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Convection currents are thought to be the main reason for movement of crustal plates or plate tectonics. The Earth's interior is very hot compared to the crust. This intense heat sets up convection cells in the mantle (the layer just underneath the crustal plats). The situation is very similar to water boiling in a hot pot. The water near the bottom of the pot gets heated up and the subsequent reduction in density causes it to move up. This hot water displaces the cold, denser water in the upper layers, which starts to sink down towards the bottom of the pot. The rising hot water ultimately cools down and is replaced by rising hot water and this cycle goes on. Something similar is happening with our crustal plates. Molten rocks from Earth's interior move up, while the denser, cooler rocks sink down. The crustal plates also have density variation- thin, lighter parts and thick, denser regions. As the hot rocks pour of the mantle, the thinner sections rise, while simultaneously pushing the thicker parts away (ridge push). The thicker, denser parts go underneath a thinner part through subduction. This whole process causes the motion of crustal plates.
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