Effect of porosity on the increase in weathering-rind thicknesses of andesite gravel

Factors affecting the increase in weathering-rind thickness, i.e. weathering rates, were investigated using andesite gravel in fluvial terrace deposits with a known emergence time. The time between the age of each terrace formation (20, 130, 290 and 660 ka) and the present was assumed to be the weathering period. Three zones were classified from measurements of the Vickers hardness number (VHN) across weathering rinds from the rock surface to the inner fresh part: (1) Zone I, the outermost brown layer, which has low VHN values of 10 to 80 gf/μm2; (2) Zone II, the inner gray layer which has increasing VHN values (100 to 500 gf/μm2) with increasing depth; (3) the innermost fresh part, which has large VHN values of 500 gf/μm2. In this paper, Zones I and I+II are defined as the brown layer and the weathering rind, respectively. Diffusion theory was applied to the relationship between the thickness of the brown layer or the weathering rind and time (weathering periods). The results show that the thickness of the brown layer depends mainly on time, whereas the thickness of the weathering rind is controlled by the porosity of the host rock as well as time. Consequently, rocks with higher porosity have thicker weathering rinds than brown layers.