The Dynamics of the 2011 Eruption of Axial Volcano: Degassing-based Geospeedometry
Adam Soule, Geology and Geophysics
Eruption dynamics (e.g., magma ascent rate, lava eruption rate) in terrestrial volcanic systems are critical to assessing the state of the underlying magmatic system and exert a fundamental control on volcanic deposition. However, as no mid-ocean ridge (MOR) eruptions have been directly observed, our knowledge of seafloor eruption dynamics is extremely limited. My recent research has led to the development of a new, degassing-based geospeedometer for quantitatively assessing eruption dynamics in mid-ocean ridge eruptions. The method involves tracking the loss of CO2 from the melt as well as the increase in vesicularity produced by the exsolving gas during transport across the seafloor. This approach is possible in deep-sea eruptions because magmas commonly erupt with elevated concentrations of dissolved CO2 (i.e., supersaturated relative to seafloor pressures) and subsequently degas as the lava is maintained at constant pressure and temperature during transport across the seafloor. This process is similar to the continued bubbling of soda water once the seal on a new bottle is released. With knowledge of the rate of bubble growth it is possible to evaluate how long the cap has been removed by measuring the CO2 remaining in the soda. The development and initial application of this method was carried out on the 2005-06 eruption of the East Pacific Rise where detailed mapping allowed collection of a sample suite along an individual flow pathway, tracking the transport of lava from the eruptive vent to the distal end of the flow. Results from this study provide the first quantitative estimates of eruption rates and flow rates of a mid-ocean ridge lava flow. The 2011 eruption of Axial Volcano, an on-axis seamount on the Juan de Fuca Ridge, has provided a new opportunity to further develop this approach. Pre- and post-eruption AUV bathymetry surveys provide a remarkably detailed map of lava distribution. In addition, pressure/temperature sensors in the caldera and a hydrophone array operating throughout the eruption provide independent estimates of eruption dynamics, against which degassing-based geospeedometry estimates can be evaluated. The key objective of this proposed work is to validate and refine a degassing-based geospeedometer for MOR eruptions with a specific application to the 2011 eruption of Axial Volcano on the Juan de Fuca Ridge. To do so, I will: 1) evaluate the volatile concentration and extent of CO2 supersaturations of samples collected along the eruptive fissure and flow paths of the 2011 Axial eruption; 2) determine the variability in vesicularity and its correlation with dissolved volatile concentrations; and 3) evaluate other extant parameters that may impact degassing (e.g., thermal evolution of the lava flow and crystal content). With this information I will apply established models of bubble growth and gas loss to evaluate eruption dynamics for comparison to independent estimates of dynamics. Based on these data I will participate in a cruise scheduled for Fall 2012 to collect additional samples from the 2011 Axial eruption in order to fully resolve the degassing paths within the eruption along multiple flow lobes. The proposed degassing-based geospeedometry method can be applied to a wide range of basaltic MOR lava flows over a range of spreading rates so that a much-improved global characterization of eruption dynamics can be achieved. Developing an ability to reliably evaluate eruption dynamics would open a new phase of MOR research, allowing a refined approach to evaluating the physical processes of melt collection in the shallow crust, the connectivity between magma bodies along ridges, and the physical processes of melt extraction and volcanic deposition on the seafloor. The results of this proposed work will serve as the basis for an NSF proposal of larger scope aimed at developing a catalog of eruption rates across the global mid-ocean ridge system in order to quantitatively evaluate how spreading rate and magma supply influence volcanic processes.