CINDER CONES WITH COMPLEX ORIGINAL FORMS AND IMPLICATIONS FOR MORPHOLOGIC DATING

Current models for morphologic dating of cinder cones focus on conical forms. However, cinder cones may be characterized by an elongate original morphology, often resulting from a fissure eruption. Their varying geometries affect typical axisymmetric morphologic analysis. Photogrammetry coupled with 3D topographic and numerical modeling illustrate the effect of complex original form on rates and styles of degradation as indicated by slope abundances.

From a survey of 77 cinder cones and 11 cone series (group of closely aligned cones) from the central San Francisco Volcanic Field (SFVF), ~84% have an eccentricity >0.5, and exhibit some degree of control from the local geologic structure. Numerical models were used to simulate linear and nonlinear slope dependent (“diffusive”) transport limited surficial processes over idealized cones. Simulated cones start with maximum initial slopes of 33°, decreasing to 31° at 10⁵ m² and 26° at 10⁶ m² morphologic ages under nonlinear diffusion for both conical and elongate forms. DEMs and derived slope distributions of 5 target cones were produced from photogrammetric data at 3 resolutions (1.0, 0.6, and 0.1 m). Lower resolution DEMs tended to underestimate slope values and resulted in an increase in histogram peakedness, while higher resolution DEMs displayed features such as steep agglutinated lava spatter on crater rims and vegetation resulting in an increased histogram range. Results from analyses of slope histograms of the target cones follow results from the simulated cones, such as the presence of an additional mode in the histograms of elongate cones.

The numerical models indicate that slope abundances vary based on cone form, but cone form alone has little noticeable effect on styles and rates of degradation. Single conic forms or conical series with close spacing produce unimodal slope histograms, with higher dispersion for the conical series. Single elongate cones or conical series with broad spacing produce bimodal slope histograms. Increasing the cone elongation results in a higher separation of the slope histogram modes, which does not occur in cone series regardless of the magnitude of the cone separation. Since cone form has little effect on rates of degradation, slope histograms provide a suitable method for morphologically dating cinder cones regardless of form.