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

摘要

Factors affecting the increase in weathering-rindthickness, i.e. weathering rates, were investigated using andesitegravel in fluvial terrace deposits with a known emergence time.The time between the age of each terrace formation (20, 130, 290and 660 ka) and the present was assumed to be the weatheringperiod. Three zones were classified from measurements of theVickers hardness number (VHN) across weathering rinds from therock surface to the inner fresh part: (1) Zone I, the outermostbrown layer, which has low VHN values of 10 to 80 gf/?m2 ; (2)Zone II, the inner gray layer which has increasing VHN values(100 to 500 gf/?m2) with increasing depth; (3) the innermost freshpart, which has large VHN values of 500 gf/?m2. In this paper,Zones I and I+II are defined as the brown layer and the weatheringrind, respectively. Diffusion theory was applied to the relationshipbetween the thickness of the brown layer or the weathering rindand time (weathering periods). The results show that the thicknessof the brown layer depends mainly on time, whereas the thicknessof the weathering rind is controlled by the porosity of the host rockas well as time. Consequently, rocks with higher porosity havethicker weathering rinds than brown layers.