Submarine Snapshots of Galápagos Magmatism

Abstract:

The Galápagos Archipelago consists of nine major islands that sit atop a broad volcanic platform that rises 2500 m from the abyssal plain to depths <500 m. The areas between the islands are poorly explored, but new mapping has revealed a large population of small (<1 km diameter) seamounts. Seamounts generated at oceanic hotspots record a short duration of volcanic activity, relative to nearby large, subaerial islands. These volcanic snapshots help to resolve the spatial and temporal variability of magmatism resulting from sampling the limited subaerial exposures. Furthermore, the morphology and lithology of the seamounts record information about the conditions in which they were formed. Two recent cruises to the Galápagos by the E/V Nautilus (7/2015) and M/V Alucia (8/2015) mapped and sampled seamounts in previously unexplored areas of the shallow volcanic platform. Areas studied include the central platform, between the islands of Santiago and Santa Cruz; the western platform, south of Fernandina Island; and the southern platform, west of Floreana Island. Mapping in each region revealed previously unknown seamounts at depths ranging from 20-900 m. We collected ~150 rock samples from >25 volcanic features in the three geographic locations. Rock samples consist of basalts, volcanic breccias and conglomerates. Major and trace elements and radiogenic isotopes in the basalts are used to evaluate the evolution of submarine volcanism on the platform, to test whether the seamounts from each region are related through a single magmatic episode, and how the magmatic plumbing system of the seamounts is related to that of the nearby islands. Preliminary results indicate that the seamounts are the result of both mono and polygenetic eruptions. Evidence for this includes variable seamount morphologies ranging from simple cones of a single composition (e.g., K₂O/TiO₂) to complex multivent structures, with variable K₂O/TiO₂. In addition, many of the seamounts in the central and southern archipelago display evidence of having been subaerial in their history, including erosional benches, flat tops, well-rounded cobbles and beach deposits. These seamounts are now at depths >200 m, providing information on the timing of their formation and subsidence history of the archipelago, relative to eustatic sea level.