Flow dynamics of rhyolite lava inferred from UAV photogrammetry

We analyzed the morphology of rhyolite lava flows using high-resolution digital elevation models (DEMs) and orthomosaics made from unmanned aerial vehicle (UAV) structure-from-motion photogrammetry. We produced new DEMs of South Coulee, Panum Crater, Wilson Butte, and Glass Creek lava flows, Mono-Inyo Craters, CA USA, using photographs captured during 19 missions comprised of 42 flights and >12,000 photographs. The newly acquired data sets are compared to DEMs of rhyolitic lava flows across the western USA, including Rock Mesa, Newberry, Obsidian Flow, Interlake Flow, and Banco Bonito. We measured pressure ridge spacing, vergence angle of the axial plane of fold of the pressure ridges, block size distribution across the lava flow, and flow thickness measurements from the DEMs. Ridge spacings across all flows range from 9 m to >120 m. Within a single flow, ridge spacing can vary by up to 86 m, whereas others are largely consistent. At Obsidian Dome, for example, ridge spacing is 10±4 m. Across all lavas, the average angle of the axial plane of pressure ridge fold hinges is 18±45º, indicating vergence back towards the vent. Some lavas display solely toward-vent vergence, whereas other exhibit variable angles alternating between towards vent vergence and towards margin vergence. Block size measurements indicate that sizes gradually decrease away from the vent in all flows. Flow thicknesses may change with position across individual flows, with the most variability shown by South Coulee flow which changes from 110 m to 48 m. Most flows are between 25 m to 90 m thick. Flows typically get thinner on steeper topography. Morphologic features form during emplacement and are controlled by the fluid dynamic properties of the lava. Ridge spacing is a function of stress, viscosity, and temperature gradients within the lava flow. Vergence is controlled by the mode of emplacement of the lava flow; either tank tread or inflation. Viscosity is a major control for all features, but it is unlikely that a simple compositional control exists for viscosity because all the lava flows are rhyolite. This suggests that the viscosities of rhyolite lavas are controlled by differences in the cooling histories during emplacement. Other factors likely include preexisting topography and effusion rate.