Lithostatic

Lithostatic pressure, also called overburden pressure, is the confining pressure exerted by the weight of overlying rock on a unit area at depth. It is transmitted through the solid rock and its pore spaces and is essentially isotropic, exerting the same force in all directions. This distinguishes lithostatic pressure from directional stresses caused by tectonic forces.

In rocks, lithostatic pressure increases with depth as P = ∫ ρ(z) g dz, often approximated as P ≈ ρ g h, where ρ is density, g is gravitational acceleration, and h is burial depth. In the crust, typical densities are about 2.7–3.0 g/cm³, which yields a lithostatic gradient of roughly 27–30 MPa per kilometer. By comparison, hydrostatic pressure, arising from pore fluids with densities around 1.0 g/cm³, increases at about 9–10 MPa per kilometer.

Lithostatic pressure is a key factor in rock mechanics, influencing fracture closure, porosity preservation, and diagenetic processes. It represents the normal stress that resists deformation, while additional tectonic or thermal processes introduce deviatoric stresses that drive shear and folding. In geology and petroleum engineering, lithostatic (or overburden) pressure is used to estimate effective stress and to assess rock stability, pore collapse, and fluid flow under buried conditions. Variations in density with depth, porosity, and fluid pressures can modify the effective stress state, occasionally leading to overpressure when pore pressures approach lithostatic values.