The contribution of rock strength to soil production

It has long been proposed¹, and then observed^2,3, that faster rates of soil production occur beneath thinner soils. It remains uncertain, however, whether soil thickness is the driving variable regulating production from the top-down^2-6, or whether soil thickness is simply responding to changes in bedrock weathering controlled from the bottom-up^7,8. Answering this question is difficult because the feedbacks between soil production and soil erosion, the processes that jointly govern soil thickness^2,9,10, respond to perturbations on timescales of thousands to millions of years^11,12, timescales that are too long for scientists to observe directly. Here we leverage a space-for-time substitution at a transient mountain range along the San Andreas Fault^13-15, where the remarkable tectonic setting allows for independent quantification of uplift, soil production and erosion. We show that, following a pulse of tectonic uplift, the conversion of rock to soil accelerates before the overlying soils thin, but at the same time that topographic stresses increase⁷ and the rock weakens¹⁶. This observation challenges the long-standing assumption that soil production rates are controlled predominantly by soil thickness^1,2, and instead lends evidence for a bottom-up, rock strength control on soil production.