What are two of the processes that move tectonic plates over the surface of the earth

One reason is that the plates float on a hot boiling mentle. when the heat rises and the mentle expands the plates above it also moves. 
the other reason is that the plates themselves move.plates have warmer inner sides that tend to rise. and colder outer side that tends to sink. this tug of war between the hotter and colder sides of the plate also helps them move.