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3.1.5.2 Plate tectonics
Plate Tectonic Theory
Our planet is made up of three main layers: crust, mantle, and core. This layered structure can be compared to that of a boiled egg. The crust, the outermost layer, is rigid and very thin compared with the other two. Beneath the oceans, the crust varies little in thickness, generally extending only to about 5 km. The thickness of the crust beneath continents is much more variable but averages about 30 km; under large mountain ranges, such as the Alps or the Sierra Nevada, however, the base of the crust can be as deep as 100 km. Like the shell of an egg, the Earth's crust is brittle and can break. (USGS). The mantle is the widest part of the the Earth (2900km), due to the thickness and the high temperatures of this layer, the mainly silicate rocks are in a thick, liquid state which become denser with depth. The core is the centre of the Earth, this is the hottest part. The core is divided up into the inner and outer core. Temperatures can reach 5000 C and made of iron and nickel. The outer core is semi-liquid and mainly iron, the inner core is solid and made of an iron-nickel alloy. ![]() |
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A rift valley is a lowland area that forms between two highland areas, where the earth's tectonic plates are shifting apart. They are found on land (East African rift valley) or on the ocean floor, due to sea floor spreading (East Pacific Rise). They can be found at transform (conservative) boundaries also, for example the San Andreas Fault. Many of the Earth's deepest rift valleys are found underwater, diving long mountain ranges. As tectonic plates move away from each other molten rock well up and hardens as it comes into contact with the frigid sea, forming new ocean crust at the bottom of the rift valley
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These are long narrow depressions on the sea floor. They are the deepest part of the ocean floor. They are formed by tectonic activity and usually found at convergent (destructive) plate boundaries, here dense lithosphere melts or slides below less dense lithosphere is a process called subduction, creating a trench. The most famous oceanic - continental trench is the Peru-Chile trench off the coast of South America, whereas the deepest Marianas Trench is created by oceanic-oceanic subduction in the Pacific Ocean,
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