The extent to which crustal processes along mid-ocean ridges are controlled by either the pattern of mantle upwelling or the mode of magma injection into the crust is not known. Models of mantle upwelling vary from two-dimensional, passive flow(1) to three-dimensional, diapiric flow(2-4). Similarly, beneath a ridge segment bounded by tectonic offsets, crustal magma chambers may be replenished continuously along the ridge(5-7) or at a central injection zone(2-4) from which magma migrates towards the segment’s ends. Here we present tomographic images that reveal the seismic structure and anisotropy of the uppermost mantle beneath the East Pacific Rise. The anisotropy is consistent with two-dimensional mantle now diverging from the rise, whereas the anomalous isotropic structure requires a three-dimensional but continuous distribution of melt near the crust-mantle interface. Our results indicate that crustal magma chambers are replenished at closely spaced intervals along-axis and that crustal systems inherit characteristics of scale from melt transport processes originating in the mantle.