[Bill S (OP)]

The following is taken from a post on another forum.  I would be fascinated to have the comments of a geologist on this.

“Those who push plate-tectonics are so amazingly stupid, that they never even bothered to check whether, or not, the hot rock at the bottom of the mantle was really lighter than the colder rock above it, as is required by their theory. And, this is though most geology books actually tell you that the hot rock, 3740 K, at the bottom of the mantle has a density of 5,560 kg/m³, and that the density decreases from 5,560 kg/m³ to 3,370 kg/m³ as one approaches the top of the mantle (3,370 kg/m³ is the density the cold rock, 930 K, at the top of the mantle, about 40 kms down).

This, totally contradicts the assumptions of the theory of mantle currents/plate-tectonics (that is, contrary to known fact, plate-tectonics assumes that the rock at the bottom of the mantle becomes hotter, and thus lighter than the colder rock above it, and consequently rises).”

[JimBob]

Yes. Plate tectonics depend on the relative VISCOSITY of the "rocks," not the heat, of the rock.

Accordin'  to ther lasted IQ test, I r knot stue-pid.

[Bass]

The poster neglects pressure.  It's not necessary for rocks at the bottom of the mantle to be less dense than rocks at the top of the mantle.  It's only necessary for them to less dense and viscous than the rocks surrounding them.  As the "hot" rocks ascend through the mantle, pressure decreases as does density.  Does the writer believe that cold rocks at the top of the mantle have a lower density than hot rocks at the top of the mantle?  If that were true, subduction slabs wouldn't sink into the mantle, as shown by seismic tomography.
Remember, MENSA for geologist only requires a 53 IQ )

[CliffordK]

When looking at ice cubes in my freezer, it is hard to think that solid ice can run like water...  well, almost.

But, perhaps one could compare the movement of rocks to that of glaciers.

I must admit that the idea of plate tectonics seemed pretty extraordinary to me at one time, but it does seem to represent what we are observing on the planet quite well.

[Bill S (OP)]

Looking at a plate tectonics world map, it seems that plates are converging on Africa from both East and West, with no area of subduction between.  How can this be?  Where is the crustal material going?