Subsidence
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In geology, engineering, and surveying, subsidence is the motion of a surface (usually, the Earth's surface) as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation. In meteorology, subsidence refers to the downward movement of air.
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[edit] Surfaces
There are several types of subsidence, listed below in order of increasing scale:
[edit] Subsidence by collapse
This commonly occurs over man-made voids, such as tunnels, wells and covered quarries. It is also frequent in karst terrains, where dissolution of limestone by fluid flow in the subsurface causes the creation of voids (i.e. caves). If the roof of these voids becomes too weak, it can collapse and the overlying rock and earth will fall into the space, causing subsidence at the surface. This type of subsidence can result in sinkholes which can be many hundreds of meters deep and can provide areas of ecological isolation which see the evolution of new branches of animal and plant life.
[edit] Subsidence by faulting
When differential stresses exist in the Earth, these can be accommodated either by geological faulting in the brittle crust, or by ductile flow in the hotter and more fluid mantle. Where faults occur, absolute subsidence may occur in the footwall of normal faults. In reverse, or thrust, faults, relative subsidence may be measured in the hangingwall.
[edit] Subsidence by isostatic rebound
The crust floats buoyantly in the plastic asthenosphere, with a ratio of mass below the "surface" in proportion to its own density and the density of the asthenosphere. If mass is added to the crust (e.g. through deposition), the crust is thought to subside minisculely to compensate and maintain isostatic balance.
[edit] Subsidence caused by extraction of natural gas
If natural gas is extracted from a natural gas field the initial pressure (up to 600 bar) in the field will drop over the years. The gas pressure also supports the soil layers above the field. If the pressure drops, the soil pressure increases and this leads to subsidence at the ground level. Since exploration of the Slochteren (Netherlands) gas field started in the late 1960s the ground level over a 250 km2 area has dropped with a current maximum of 30 cm [1]. See also this subsidence lecture.
This type of subsidence can similarly be caused by extraction of other resources, e.g. ground water, petroleum or rock salt.
[edit] Subsidence induced by water table management
The habitation of lowlands, such as coastal or delta plains, requires drainage. The resulting aeration of the soil leads to the oxidation of its organic components, such as peat, and this decomposition process may cause significant land subsidence. This applies especially when ground water levels are periodically adapted to subsidence, in order to maintain desired unsaturated zone depths, exposing more and more peat to oxygen. In addition to this, drained soils compact as a result of pore-tension reduction (see compaction). In this way, land subsidence has the potential of becoming self-perpetuating, having rates up to 5 cm/yr. Water management used to be tuned primarily to factors such as crop optimisation but, to varying extents, avoiding subsidence has come to be taken into account as well.
[edit] Atmosphere
The most common cause of subsidence in the atmosphere is cold temperatures: as air cools, it becomes more dense and moves towards the ground, just as warm air becomes less dense and moves upwards. Subsidence generally causes high barometric pressure as more air moves into the same space: the polar highs are areas of almost constant subsidence, as are the horse latitudes, and these areas of subsidence are the sources of much of the world's prevailing wind. Subsidence also causes many smaller-scale weather phenomena, such as morning fog. An extreme form of subsidence is a downburst, which can result in damage similar to that produced by a tornado. A milder form of subsidence is referred to as downdraft.
