tectonic plates float on amass of melted lava. its very hot and produces gases. these gases tend to move the plates with the compression becomes very high and the gases do not find an exit.the other reason is the plates themselves tend to move. because of the temprature gradient. the plates are hotter on the lower side which forces the plates to move up. and they are colder on the outside,which forces the plates to move downward..this is the other reason why the plates move
