Rates of hornblende etching in soils in glacial deposits of the northern Rocky Mountains (Wyoming-Montana, U.S.A.): Influence of climate and characteristics of the parent material

Abstract

Hornblende grains in the soil matrix of glacial deposits in the Northern Rocky Mountains etch at rates that depend primarily upon the influences on soil moisture of the climate and the texture of the parent materials. Etching is a logarithmic function of age of the parent material, and etching rates decline rapidly after a period of initial high rates over the first 10–15 ka after deposition. In this study, a comparison of etching rates was made among four chronosequences in the Wind River Range of western Wyoming and the Tobacco Root Range of southwestern Montana which have differences in mean annual precipitation (MAP) and texture of the till parent materials. Using rates calculated for the first 12 ka after deposition, etching is slowest (0.02 μm ka −1) in coarse-textured granitic parent materials where the MAP is 25–40 cm. In contrast, etching is faster by an order of magnitude (0.21 μm ka −1) where the MAP is 110–150 cm and the parent material is finer textured because ∼ 15% sedimentary rock material has been mixed with the granitic component. Within individual chronosequences, deposits at higher elevations have accelerated etching rates due to higher orographic precipitation or the influence of late-lying snow.