Effect of Cinder Cone Lithology on Erosional Morphology in a Post-Fire Setting

Abstract:

Sugarloaf Peak is a cinder cone on the eastern flank of the San Francisco Peaks approximately 18 km north of Flagstaff, Arizona. We observe that the rhyolitic Sugarloaf Peak is heavily gullied for a cinder cone of its age (91 ka), whereas nearby basaltic cones of a similar age do not display these features. Sugarloaf Peak thus represents a potential natural experiment for understanding the impacts of lithology on gully development. Specifically, we hypothesize that Sugarloaf Peak may be more heavily gullied as a result of its lithology. Due to the typically lower porosity and permeability of rhyolite, we expect the infiltration rate of rhyolite to be less than that of basalt; subsequently, we expect areas underlain by rhyolite to generate more overland flow than equivalent areas underlain by basalt. Comparison between Sugarloaf Peak and nearby basaltic cones is confounded by the 2010 Schultz Fire that impacted only Sugarloaf Peak. Given these observations, we aim to determine the extent to which the gully development on Sugarloaf Peak is due to lithologic effects, while controlling for post-fire effects.

We used Google Earth imagery from a period of 8-10 years to estimate rates of gully-head advancement over time. Gullies on Sugarloaf Peak increased in length by 16-25 m in the 8 years post-fire. The rate of erosion has varied over time; 10-20 m of erosion occurred between January and April in 2015 and is attributed to an early March precipitation event that was 165% above average for that time period. These aerial imagery observations indicate that while gully development was greatest within the first few years post-fire, gully sizes have continued to increase well beyond the expected time scale for post-fire soil alteration. Using our observations of gully headcut propagation, we predict that although wildfire likely accelerated growth by increasing post-fire runoff, lithology is the main factor in the extent of erosion. To further investigate the cause of gullying on Sugarloaf Peak, future analyses will include measurement of infiltration rates on Sugarloaf Peak and nearby basaltic cones, as well as the use of aerial imagery to reconstruct pre-fire topography. Additional methods will include assessment of LiDAR data to estimate depths of erosion.