text: There are only two driving forces:\n\nSlab pull (from the descending slab)\nRidge push (from the topographic high at the rift)\n\nThese are the only driving forces.  Often the effect of the mantle below the plate is believed (sometimes even taught in first and second year courses! - something that frustrates us TAs to no end...) to be a driving force, but in reality, it is a resistive force referred to as "mantle drag".  This is because the rate of movement of the lithospheric plates is in general, far greater than the rate of convection of the asthenosphere - that is, if they are in the same direction in the first place (which is often not the case!)  However, IF the rate of mantle movement in the area WAS in the direction of plate movement (only an assumption - and this would likely be a rare instance), AND the rate of mantle convection exceeded that of the lithospheric plate, only THEN would mantle drag be considered a driving force.\n\nHope that helps - and hope you don't have a prof that is "oversimplifying" things to the point of being in error.