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DIRECTION Institute for IAS 1 Sample Question Answer (300 Words) Que. ‘‘Plate tectonic has evolved as a grand framework for understanding endogenic forces in long sequence of time.’ Elaborate’’ Ans. Plate tectonics provides us with a grand framework for understanding the extensive crustal rearrangement that apparently has taken place during the relatively recent history of Earth. Wegener’s Pangaea is now generally accepted as having existed. There is substantial evidence to indicate that, 450 million years ago, five continents existed; these continents sutured to form Pangaea. For the next 200 million years or so, Earth had but a single major continent and single world ocean. About 250 million years, Pangaea began to break up, first into two massive pieces – Laurasia in the Northern Hemisphere and Gondwanaland in the Southern Hemisphere – and then into a number of smaller bits. One of the great triumphs of the theory of plate tectonics is that it explains topographic patterns. It can account for the formation of many mountains, midocean ridges, oceanic trenches, island area, and the associated earthquake and volcanic zones. Where these features appear, there are usually plates either colliding or separating. Despite the correlation of mountain building with plate convergence, the contemporary pattern of plates does not explain all mountain belts. Many of the major mountain ranges of North America and Europe are in the middle of plates rather than in boundary zones. The genesis of such midplate ranges is not fully understood but presumably is related to changing tectonic conditions in the past. There is convincing evidence that during some past eras there were fewer plates than there are today and during other eras there were more plates than there are today. In addition, the sizes and shapes of past plates different from the current sizes and shapes. For example, there was no Atlantic Ocean 250 million years ago, and yet today it is a major feature of the planet, DIRECTION Institute for IAS 2 widening at a long-term average rate of about 2 inches (5 centimeters) per year. Seafloor spreading is proceeding even faster in parts of the Pacific. Indeed, some geophysicists have postulated that oceans are being created and removed by crustal rearrangement on about a 100-million-year cycle. Modifications to the Original Theory: With each passing year, we learn more about plate tectonics. Two examples of recently acquired knowledge are accreted terranes and mantle plumes. Accreted Terranes: A terrane is a small-to-medium mass of lithosphere carried a long distance by a drifting plate that eventually converges with another plate. The terrace is too buoyant to be subducted in the collision and instead is fused (“accreted”) to the other plate, often being fragmented in the process. Terranes are distinctive geologically because their lithologic complement (types of rock) is generally quite different from that of the plate to which they are accreted. It is generally believed that every continent has grown outward by the accumulation of accreted terranes on one or more of its margins. North America is a prominent example. Most of Alaska and much of western Canada and the western United States consist of a mosaic of several dozen accreted terranes, some of which have been traced to origins south of the equator. Mantle Plumes: There are many places on Earth where magma from deep in the mantle comes either to or almost to the surface at locations that are not anywhere near a plate boundary; these leaky spots in the interior of a plate are referred to as mantle plumes or hot spots. They are believed to be relatively stationary over long periods of time (tens to hundred of millions of years). We do not yet know the cause of these plumes, but more than 100 of them have thus far been identified. As the magma rises through the crust, it creates volcanoes and/or hydrothermal (hot water) features on the crust surface. The plate containing the hot spot is on the move, however, and so the volcanoes or other DIRECTION Institute for IAS 3 plume features appear to drift away from the plume generating a “hot spot train.” From a surface perspective, the hot spot appears to migrate in the direction opposite the direction of plate movement, the volcanoes produced by hot spots because younger in the opposite direction from that which the plate is moving. Thus these traits can be used to determine absolute plate motions. One striking result of mantle plume is the Hawaiian Islands, where the ancient volcanic remnants of Midway Island are now 1600 miles (2500 kilometers) northwest of the presently active volcanoes on the big island of Hawaii, although both developed over the same mantle plume, separated in time by several million years. The volcanoes of the Hawaiian chain are progressively younger from west to east, as the Pacific plate drifts northwestward and while new volcanoes are produced on an “assembly line” moving over one persistent hot spot. Mantle plumes help explain both topographic development and the rate and direction of plate movement. Because the plumes are effectively fixed in position, the “traits” they produce indicate absolute plate motions. Despite the intellectual and scientific enrichment we have received from plate tectonics theory, we still have many unanswered questions.