The main reason why the tectonic plates move is the conversational currents. There is magma between the upper mantle and the asthenosphere that heats up at the upper mantle. The weight is so small that the magma rises with the help of these conversational currents. When the magma has ascended towards the asthenosphere, it has nowhere to go so the magma lies along the asthenosphere. There the magma cools down again because the asthenosphere is colder than the upper mantle, which makes the magma heavier and goes back down to the upper mantle, where the magma heats up again and everything starts all over again and so it goes on over and over again. Scientists have concluded that the earth's gravity causes two other processes that also cause the tectonic plates to move.
Ridge push - in the spreading zones, the sides of the tiles are raised and slide to the side as in a slide. Slab pull - when a plate is pulled down by gravity in a subduction zone like an anchor.
