The heat of the Earths Core causes magma to heat up+rise
When the hot magma reaches the underneath of the crust. It spreads out in all directions, dragging the crust with it
The magma then cools slightly, as it is furthest from the core. This makes it heavier
This cycle is repeated again and again in a circular motion, causing convection currents
The heavier magma sinks back down towards the core, where it is heated again