Thermal Weathering and Distribution of Mountain Rockwalls

Abstract

Thermal weathering assessment is essential in establishing present and past rock surfaces dynamics in mountain areas. Though for the Romanian Carpathians most studies rely on air temperature when estimating the intensity of freeze-thaw cycles, comparative analysis of in situ ground thermal measurements shows that important differences occur in the thermal behaviour of mountain sectors and landforms. Specifically, air temperature fails to account seasonal frost cycles and the duration of snow cover, which, in most situations leads to a misleading evaluation of freeze-thaw magnitude. Virtually snow-free surfaces such as steep rockwalls would thus be subject to intense manifestation of freeze-thaw weathering. Based on intensive rock temperature measurements performed above 1800 m elevations in the Southern Carpathians, we here describe the occurrence patterns of both diurnal and seasonal frost in different topographic conditions (exposure, altitude) as a first step in evaluating frost weathering susceptibility. Important differences are noticed between north and south-facing rock surfaces, as the first experiences deep continuous freezing throughout most of the cold season whereas the latter is subject to high day–night thermal amplitudes and up to 120 diurnal freeze-thaw cycles per year. This reflects into the characteristics of weathered rock fragments, the rate of weathering and into the resultant long-term configuration of the slope. In order to report these patterns to the present distribution of rockwalls in the Romanian Carpathians, an inventory was obtained comprising 788 rock surfaces mapped in the Eastern and Southern Carpathians, by the use of available satellite imagery. Both frequency and coverage of rock surfaces highlight their development mainly on the northern slopes of the Southern Carpathians (above 2000 m) in the detriment of the southern ones (5:1 ratio in terms of total area). This could, in part, reflect the differences of frost propagation intensity (mainly expressed in the size of the removed particles and of the resulting debris) and implicitly of freeze-thaw weathering magnitude, but should also integrate the control of lithology and structural imprint on a local scale.