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It has long been theorized that the driving forces of plate movements are deep convection currents that well up beneath the mid-ocean ridges and then circle back down beneath the ocean trenches.
The problem with this theory is that the mantle appears to be horizontally layered.
Evidence for significant amounts of submerged continental crust in the present-day oceans provides another major challenge to plate tectonics.
Such convection currents do not seem to be consistent with such layering.
It was hoped that seismic tomography would give clear evidence of such convection-cell patterns.
It seems like crust is being produced in more areas than it is being subducted. Certain specific examples are also interesting, such as the African plate.
Africa is allegedly being converged on by plates spreading from the east, south, and west, yet it exhibits no evidence whatsoever for the existence of subduction zones.