when seafloor spreading was first introduced, geologists proposed that the main driving force for plate motion was up-welling that came from deep in the mantle. upon reaching the base of the lithosphere, this flow was thought to spread laterally and drag the plates along. thus, plate were viewed as being carried passively by convection flow in the mantle. Based on physical evidence, however, it became clear that up-welling beneath oceanic ridges is quite shallow and not related to deep circulation in the lower mantle. it is the horizontal movement of lithospheric plates away from the ridge that causes mantle up-welling, not the other way around. thus, modern models have plates being integral part of mantle convection and perhaps even most active component.