http://www.platetectonics.com/book/page_4.asp

“One idea that might explain the ability of the athenosphere to flow is the idea of convection currents. When mantle rocks near the radioactive core are heated, they become less dense than the cooler, upper mantle rocks. These warmer rocks rise while the cooler rocks sink, creating slow, vertical currents within the mantle”.

Whilst this kind of simplified explanation may be all that is sought by the majority of people who might read this, it is, with some justification, open to the sort of criticism leveled by Pre.

I would suggest that the idea of mantle convection should be considered in greater detail than this before it is either thrown out or unconditionally accepted.

There never was nothing.