EPSS Colloquium winter-2020

The existence of two large low shear velocity provinces (LLSVPs) at the base of the earth's mantle with a strong equatorial "degree 2" signature has been known since the development of the first global seismic tomographic models of the earth's lower mantle. In the last few decades, their existence has been confirmed in many studies, and they are generally associated with the global upwelling flow in the lower mantle, but their precise nature and role in the earth's evolution and present-day dynamics is still debated. Several indirect clues point to their stability for the last 250-300 Ma, and possibly much further back in geological time. It has been suggested that they may be of a different composition than surrounding regions and some authors have argued that they may be denser than the ambient mantle to a significant height above the CMB. One of the key questions in furthering our understanding of the LLSVPs is how high they extend as coherent and compositionally distinct structures above the core-mantle boundary (CMB). Because of their very strong long wavelength signature, which persists to at least 1500 km above the core-mantle boundary, a common assumption is that they are compact structures across that depth range. I will discuss these ideas in the light of recent full waveform tomography images that shows the presence of ~20 broad low velocity plumes, rooted in the LLSVPs, that rise quasi-vertically from the CMB to around 1000 km depth, and then meander through the upper part of the mantle towards major hotspots.