Lowell's comments on driving forces on plates sounded correct to me. I agree that a lot of what was being said by David T. on Terracycles was not correct (or, maybe since we are supposed to be open-minded on this page-my objection is that much of what he was saying is not plausible but was presented as "facts"...), and am surprised that Roger bothered to spend that much time on that debate. I only bothered to read the first few posts. Concerning convection: I'll check with my geophysicist wife on the latest, but there is probably large scale convection-but Lowell is correct that many think the convection is being dragged by the plate. A poster in the "U" section by Warren Hamilton at AGU may be interesting on this-things like sinking plates causing slab rollback and forcing asthenosphere (less viscous mantle) out of the way. -Chris-back from AGU