In rheology, Byerlee's law, also known as Byerlee's friction law[1] concerns the shear stress (τ) required to slide one rock over another. The rocks have macroscopically flat surfaces, but the surfaces have small asperities that make them "rough." For a given experiment and at normal stresses (σn) below about 2000 bars (200 MPa) the shear stress increases approximately linearly with the normal stress (τ = 0.85 σn, where τ and σn is in units of MPa) and is highly dependent on rock type and the character (roughness) of the surfaces, see Mohr-Coulomb friction law. Byerlee's law states that with increased normal stress the required shear stress continues to increase, but the rate of increase decreases (τ = 50 + 0.6σn), where τ and σn are in units of MPa, and becomes nearly independent of rock type.[2]
The law describes an important property of crustal rock, and can be used to determine when slip along a geological fault takes place.