Seismic structure and origin of hotspots and mantle plumes

A new global tomography approach has been used to study the deep structure and dynamics of hotspots and mantle plumes. In this approach, depth variations of the Moho, 410 and 660 km discontinuities are considered, the Earth structure is expressed by seismic velocities at grid nodes instead of blocks, and ray paths and travel times are calculated with a 3-D ray tracing technique. Prominent plume-like slow anomalies are detected under major hotspot regions from the crust down to the core¯mantle boundary (CMB). Hotspots in Hawaii, Iceland, South Pacific and East Africa are located above slow anomalies in the lower mantle down to the CMB, suggesting that the mantle plumes under those hotspots may originate from the CMB. However, there may be some small-scaled, weak plumes originating from the transition zone or middle mantle depths. The slow anomalies under hotspots usually do not show a vertical pillar shape, which suggests that plumes are not fixed in the mantle but can be deflected by the mantle flow. As a consequence, hotspots are not fixed but can wander on the Earthʹs surface, as evidenced by recent geomagnetic and numerical modeling studies.