Continental Drift theory

CHAPTER-V

CONTINENTAL DRIFT AND PLATE TECTONICS

The possibility of drifting of continents was first suggested by the French scholar Antonio Snider in 1858, but was opposed.  In 1910 F.B.Taylor of America invoked the hypothesis of horizontal displacement of continents, with a view of explaining the distribution of mountain ranges, but Taylor received a scant attention.  German professor Alfred Wegener was the first to put forward this idea in the form of a theory in 1912,its English translation was made in 1924 since then it has attracted much attention and publicity, and a huge literature has grown around this theory.

     According to the Wegener, all the continental mass which he called ‘Pangaea’ was united.  This super continent was surrounded by a mega ocean called ‘Panthalassa’, meaning all water.  He argued that, around 200 million years ago, the super continent Pangaea began to split.  First it broke into two large continents called Laurasia and Gondwana forming the northern and southern components respectively.  These two blocks were separated by a long shallow inland sea called “Tethys”.  The super continent Pangaea started breaking and the present shape and relative position is the result of fragmentation of Pangaea by rifting and the drifting apart of the broken parts.  According to this theory the continents are made of lighter SIAL and are floating on the denser SIMA.  The drifting of Pangaea was made possible chiefly due to differential gravitational forces.  The continents drifted in two directions – one towards Equator and other towards West.  On account of equatorial drift Africa and Eurasia were pushed closer together and the Tethys marine deposits located in between the two raised up in the form of mighty fold mountains extending from the Pyrenees and the Alps, and the Atlas mountain of N.Africa to the extensive Himalayan ranges of Asia.  On account of this equator ward drift Peninsular India and Africa separated from Antarctica and Australia, and as a result of their further drift in the course of time a portion of panthalasa got converted into Indian ocean.  The reason for the equatorial movement was the gravitational attraction exerted by the earth’s equatorial buldge.  The other movement of the continents was towards the west, the main reason for this drift as given by Wegener was the tidal force of the moon and the sun on the continents.  North America and the South America got separated from Europe and Africa respectively and the Atlantic ocean came into existence.

Evidence in favour of the Drift theory

Jig-saw Fit evidence:  He was struck by the geographical similarity between the opposing coasts of the Atlantic Ocean.  The outlines of the coast on two sides of the Atlantic are such that they can be easily joined together and one appears to be a detached portion of the other.  The eastern coast of South America can be fitted into the western coast of Africa. Similarly the eastern coast of North America can be fitted against the western coast of the Europe.

Geological structure:  There is remarkable similarity in the geological structure of the lands located on the two coasts of the Atlantic Ocean.  One, the Appalachian mountains of the North America which come right upto the coast and then continue their trend across the North Atlantic Ocean in the fold mountains of South West Ireland, Wales and Central Europe.

Paleo-climatic evidence:  The distribution of the Permo-carboniferous glaciations presents a powerful proof of the fact that at one time these landmasses were assembled together, since the evidences of these glaciations are found in Brazil, Falkland, South Africa, Peninsular India as well as in Australia. It is difficult to explain the distribution of glacial on land and water.  In the opinion of Wegener, all these land masses were united together to form one super mass of land.

Paleontological evidences:  Fossil remains of land animals and plants and of fresh water species in distant lands provide good evidence as they are now separated by oceans.  These species could migrate freely across united continents but not across as intervening ocean.

Polar wandering:  Paleomagnetic studies have shown that there has been periodic change in the position of magnetic pole that it recorded in the rocks by way of permanent magnetism.  It shows the changing position of the earth’s poles in geological time scale.  This is known as polar wandering.  This clearly demonstrates that the continents have frequently moved and changed direction of their motion from time to time.