Differential weathering and erosion in an inselberg landscape in southern Zimbabwe: A morphometric study and some notes on factors influencing the long-term development of inselbergs

摘要

A morphometric analysis of inselberg systems in southern Zimbabwe indicated that there are two types of inselberg systems that differ in the mode of development. Inselberg systems characterized by backwearing tend to occur on granitoid rocks that are foliated, highly fractured, and prone to grusification. The inselbergs are mostly of the koppie-type and are covered to a great extent with regolith or debris. The second type of inselbergs consists of sparsely fractured granitoid rocks, and areas underlain by these rocks are characterized by clusters of closely spaced rock domes. In the case of these inselbergs, backwearing is not indicated. They appear to be the result of the exposure of structurally pre-designed, highly resistant rock compartments. The hypothesis that differences in lithology and structure may influence the rate and style of inselberg development was tested by means of a simulation model. The simulations are run under the same boundary conditions with differing assumptions concerning role of divergent weathering. The results show that both modes of inselberg development may occur alongside one another as long as a critical relief is maintained. The modelling provided also indicates that, although the inselberg systems must have integrated several sequences of environmental changes during their development, these changes were not able to divert the general trend of the process-response system away from its steady course. Lithological and structural factors, which are responsible for the differences in the mobility of material on the inselberg sideslopes, are thus considered to be the primary source of influence of the different modes of inselberg development. It is suggested that the compositional and structural diversity in granitoid rock terrains is what enables inselbergs to pass through different development modes and sequences as rocks of differing resistance become subject to weathering during the development of the landscape.