The notion that thermal convection currents must exist in the Earth's mantle occupies a central position in theories which have been proposed to account for the occurrence of continental drift and sea-floor spreading. Apart from the controlling influence which these currents exert on surface geological processes such as mountain building, they also govern the rate at which heat is transferred from the hot interior of the Earth to its surface and so are crucial to understanding the thermal history of the planet. Mantle convection serves as the guiding principle in the geological and geophysical sciences.