The project will use the submersible ALVIN and the AUV SENTRY to obtain samples on-and off-axis at the intermediate-spreading Galapagos Spreading Center to study the effects of variable magma supply on volcanic processes at the intermediate-spreading Galapagos Spreading Center. The project will obtain important new constraints on the relationships between magma supply, magma chamber properties and processes, eruption dynamics and the development of volcanic landforms at this intermediate-spreading mid-ocean ridge.
This project involved geologic mapping of lava flow fields produced during volcanic eruptions along the Galapagos Spreading Center. The field investigation used the Alvin human-occupied research submersible as the principal vehicle, where geologists descended to the seafloor to make critical observations and collect rock samples for laboratory study. In addition we used an autonomous, very high-resolution bathymetric mapping tool (Sentry), which was deployed at night when no submerisble dives were taking place. This work demonstrated that it is possible to determine the eruptive history of a region of mid-ocean ridge, even where no recent volcanic eruptions had been detected by remote seismic or other means. We identified the most recent 6-10 flow fields in each of two main study areas: one near 91°W where overall magma supply is enhanced because of proximity to the Galapagos hotspot, and a second area 450 km farther west, where the magma supply is normal (lower) for the spreading rate there. We learned that there are profound differences in the nature of volcanic eruptions in the two areas. At low magma supply eruptions are characterized by slow effusion from point sources that mainly build hills called seamounts. In contrast, at higher mamga supply more frequent eruptions tend to occur along fissures up to ~10 km long; effusion rates are higher and the resultant topography is more subdued. In addition to differences in the seafloor opography produced, we learned that the actual lava material is substantially different. In particular, high magma supply lavas are nearly devoid of crystals and have been erupted from melt-dominated magma reservoirs that reside at shallow depths in the crust. In contrast, lavas erupted at low magma supply tend to have more crystals and show evidence of magma mixing between crystal-rich and crystal-poor magmas having occurred deeper in the crust. This is the first study that explicitly separates the effects of magma supply from those of spreading rate in studying mid-ocean ridge magmatic and volcanic processes. An important outcome is the production of geologic maps at the scale of individual eruptive events on the deep seafloor and detailed petrological study of the returned samples.
