Using local body wave arrival-time tomography methods to determine 3-D seismic velocity structure, we imaged the plumbing system of Sierra Negra Volcano, Galápagos. This hot spot volcanic chain includes some of the fastest deforming volcanoes in the world, making this an ideal location to study shield volcano plumbing systems. We inverted P and S wave arrivals recorded on a 15-station temporary array between July 2009 and June 2011 using an a priori 1-D velocity model constrained by offshore refraction studies. With local seismicity from nearby volcanoes as well as the ring fault system, the model resolution is good between depths of 3 and 15.5 km. The propagation of S waves throughout this volume argues against any large high-melt accumulations, although a shallow melt sill may exist above 5 km. We image a broad low-velocity region (>25 km laterally) below Sierra Negra at depths~8-15 km. No large, regional velocity increase is found within the limits of good resolution, suggesting that crust is thicker than 15 km beneath the western Galápagos archipelago. Our results are consistent with crustal accretion of mafic cumulates from a large-volume magma chamber that may span the boundary between preplume and accreted crust. The similarity between our results and those of Hawaii leave open the possibility that the crust has also been thickened by under-plating.

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P and S wave tomographic models have been developed for the northern Malawi rift and adjacent Rungwe Volcanic Province (RVP) using data from the Study of Extension and maGmatism in Malawi aNd Tanzania project and data from previous networks in the study area. The main features of the models are a low‐velocity zone (LVZ) with δVp = ~−1.5–2.0% and δVs = ~−2–3% centered beneath the RVP, a lower‐amplitude LVZ (δVp = ~−1.0–1.3% and δVs = ~−0.7–1%) to the southeast of the RVP beneath the center and northeastern side of the northern Malawi rift, a shift of the lower‐amplitude anomaly at ~−10° to −11° to the west beneath the central basin and to the western side of the rift, and a fast anomaly at all depths beneath the Bangweulu Craton. The LVZ widens further at depths >~150–200 km and extends to the north beneath northwestern Malawi, wrapping around the fast anomaly beneath the craton. We attribute the LVZ beneath the RVP and the northern Malawi rift to the flow of warm, superplume mantle from the southwest, upwelling beneath and around the Bangweulu Craton lithosphere, consistent with high 3He/4He values from the RVP. The LVZ under the RVP and northern Malawi rift strongly indicates that the rifted lithosphere has been thermally perturbed. Given that volcanism in the RVP began about 10 million years earlier than the rift faulting, thermal and/or magmatic weakening of the lithosphere may have begun prior to the onset of rifting.