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‘Es gibt nichts praktischeres als die Theorie ’ (ludwig boltzmann—Vorlesungen 1895) The possible causes of convection in the Earth’s mantle are examined, and it is concluded that radiogenic heating together with thermal conduction is most likely to provide the driving force for any convection present at this stage of the Earth’s history. The theory of convection in a medium with internal heat generation is discussed semi-quantitatively. It is concluded that a mantle convection pattern based on calculations of marginal stability in an incompressible fluid is a gross oversimplification of the real situation. A more realistic theory indicates that thermal convection is probably confined to the outer mantle and is an unstable flow. There is evidence that it would have a character that would explain the narrowness and linearity of the pattern presented by the ocean ridges and some orogenic belts.

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