Role of porosity in rock weathering processes: a theoretical approach

Abstract

Under surface and subsurface conditions, porosity is a major factor in rock weathering, for it controls not only the movement of the fluids throughout the rock mass but also the processes at work. Depending on the pore dimensions and pattern, mechanisms such as hydroxylation, diffusion or flow play their role in the weathering process. Based mainly on theoretical arguments, three examples of silicate parent rocks replaced by carbonate, oxyhydroxides and silica are analysed, in terms of water activity, pF, specific surface, and molar volume. In central Spain, Ca and Mg carbonates replace silicates and quartz in both calcretes and sandstones. Where the two cations are present, Mg mainly is associated with small pore spaces whereas Ca is dominant within larger pores, and there is a transition between the two extremes. The replacement of the quartz grains in gravels and pebbles of quartzite by Fe oxyhydroxides is a typical feature of the old Rañas surfaces of central and western Spain. This replacement is the result of the alternating wet and dry seasons combined with poor drainage conditions that allow the xerolysis to be active over long periods. In western Spain, remnants of a siderolithic sedimentary cover preserve a deeply weathered Hercynian basement. An upward enrichment in CT opal and in kaolin affects to both the weathered basement and the siderolithic cover. Recent studies show that these traits were caused by the weathering of an old land surface that was disrupted by Alpine tectonism. Weathering occurred under acid conditions which led to the release of most of the elements, Fe and Al included, but with the most of silica remaining as opal.