A porosity-related diffusion model of weathering-rind development

Abstract

A model of weathering-rind growth is proposed based on the study of andesite cobbles sampled from a modern floodplain and four fluvial terraces (0, 20, 130, 290 and 660 ka) in central Japan. The times elapsed since each of the above ages can be assumed to be the weathering periods. The 0-ka rocks are not visibly weathered. The 20-ka rocks have an alteration layer with white or brown colours and a mean thickness of 0.7 mm on their surface. The 130-, 290- and 660-ka rocks have brown weathering rinds with average thicknesses of 3–6 mm. Based on the measurements of physical and mechanical properties throughout from the rock surface to the interior, weathering rinds are divided into two bands. The outer brown band is characterized by ferric hydroxides, which experienced both oxidization and leaching. The inner band is formed by leaching alone of alkalis/alkaline earths. The inner border of this band is located at the position where Vickers' hardness numbers start decreasing with decreasing depth. The thickness of each band was determined from its physical and mechanical properties. The results show that brown band thickness ( L I) is a function of time. However, total thickness of the two bands, i.e. total weathering-rind thickness ( L I+II), is also controlled by initial rock porosity ( n). Their relationships are represented by diffusive equations: L I=( Dt) 1/2=(0.0283 t) 1/2 and L I+II=( Dt) 1/2={0.0431 exp(0.4287 n)· t} 1/2. These findings indicate that rind-characterization and original rock porosity should be taken into consideration when weathering-rind thickness is used as a dating tool.