The main point of Table 6.2 is that for any variety of convection styles that may exist, the Rayleigh number associated with heating from below, easily exceeds the critical value of 600 or so found in the Rayleigh-Benard experiments and theory. The Nusselt Number, the ratio of heat transport by convection to conduction is 20-100, hence convection dominates as the mechanism of heat transport.
Several features indicate that the Rayleigh-Bernard convection model is inadequate.
In the first place, there is widespread evidence for hotspots, interpreted to be mantle plumes. If these hot spots remained relatively fixed as the crustal plates slide over them, then the motion of the mantle is relatively slow compared to that of the plates, sliding on the lubricated zone we call the asthenosphere. The fast motion of the plates and the high shear region are not features of simple mantle convection in the Rayleigh-Bernard styel. Furthermore, the density driven flow is dominated by slab pull not by the pull of downgoing cold mantle in general, thus the down welling is very concentrated in the subduction zones. The upwelling is not likely to be so concentrated, though it may seem so as the ridges are narrow.