Rheology of bubbly rhyolites and its effect on the emplacement of Obsidian Dome

Obsidian rhyolite lava flows are abundant in the geologic record, but modern eruptions have rarely been observed. Emplacement behaviors witnessed during the effusive eruptions of Chaitén (2009), and Cordón Caulle (2011-2012), such as inflation and breakout lobe development, cannot be explained by current models of silicic lava emplacement. The goal of this study is to infer the emplacement style of Obsidian Dome, Inyo Craters, Long Valley, CA, by constraining the rheology of vesicular and dense obsidian. Samples were collected from outcrop and drill core, allowing comparisons between the flow interior and exterior. We used relaxation geospeedometry, Fourier-transform infrared spectroscopy (FTIR), and uniaxial parallel-plate viscometry to measure natural cooling rates, H₂O contents, and apparent viscosities of pumiceous and dense obsidians. Our measurements quantify the effect of bubble fraction, temperature, and dissolved H₂O on the viscosity of obsidian, and constrain the evolution of the rheology of an advancing obsidian lava. Modeled natural cooling rates from drill core vary by ~3.5 orders of magnitude, with fine pumiceous obsidian on the surface of the lava flow cooling quickly at ~5°C/min and dense obsidian within the flow cooling more slowly at ~0.002°C/min. H₂O contents vary little throughout the core (0.17 to 0.23 wt.% H₂O), with the exception of the basal samples from the drill core closest to the vent (~0.6 wt.% H₂O). Bubble volume fraction decreases as a function of depth, because higher pressures suppress bubble nucleation and growth, and collapse any pre-existing bubbles. These observations demonstrate that the base of Obsidian Dome cooled slowly, and was relatively H₂O-rich and dense. Our results can be used to estimate the rheological contrast between the crustal and basal lava by using conservative values of 700°C and 0.2 wt.% H₂O at the surface and 800°C and 0.6 wt.% H₂O at the base resulting in a crustal viscosity of 10^12.3 Pa s and basal viscosity of 10^8.6 Pa s, a factor of ~5000 different. This rheological contrast allows overlying lava to advance by accommodating strain in a narrow basal shear zone, suggesting Obsidian Dome advanced in a modified "tank-tread" style of emplacement, where a cool brittle crust advances, cascades over the flow margin, and is eventually overridden by the advancing flow.