The movement of earth's tectonic plates relate to many factors, including density differences in the plates

The movement of Earth's tectonic plates relates to many factors, including mantle convection and density differences in the plates. Earlier work on plate tectonics theory suggested that mantle convection is the driving force behind plate motion. However, recent studies suggest that, although mantle convection was likely a key factor in beginning plate motion long ago, plate motion may actually be the main driving force behind present mantle convection. 

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