The normal stress dependence of rock friction

Rock friction governs lithospheric strength and earthquake mechanics, yet its fundamental characteristics remain poorly understood. A survey of experimental data on the frictional resistance of bare contacts or powdered gouge from 41 published studies encompassing 119 rocks and synthetic materials reveals a power-law dependence between sliding friction and effective normal stress from 10 Pa to 1 GPa. For framework silicates, ice, and most synthetic materials considered, the friction coefficient is a decreasing function of effective normal stress, in contrast with water-saturated phyllosilicates. The normal stress dependence of the friction coefficient is controlled by the real area of contact, possibly modulated by osmotic pressure near contact junctions for phyllosilicates. Cohesion is negligible at macroscopic scales for fractures with rough contacts. These experimental findings challenge the conventional representation of rock friction as a linear function of normal stress. These results provide critical constraints for advancing physical models of rock friction and fault mechanics.