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
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).
The pressure experienced by a rock during metamorphism is due primarily to the weight of the overlying rocks (i.e.,lithostatic pressure) and is generally reported in units of bars or kilobars.
The pressure exerted on a floor by a 42-pound box the bottom of which has an area of 84 square inches is equal to the force divided by the area over which it is exerted; i.e., it is one-half pound per square inch.
pressure exerted by a vapor when the vapor is in equilibrium with the liquid or solid form, or both, of the same substance; increases with increasing temperature; the temperature at which the vapor pressure at the surface of a liquid equals the pressure exerted by the surroundings is called the boiling point of the liquid.
The lithostaticpressure is derived by numerical integration of the density log.
The reduced lithostaticpressure is defined as the lithostaticpressure minus the hydrostatic pressure.
Below the seal, the fluid overpressure is observed to be close to the reduced lithostaticpressure, which indicates the presence of natural fracturing in the transition zone.
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