4.6 Weathering Rinds: Formation Processes and Weathering Rates

Abstract

Models of weathering rind development have been discussed by many researchers. To understand this phenomenon, considering suitable rock properties is important because weathering rates differ from rock property types. This chapter briefly reviews the modeling types such as logarithmic, power, and relaxation function of previous researches, and then, describes rock property changes by giving an example of an andesite rock type. The properties are major 10 elements, L⁎, a⁎, and b⁎ chromatic values, bulk density, and porosity determined by mercury porosimetry, and Vickers microhardness. This chapter also considers a formation process model of weathering rind development and a growth model of weathering rind thickness. A conceptual model of weathering-rind formation suggests that the inner white band is produced by dissolution of alkali/alkaline earth metals related to the inward and subsequent outward movement of water. The brown band is probably formed by both leaching of these metals and oxidation of irons. A porosity concerned growth model, consisting of two diffusion equations based on the two bands, was proposed. One is a normal diffusion equation showing the development of strongly weathered brown bands due to both oxidation and leaching. The other is an equation with the diffusion coefficient exponentially related to porosity of the host rock showing the growth of total weathering rinds mainly due to leaching. The important properties to consider weathering-rind developments are contents of iron and alkali/alkaline earth metals as well as porosity of rocks. Environmental conditions are also important factors to determine the degree of oxidation or dissolution of sub-bands of weathering-rinds, although these studies are expected in the near future.