Undergraduate Honors Thesis


Effect of volcanic lithology on erosional morphology in a post-fire setting Public Deposited

  • Though much research has been performed on hazards of large volcanoes, sedimentation hazards on smaller volcanic structures like rhyolite domes and cinder cones have largely been ignored. One such structure, Sugarloaf Peak, is a ~91 ka rhyolite dome on the eastern flank of the San Francisco Peaks approximately 18 km north of Flagstaff, Arizona. Sugarloaf Peak is heavily gullied compared to nearby basaltic cinder/scoria cones of a similar age, which show no obvious gullying. Sugarloaf Peak thus represents a potential natural experiment for understanding the impacts of lithology on gully development. Specifically, I hypothesize that Sugarloaf Peak may be more heavily gullied as a result of its lithology. This question is best framed by considering whether the gully features are actively growing or are a relic of past events. Due to the typically lower porosity and permeability of rhyolite, infiltration rates of rhyolite rock and soils derived from rhyolite parent material are expected to be less than that of basalt scoria; subsequently, areas underlain by rhyolite are expected to generate overland flow and propagate gullying more than equivalent areas underlain by basalt. By observing recent historical imagery of the area over a timespan of several years, I can establish whether the gullies on Sugarloaf are a product of present-day erosional processes. An opportunity to investigate the impacts of wildfire on gully formation is provided by the 2010 Schultz Fire, which severely burned much of Sugarloaf Peak but did not reach the nearby cinder cones. Given these observations, I aim to determine whether gully development on Sugarloaf Peak is due to lithologic effects, while observing and controlling for post-fire effects.

    To determine whether the gullies on Sugarloaf are actively developing, and thus whether gully development on small volcanic structures is heavily impacted by lithology, Google Earth imagery from 2011-2018 was used to estimate rates of gully headcut advancement over time. Gullies on Sugarloaf Peak were found to increase in length by approximately 5-25 m in the eight years post-fire. Their rates of erosion have varied over time; 3-25 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. The question of wildfire impact was addressed by taking infiltration measurements on the burned and unburned sides of Sugarloaf, as well as on the unburned cinder cone. This combined dataset of aerial imagery observations and infiltration rates indicates that while gully development was most rapid within the first few years post-fire, gully sizes have continued to increase well beyond the expected time scale for post-fire soil alteration. This not only supports the hypothesis that the lithology of these structures is a driving factor of their morphological differences, but also suggests that the established time scale for fire-induced soil alteration may not be applicable to this area.

Date Awarded
  • 2021-04-02
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Last Modified
  • 2021-04-13
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