Granite Weathering

Weathering is a necessary precursor for landform development. However, in the context of granite it acquires a particular importance for various reasons. First, many granite terrains show an extensive development of deep weathering profiles, which can be extremely varied in terms of their depth, vertical zonation, degree of rock decomposition, and mineralogical and chemical change. Moreover, the transitional zone between the weathering mantle and the solid rock, for which the term ‘weathering front’ is used (Mabbutt, 1961b), may be very thin. There is now sufficient evidence that many geomorphic features of granite landscapes, including boulders, domes, and plains, have been sculpted at the solid rock/weathering mantle interface and they are essentially elements of an exposed weathering front. Therefore, the origin of granite landscapes cannot be satisfactorily explained and understood without a proper understanding of the phenomenon of deep weathering. Second, granites break down via a range of weathering mechanisms, both physical and chemical, which interact to produce an extreme diversity of small-scale surface features and minor landforms. In this respect, it is only limestones and some sandstones which show a similar wealth of weathering-related surface phenomena. Third, both superficial and deep weathering of granite act very selectively, exploiting a variety of structural and textural features, including fractures, microfractures, veins, enclaves, and textural inhomogeneities. In effect, the patterns of rock breakdown may differ very much between adjacent localities, and so the resultant landforms differ. In the context of deep weathering, selectivity is evident in significant changes of profile thickness and its properties over short distances, and in the presence of unweathered compartments (corestones) within an altered rock mass. Fourth, it is emphasized that granites are particularly sensitive to the amount of moisture in the environment (Bremer, 1971; Twidale, 1982). They alter very fast in moist environments, whereas moisture deficit enhances rock resistance and makes it very durable. Hence, a bare rock slope shedding rainwater and drying up quickly after rain will be very much immune to weathering, whereas at its foot a surplus of moisture will accelerate decomposition.