The convection currents are what causes the movement of the tectonic plates. These currents occur in the mantle layer. Basically, the convection currents are a circular movement of the magma in the mantle which is cause by the difference in temperature and density. The hotter magma moves up because it is less dense, while the cooler magma moves down because it is more dense. This movement continues all the time. The magma that moves up comes in touch with the crust above it. At certain places where the crust is thinner, or there are cracks, under the big pressure from the mantle, the magma manages to penetrate through it, filling up the gaps, and also pushing the plates away. As the plates are pushed more and more, their opposite ends collide with other plates, resulting in subduction, so the crust is getting into the mantle and melts, moving down in the mantle as it is becoming cooler and denser magma, and the process continues on and on.
