Influence of snow upon rates of granodiorite weathering, Colorado Front Range, USA

Measurements of phenocryst relief in 1‐m 2 quadrats placed randomly on southwest‐facing granodiorite outcrops exposed by late Pleistocene deglaciation of a high‐altitude Front Range col suggest that weathering is strongly influenced by snow. Depending upon local differences in snow‐cover duration (inferred from differences in lichen cover), rates of bedrock‐surface lowering vary from 0.0001 to 0.0013 mm/year. A quadratic polynomial best describes the relationship. Weathering is least effective beneath late‐lying snowbanks that protect the rock from freeze‐thaw and wetting‐drying cycles for most of the year, and in winter snow‐free sites where dryness and the binding effects of crustose lichens discourage granular disintegration. Weathering is most effective in areas of thin to moderate winter snow cover where meltout occurs early, moisture is available at critical seasons, and episodic lichen snowkill ensures that bare rock is exposed. If granite‐weathering data are to be used for dating, measurements should be restricted to microenvironments with comparable snow‐accumulation histories.