1. The earth's core heats the magma above. As a result, the magma rises slowly upwards towards the crust.
2. As the magma rises it then begins to cool. Subsequently, this cooling results in the magma becoming heavier and sinking back down towards the core.
3. This cycle continuously repeats and forms a circular movement of magma within the mantle. This causes, in these convection currents to move very slowly so plates are either dragged apart (separate), pushed together (collide) or slide past each other.