1) heat from radioactive decay in the core of the Earth heard the mantle.
2) Plates move averagely only a few cm a year, this has been happening ever since the planet formed and cooled to give a thick crust so this movement has added up over many years to move plates far from where they started.
3) because the mantle is heated and is able to flow slowly, convection currents are set up which cause the plates on the crust to move.
