 1: Direction of Continental Drift

Forces Causing Continental Drift

  • The continents drifted in two directions/ways i.e. 1. Westwards and 2. Equatorward.
  • According to Wegener, there are three types of forces which lead to westward and equatorward movement of continents.
    1. Tidal Force of the Moon and Sun: The moon exerts gravitational pull on every object on Earth. This pull generates tidal waves in the oceans which flow from east to west. Over millions of years, these tidal waves lead to westward movement of continents. The gravitational pull of moon and sun also slow down the speed of rotation of continents from west to east. So, the underlying layers of earth rotate faster than the continents. Therefore, the continents lag behind and seem to move westward.
    2. Centrifugal Force of Earth Rotation: The rotational force of the earth exerted centrifugal force on the Pangaea which was located at south pole. This caused the fleet of continents from the pole towards the equator. This process is similar to outward movement of a dice thrown at rotating Roulette.
    3. Buoyance Force: Buoyancy force is the upward force exerted by any liquid on any object floating on it. In case of continents, the lava in the mantle layer pushes continents upwards at their center of gravity.  The center of gravity and buoyant force is aligned in such a manner that the net movement of continents is equatorward with slow anticlockwise rotation.

These forces lead to continental drift. When the continents started drifting, the friction between lithosphere and mantle, lead to formation of mountains on the western coasts of continents i.e. Rockies and Andes. Similarly, some pieces landmass of the continents lagged behind due to friction. These are known as archipelagos, group of island which are usually found on the east of continental shores

Evidence in Favour of Wegner’s Theory

The following evidences support the continental drift theory.

  1. Jigsaw fit of continents: The shape and size of the continents is such that different continents seem to fit with each other like a jigsaw puzzle.
  2. Rock Types and Structure: Wegener found the same rock type with similar structure and age on the coast of West Africa and East Latin America. This points towards their past proximity.
  3. Mountain Ranges: The Appalachian mountain ranges have similar structure and age as the Caledonian Mountains of Scotland.
  4. Animal Fossils: Wegener found the fossils of Mesosaurus and Cynagnathus on the coast of West Africa and East Latin America. These coasts are thousands of kilometers apart and these animals could not swim from one coast to another.
  5. Plant Fossils: Similarly, the plant fossils found in the Svalbard region are of those plants which can not survive in cold and frigid environment of Svalbard.
  6. Coral Deposits: Coral polyps are tropical organisms. Geologist have found calcium carbonate deposits by coral polyps in frigid environments where polyps could not survive which points towards the continental drift.
  7. Formation of Archipelagos: An archipelago is a group of islands. For instance, Japan, an archipelago, is located towards east of Asia. These were formed when some of the landmass  could not keep up with the westward moving continents.

Assessment of Continental Drift Theory

Wegener was far ahead of his times in terms of his ideas. His peers made fun of his theory. Scientists rejected his theory on following grounds.

  1. The continents don’t fit into each other perfectly.
  2. The gravitational pull of the moon is not so strong that it can move continents. If it was so, the earth would have stopped rotating by now.
  3. Tidal currents are insufficient to move continents.
  4. The centrifugal and buoyancy force is insufficient to move continents.
  5. This theory does not explain the origin of Pangaea and conditions prior to formation of Pangaea.
  6. This theory is self contrary. Wegener first argued that Pangaea floats on liquid mantle without any friction but later said that the friction between Pangaea and mantle lead to formation of mountains and archipelagos.

Conclusion

Apparently, Wegener could not give proper explanation to his critics regarding the forces causing continental drift and origin of Pangaea. However, the modern evidences in Plate Tectonic Theory compliment Wegner’s ideas on continental drift.

  • Firstly, the evidences from Paleomagnetism, Sea Floor Spreading and Polar Wandering clearly points towards the continental drift.
  • Secondly, the primary force of continental drift is convectional current in upper mantle.
  • Lastly, if we take million of years of weathering and erosion into account, the continents will fit with each other. Therefore, Wegener was truly a pioneer.