Computer-simulation models of scoria cone degradation

Abstract

Long-term erosional modifications of the relatively simple morphology of scoria (`cinder') cones are ideally suited for study by field and computer-simulation methods. A series of temporally-distinct cones in the San Francisco and Springerville volcanic fields of Arizona provides the foundation for documenting the degradational evolution of scoria cones in a semi-arid climate. Progressive changes due to erosion are illustrated by the systematic decrease with increasing age of various morphometric parameters, including scoria cone height, cone height/width ratio ( H co/ W co), crater depth/width ratio, and slope angle. For example, Holocene–latest Pleistocene cones in the San Francisco field have a mean H co/ W co value of 0.178±0.041, a mean maximum slope angle of 29.7±4.2°, and a mean average slope angle of 26.4±7.3°, whereas the group of Pliocene cones have values of 0.077±0.024, 20.5±5.8°, and 8.7±2.7°, respectively. Comparative morphology of scoria cones is a potentially useful dating tool for mapping volcanic fields. In order to better understand the degradational modifications of these volcanic landforms, we have developed a numerical approach to simulate the surficial processes responsible for the erosion of a typical scoria cone. The simulation algorithm can apply either a linear diffusion-equation model or a model with a nonlinear transport law. Using a finite-difference formulation, the simulation operates upon a three-dimensional scoria cone input as a matrix of elevation values. Utilizing both field and model results, the correlation between changing H co/ W co value, cone age, and computer time step was expressed graphically to derive comprehensive values of the transport or diffusion coefficient ( D f) for both volcanic fields. For the San Francisco volcanic field, D f had a calculated value of 21.4 m 2/kyr for the linear model and 5.3 m/kyr for the nonlinear model, while for the Springerville volcanic field D f had a calculated value of 24.4 m 2/kyr for the linear model and 6.3 m/kyr for the nonlinear model.