Hot spots have been defined1 as centres of volcanism not associated with plate boundaries. Some authors2,3 have regarded them as surface expressions of upwellings of relatively high temperature material in the mantle. These upwellings, either deep mantle plumes2,4, or shallow convection cells5, create a hot spot on the surface of the Earth only where they have molten and broken their way to the surface. I suggest here that there is a uniform sublithospheric upwelling distribution, and that the ability of upwellings to make themselves evident on the surface, and, therefore, the difference in surface hot spot density among the various plates, depends on the velocity and thickness of each lithospheric plate. From these characteristics and the rate of magma entry into the lithosphere, a normalization of the areal hot spot density can be made to calculate the distance between sublithospheric upwellings.