Continental drift GEO-09

Transcription

1 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 (CW) Content Reviewer (CR) Dr. Syed Zaheen Alam Dayal Singh College, Delhi University Language Editor (LE) Component-I (B) - Description of Module Items Description of Module Subject Name Geography Paper Name Module Name/Title Module Id Geomorphology Continental drift GEO-09 Pre-requisites Objectives Keywords

2 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 ( ) 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)

3 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 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 ( ), 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,

4 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).

5 Figure 4: Gondwana Land during Paleozoic times. Figure 5: Map representing the present position of drifted continents.

6 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

7 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 polefleeing or centrifugal force and tidal force are responsible for the journey of continents. He left many questions unanswered in context to mechanism of drifting.

8 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 greatgranduncle s friend Alexander von Humboldt was already aware about this jigsaw puzzle (fig. 6).

9 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 ( 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.

10 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.

11 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. Similarly remains Lystrosaurus (A Triassic land reptile) are excavated in Africa, India and Antarctica. The Glossopteris (Gondwana plant fossil) can be found in widely separated present day continents. Therefore more or less the identical fossils of plants and animals found in different part of Gondwanaland validates his argument for all land i.e. PANGAEA.

12 continents. Figure 14: The Glossopteris (Gondwana plant fossil) can be found in widely separated present day The identical fossils of Plants and Animals in Europe and northeastern North America were not very apparent, but Wegener noticed that earthworms are found in Europe and the northeastern areas of North America. The Earthworms cannot swim across Atlantic Oceans. It suggests that Atlantic Ocean did not exist in the past. 4.3 Comparative Stratigraphy: A Similar Sequence of Rocks at Numerous Locations The comparative stratigraphy is the study of similar sequence of rocks at numerous Locations (Fig. 15). Layers of same rocks can be recorded many kilometers apart. By sequencing and mapping them geologists can figure out how they extend underground or split apart because of continental drift. Figure 15: Comparative stratigraphy. In this regard Wegener s most compelling support came from the eminent South African field geologist Alexander Du Toit. After spending five months in Brazil, Uruguay and Argentina he wrote in his book A Geological Comparison of South America with South Africa (1927) that The Brazilian strata [layers of sedimentary rock] correspond perfectly in every detail to the strata series of the southern African shield. Du

13 Toit further wrote that he had great difficulty in realizing that this was another continent i.e. South America. The mixed sequence of glacial and marine deposits in both South Africa and South America also confirmed two landmasses had once been joined. Alexander Du Toit produced a considerably more detailed body of evidence than Wegener himself had. Figure 16: Similar mountain Chain across Atlantic Ocean. The Caledonian mountains of North America reappear in Scotland, Northern Ireland and matches in terms of age and structure with the Appalachian Mountains of North America (Fig. 16). Similarly the radiometric dating method together with stratigraphic principles also reveals that broad belts of rocks in Africa and South America are the same type. 4.4 The Ice Matches: Glaciers and Tillite Wegener was also expert in the field of paleoclimatology. He had written a book on paleoclimatology under the guidance of his father-in-law, Wladimir Köppen. Wegener was aware that a continental ice sheet covered parts of South America, southern Africa, India, and southern Australia about 270 million years ago during late Paleozoic times (fig. 17). Glacial striations on rocks show that glaciers radiated outward from glacial centre over the South Pole in Africa toward the Atlantic Ocean and from the Atlantic Ocean onto South America. Therefore, the continents must have been joined so that the ice moved across a single landmass Pangaea. The presence of glacier evidence in turn supported the idea of polar wandering.

14 Figure 17: The region of Paleozoic glaciations in Gondwanaland indicates South Pole was located where South America and Antarctica reassemble. The presence of tillite that is, sedimentary rock form out of deposits of glaciers in India, Madagascar, Africa, Antarctica and Australia further revealed that these landmasses had amazingly parallel pasts. Wegener also raised an interesting question, Why there is a huge stock of coal across present day cool middle latitude parts of earth? The Coal can be found in North America, Europe, Asia Minor, China and even under the Antarctic ice cap. This huge stock of coal can only form in equatorial climatic conditions. It would have been if the poles were in the location Wegener described. Similarly, the remains of Coral reef which requires warm water to survive can be found in the cold North Polar Region. Therefore It also supported drifting hypothesis. (Fig. 18)

15 Figure 18: Other evidence Why his theory was considered as an impossible hypothesis? Wegener's Continental Drift theory was not readily accepted by the scientific community of his day. Generally scientific hypotheses which contradict dominant theory, principle or prevailing contemporary view invites host of criticism. Wegener s theory was no exception to it. Wegener failed to devise a sound mechanism for the movement of the continents. For Wegener the drifting mechanism was the most difficult question to solve. He tried to attribute the westward drift of the Americas to feeble lunar-solar drag. J.A. Steers (1992) said that such a force is extraordinarily small. Even if we accept that tidal force was powerful enough to drag Americas then it would have stopped the rotation of earth within a year. It was also difficult to imagine that how compression could be possible to raise Andes and Rockies (mountain chain) due to lunar solar drag of sialic landmass over the rigid sima-floored ocean (Panthalassa). Geologists also raised question about the exact fitting of the opposing coasts of Atlantic. It was also assumed that approximate fit of the continents may be a coincidence created by erosion process. His theory was vehemently rejected by American geologists. For example, Harold Jeffrey ( ), an eminent geophysicist at Britain s Cambridge University stated that Pohlflucht and tidal forces were a million times too weak to move the continents. Chester Longwell of Yale University also doubted much of Wegener s evidence. (Yount,2009) Modern developments and Continental drift theory

16 Within 40 years of the denial of Continental drift theory by contemporary much of the geological Community the main idea of horizontal movement of continents has become part of plate tectonics theory. The technological developments leading to the opening up of ocean basin geology has uncovered Mid Oceanic Ridge system in the middle part of Atlantic Ocean. The discovery of Mid Oceanic Ridges added new dimension in drifting mechanism. Figure 19: Convection Current Mechanism in Boiling Water In the shadow of continental drift British geologist Arthur Holmes ( ) proposed the concept of convection currents mechanism to explain drifting of the continents. To understand the convection current mechanism, take a pan of boiling water and put a piece of cork in it. You will observe that cork on the surface of the boiling water will be pushed sideways (figure 19).

17 Figure: 20: Convection Currents and sea Floor Spreading In his well-known textbook The Principles of Physical Geology, Holmes suggested that in the similar manner, the convection currents in the upper part of the mantle may perhaps cause movement of the continents. Later in 1962 professor Harry H. Hess in his seminal paper on the concept of seafloor spreading agreed with Arthur Holmes idea of convection current mechanism (Fig. 20). Recently the Palaeomagnetic (fossil magnetism) evidence from the rocks found along the Mid Oceanic Ridge system revived interest in original single sialic land-mass i.e., PANGAEA. Do you know? 1. Additional evidence of seafloor spreading came from an unexpected source that is petroleum exploration. 2. The drifting of the present day continents can be recorded directly through satellite linked Global Positioning System. The Journey of India

18 In 1950s the sensitive magnetic inclination samples collected from the instrument called magnetometer in Deccan Plateau region of peninsular India showed amazing findings. The magnetic inclination samples confirmed that the region had previously been in the Southern hemisphere as proposed by Wegener. For example, the 150 million years old rock manifested a magnetic inclination of 64 degrees south. The geologists were surprised to note that rocks dating 25 million years old in Deccan Plateau region entirely inverted their magnetic inclination. This sensitive magnetic inclination record confirmed that India used to be part of Wegener s Gondwanaland. Figure 20: The anticlockwise journey of India from Southern Hemisphere. The anticlockwise journey of India from Southern Hemisphere reveals that it rammed into Asia about 40 to 50 million years ago. The northward advance of India caused rise of the earth s tallest mountain i.e. The Himalayas. In just 50 million years of Earth s reconstructed history, peaks such as Mt. Everest have risen to heights of more than 8 kilometers. (Fig. 20)

19 What can a young geographer learn from Alfred Wegener? The perseverance, patience with empirical observations was the most important attribute of his personality despite initial mockery of his theory. It took nearly 60 years for the idea of continental drift to be scientifically confirmed in the form of plate tectonic theory. Ultimately it added new dimension in the understanding of the global distribution of earthquakes, volcanoes and identification of disaster prone areas.