Two older theories had to be discounted first. Largest hotspot, Hawaii, should be elevated. Direct viscous drag on lithosphere base by mantle convection cell. Thermal boundary layer is the asthenosphere.
Convection generally follows ridge and trench location.
No cells beneath continents. Tension at ridges, compression at trenches. Mechanism Top of convection cell is the oceanic lithosphere. Thermal boundary layer is the lithosphere. Ridge push from asthenospheric upwelling.