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Evaluate the evidence that lead to the formation of the concept of which developed the theory of plate tectonics.

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Introduction

Evaluate the evidence that lead to the formation of the concept of which developed the theory of plate tectonics. The earth is made up of 4 parts: Crust, Mantle, Outer core and the Inner core. The crust is compared to the earth a thin 'skin' it floats upon the mantle and is split into 2 different parts. The oceanic crust (sima) is a layer consisting mainly of basalt, averaging 6-10km in thickness, the continental crust (sial) can be 70km thick, and the crust is separated from the mantle by the Moho discontinuity. The crust and the rigid top layer of the mantle are collectively known as the lithosphere. The mantle is mainly composed of silicate rocks, rich in iron and magnesium. Apart from the top layer the rest of the rocks, the aesthenosphere, are kept in a semi-molten state. The mantle extends to a depth of 2900km where temperatures may reach 5000oC. These high temperatures form convection currents. The core consists of iron and nickel. The outer core is semi-molten while the inner core is solid; the temperature of the core, which is 6371km deep, is 5500oC. ...read more.

Middle

All these pieces of evidence have pointed towards the continents once being joined or by having some sort of bridge between them. Wegener's theory of continental drift was dismissed by several scientists as he was not considered a specialist in any of the sciences he had used to develop his theory and he could not explain how solid land masses had managed to change their positions, he was unable to suggest the mechanism for the drift. Wegener's theory of Continental drift. Since Wegener put forward his theory of continental drift there have been several pieces of new evidence that support his unproven theory. The discovery of the Mid-Atlantic ridge, which is a range of mountains that run the length of the Atlantic Ocean, found by Ewing in 1948 during an investigation of islands in the Atlantic has shown that the mountain range consists of fairly new volcanic rock - not ancient rock as previously presumed. This evidence proves that there is still physical activity of the earth's crust still occurring today. Studies of palaeomagnetism in the 1950's showed that the rocks that were either side of the Mid-Atlantic ridge had small deposits of iron that were aligned along the earth's magnetic field. ...read more.

Conclusion

The continental plates are older, less dense and consist of silica and aluminium. As a result of the convection currents these plates are moved about either towards, away or sideways to other plates resulting in Fold Mountains, volcanoes, earthquakes and other major landforms. Due to the lower density of the continental crust when the continental and oceanic crusts collide the oceanic crust is forced downwards into a subduction zone where the rock is heated and turned back into a semi-molten state, which rejoins the mantle or increases the pressure in the mantle and causes eruptions. This has occurred in the Pacific Ocean around Asia. When plates move apart to form constructive plate boundaries magma from the mantle surfaces and cools to form new rock between the plates forming mid-ocean ridges and volcanoes. Plate tectonics Using all the evidence that has been collated over the years has let us contrive a reliable theory which looks at all possible ideas that may have been overlooked it has lead us to complete a detailed model of the earth's surface and the explanations for landforms and major natural hazards i.e. Earthquakes and volcanoes. Gary Woodsford Geography 02/05/2007 1 ...read more.

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