Rhyolite lava emplacement dynamics inferred from surface morphology

Abstract

The morphology of a silicic lava is created by the fluid dynamic conditions that operated during flow. Careful analysis of these features can thus be used to reconstruct emplacement processes. We used unoccupied aerial system and structure-from-motion photogrammetry techniques to collect centimeter-scale spatial resolution imagery of the surfaces of South Coulee and Obsidian Dome rhyolite lavas (Mono-Inyo Craters, USA). We supplement the newly acquired orthomosaics and digital elevation models with existing imagery of other Holocene rhyolite lavas from across the western USA, including Rock Mesa Dome (South Sister, OR), Newberry Flow (South Sister, OR), Obsidian Flow (Newberry, OR), Interlake Flow (Newberry, OR), and Banco Bonito Flow (Valles Caldera, NM). Although many morphologic features exist, we restrict our quantitative analyses to the recurrent ridges and blocks of loose rubble that are common to the flows. Ridge spacing and ridge vergence indicate the ridges are likely folds produced by compression during emplacement. Pressure ridge spacings range from 27 to 45 m at South Coulee and 18 to 34 m on Obsidian Dome. Spacing generally decreases with distance from the vent across both lavas. Ridge amplitudes range from 4 to 17 m at both lavas and show little correlation with distance from the vent. The vergence of the crest of the ridges in the flow interiors points back to the source vent, which provides strong evidence for endogenous flow and the backward rotation of folds produced by undertow of the underlying lava. Ridges near flow margins verge towards the flow front, demonstrating that exogenous flow becomes increasingly important at the point of advance. In both South Coulee and Obsidian Dome block sizes are largest near the vent, and gradually decrease towards the flow front. The gradual decrease in block size with increasing distance from the vent likely reflects decreasing effusion rates from the conduit. We interpret our high-resolution field measurements under the lens of analytical solutions to fluid dynamic models to estimate emplacement timescales and associated rates. We calculate relatively abbreviated eruptive timescales ranging from 1 month to <2 years. Such values predict eruptive fluxes <135 m3 s−1 and velocities <100 m day−1, providing helpful criterion for volcanic monitoring or hazard forecasting efforts.