Continental drift theory

The theory that the present day position of continents is not permanent but the continents constantly drift and change positions, was first proposed by Alfred Wegener (1904, 1912, 1924) who from close studies of maps, discovered that coastlines of all continents fitted together like a jigsaw puzzle, suggesting that all continents in the past were placed together in a mass of supercontinent. Wegener believed that continents were made of light material called Sial (made of silica and aluminium) that floated over a heavier material Sima (made of silica and Magnesium) that makes ocean floors, like icebergs floating in ocean water.

Later, in 1937, A.L. Du Toit elaborated on this theory in his book, On Wandering Continents, which explained many of the puzzles of discontinuous distribution of animals. But it was much later in the late 1960s that detailed studies of ocean floors revealed that the earth’s crust, which is made of hard rocks and soil, is only 30-60 km thick and is broken into several plates that float over the molten mass of the interior of earth that is constantly in circulation. The theory is now known as The plate tectonic theory which postulates that the continental plates move at a very slow pace to change positions that can be measured in thousands of kilometres.

The innermost part of earth is called Barysphere or core or nife which is about 4,400 km in diameter, surrounding which is Pyrosphere or mantle, which is 2,500 km thick molten mass and like a wrapper is a thin crust or Lithosphere of only 30-60 km thickness that is made of rocks and soil and which supports life on its surface.

Continents move because the crust is broken into blocks or plates that float on the hot molten lava of mantle that keeps in circulation due to slow convection currents produced by the heat emanating from barysphere or core. Due to this heat the molten basalt rises upwards and spreads horizontally just under the crust and cools down. hen there are subduction zones where the crust sinks down and melts into the mantle due to the pressure of spreading sea floor. This process of spreading and sinking of the earth’s crust makes the continental plates move.

During the Cambrian period major land masses occurred south of equator and Africa, South America, Australia, Antarctica and parts of Asia formed a single continental mass, Gondwanaland, that extended up to the South Pole but the position of continents was reverse of what it is today. There are evidences of major mountain building in North America and Europe and of glaciation during Devonian period, when much of Gondwanaland was covered with ice sheets. There was sharp decrease in the atmospheric CO2 during this period perhaps because of invasion of land by plants. Invasion of land by plants attracted invertebrates and then vertebrates on land, pioneers among them must have been detrivores rather than plant feeders, such as springtails, millipedes and mites. 

By Carboniferous period even the northern land masses started to move southwards to join Gondwanaland, forming a single supercontinent called Pangaea towards the end of this period. Most of the present day coal beds were formed when these forests were buried during this period (carboniferous means coal-bearing). High levels of atmospheric oxygen, low levels of CO2 and abundance of food enabled invertebrates to diversify and become giants. Pangaea continued to exist till Permian when it again broke and started to drift apart.

During Permian, about 270 million years ago, a single large continent Pangaea started to split into two land masses, the northern Laurasia and southern Gondwanaland, separated by the sea of Tethys. The splitting perhaps caused what is known as the worst mass extinction in the history of evolution. 

During Jurassic, the continental masses began to break up leading to the formation of Atlantic Ocean. Americas began to drift westward, Antarctica and Australia southwards and India towards the northeast. 

North and South Americas were connected together by a narrow corridor, through which migrated reptiles and primitive mammals. Towards the end of Cretaceous some catastrophe struck the earth resulting in another mass extinction and extermination of the most powerful of all animals that ever existed-the dinosaurs.

During Eocene, North and South Americas got disconnected by the submergence of panama connection, isolating the South American continents from the others for a very long time. Almost at the same time North America also got disconnected from Europe for ever, although a filter bridge may have existed between the two continents for some time.

During Miocene Indian plate had moved sufficiently northwards to collide with the Eurasian plate, obliterating the Sea of Tethys and starting a process of mountain-building that continued well into the Pliocene. In the beginning, the rising Himalayas did not produce an effective barrier between the Palaearctic and Oriental Regions but they gradually rose to great heights of today and effectively checked the movement of animals between the two regions.