Geography - Geomorphology - Continental Drift Theory – Tectonics October 29, 2017 Continental Drift Theory – Tectonics Tectonics == Large scale movement of lithospheric plates. Introduction During WW II, scientists discovered that the ocean floor was not a flat surface but had some unique relief features like ridges, trenches, seamounts, shoals etc. The most important discoveries were ridges and trenches which gave insights into natural boundaries between various lithospheric plates (sometime called as crustal plates or tectonic plates) These important discoveries led to the theory of Plate Tectonics. Plate Tectonics Plate tectonics is the large scale movement of lithospheric plates due to forces emanating from earth’s interior. Prior to the theory of ‘Plate Tectonics’, there were other theories like ‘Continental Drift Theory’ and ‘See Floor Spreading Theory’ which tried to explain the large scale movements on earth’s surface. In this post, we will study about ‘Continental Drift Theory’. Important theories Polar wandering (Similar to Continental Drift Theory) Continental DriftTheory (CDT) Convectional Current Theory (CCT) Sea Floor Spreading Theory (SFST) Plate Tectonics (PT) Polar wandering is the relative movement of the earth’s crust and upper mantle with respect to the rotational poles of the earth. Continental drift refers to the movement of the continents relative to each other. Convectional current theory forms the basis of SFST and PT. Sea floor spreading describes the movement of oceanic plates relative to one another. Plate tectonics is simply the movement of crustal plates relative to each other. Continental Drift Theory (Alfred Wegener, 1922) This theory was suggested by Alfred Wegener in 1920’s. According to Wegener’s Continental Drift Theory, there existed one big landmass which he called Pangaea which was covered by one big ocean called Panthalassa. A sea called Tethys divided the Pangaea into two huge landmasses: Laurentia (Laurasia) to the north and Gondwanaland to the south of Tethys. Drift started around 200 million years ago (Mesozoic Era), and the continents began to break up and drift away from one another. To get a rough idea about Geologic timescale, refer the image below. Force for Continental Drift The drift was in two directions- equator wards due to the interaction of forces of gravity, pole-fleeing force and buoyancy (ship floats in water due to buoyant force offered by water), and westwards due to tidal currents because of the earth’s motion (earth rotates form west to east, so tidal currents act from east to west. Watch video for better understanding). Wegener suggested that tidal force also played a major role. The polar-fleeing force relates to the rotation of the earth. You are aware of the fact that the earth is not a perfect sphere; it has a bulge at the equator. This bulge is due to the rotation of the earth. [Greater Centrifugal force at the equator. Centrifugal force increases as we move from poles towards equator. This increase in centrifugal force has led to pole fleeing]. Tidal force is due to the attraction of the moon and the sun that develops tides in oceanic waters. Wegener believed that these forces would become effective when applied over many million years. According to Wegener, the drift is still continuing. Evidence in support of Continental Drift Apparent Affinity of Physical Features South America and Africa seem to fit in with each other, especially, the bulge of Brazil fits into the Gulf of Guinea. Greenland seems to fit in well with Ellesmere and Baffin islands. The east coast of India, Madagascar and Africa seem to have been joined. North and South America on one side and Africa and Europe on the other fit along the mid-Atlantic ridge. The Caledonian and Hercynian mountains of Europe and the Appalachians of USA seem to be one continuous series. Criticism Coastlines are a temporary feature and are liable to change. Several other combinations of fitting in of landforms could be attempted. Continental Drift Theory shifts India’s position too much to the south, distorting its relation with the Mediterranean Sea and the Alps. The mountains do not always exhibit geological affinity. Causes of Drift Gravity of the earth, buoyancy of the seas and the tidal currents were given as the main factors causing the drift, by Wegener. Criticism This is illogical because for these factors to be able to cause a drift of such a magnitude, they will have to be millions of times stronger. Polar wandering (Shifting of Poles) The poles drifted constantly. Criticism Poles may have shifted, not necessarily the continents (don’t think deep). Distribution of Fossils 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 South Africa and Iraver formations of Brazil. The two localities presently are 4,800 km apart with an ocean in between them. Botanical Evidence Presence of glossopteris vegetation in carboniferous rocks of India, Australia, South Africa, Falkland Islands (Overseas territory of UK), Antarctica, etc. can be explained on the basis of the fact that parts were linked in the past. Criticism Such vegetation is also found in the northern parts like Afghanistan, Iran and Siberia. Similar vegetation found in unrelated parts of the world. Rocks of Same Age Across the Oceans The belt of ancient rocks of 2,000 million years from Brazil coast matches with those from western Africa. Criticism Rocks of same age and similar characteristics are found in other parts of the world too. Tillite deposits It is the sedimentary rock formed out of deposits of glaciers. The Gondwana 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 Gondwana type sediments clearly demonstrates that these landmasses had remarkably similar histories. The glacial Tillite provides unambiguous evidence of palaeoclimates and also of drifting of continents. Placer Deposits Rich placer deposits of gold are found on the Ghana coast (West Africa) but the source (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. Drawbacks of Continental Drift Theory Wegener failed to explain why the drift began only in Mesozoic era and not before. The theory doesn’t take oceans into consideration. Proofs heavily depend on assumptions and are very general in nature. Forces like buoyancy, tidal currents and gravity are too weak to be able to move continents. Modern theories (PT) accept the existence of Pangaea and related landmasses but give a very different explanation to the causes of drift.