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.
As tectonic plates are pulled apart, the underlying mantle begins to melt and produce magma. As this magma rises to the seafloor it produces giant, long volcanoes knowns as mid-ocean ridges. Mapping and sampling individual mid-ocean ridge eruptions tells us about the melting of the mantle, magma storage prior to eruption, and the eruption process including eruption size and frequency. In Spring 2010, we embarked on the Galapagos Ridge Undersea Volcanic Eruptions Expedition (GRUVEE) dedicated to deciphering the eruptive history of a mid-ocean ridge where the magma supply to ridge doubles while the spreading rate remains constant. We used the submersible Alvin, the autonomous Sentry, and the Woods Hole towed camera system to make geologic maps of two areas of contrasting depth, bottom roughness, and crustal magma supply. We investigated two study sites on Galapagos Ridge, near and far from hotspot influence at 92W and 95W, respectively. Ultimately, we distinguished a series of individual eruptive units in each study area through a combination of high-resolution sonar mapping, geologic observations from submersibles and camera tows, and petrologic analyses of lava samples. We added 18 new eruptive units to the total catalog of known mid-ocean ridge eruptions. Our initial line of research focused on using high-resolution sonar data to discriminate between lava flow morphologies at the Galapagos Ridge, which can be used to infer lava flow processes. A novel classification algorithm was developed that used sonar-derived attributes to describe the surface geometry, acoustic properties, and texture of the lava flows. The result is a map of lava morphology at unprecedented scale (>45km2) and detail (2m x 2m resolution) with a high level of user accuracy (~90%). The digital maps enabled spatially-comprehensive measurements of the coverage areas of each lava morphology and development of geologic maps. We applied this method to map lava flows within each of the Galapagos Ridge lava flow fields, producing lava morphology maps that were used to examine the abundance and distribution of lava flow morphologies and interpret volcanic structures in the context of lava flow rates and emplacement processes. This work compared lava flow fields at lower- and higher-magma-supply Galapagos Ridge study areas and showed that average flow rates increased along with magma supply, but also revealed significant variability of eruption styles within each setting. Additionally, we have compiled 25 years of multibeam bathymetry collected during 16 oceanographic expeditions by nine different research vessels to construct the first continuous bathymetric map covering the Galapagos Spreading Center (GSC) from 104°-84°W, revealing the terrain along the axial rift valley and ridge flanks out to a distance of 10-15km for over 50% of the GSC. This new bathymetric compilation provides the opportunity to quantitatively analyze variations in the morphological characteristics of the GSC along its nearly 2200km length and compare to other geological and environmental data. The compilation reveals areas of unusual bathymetry as well as providing a valuable regional base map to guide future work and regional synthesis studies. The last phase of work focuses on the crystal content of Galapagos Ridge lavas, which has important consequences for their flow properties and eruption style. This aspect of the project uses images of Galapagos Ridge rock samples with image processing techniques that automatically isolate the relative proportion of the crystal phase from the glass matrix in a quantitative and statistically robust analysis. The goal of this work is to compare the crystal contents of samples to their flow morphology setting, discuss the effects of crystallinity on submarine lava viscosity, and compare samples from lower- and higher-magma-supply Galapagos Ridge study areas. Major conclusions of the project are: seafloor lava morphology records the emplacement of ocean crust, prior eruptions can be mapped on the seafloor with the right tools and blend of personnel, and magma supply at mid-ocean ridge plate boundaries exerts an important first-order control on the architecture and shape of the ocean basins.
