Rock weathering and ultrasonic pulse velocity : an analysis using South African cemeteries as time-series laboratories

Abstract

The study of rock weathering poses unique challenges due to the operation of weathering processes on small spatial and large temporal scales. Many, although not all, studies that focus on a specific weathering process tend to confine themselves to regional climates in which the processes are expected to function most efficiently; for example, thermal weathering studies tend to take place predominantly in arid or polar regions and many chemical weathering studies are in tropical regions. The aim of this study is to develop and assess an experimental measurement technique for the quantification of weathering in different regions. This technique, which makes use of state of the art ultrasonic technology, can potentially be used to quantify weathering rates in a manner that allows results to be compared for different environmental or climatic regions. Ultrasonic Pulse Velocity (UPV) measurements were recorded in six cemeteries at different locations across South Africa using a variety of field measurement protocols and apparatus configurations. The results show that rock weathering can be assessed in this way, but the weathering signal is extremely small. This method thus requires a high degree of measurement precision from the field operator when data are being gathered. Overall, the results show that climate does exert a general influence on the weathering regime of a specific region, but that the rock properties of any specific tombstone can cause weathering to occur very differently to what may be regarded as typical for a region. The specific weathering regime for a given area is, therefore, not determined by either climatic forcings or rock properties, but rather is the result of tension between these two elements, the nature of which will be unique to each regional environment.