Bill S. has supplied the following;
Some "answers" for Pre
“Plate-tectonics, is the belief that many of Earth's geological features, such as mountains, are caused by currents of solid rock which circulate in the mantle.“
Current belief is that many of the Earth’s geological features are caused, or at least influenced, by the movement of tectonic plates. For some time the belief was quite widely held that these movements were driven by mantle convection, such is no longer the case. Gravity is now regarded as a more important influence.
“The convection is claimed to be due to the temperature difference (about 3,000 degrees) between the top and the bottom of the mantle.“
Although there are still discussions about the possibility of such features as mantle plumes; if convection occurs, it is much more likely to be on a smaller scale, and would be driven by local differences in temperature and density, not by the difference between top and bottom, which would be influenced by intervening discontinuities.
“The basic idea, is that the rock at the bottom of the mantle, on being heated by the core, becomes lighter, and thus, rises (in a gigantic up-welling) to the top of the mantle”.
This somewhat outdated idea is covered in the previous response. “gigantic up-wellings” may still be favoured by mantle plume supporters, but are not “flavour of the month”.
“The rock current, then flows (away from the up-welling and) under the Earth's surface, but parallel to it (carrying the continents with it), until it cools”
This too is an outdated concept.
“On cooling sufficiently, the rock becomes heavier and sinks (in a gigantic down-welling) back to the bottom of the mantle, and on doing so, completes one lap of a circuit”.
There is some evidence, which I have indicated, that sinking , cold rock can be traced to considerable depth, but that this has anything to do with a clearly defined “lap of a circuit” is an assumption too far.
