Rock weathering rates at the soil surface in an alpine environment

Mass loss from 60 g samples of crushed rhyodacite exposed on 71 plots at two localities above treeline in the San Juan Mountains, Colorado are used to estimate weathering rates at the soil surface. The results define an average rate of loss of 0.079 ± 0.012 %/yr (mean of 42 plots over 5 years, ± 2 standard errors), with an apparent slowing of the rate as the period of observation increased. Two features of the soil surface environment influence the observed weathering rates: the soil pH (which is dependent on the vegetation cover); and the volume and residence time of water moving through and over the soil. Both of these are controlled basically by the distribution of the winter snow cover.