Rock weathering on the eastern mountains of southern Africa: Review and insights from case studies

Abstract

a b s t r a c tThe mountains in the eastern region of southern Africa are of significant regional importance, providing for a diverse range of land use including conservation, tourism and subsistence agriculture. The higher regions are comprised of flood basalts and are immediately underlain by predominantly aeolian-origin sandstones. Our understanding of the weathering of these basalts and sandstones is reviewed here, with particular focus on the insights gained from the Lesotho Highlands Water Project and an ongoing study into the deterioration of rock art. While the chemical weathering attributes of the basalts have been substantially investigated, it is evident that the environmental surface conditions of rock moisture and temperature, as affecting weathering processes, remain largely unknown. Within the sandstones, studies pertaining to rock art deterioration present insights into the potential surface weathering processes and highlight the need for detailed field monitoring. Outside of these site-specific studies, however, little is understood of how weathering impacts on landscape development; notably absent, are detail on weathering rates, and potential effects of biological weathering. Some palaeoenvironmental inferences have also been made from weathering products, both within the basalts and the sandstones, but aspects of these remain controversial and further detailed research can still be undertaken.

A multi-technique characterization and provenance study of the pigments used in San rock art, South Africa

“…This crystallization will then dissociate the pigment from the rock surface and so break up the painting, causing holes to appear in the paint layer. It is wellrecorded that gypsum is a chemical alteration product of Clarens Formation sandstone and, because of weathering stresses, the size of the gypsum crystals changes over time, which may lead to the break-up the pigment layer (Sumner et al 2009). Here we point to a further deterioration mechanism of San art already seriously damaged because of rock degradation.…”

Dehydration-hydration reactivity of laumontite: analyses and tests for easy detection

“…4, Table 2). The cores studied displayed a greater laumontite content than the basaltic sequence from Lesotho (Summer et al, 2009), where laumontite reached only 11 wt.% (based on XRD analysis) but also generated rock detachments in a water-transfer tunnel.…”

“…Of particular relevance to the present study are the reports on laumontite-filled fractures in rocks from the Gotthard railroad tunnel (Bucher et al, 2006) and from altered basalt of the water-transfer tunnel of the Lesotho Highland Water Project (Summer et al, 2009). For the latter site, Bell & Haskins (1997) reported difficulty in predicting the expansion pressure of the laumontite-bearing basalt with maximum laumontite contents of 11% over 60 days of ethyleneglycol exposure tests.…”

The role of rock surface hardness and internal moisture in tafoni development in sandstone

“…As Sumner et al (2009) describe, the Clarens sandstone is the uppermost section of the Karoo Sequence deposited between 200 and 160 MA ago. This medium-to fine-grained sandstone was deposited when aridity reached a maximum (Visser, 1984).…”