COMPUTER-SIMULATION MODELS OF SCORIA CONE DEGRADATION

Long-term erosional modifications of the relatively simple morphology of scoria ('cinder') cones are ideally suited for study by field and c omputer-simulation methods. A series of temporally-distinct cones in t he 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 i llustrated by the systematic decrease with increasing age of various m orphometric parameters, including scoria cone height, cone height/widt h ratio (H-co/W-co), crater depth/width ratio, and slope angle. For ex ample, Holocene-latest Pleistocene cones in the San Francisco field ha ve a mean H-co/W-co value of 0.178 +/- 0.041, a mean maximum slope ang le of 29.7 +/- 4.2 degrees, and a mean average slope angle of 26.4 +/- 7.3 degrees, whereas the group of Pliocene cones have values of 0.077 +/- 0.024, 20.5 +/- 5.8 degrees, and 8.7 +/- 2.7 degrees, respectivel y. Comparative morphology of scoria cones is a potentially useful dati ng tool for mapping volcanic fields. In order to better understand the degradational modifications of these volcanic landforms, we have deve loped a numerical approach to simulate the surficial processes respons ible for the erosion of a typical scoria cone. The simulation algorith m can apply either st 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 ma trix of elevation values. Utilizing both field and model results, the correlation between changing H-co/W-co value, cone age, and computer t ime step was expressed graphically to derive comprehensive values of t he 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 valu e of 24.4 m(2)/kyr for the linear model and 6.3 m/kyr for the nonlinea r model. (C) 1998 Elsevier Science B.V. All rights reserved.