We discuss the space relationship between upper mantle plumes revealed earlier from analysis of long-wavelength isostatic gravity anomalies and the subducting Pacific slab. According to global seismic tomography, the oceanic slab in its segments corresponding to the Japan and Izu–Bonin island arcs flattens out at the bottom of the mantle transition zone, extends horizontally far beneath Eurasia, and then resumes sinking into the lower mantle. The upper mantle plumes are located beyond the western endpoint of the slab sector that advances the farthest beneath the continent.

A considerable part in the plume material may belong to fertilized (enriched with incompatible elements) peridotite. A layer of fertilized peridotite forms at depths between 200 and 600 km under the effect the melts produced by partial melting of the slab oceanic crust cause on the overlying depleted mantle. The peridotite layer integrates into the slab and heats up by friction along the slab top during the horizontal motion of the latter in the transition zone where the mantle material is of relatively high strength. Portions of hot fertilized peridotite detach from the slab as it sinks into the lower mantle, rise by buoyancy through the upper part of the transition zone, and become entrained into an elongate asthenospheric convection cell which arises beneath the continent behind the subduction zone. The ascending convection flow splits into separate streams which are the upper mantle plumes.