HOW LOW CAN YOU GO? THE EFFECTS OF RESIDUAL H2O ON THE ADVANCE OF OBSIDIAN LAVA FLOWS OF THE WESTERN UNITED STATES

The emplacement style of large obsidian lava flows, domes, and coulées is a function of a complex relationship between volatile content, temperature, and time. Conduit processes are efficient at removing volatiles from magmas during volcanic eruptions. However, some water (H₂O_tot) remains in effusive lavas, which controls the apparent viscosity of the lava and drives the formation of macro morphological features and textures. The primary goal of this study is to better understand the effects of residual magmatic water on the advance of obsidian lava flows.

Spatially well constrained samples were collected from three drill cores produced during the Continental Scientific Drilling Program (CSDP) in 1984. Two drill cores were collected from the ~0.6 ka Obsidian Dome, Inyo Craters, California, including one drill core near the vent (RDO-2B) and another near the flow margin (RDO-2A). The third drill core (VC-1) was collected from the ~68.3 ka Banco Bonito flow in Valles Caldera, New Mexico. The volatile content, apparent viscosity, and texture have been determined for samples using Fourier-transform infrared spectroscopy (FTIR), parallel-plate viscometry, and density measurements respectively.

We find that the water content of the three drill cores vary little as a function of depth in the lava flow with an average of 0.32 ± 0.35 (1σ) wt. % H₂O_tot (RDO-2B), 0.19 ± 0.09 (1σ) wt. % H₂O_tot (RDO-2A), and 0.27 ± 0.14 (1σ) wt. % H₂O_tot (VC-1). However, there is a significant difference (P = 0.001) between the average H₂O_tot from the vent proximal drill core, to the distal drill core from Obsidian Dome, suggesting additional degassing occurs during emplacement. Additionally, samples with low H₂O_tot (≥0.1 wt. %) can still vesiculate at ambient pressures and reasonable emplacement temperatures. Thus, pumiceous textures observed in obsidian lava flows can form at the surface, simultaneously removing water and increasing the viscosity by up to a factor of 4 to 5. Additionally, vesiculation introduces bubbles to the lava and increases the apparent viscosity by a factor of 3 to 30, when compared to a bubble-free melt with a comparable water content. We conclude that water contents as low as ~0.1 wt. % H₂O_tot can continue to influence the evolution and advance of obsidian lava flows.