THE GLOBAL HOTSPOT DISTRIBUTION AND INSTABILITY OF D''

We present a new spherical harmonic expansion of the global hotspot distribution, with the contribution of each hotspot weighted by its buoyancy flux. The resulting power spectrum, normalized to that expected for a random distribution, has statistically significant peaks only at degrees 1 and 2. To explain this, we study a simple model for the origin of mantle plumes in the D'' layer at the base of the mantle: the Rayleigh-Taylor instability of two superposed fluid layers with different viscosities and densities in a spherical shell. The wavelength of the fastest growing instability greatly exceeds the thickness of D'' and increases as the viscosity contrast gamma across this layer increases. These results are consistent with the hotspot spectrum if gamma is similar to 10(6).