The Weathering Environment

Abstract

The alteration of rocks at the earth’s surface is shown by the development of saprolites and soils. Rocks vary in composition and therefore in their susceptibility to alteration by weathering processes. Furthermore, the chemical environment in which rocks are altered varies with position on the earth’s surface and also with local conditions. The environments under which most atmospheric weathering takes place are oxidizing with an oxidation potential (Eh) of + 600 mV and a pH ranging from pH 4 to pH 10. Environments isolated from the atmosphere may have Eh ranging from + 400 mV to − 400 mV within the same pH range as above. Many chemical reactions between minerals and water take place in the contact of groundwater, water in rocks, waterlogged rocks or soils, and in organic-rich fresh and saline waters. Baas-Becking et al. (1960) compiled the results of many hundreds of published analyses of the pH and Eh values of soils, shallow groundwater, and fresh water to obtain the natural weathering environment as it actually occurs. Figure 2 shows this environment.