Micro-weathering processes, rates and morphology:
Evidence from a high mountain plateau

D. T. Nicholson*1, F. H. Nicholson2 and E. Webber1

*Corresponding author: Department of Environmental and Geographical Sciences, Manchester Metropolitan University, Chester Street, Manchester, UK. E-mail: d.nicholson@mmu.ac.uk Tel: +44 161 247 6232. 1Manchester Metropolitan University, 2Liverpool John Moores University

Abstract

Research on Hardanger mountain plateau in southern Norway is being undertaken to examine the influence of post-glacial weathering on landscape evolution in an active periglacial environment. Work is based around several locations in the south west corner of Hardangervidda (59oN) at altitudes ranging from 1200 to 1450m. The region experiences a mean annual temperature of -2oC and mean annual rainfall of 1000mm. This part of the plateau has sparse tundra vegetation, minimal soil development and a high percentage of bare rock. The area was covered by ice in the last glacial period and therefore evidence for weathering on ice-moulded surfaces can be regarded as post-glacial.

The landscape appears to be dominated by frost shattering. However, closer examination of ice-scoured surfaces reveals evidence of granular disintegration and chemical weathering. Many rock surfaces display distinctive weathering rinds typically 5mm thick but up to 30mm thick in fractures. This indicates chemical weathering probably plays an active role in fracture enlargement and may contribute to macro-breakdown of intact rock (and ultimately to the development of blockstreams). Geochemical analysis of these rock weathering rinds will determine if they represent a comparable solutional loss of key mineral constituents (eg Si, Ca, Mg, K) as found by Dixon et al (2002) in comparable conditions.

Many ice-smoothed rock surfaces display a small scale weathering morphology typical of granular disintegration. This includes surface spalling, pitting and rounding of fracture edges. Discolouration and visible decomposition associated with these features suggests chemical weathering also contributes to their development. Further evidence for chemical weathering includes the presence of pseudokarren on granitic gneiss such as rounded solution pits up to 80cm in length and 10cm in depth.

Preliminary results from a study of ice polished quartz veins (using the method of Dahl, 1967) indicate a mean rate of post-glacial lowering of 0.8mm ka-1, ranging from 0.0 to 2.3mm ka-1 for different lithologies. These results demonstrate the influence of mineralogy and rock fabric on micro-weathering and are comparable with those obtained by André working on a similar range of rocks in the Abisko Riksgränsen region in north Norway (André 1995, 1996, 2002).

Further evidence for chemical breakdown is being sought via analysis of the solute content of surface runoff (streams, ponds, springs and late-lying snow). Analysis of micro-structural weathering-related features (micro-fractures, changes in porosity) is also being conducted using SEM and Schmidt hammer rebound is being used in the field to detect spatial variability in rock surface hardness.