Wietrzenie mrozowe wybranych skał tatrzańskich w świetle badań laboratoryjny = Frost weathering of selected Tatra rocks in the light of laboratory tests

Abstract

The aim of the work described here was to determine the rate of frost weathering for selected types of rock, and the manner in which this proceeds. The authors attempts to answer questions regarding progress with the disintegration of a given type of rock over time; the size and shape of weathered grains; the role played by fissures in rock and rock texture; and further relevant properties like compressive and tensile strength, porosity and water absorption. The rock samples used in laboratory testing were collected in the catchment area of the Chochołowski Stream in the Western Tatra Mountains of Poland. The seven types of rock analysed were white and brown granite, organodetric limestone, fine-grained conglomerate, dolomite breccia, quartzite sandstone and amphibolite. Samples were subjected to simulated frost weathering via the impact of repeated thermal cycles across a temperature range of -5 to +10°C. The simulation was carried out at the Low Temperatures Laboratory of the Institute of Geography and Spatial Management of Kraków’s Jagiellonian University, using a CI/1400/LT/2D cooling device. As testing was in progress, changes in the states of samples were determined through the measurement of dry and saturated mass and water absorption, as well as the speed at which an ultrasonic wave passed through. By reference to results for these measures, it was possible to calculate the frost weathering index after Matsuoke, i.e.:Rf= (Vp0-Vpk)/(Vp0*k) [cykle -1],where Vp0 is wave speed at cycle 0 (in km/s), Vpk is wave speed at cycle k (km/s) and k is the number of cycles. Rock dissolution tests and measurements of the products of weathering were also carried out. Values obtained for the index were used to rank the rocks tested for their resistance to frost weathering, as was the percentage of material in the initial mass that became subject to rock weathering. The least-resistant rock proved to be dolomite breccia, and the most-resistant amphibolites and quartzite sandstones. The rankings of other rocks varied in line with the indicative parameter referred to. The testing of physical properties suggested several reasons for high resistance to frost weathering among the analysed types of rock from the Western Tatras, i.e. the limited (<5%) open porosity noted for all types, limited water absorption, high compressive and tensile strength, compactness and homogeneity, low densities of fissures in samples in their initial state, almost complete filling of pores with matrix (e.g. in sandstone and conglomerate), re-filling of cracks (e.g. in limestone), and a significant component of resistant quartz. No effect of rock texture on resistance to frost weathering could be observed, but the presence of carbon matrix and carbonate rock fragments is important. The latter dissolve steadily, creating more favourable conditions for physical weathering (e.g. of amphibolites and conglomerates). The occurrence of mineral veins within rocks determines disintegration routes (e.g. in conglomerates). Given that rocks were subjected to an average of 850 repeated thermal cycles in the laboratory, the simulation achieved was of approx. 50 years of frost weathering under natural conditions in the study area. Experiments of this kind thus offer insight into processes running very slowly under natural conditions.