Recent tomographic imaging suggest that mantle plumes are much thicker than previously thought. Using new laboratory experiments and scaling laws, we show that thermal plumes developing in a visco-plastic fluid present much larger diameters than plumes developing in a newtonian fluid. Such a rheology requiring a yield-stress is consistent with a lower mantle predominantly deforming by pure dislocation climb. Yield stress values between 1 and 10 MPa, implying dislocation densities between 10⁸ and 10¹⁰ m^-2 would be sufficient to reproduce the plumes morphology observed in tomographic images.

A. Davaille, Ph. Carrez & P. Cordier (2018) Fat plumes may reflect the complex rheology of the lower mantle. Geophysical Research Letters, 45, 1-6. https://doi.org/10.1002/2017GL076575