Seismic imaging of melt in a displaced Hawaiian plume

The Hawaiian Islands are the classic example of hotspot volcanism: the island chain formed progressively as the Pacific plate moved across a fixed mantle plume(1). However, some observations(2) are inconsistent with simple, vertical upwelling beneath a thermally defined plate and the nature of plume-plate interaction is debated. Here we use S-to-P seismic receiver functions, measured using a network of land and seafloor seismometers, to image the base of a melt-rich zone located 110 to 155 km beneath Hawaii. We find that this melt-rich zone is deepest 100 km west of Hawaii, implying that the plume impinges on the plate here and causes melting at greater depths in the mantle, rather than directly beneath the island. We infer that the plume either naturally upwells vertically beneath western Hawaii, or that it is instead deflected westwards by a compositionally depleted root that was generated beneath the island as it formed. The offset of the Hawaiian plume adds complexity to the classical model of a fixed plume that ascends vertically to the surface, and suggests that mantle melts beneath intraplate volcanoes may be guided by pre-existing structures beneath the islands.