Solidification and Convection in the Core of the Earth

Abstract

The Earth can be divided into three regions: a solid inner core, whose current radius is 1221 km; a liquid outer core, which extends to 3480 km; and a generally solid mantle. Concomitant with heat being lost from all these regions of the earth, the fluid outer core gradually solidifies to extend the radius of the inner core which is made up of almost pure iron. The residual fluid released by the solidification is poor in iron and is thereby less dense than the original melt. This compositional difference, along with thermal differences across the boundary between the core and the mantle, drives vigorous convection in the liquid outer core at a Rayleigh number which is extremely large, and may even exceed 1025. The motion in the outer core is influenced by the rotation of the earth and, because the fluid in the outer core is a good conductor, it is also influenced by electromagnetic effects. Indeed, it is generally believed that the all-important external magnetic field of the earth is maintained by the interaction of the convection within the rotating outer core and the magnetic fields which permeate it. As described, this is a very complicated, highly nonlinear coupled system, concerning which there have been a vast number of intensive studies over many decades (see, for example, Gubbins 1991 and references therein).