In certain places the hot core heats the cooler mantle above it.
This causes the heated mantle to rise.
As this hot mantle reaches the solid crust it cannot rise further and so it moves sideways.
This causes the plate above it to also move.
When the mantle cools it will sink back down.
This movement is known as a convection current.
Theory 2: Ridge Push and Slab Pull
An alternative explanation is called ridge push and slab pull.
At constructive margins oceanic ridges form high above the ocean floor.
Beneath ocean ridges the mantle melts, the molten magma rises as the plates move apart and cools down to form new plate material.
As the crust cools it becomes denser and starts to slide down, away from the ridge, which causes plates to move away from each other. This is called ridge push.
Additionally, at destructive margins the denser plate sinks back into the mantle under the influence of gravity, which pulls the rest of the plate along behind it. This is called slab pull.