http://www.sciencedirect.com/science/article/pii/S1674987111000181

This study investigates the mechanism of formation of convection plumes of mushroomshape in sub-solidus mantle and their prediction. The seismic-tomographic images of columnar structuresof several hundreds kilometers in diameter have been reported by several researchers, while the muchcherished mushroom-shaped plume heads could only be found in computational geodynamics (CGD)models and simple small-scale laboratory analogue simulations. Our theory of transient instability showsthat the formation of convection plumes is preceded by the onset of convection caused by unsteady-stateheat conduction at the boundaries, from which filamentous plumes first appear. The plumes generated atthe Core Mantle Boundary (CMB) and lithosphere rising and falling through the mantle have been predictedsimply with our theory for various heat fluxes and viscosities, which still remain uncertain amongstgeoscientists. The sizes of mushroom plumes in the sub-solidus mantle caused by heat fluxes of 20 and120 mW/m2 at the CMB are found to be 1842 km and 1173 km with critical times over 825 Myr and334 Myr respectively. They are comparable to some large continental flood basalt provinces, andthey number between 17 and 41. The thickness of the thermal boundary layers at the CMBfrom which convection plumes evolved are found to be 652 km and 415 km for 20 and 120 mW/m2respectively.Top cooling may produce plunging plumes of diameter of 585 km and at least 195 Myr old. Thenumber of cold plumes is estimated to be 569, which has not been observed by seismic tomography or as cold spots. The cold plunging plumes may overwhelm and entrap some of the hot rising plumesfrom CMB, so that together they may settle in the transition zone.