The idea of lateral movement of continents or continental drift was put forward in 1912 by Alfred Wegener in a book ‘The Origin of Continents and Oceans’. The continental drift theory of Wegener ‘grew out of the need for explaining the major variations of climate in the past’.
According to A. Wegener, The climatic changes which have occurred on the globe may be explained in two ways.
- If the continents remained stationary at their place throughout geological history of the earth, the climatic zones might have shifted from one region to another and thus a particular region might have experienced varying climatic conditions from time to time.
- If the climatic zones remained stationary, the landmasses might have been displaced and drifted.
Wegener opted for the second alternative as he rejected the view of the permanence of continents and ocean basins.
According to Wegener, the Earth is made of SIAL, SIMA, and NIFE. SIAL, comprising the continents, is floating over SIMA, which is the upper portion of the seafloor.
He assumed based on the evidence of geology, paleo-ontology, and paleo-climatology, that all the landmasses were united together in the form of one landmass, which he called Pangaea, which was surrounded by one big ocean called Panthalassa.
According to Wegener, the drift started around 200 million years ago, and the Pangaea began to split and drift away from one another. Pangaea first broke into two large continental masses as Laurasia and Gondwanaland forming the northern and southern components respectively. Subsequently, Laurasia and Gondwanaland continued to break into various smaller continents that existed today.
According to A. Wegener, the continental drift was in two directions e.g. equatorward and westward.
- The equatorward movement was caused by gravitational forces, i.e., because of the intense gravitational pull of the Poles.
- The westward movement was caused by tidal forces. The westward movement led to the formation of Andes and Rockies and the Caribbean Island arcs.
Evidence in Support of the Continental Drift Theory:
Wegener has successfully attempted to prove the unification of all landmasses in the form of a single landmass. The following evidences support the concept of the existence of Pangaea.
The Matching of the Continents (Jig-Saw-Fit):
According to Wegener, there is geographical similarity along with both coasts of the Atlantic Ocean. The continental margins of the sub-equatorial portions of Africa and South America can be fitted together.
Geological evidence denotes that the Caledonian and Hercynian mountain systems of the western and eastern coastal areas of the Atlantic are similar and identical. These mountain systems have large deposits of coal in the same period.
But these regions don’t have a climate suitable for thick forest cover (high temperature and heavy rainfall). So this region could be located on the equatorial zone and slowly drifted away from the equatorial area.
The belt of ancient rocks of 2000 million years from Brazil coasts matches with those from Western Africa. The earliest marine deposits the coastline of South America and Africa are of the Jurassic age. This suggests that the ocean did not exist before that time.
The occurrence of rich placer deposits of gold in the Ghana coasts and the absolute absence of source rock in the region is an amazing fact. The gold-bearing veins are in Brazil and it is obvious that the gold deposits of Ghana are derived from the Brazil plateau when the two continents lay side by side.
Till is the sedimentary rock formed out of glaciers. At the base of the Gondwana system of sediments from India, Africa, Falkland Island, Madagascar, Antarctica and Australia have thick tillite indicating extensive and prolonged glaciation. The overall resemblance of the Gondwana type sediments demonstrates that these landmasses had remarkably similar histories.
According to Wegener, all the continental blocks were united together in the form of one landmass and the South Pole was located near the present position of Durban (South Africa). Thus the South Pole was located in the middle of Pangea.
Consequently, ice sheets might have spread from the South Pole outward at the time of glaciation and the aforesaid land areas which were closer to the south pole might have been covered with thick ice sheets. At much later date these land areas might have parted away a due to disruption of Pangea and related continental drift.
This glacial tillite provides unambiguous evidence of paleoclimate and also supports the drifting of the continents.
Distribution of Fossils:
Mesosaurus was a small reptile adapted to shallow brackish water. The skeletons of these are found only in two localities. The southern Cape province of South Africa and Iraver formation of Brazil. The two localities presently are 4,800 km apart with an ocean in between them. Fossil remains of Cynognathus (a Triassic land reptile about 3m long) are found in South America and equatorial Africa. Fossil remains of Lystrosaurus (a Triassic land reptile) are found in East Africa, South India, and North Antarctica.
The observations that lemurs occur in India, Madagascar and Africa led some to consider a contiguous landmass ‘Lemuria’ linking these three landmasses.
Fossilized plants such as fern-like glossopteris are found in similarly aged rocks in South America, South Africa, Australia, India and Antarctica — its seeds two large and heavy to have been carried across the expense of the present — day oceans by winds. Which support the idea of continental drift.
Evidence from Paleomagnetism:
Paleomagnetism is the study of the direction of the pole through ages. Magnetically susceptible minerals like hematite, pyrrhotite magnetite, etc. get aligned with the magnetic pole of the earth and recorded in the solidification of magma during that time. It is found that periodic changes have occurred and poles have wandered, which is not possible for the entire earth. Hence, it is the twist and turn of the land block and not for the entire earth which has again explained that the continents have shifted their positions.
Evaluation of the Continental Drift Theory:
- The concept of jig-saw-fit cannot be validated because both the coasts of the Atlantic Ocean cannot be completely refitted.
- The force applied by the Wegener (Gravitational Force and Tidal Force) is not sufficient enough to drive the continents so apart. For these factors to be able to cause a drift of such a magnitude, they will have to millions of times stronger
- According to Wegener SIAL is floating on SIMA, but in the latter part of his theory, he has described forceful resistance offered by SIMA in the free movement of sialic blocks to explain the origin of mountains along the frontal edge of floating continents.
- Though the most point of the continental drift theory was rejected, but its central theme of horizontal displacement was retained.