Dynamic Magma Storage at Near-Ridge Hot Spots: Evidence from New Galápagos Gravity Data

A morphological dichotomy exists between the active western and older (\u3e1–2.5 Ma) eastern volcanoes of the Galápagos Archipelago. All of the young shield volcanoes in the west have calderas, but none are present on the older volcanoes in the east. This striking difference suggests that there has been a change in volcanic construction and magmatic supply processes, a finding dissimilar to the prevailing Hawaiian model of hot spot evolution. Bouguer anomaly highs (30–50 mGal) consistent with dense cumulate bodies are measured over these calderas. In contrast, we present new gravity data from Santa Cruz and San Cristóbal islands that lack comparable gravity highs. We propose that formation of shallow magma reservoirs and their associated Bouguer anomaly highs were inhibited during evolution of the eastern volcanoes because the proximal Galápagos Spreading Center diverted plume material more efficiently ~1–2 Ma when the spreading center was ~100 km closer to the plume. The difference in caldera expression between the western and eastern islands may be a surface expression of this process. These results suggest that plume-ridge interaction may play a first-order role in the evolution of magmatic plumbing systems of near-ridge ocean islands, which account for one third of hot spot systems