In the previous chapter, you have studied the interior of the earth. You are already familiar with the world map. You know that continents cover 29 per cent of the surface of the earth and the remainder is under oceanic waters. The positions of the continents and the ocean bodies, as we see them in the map, have not been the same in the past. Moreover, it is now a well-accepted fact that oceans and continents will not continue to enjoy their present positions in times to come. If this is so, the question arises what were their positions in the past? Why and how do they change their positions? Even if it is true that the continents and oceans have changed and are changing their positions, you may wonder as to how scientists know this. How have they determined their earlier positions? You will find the answers to some of these and related questions in this chapter.
CONTINENTAL DRIFT
Observe the shape of the coastline of the Atlantic Ocean. You will be surprised by the symmetry of the coastlines on either side of the ocean. No wonder, many scientists thought of this similarity and considered the possibility of the two Americas, Europe and Africa, to be once joined together. From the known records of the history of science, it was Abraham Ortelius, a Dutch map maker, who first proposed such a possibility as early as 1596. Antonio Pellegrini drew a map showing the three continents together. However, it was Alfred Wegener—a German meteorologist who put forth a comprehensive argument in the form of “the continental drift theory” in 1912. This was regarding the distribution of the oceans and the continents. According to Wegener, all the continents formed a single continental mass, a mega ocean surrounded by the same. The super continent was named PANGAEA, which meant all earth. The mega-ocean was called PANTHALASSA, meaning all water. He argued that, around 200 million years ago, the super continent, Pangaea, began to split. 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 exist today. A variety of evidence was offered in support of the continental drift. Some of these are given below. Evidence in Support of the Continental DriftThe Matching of Continents (Jig-Saw-Fit) The shorelines of Africa and South America facing each other have a remarkable and unmistakable match. It may be noted that a map produced using a computer programme to find the best fit of the Atlantic margin was presented by Bullard in 1964. It proved to be quite perfect. The match was tried at 1,000- fathom line instead of the present shoreline. Rocks of Same Age Across the Oceans The radiometric dating methods developed in the recent period have facilitated correlating the rock formation from different continents across the vast ocean. The belt of ancient rocks of 2,000 million years from Brazil 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
It is the sedimentary rock formed out of deposits of glaciers. The Gondawana system of sediments from India is known to have its counter parts in six different landmasses of the Southern Hemisphere. At the base the system has thick tillite indicating extensive and prolonged glaciation. Counter parts of this succession are found in Africa, Falkland Island, Madagascar, Antarctica and Australia besides India. Overall resemblance of the Gondawana ype sediments clearly demonstrates that these landmasses had remarkably similar histories. he glacial tillite provides unambiguous evidence of palaeoclimates and also of drifting of continents.
Placer Deposits
The occurrence of rich placer deposits of gold in the Ghana coast 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 the Ghana are derived from the Brazil plateau when the two continents lay side by side.
Distribution of Fossils
When identical species of plants and animals adapted to living on land or in fresh water are found on either side of the marine barriers, a problem arises regarding accounting for such distribution. The observations that Lemurs occur in India, Madagascar and Africa led some to consider a contiguous landmass “Lemuria” linking these three landmasses. 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 formations of Brazil. The two localities presently are 4,800 km apartwith an ocean in between them.
1. Multiple choice questions.
(i) Who amongst the following was the first to consider the possibility of Europe, Africa and America having been located side by side.
(a) Alfred Wegener (c) Abraham Ortelius
(b) Antonio Pellegrini (d) Edmond Hess
(ii) Polar fleeing force relates to:
(a) Revolution of the Earth (c) Rotation of the earth
(b) Gravitation (d) Tides
(iii) Which one of the following is not a minor plate?
(a) Nazca (c) Philippines
(b) Arabia (d) Antarctica
(iv) Which one of the following facts was not considered by those while discussing the concept of sea floor spreading?
(a) Volcanic activity along the mid-oceanic ridges.
(b) Stripes of normal and reverse magnetic field observed in rocks of ocean floor.
(c) Distribution of fossils in different continents.
(d) Age of rocks from the ocean floor.
(v) Which one of the following is the type of plate boundary of the Indian plate along the Himalayan mountains?
(a) Ocean-continent convergence
(b) Divergent boundary
(c) Transform boundary
(d) Continent-continent convergence
2. Answer the following questions in about 30 words.
(i) What were the forces suggested by Wegener for the movement of the continents?
(ii) How are the convectional currents in the mantle initiated and maintained?
(iii) What is the major difference between the transform boundary and the convergent or divergent boundaries of plates?
(iv) What was the location of the Indian landmass during the formation of the Deccan Traps?
3. Answer the following questions in about 150 words.
(i) What are the evidences in support of the continental drift theory?
(ii) Bring about the basic difference between the drift theory and Plate tectonics.
(iii) What were the major post-drift discoveries that rejuvenated the interest of scientists in the study of distribution of oceans and continents?
Please refer to attached file for NCERT Class 11 Geography Distribution of Oceans and Continents
