Weathering and Erosion in the Humid Tropics

Abstract

In the humid tropics, the primary factor which distinguishes different weathering regimes is tectonic setting, especially the contrast between tectonically active areas and stable cratonic regions. In tectonically active environments, most of the material that is exposed to weathering has undergone rapid uplift involving brittle deformation, and volcanism is sometimes important. Uplift rates are on the order of 103–104 mm/ka, while erosion rates are about the same to an order of magnitude less. As a result, easily weathered lithologies are exposed on steep slopes and weathering rates are lithology dependent. Where carbonates and evaporites are present, their weathering products dominate the river solution chemistry. Similarly, weakly cemented clastic rocks dominate solid load inputs. Aluminosilicate rocks do not weather completely to phases depleted in the major cations (Na, K, Mg, Ca); instead, some unweathered primary minerals remain, and various secondary cation-rich clays frequently form. On the cratons, sea level changes exert a major control on long term weathering rates and landscape development. When sea level is high, sediment is deposited on the passive margins, continental platforms, and intracratonic basins. At the same time near-sea-level planation surfaces develop on shield areas. As sea level drops erosion is not regionally uniform, but is localized on scarps which advance into the older erosion surfaces. For South American shields average uplift rates approach 10–20 mm/ka, and it seems reasonable that their long-term average erosion rates are close to this value. Weathering on the craton is much more complete, and the solid weathering products are cation depleted: quartz, kaolinite, and sesquioxides. After cyclic salt corrections, major cations in rivers are in bedrock proportions.