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Overburden, or lithostatic pressure, is a term used in geology to denote the pressure imposed on a stratigraphic layer by the weight of overlying layers of material. If a given layer is in hydrostatic equilibrium, the lithostatic pressure at a depth z is given by ho(z) , dz where ρ(z) is the density of the overlying rock in (kg.m-3) at depth z, g is the acceleration due to gravity (m.s-2) and z is the depth in metres. p0 is the datum pressure, e.g. pressure at the surface. It is common for parts of stratigraphic layers to be isolated, or sealed, such that they are no longer in hydrostatic equilibrium. In such cases, the layer, or part of layer, is said to be in a condition of overpressure (if local pressure is greater than hydrostatic) or underpressure (if local pressure is less than hydrostatic). See also: sedimentary rock
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