Weathering potential of wetting and drying of sandstone and basalt : a laboratory isolation study

Abstract

The cyclic wetting and drying of a rock is considered to be one of a number of physical weathering processes that have an effect on the weathering of rock. While the presence of moisture is known to be of critical importance for the activation and enhancement of a number of other known weathering processes, such as cryogenic weathering, salt weathering and slaking it is possible that the mere cyclic application and removal of moisture over time may also have an effect on the physical structure of a rock. The precise nature of the process is not well understood, however. This document begins by investigating the studies that have previously been undertaken to determine how the wetting and drying weathering process is defined and to ascertain the current state of knowledge regarding this process. After an establishment of background context, a physical experiment is carried out on Clarens Formation sandstone and Marion Island basalt to note the relationship between cyclic wetting and drying and the changing physical properties of the rocks. The rocks were subjected to 105 wetting and drying cycles over a period of 21 weeks. At the beginning of the experiment, physical rock properties were measured by way of the method laid out by Cooke (1979) and again at the end of the experiment. Since the experiment was carried out under static environmental conditions, the comparison of physical rock properties gives a good indication of how the rocks have altered their structure over the experimental time period. The results obtained in this experiment show that different rock types will change in different ways when exposed to a common weathering process. The basalt samples experienced no mass loss at, while the sandstones did. The sandstones, which are rocks that are of common lithology and of very similar physical and chemical structure reacted to the wetting and drying weathering process in ways that could not be predicted without experimentation. The data does show a clear causal link between the application of external stimuli and rock property change, however. Changes in physical rock properties are not always straightforward and linear, but evolve dynamically over time, often yielding results that appear to oppose those intuitively predicted. A number of questions are asked regarding the philosophical approach that is taken to process isolation studies, with emphasis given to the careful consideration of the place that such studies have in the realm of process geomorphology. While process isolation studies may give an excellent indication of what a particular weathering process may be capable of under certain conditions and on certain rock types, they should not be regarded as indicative of what is occurring in the field. Additionally, it has become clear that it is not possible to predict how a specific rock type may respond to a specific weathering process without physical experimentation since the number of variables present in a typical weathering system are simply too vast to easily categorise.