Lithostatic Pressure
Pressure from the weight of overlying rock.
Formula
Variables
Example
3 km of granite (2700 kg/m³) ⇒ ≈ 79 MPa
How it works
Lithostatic pressure is the pressure created by the weight of rock sitting above a point underground. The deeper you go, the more rock is piled on top, so the force per unit area increases roughly in direct proportion to depth. The formula P = ρgh comes from the hydrostatic pressure relationship, using rock density instead of water density, and it assumes the overlying rock acts like a uniform column. It gives an estimate of the normal stress from overburden, not the extra pressure caused by tectonics, fractures, or fluids.
When to use it
Use this when estimating the pressure at depth in the crust, such as in geology, geophysics, geothermal studies, or mining. It is also useful for comparing how overburden changes with depth in different rock types.
Worked example, step by step
Find the lithostatic pressure at 2.5 km depth in basalt with density 3000 kg/m³. P = ρgh P = (3000 kg/m³)(9.81 m/s²)(2500 m) P = 73,575,000 Pa P = 7.36 × 10^7 Pa P ≈ 73.6 MPa
Common mistakes to avoid
- Using metres instead of kilometres incorrectly in the depth conversion
- Forgetting that the answer in pascals can be converted to MPa by dividing by 1,000,000
- Treating lithostatic pressure as the same as fluid pore pressure in rocks
Did You Know?
At the base of the continental crust the pressure exceeds ten thousand times the atmosphere at sea level.
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