It is the rising of hot, light fluids and sinking of cool, dense fluids in a gravitational field. Convection in the solid silicate mantle drives plate motion. Given high temperatures and lots of time, rocks flow like silly putty. This forms a divergent plate boundary . An example of a divergent boundary is the Mid-Atlantic Ridge.
Oceanic crust is subducted back into the mantle.
Continental crust is too light to subduct and accumulates on the surface. Thus, continents have grown over geologic time. Examples of convergent plate boundaries are the west coast of South America, the Fiji-Tonga Trench, Phillipine Trench, and the Japan Trench. If plates slide past one another, this forms a transform boundary. An example of a transform plate boundary is the west coast of North America at the San Andreas Fault.