Mantle plumes rooted at the core-mantle boundary: evidence from seismic waveform tomography

Barbara Romanowicz

UC Berkeley

Date & Time
Building 3, Rambo Auditorium

Many questions remain on the detailed morphology of convection patterns in the earth's mantle. In particular, the existence of deep mantle plumes has been the subject of debate ever since they were proposed to explain the presence of hotspot volcanoes. With the advent of numerical methods for accurate seismic wavefield computations, it is now possible to apply the tools of waveform tomography to better detect structures in the deep mantle, previously "hidden" by wavefront healing effects not captured by approximate wave propagation methods. Taking advantage of these new tools, we have recently constructed a global shear velocity model of the mantle, which shows better focused, finer scale structures both in the upper and in the lower mantle, and in particular vertically elongated structures that form discrete columns rooted at the base of the mantle, positioned in the vicinity of some 25 major hotspots. At least some of the roots of these fat plumes contain large quasi-circular "ultra-low velocity zones", as I will illustrate in the case of Hawaii and Iceland. The vertical conduits are quite straight from the base of the mantle to 1000 km depth, but wider (500-1000 km) than expected from the standard "plume" model. Their character changes above this depth, as they seem to become narrower and meander across the upper mantle, where they interact with secondary scale convection set off by plate motions. I will also discuss the significance of the apparent rheological boundary around 1000 km depth.

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