HIGH-RESOLUTION UAV MAPPING OF SOUTH COULEE, MONO CHAIN, CALIFORNIA: CONSTRAINTS ON EMPLACEMENT CONDITIONS

We analyzed the surface morphology of South Coulee lava flow in California using high-resolution DEMs and orthomosaic derived from UAV photogrammetry. Flow morphologies can be used to infer emplacement characteristics, which is important because few rhyolite flows have been observed scientifically. The average ridge spacing for South Coulee is 34±6 m. The average slope of the flow front of South Coulee is 44±5^o, and stays consistent from margins to toe. The block sizes across the surface South Coulee range from 0.3 m to 0.6 m. We compared those measurements of South Coulee to a subset of rhyolitic lava flows across the western USA, including Rock Mesa, Newberry, Obsidian Flow, Interlake Flow, and Inyo Chain lava flows. Ridge spacing across those flows ranges from 9 m to 50 m. Flow front slopes are also highly variable and range from 38 ^o to 68 ^o. Block size measurements across most of the Inyo Chain lava flows have been shown by Anderson to range from 0.2 m to 0.7 m and gradually decreases away from the vent. All lava flows in this study are rhyolitic so we would expect the viscosity and other fluid dynamic properties to be comparable, and thus produce similar surface features. When compared to South Coulee, we find that the other rhyolitic lavas flows have lower flow front angles and a different mean ridge spacing. A gentler flow front slope indicates that the lava flow is less viscous, but a reduced viscosity is not expected since all the lava has a similar whole rock composition. We hypothesize that the controlling factor of these geomorphic features on rhyolites includes preexisting topography and eruption rate.