Hot liquids are less dense than cold liquids.
Hot liquids are less dense than cold and will rise. In the earth's deep mantle and outer core, the magma that is closer to the exteremely hot inner core rises because it's less dense and then pushes the cooler magma that is further from the intense heat down. When the cooler magma is pushed down, it is heated more and rises. The somewhat less hot magma is more dense and so sinks. This motion is convection currents and is what causes the plates to move that are riding on the surface of all of this.