EROSION OF CINDER CONES IN THE SAN FRANCISCO VOLCANIC FIELD (SAN FRANCISCO VOLCANIC FIELD REU)

The topographic forms of cinder cones indicate both their eruptive and post-eruptive history and relate directly to the action of surface processes over time. Aerial photography of northern Arizona’s San Francisco Volcanic Field was conducted and used for digital photogrammetry to build digital elevation models of the cones. Numerical modeling and analogue experiments were also used to study the post-eruptive geomorphic evolution of cinder cones by applying a mathematical erosion model and water erosion, respectively, to idealized cones in both cone models. Slope histograms, a metric used to compare different cone forms, were taken from the topographic profiles or DEMs that were produced by these techniques. These histograms show from that as a cinder cone is modified by surface processes, the slope of its flank decreases and nonzero slopes appear in previously zero-dominated areas, representing the deposition of sediment around the cone base. The slope distribution changes overall from high concentrations at zero and at the angle of tephra repose (~32°) to a more distributed range of shallower slopes, indicating a convex-concave profile. Cinder cones with a thin layer of resistant spatter agglutinate on their rims, however, acquire steeper slopes high on their flanks as well as slopes lower than the angle of repose lower on their flanks, eventually reaching an overall concave flank profile and exceeding the previous maximum slope of ~32° on the histogram. The resistant agglutinate protects itself and the material directly underneath it from erosion; this material stays in place while the sediments around it are transported downslope. In the numerical model, an annulus of lower diffusivity on an idealized cone creates a neck-like form, as does a ring of spray adhesive simulating agglutinate on a sand cone in the analogue experiments. Accounting for the complex erosion of agglutinated cones contributes to an improved understanding of age constraints on cinder cones, refining knowledge of eruption recurrence intervals that is useful for hazard and risk assessment.