Component-I(A) - Personal Details Role Name Affiliation Principal Investigator Prof. Masood Ahsan Siddiqui Department of Geography, Jam Jamia Millia Islamia, New Delhi Paper Coordinator, if any Dr. Syed Zaheen Alam Dayal Singh College, Delhi University Content Writer/Author Dr. Syed Zaheen Alam Dayal Singh College, (CW) Delhi University Content Reviewer (CR) Language Editor (LE) Component-I (B) - Description of Module Items Description of Module Subject Name Geography Paper Name Geomorphology Module Name/Title Continental drift Module Id GEO-09 Pre-requisites Objectives Keywords 1 Introduction Less than 420 years ago, many scientists thought the continents always had been in the same place. It was difficult to accept that the continents and oceans are moving across the surface of the Earth although the idea about united continent was not new. A few scholars used to wonder that South America and Africa appear to fit together. For example Abraham Ortelius in 1596, Sir Francis Bacon in 1620 and Antonio Snider Pellegrini in 1858 in his book ‘Creation and Its Mysteries Unveiled’ (La créationet ses mystéres dévoilés), noted that the eastern coastline of South America and the western coastline of Africa looked as if they could fit together (fig. 1). Alexander von Humboldt one of the founder of modern Geography during his scientific expedition (1799-1804) across the Atlantic further grasped that this similarity is more than the apparent “fit”. The mountains of Brazil in South America were the same as those of the Congo in Africa. Therefore similarity between eastern coastline of South America and the western coastline of Africa is not accidental. Figure 1: In 1858 Antonio Snider-Pellegrini made this map showing how the American and African continents may once have fit together during Carboniferous time. (Based on Antonio Snider-Pellegrini’ s map) However it was Alfred Lothar Wegener 1912 – a 32 year old lecturer in meteorology and astronomy in Germany who propounded the Continental Drift theory with sound evidence (fig. 2). He delivered a lecture on “The Formation of the major Features of the Earth’s Crust (Continents and Oceans)” at Frankfurt in an eminent Geological Association. In his lecture he suggested that continents had once been combined in the form of original single sialic land-mass and had afterward broken apart and drifted to their present positions. Later in 1915 he circulated exposition of his theory of continental drift titling The Origin of Continents and Oceans (Die Entstehung der Kontinente und Ozeane). He revised the book extensively three times. The revised editions with further elaborations appeared in 1920, 1922, and 1929. His theory attracted little notice by the scientific community of his day and publication had a small circulation. Later plate tectonic theory revived and revised interest in his theory. Figure 2: Alfred Lothar Wegener (1880-1930), the Propounder of the theory of continental drift. 2. Continental Drift theory Wegener said that all the continents could be joined together like giant puzzle pieces to create one primordial super continent i.e. PANGAEA means all land (fig. 3). It occupied about half of the planet’s surface area and existed 300 million years ago. This gigantic continent was surrounded by shallow single vast ocean called PANTHALASSA meaning “all sea”. In the carboniferous age the South Pole was near the South African coast and the North Pole was situated in Pacific Ocean. He said 200 million years ago during the Mesozoic era PANGAEA began to separate into two proto-continents viz. northern continent (Laurasia = North America, Europe, Greenland and Asia excluding India), and a southern (Gondwanaland = South America, Africa, India, Australia, New Zealand and Antarctica) (fig. 4). The two proto-continents were separated by Tethys sea. The exact boundaries of these two domains are still in question. Opinions are also divided on the date of the splitting-up of the PANGAEA. Figure 3: PANGAEA surrounded by PANTHALASSA during Upper Carboniferous period. The continents drifted away from the contemporary poles. Wegener described this phenomenon as Polflucht or flight from the poles. By the end of the Cretaceous period, the continents were separating into land masses that resembled the shape of our present-day continents. Wegener assumed that land masses horizontally moved on viscous sea floor. The modern-day position of continents is the outcome of this horizontal drifting. Wherever the drifting continent faced resistance in the way of movement it resulted in huge fold mountains. Therefore, his book provided a new reasoning for the construction of mountain chains. It is interesting to note that “Wegener’s term for this motion, the German word verschiebung, would be most accurately translated as displacement. The theory nevertheless became known in English as continental drift” (Yount, 2009). Figure 4: Gondwana Land during Paleozoic times. Figure 5: Map representing the present position of drifted continents. Figure 6: Future Continents some 200 million years from now (After Meadows, 2007). The continents in the present time are attaining their maximum distances. Their continued motion in the future must bring them closer together again. In future 200 million years from now the South America, Antarctica, and Australia likely to join together in the southern hemisphere (Fig. 6). Similarly, Africa, Europe, and Asia are likely to unite in future (Meadows, 2007). 3. Mechanism of Drifting Wegener knew that his drifting hypothesis must also explain mechanism of Drifting or what moved them. Wegener thought that the oceans are composed of a heavier material than the continents. He thus deduced that the less dense continents are floating on the heavier subsurface. It doesn’t mean that he was advocating for isostatic balance between the different crustal segments of the upper portion of earth, rather he was interested in the logic of horizontal drifting. It was clear to him that drifting would require enormous amount of energy. Therefore he attempted to attribute the drifting process to “Eotvos” or pole-fleeing and external tidal force. Pole-fleeing or centrifugal force: The spinning of Earth on its own axis creates a centrifugal force i.e. force oriented away from the axis of rotation towards the equator. Wegener believed the centrifugal force of the planet caused the super continent to break apart and pushed continents away from the Poles toward the equator. Therefore, He called this drifting mechanism as the "pole-fleeing or centrifugal force". Figure 7: the pole-fleeing or centrifugal force. Tidal force. Wegener tried to attribute the westward drift of the Americas to lunar-solar drag i.e. by invoking tidal force that is the gravitational forces of the sun and the moon (Fig. 8). He also admitted that “it is probable” that pole- fleeing or centrifugal force and tidal force are responsible for the journey of continents. He left many questions unanswered in context to mechanism of drifting. Figure 8: The tidal force responsible for westward journey of continents. Do you know? Wegener’s suggestion to relate the drifting process to centrifugal force and external tidal force was the weakest point of his theory. It was also an idea of “impossible hypotheses for geophysicist Harold Jeffery. In his book The Earth, Its Origin, History and Physical Constitution Jeffery asserted that crust of the earth is too strong to be affected by these forces. Figure 9: Weakest point of his theory i.e. drifting mechanism 4. Evidence in Support of the Continental Drift Science requires the use of methods that are systematic, logical, and empirical. To support his theory Wegener gathered empirical evidences from variety of scientific methods. He knew that truth of drift can only be achieved by combining all possible evidences. These scientific evidences are as follows: 4.1 The Shapes Match: Jig-Saw-Fit In a letter to Else the daughter of climatology expert Wladimir Köppen Wegener raised a question i.e. “Doesn’t the east coast of South America fit exactly against the west coast of Africa, as if they had once been joined?” (Yount, 2009). Later Wegener also married to Else. Wegener’s idea was not new since his great- granduncle’s friend Alexander von Humboldt was already aware about this jigsaw puzzle (fig. 6). Figure 10: Jigsaw fit of East coast of South America and West coast of Africa. 4.2 The identical fossils of Plants and Animals Snider-Pellegrini recorded that fossil plants from the Carboniferous period (354–290 million years ago) in Africa and the Americas are largely identical. Later Paleontologists were satisfied to accept that present-day continents were connected by land bridges and later theses land bridges sank beneath the sea. The land bridges in the past would have been helpful for the animals to cross since it was not possible to cover the distance via Atlantic Ocean. Another possibility for migration of specifies across the Atlantic was ruled out by the balancing act of isostasy. Wegener’s theory became controversial because it presented alternate reason for fossils similarity. He said continents were joined together in the form of single landmass. Therefore there was no barrier in the form of present day Atlantic Ocean. This means that they evolved together on a single large landmass Gondwanaland. He reasoned that it was physically impossible for most of these organisms to have traveled or have been transported across the vast oceanic barrier. Figure 11: More or less identical fossils of certain plants and animals. For instance, Lemurs found in India, Madagascar and Africa validates that India was connected with Africa via Madagascar in the past (Fig.12). Figure 12: Lemurs found in India, Madagascar and Africa. The remains of Cynognathus a Triassic land reptile and Mesosaurus a type of Freshwater reptile are found in South America and Africa. Figure 13: The remains of Cynognathus a Triassic land reptile and Mesosaurus a type of Freshwater reptile.