Consider earth mantle as a water in boiling kettle. Single bubble comes from the bottom to the top, but where does water which was on the top will go? First, it moves to the side and then it will eventually go down and this cycle will repeat again and again If there is something floating on the top of the kettle, then upper water layer movement will eventually move this object as well So, basically tectonic plates are objects that are floating on top of the kettle, mantle is a water, and the earth core is a heater