A simple model for regolith formation by chemical weathering

We present here a new model for the formation of regolith on geological timescales by chemical weathering based on the assumption that the rate of chemical weathering is primarily controlled by the ability of groundwater to transport solute away from the reacting solid-fluid interface and keep the system from reaching equilibrium (saturation). This allows us to specify the rate of propagation of the weathering front as linearly proportional to the pore fluid velocity which we obtain by computing the water table geometry in the regolith layer. The surface of the regolith layer is affected by mass transport and erosion. The main prediction of the model is that the geometry of the regolith, i.e., whether it is thickest beneath topographic highs or topographic lows, is controlled by the value of a dimensionless number, which depends on the square of the surface slope, the hydraulic conductivity, and local precipitation rate, but is independent of the chemical weathering rate. In orogenic environments, where regolith formation by chemical weathering competes with surface erosion, the model predicts that the existence and thickness of the regolith layer are controlled by the value of another dimensionless number which is the ratio between the timescale for erosion and the timescale for weathering. The model also predicts that in anorogenic areas, regolith thickness increases as the square root of time, whereas in orogenic environments, a steady state regolith thickness can be achieved, when the propagation of the weathering front is equal to erosion rate.