We examine the interplay between the Cobb hotspot and the Juan de Fuca ridge by mapping the base of the crust at their intersection near Axial Volcano. PmP traveltimes from an extensive active source seismic experiment are used in conjunction with prior crustal tomography to map crustal thickness. A cross-section along the ridge axis shows two distinct trends. In addition to 1-2 km of excess crust, inferred from gravity along a 100 km section of the central ridge, we find a narrow 20-40 km diameter root extending to 11 km directly beneath the volcano. Focused magma flux from the Cobb hotspot (0.3-0.8 m(3)/s) is inferred to be the cause of both the narrow root, and the large magma reservoir, at Axial. The rapid return to thinner crust, away from the intersection, marks a transition back to ridge-dominated magmatism.
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