World’s Physical Geography

Answer: Continental drift is the movement of the Earth’s continents relative to each other. The hypothesis that continents ‘drift’ was fully developed by Alfred Wegener in 1912. However, it was not until the development of the theory of plate tectonics in the 1960s, that a sufficient geological explanation of that movement was understood. According to Wegener, all the continents formed a single continental mass named Pangaea, which was surrounded by a mega-ocean Panthalassa. He argued that, around 200 million years ago, the supercontinent, Pangaea, began to split and the drift started. Pangaea first broke into two large continental masses Laurasia and Gondwanaland forming the northern and southern components respectively. Subsequently, Laurasia and Gondwanaland continued to break into various smaller continents that exist today. Wegener suggested that the movement responsible for the drifting of the continents was caused by pole-fleeing force and tidal force. Following evidence in support of the continental drift theory was given by him:

• Matching of continents (jig-saw-fit): The shorelines of Africa and South America facing each other has a remarkable and unmistakable match. Similarly, when matched, Africa, Madagascar, and India’s east coast all fit together.
• Rocks of the same age across the oceans: The belt of ancient rocks of 2,000 million years from Brazil’s coast matches with those from western Africa. The earliest marine deposits along the coastline of South America and Africa are of the Jurassic age. This suggests that the ocean did not exist prior to that time.
• Tillite deposits: The glacial tillite found in the Gondwana system of sediments from India has a resemblance to six different landmasses of the Southern Hemisphere. Counterparts of this succession are found in Africa, Falkland Island, Madagascar, Antarctica, and Australia besides India.
• Placer deposits: Rich placer deposits of gold are found on the Ghana coast despite the absolute absence of source rock in the region. The gold deposits of Ghana seem to have been derived from the Brazil plateau when the two continents were once joined.
• Distribution of fossils: Identical species of plants and animals adapted to living on land or in freshwater are found on either side of the marine barriers. For example, skeletons of Mesosaurus a small reptile adapted to shallow brackish water are found only in the Southern Cape province of South Africa and the Iraver formations of Brazil. But, the two localities are presently 4,800 km apart with an ocean in between them.

Wegener’s theory was criticized on the grounds of forces such as pole-fleeing force and tidal force, which are considered to be too weak to be able to move continents. Plate tectonics accommodated continental motion through the mechanism of seafloor spreading. There is now considerable evidence that the continents are parts of lithospheric plates and these plates move over the asthenosphere and cause continents to drift. New rock is created by volcanism at mid-ocean ridges and returned to the Earth’s mantle at ocean trenches..