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Chapter 5

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In this topic I have learned that continents are not static but they move. When continents come closer to each other than mountains are formed and when they move away then new basins are formed. A theory called continental drift theory explain the concept of movement of continents. Early geologists do not believe this concept. It was difficult for the North America geologists to believe this concept because all the evidences were from other parts of world and North America geologists were unfamiliar with them. In 1968 plate tectonics theory was proposed. Alfred Wegener proposed this theory and said that once all the continents were joined together called Pangaea. But they were separated in a period called Mesozoic era. But other geologists said that boundaries of all continents were not same. But after some time they found that outer boundariesare submerged many meter below the surface. Paleontologists also believed that fossils on different continents have similarities. Wegener said that South America and Africa were once joined together as he gave the example of reptile Mesosaurus whose fossils are present in both countries. He also gave the example of Glossopteris as evidence of Pangaea. He also gave the example of rocks that similar rocks that are present in Brazil and in Africa. He also said that climate of continents was same. Wegner said that gravitation force of sun and moon causes continents to move which is not true because it that forces are strong too much then they could halt planet rotation which has not happen. The main idea of Wegner was true but he could not provide correct evidences.After World War II, it was found that large earthquakes were occurred at great depths. According to plate tectonics theory the lithosphere varies in density. It consists of oceanic crust and continental crust and the overall density of oceanic crust is more than continental crust. Below lithosphere there occurs asthenosphere. Asthenosphere is relatively weak. Lithosphere has divided into several plates like North American, South American, Pacific, African, Eurasian, Australian-Indian, and Antarctic plates. When plate moves the major interactions occurs at their boundaries. The boundaries are of three types. The boundaries are convergent, divergent and transform plate boundaries. When divergent boundaries moves apart then magma comes from subsurface and form new ocean floor. Majority of divergent plate boundaries are associated with oceanic ridges. Along some ridges segments there is canyon like structure called valley rift. Seafloor spreading is a mechanism of creating new seafloor. Along convergent plate boundaries, subduction occurs. When their occur interaction between oceanic-continental then ocean crust sinks that’s why continental crust are older than oceanic crust. When there occurs oceanic-oceanic then one plate comes over another and forms subduction zone. The subducting plate forms a large depression in ocean floor called trench and when there occurs continental-continental interaction then the both plate uplift and form mountain. When the two plates slides past each other than there occur transform plate boundary.I have also learn that total earth surface do not change but plates changes regularly. Due to change in forces in lithosphere, plates creates and destroys. Modern geologists by using new tools have proved Wagner’s theory and establish their relative motions. For the support of seafloor spreading, the Deep Sea Drilling Project have been started. It is believed that cylindrically shaped upwelling which is called mantle plume is at island of Hawaii. Hotspots are the areas with temperature more than other places. Simple bar magnet produces same magnetic field as earth produces. Rocks have record of time when formedare called fossil magnetism. It is clear that major driving force of plate motion is due to the subduction of cold, dense slabs of oceanic lithosphere. Convection is the process in which warm rocks rise while cool, dense lithospheric plates sink. I have also studied about Whole-Mantle Convection, Layer Cake Model.I have learnt that the plate tectonics theory is very important because in thisprocess, new rocks are formed by magma or lava. We know that mostly lava causes problems in different countries. Although lava have benefits also. But if we study plate tectonics of a certain area then we can know about coming of lava when plates moves apart each other.I have a question for you that what will happen if all the continents merge

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