Abstract
Hotspots are commonly attributed to mantle plumes and to a first order, the hotspot distribution is correlated with long-wavelength geoid anomalies and with higher temperatures in the lower mantle, suggesting that the plumes are generated in the lower mantle. I show that the second-order distribution is periodic with a typical spacing of 800–1000 km. Such a spacing would be too narrow for plumes to ascend directly from the lower mantle, but is close to the thickness of the upper mantle. The alignment of convection cells in the upper mantle seems to control the periodicity. To reconcile the first-a and second-order distributions, assuming limited mass fluxes across the 670-km boundary, a lateral deflection of plumes as they breach the transition zone from below is required.
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