Influence of size, shape, and proportion of suspended crystals on the viscosity and morphology of submarine basaltic lava flows

Despite being one of the planet's most ubiquitous and prolific processes, only two deep submarine eruptions have been directly observed, leaving the eruptive products as the only clues to eruption and emplacement processes. For most submarine eruptions, lava flow morphology (e.g. pillows, lobates, sheets) is used as a proxy for volumetric flow or areal advance rates; however, lava viscosity, a critical parameter that fundamentally influences eruption and lava flow transport, is rarely considered. An estimation of flow viscosity is essential for the interpretation of submarine lava flow morphology as a proxy for flow rates, and by extension, the modeling and understanding of submarine volcanic processes. We have performed textural analyses on high-resolution backscattered electron images of basaltic glass from 25 samples collected at the Galápagos Spreading Center (GSC). Samples were collected from a range of emplacement settings, enabling comparisons of texture, viscosity, and flow morphology. Quantitative measurements of crystal size, shape, and proportion were used in conjunction with existing predictive models of suspension rheology to estimate a relative flow viscosity and predict flow behavior. Additionally, crystallinity measurements were integrated with existing petrologic data to estimate an overall flow viscosity that incorporates compositional variations. Our results highlight the role of viscosity in the formation of submarine lava flow morphology: pillows form from magmatic suspensions with higher crystal content, finer crystal texture, and lower aspect ratio crystals which collectively lead to higher viscosity values that inhibit flow, while sheet flows form from suspensions with lower crystal contents, coarser crystal textures, and higher aspect ratio crystals that allow higher flow rates to be maintained over greater distances. Transitional lobate lavas have a wide range of crystallinity and viscosity values suggesting a strong local topographic control on their morphology. Significant variations in texture and viscosity values occur within the same lava flow field. Differential magma supply along the GSC related to the influence of the nearby Galápagos hotspot also allows for an examination of the effect of magma supply on the intrinsic textural characteristics of erupted lavas. Samples from a lower-magma-supply area at 95°W show a strong relationship between viscosity and the suspended crystal fraction, reflecting relatively uniform, low degrees of fractionation prior to eruption. In contrast, the viscosity values of samples from a higher-magma-supply area at 92°W are less sensitive to the properties of the suspended crystals than variations in composition-temperature-H₂O space. In addition, there are significant variations in textural properties and flow morphology among samples with similar viscosity values, highlighting the compositional diversity created by higher and more variable degrees of melt fractionation. These results indicate that magma supply not only influences the nature and character of submarine eruptions, but also the intrinsic characteristics of the volcanic deposits that build the upper oceanic crust.