Holocene Chemical Weathering, Surface Lowering and Rock Weakening Rates on Glacially Eroded Bedrock Surfaces in an Alpine Periglacial Environment, Jotunheimen, Southern Norway

Indices of the rate of Holocene chemical weathering of pyroxene‐granulite gneiss and peridotite are estimated from bedrock surfaces deglaciated c. 9700 years ago. A mean surface lowering rate for pyroxene‐granulite gneiss of 4.8 ± 1.0 mm ka−1 is estimated from the mean height of upstanding quartz veins, and Schmidt hammer R‐values are used to validate the reference‐surface method. Microrelief development and crack‐widening rates for peridotite are 2.6 ± 1.1 mm ka−1 and 1.8 ± 0.4 mm ka−1, respectively. Well‐developed weathering rinds indicate a mean rind development rate for peridotite of 0.6 ± 0.1 mm ka−1. New indices of rock surface weakening (IRW) and the rock surface weakening rate (WR) are defined in relation to Schmidt hammer mean R‐values. IRW values from weathered and unweathered surfaces of known age are 38.7 per cent for gneiss and 34.1–59.2 per cent (depending on the texture of the weathered rock surface) for peridotite: corresponding WR values are 2.7 R‐units ka−1 and 2.1–3.7 R‐units ka−1, respectively. Our methods and results are evaluated, particularly in relation to previous research and other rock types in Scandinavia. The various indices, being affected in different ways by biochemical and biological mechanical weathering and micro‐erosion, are regarded as complementary aids in the investigation of both the effects of chemical weathering and its rate. Copyright © 2010 John Wiley & Sons, Ltd.