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5.4 Human Contribution to Earth’s Evolution
Due to the always growing interest in space that humans have we have learned many things about the Earth. The American satellite, Explorer 1,found out Earth has an strong radiation zone. Humans have caused a lot of pollution to the Earth’s atmosphere, which has been noticed during the recent years. The gases released by ushave left the atmosphere very weak. 85% of the air on earth is kept in the troposphere, it also protects us from UV Rays and meteorites. The polar ice caps, (which took thousands of year toform) have also started to melt, because humans have not taken care of the Earth. They have also wasted and used up many natural resources like oil.


5.5 Movement of the Plates
The outer layer of the Earth which is made up of the crust and upper mantle iscalled the lithosphere. The oceanic lithosphere is created through volcanism in the form of cracks in the ridges, which go all around the world. Heat escapes as new lithosphere come upfrom below the old one. As it cools down it contracts and breaks apart from the ridge. When this happens the plate moves across the ocean floor to subduction zones, which is known asseafloor spreading. Over time the old lithosphere becomes denser and sinks into the earth. The continental lithosphere is 150km thick, with a crust and upper mantle that float. The continentsmove sideways to get to colder parts of the mantle, this movement is called continental drift.

Beatty, J. K. and A. Chaikin, eds. The New Solar System. Massachusetts: SkyPublishing, 3rd Edition, 1990.
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Seeds, Michael A. Horizons. Belmont, California: Wadsworth, 1995
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