Rock control on microweathering of bedrock surfaces in a periglacial environment.

D. T. Nicholson

School of Science and Environment, Manchester Metropolitan University, Chester Street, Manchester, UK. E-mail: d.nicholson@mmu.ac.uk Tel: +44 161 247 6232.

Abstract

Microweathering of ice-smoothed bedrock surfaces was investigated in the Røldal area of Hardanger Plateau
(60°), southern Norway. Postglacial rates of weathering were determined from surface lowering using quartz
veins as reference surfaces. Weathering processes are inferred from assessment of weathering rind
formation, surface hardness, and the preservation of small-scale glacial erosional features.
Surface lowering rates for a range of metamorphic rocks vary from 0.05 to 2.20 mm ka−1 and are broadly
comparable with those obtained from crystalline rocks in other periglacial environments. The mean rate of
surface lowering at 0.55 mm ka−1 is low and demonstrates the relatively small impact of microweathering on
postglacial landscape evolution. Variations in bedrock microweathering can be explained by lithological
variation. Amphibolite and mica-rich bedrock surfaces experience greater denudation and weakening, least
weathering rind formation, and abundant preservation of glacial striae, despite greater surface lowering.
Conversely, quartz-rich bedrock surfaces are most resistant to denudation and weakening, but have greater
weathering rind formation and fewer preserved striae. Postglacial microweathering is achieved primarily
through granular disintegration involving detachment and removal of mineral grains and weakening from
increased porosity. Granular decomposition is manifest in the formation of weathering rinds. Analysis of
interactions between weathering indices indicates that rind accumulation is limited by microerosion.
A conceptual model is proposed that illustrates the temporal interrelationships between in situ and erosional
facets of microweathering in two contrasting mineral assemblages. The model proposes that cyclic processes
of in situ disintegration, decomposition, and erosion are at work. The relative balance between these
processes varies with lithology so that in more resistant quartz-rich rocks the net effect is minimal surface
lowering and accumulation of weathering rind. In weaker, amphibolitic and micaceous rocks, the net effect is
greater surface lowering and minimal accumulation of weathering rind. The results of the research
demonstrate the important influence of rock properties, notably mineral composition, in postglacial
microweathering of crystalline bedrock in a periglacial environment.