Degradation of Quaternary cinder cones in the Cima volcanic field, Mojave Desert, California

Abstract

Basaltic cinder cones in the Cima volcanic field record a detailed history of progressive erosion in the arid environment of the eastern Mojave Desert. These cones range in age from ∼0.015 m.y. to 1.09±0.08 m.y., as dated by radiocarbon or K-Ar analyses of the youngest lava flows from each cone. Cone heights range from 50 to 155 m, and basal widths range from 400 to 915 m; on younger cones, height/width ratios average 0.17, and crater-width/cone-width ratios average 0.42. The degradational morphology of these cones displays several trends that are closely related to cone age. (1) Crater-width/cone-width ratios decrease from 0.48 on the youngest cone to 0.21 on the oldest cone with a preserved crater. (2) Mean maximum side slopes (Tan Sc) decrease from 0.575 on the youngest cone to an average of 0.41 on the oldest cones studied. (3) Debris-apron–height/cone-height ratios increase from <0.10 on the youngest cone to an average of 0.34 on the oldest cones. (4) Cone drainage evolves from irregularly spaced rills and gullies on the youngest cone to regularly spaced gullies on 0.20- to 0.35-m.y.-old cones to valleys as much as 110 m wide and 10 m deep on cones 0.59 m.y. old and older. These trends form the basis of an empirical model of cinder-cone degradation in arid environments. This model documents (1) an erosional loss of ∼15% of cone volume during the first million years; (2) progressive decline of cone slope (at an average rate of 0.006°/103 yr) and cone height (at an average rate of 2.25 cm/103 yr); (3) initial rapid stripping of the loose cinder mantle from upper-cone slopes accompanied by rapid debris-apron formation; and (4) a gradual transition, between 0.25 and 0.6 m.y., from relatively uniform stripping of upper slopes to localized fluvial dissection of both cone slopes and debris apron. This transition is apparently controlled by a concomitant change from diffuse subsurface drainage within the pervious cinder mantle to concentrated surface flow across the heterogeneous assemblage of agglutinate layers, dikes, and ponded flows of the cone interior.