Introduction
Continental drift refers to the horizontal movement of the continents on a vast-scale. Our earth can be divided into two major units :(i) the continental land masses, essentially composed of sial.
(ii) the oceans which are floored by sima.
It is assumed that the continental masses are floating above tthe viscous sima layer. It is well known that there is a remarkable accumulation of land masses in the northern hemisphere of the earth, while there is appreciable concentration of water bodies in southern-hemisphere. It has been further noticed that some of the continental masses like Africa, South America etc. exhibit southerly tapering edges. The major problem of geology is to what extent ‘these arrangements have been stable during the geological past.
The evidences in favour of the ‘Continental-drift’ hypothesis ‘were collected from the physiographic, geological and meteorological observations. The following evidences strengthen the idea of ‘Continental-drift’ :
1. Geological Evidences
These are as follows :
(i) Continental boundaries on either side of the Atlantic-ocean are matching with each other.
(ii) Similarity in fossil contents i.e., faunal and floral similarity.
(iii) Lithological similarity.
(iv) Stratigraphic similarity, with relation to age.
2. Geo-physical evidences
Palaco-magnetic studies and Zeodetic survey gives evidences in support of the phenomenon of Continental drift.
3. Tectonic evidences
These are as follows :
(i) Geological structures.
(ii) Development of mid-oceanic ridges.
(iii) Development of fold-mountains.
(iv) Development of island-arcs.
4. Meteorological evidences
Information obtained from palaeo-climatic and palaeo-geographic analyses, indicate that drifting of continents had taken place during the geological past.
The idea of continental drift was develeped by F.B. Taylor im America, and Alfred Wegener in Germany. Their theories are as follows :
Taylor’s hypothesis
According to him long back there were two great landmasses in the southern and northern hemisphere known as Gondwana land and Laurasia, respectively. He supposed that in due course of time those land masses started spreading towards the equator, more or lese radially from the polar regions.
He assumed that the sudden increase of tidal action of the moon in late cretaceous time, increased the rate of rotation of the earth which dragged the continents away from the poles. Accordingly he cited certain evidences in support of his theories, that
(i) Eastcoast of South America is similar in shape to the West coast of Africa and they fit like the parts of a ‘Zig-saw’.
(ii) Location of most of the tertiary mountains are found more or less towards the equator side of the continental masses.
But his assumptions fail to justify the exact happenings, for example tidal forces never increase the earth’s rotation, but the assumption is just the reverse of it. Besides, the equatorial movement of continents as assumed by Taylor fail to explain the drifting of South America from Africa.
Wegener’s hypothesis
According to him all the sialic layer was concentrated in a large continent called the ‘Pangea’ before the Silurian period. This block floated in a universal basaltic layer. Pangea was surrounded by the world-ocean called ‘the Panthalassa’.
In the late Palaeozoic period, probably during permian or Mesozoic era, the Pangea broke into pieces under the influence of the tidal force, and the force generated by the movements of the earth’s axis of rotation and revolution.
The southern parts of the Pangea broke apart during Mesozoic and the northern in the Tertiary periods. The Continental drift was caused by the differentials gravitational forces which acted upon the protruding block of sial. One force caused the drift towards the equator and the other towards the West.
The African block (the Gondwanaland) and the Furasianblock (the Laurasia) moved towards the equator. When the drift towards the equator was taking place, the Americas drifted towards the West. Thus the Atlantic Ocean was created between North and South America in the West and Europe and Africain the East. Australia was left behind in the beginning. Later it swung to the east. Only recently, in terms of geological time periods Antarctica separated from South America.
Before drifting, North and South America formed one unit. They rotated about a point in North America. Then they were drawn apart. This produced the narrow land of central America and the scattered fragments of the West Indies. Labrador and New Foundland separated from Europe during Quaternary. They swung South-West and Greenland was left behind as a separate block. At the same time the Indian part of the Gondwanaland moved nortb against the mass of the Asian main continent. It separated from. Africa. Madagascar was left behind. By the compression of the Indian part against the Angara shield (i.e., Asian maia continent) the mountain chains of the Himalayas were created.
Wegener and his followers, in favour of this theory, have put forwarded the following evidences :
(a) Similarities of the coast-lines once thought to be adjacent, especially South America and Africa.
(b) The similarity of pre-Cambrian (before 500 million years ago) rocks of Central Africa, Madagascar, Southern India, Brazil and Australia.
(c) The continuity of tectonic trends of the blocks of these countries across their present boundaries.
(d) There are wide-spread occurrences of carboniferous coal deposits implying a tropical humid-climate. The coal bearing formations are now found in South America, South Africa, Madagascar, India, Australia and Antarctica.
(e) Unmistakable evidence of wide-spread glaciation towards. the end of the Palaeozoic era is found at the southern extremity of South America, the southern half of Africa, in the peninsular India, extending to the Himalayan regions and in Australia.
Although the theory of continental drift has been widely discussed and accepted, serious doubts have been raised about the period during which the force causing the drift had operated and also about the direction and the amount of force. However the recent theories of ‘Plate-tectonics’ and information obtained from palaco-magnetic studies, lends support to the theory of Continental drift.
Palaeo magnetism
According to the studies on palaeo-magnetism, it is possible to find out the direction and dip of the earth’s Magnetic field during different geologic periods. It has been Observed that the pole-positions of the present globe are different during the geologic past and by joining these poles, a curve is Obtained, which is known as ‘polar-wandering curve’, It is seen that the polar wandering curves drawn for different continents are not parallel or sympathetic, which confirms ‘Continental-drift’.
Palacomagnetic works on the Deccan plateau-basalt shows that the average rate of movement with refrence to India is about 7cm/year.
Plate tectonics, The concept of the movement of lithospheric plates and the sea-floor spreading, along with their supporting evidenes prove that there was drifting of continents.
A general concept of continental drift may be obtained from the following stages ;
