Subsidence
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Subsidence is a term used in geology, engineering and surveying to denote the motion of a surface (usually, the earth's surface) downwards relative to a datum such as sea-level. In meteorology, subsidence refers to the movement of air downwards.
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Surfaces
There are several types of subsidence, listed below in order of increasing scale:
- Subsidence caused by collapse into an underlying space.
- Subsidence caused by motion along geological faults.
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 metres deep and can provide areas of ecological isolation which see the evolution of new branches of animal and plant.
Subsidence by faulting
When differential stresses exist in the Earth, these can 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.
Subsidence by thermal contraction of the lithosphere
When the lithosphere is stretched, perhaps due to slab-pull, the lithosphere is thinned and hot asthenosphere rises into the space that is created. This causes heating of the overlying crust and mantle and thermal expansion of these materials. Over time, heat is lost through radiation from the earth surface and the thermal gradient relaxes. As the temperatures fall, the lithosphere will contract, often causing subsidence at the surface.
On the scale of the lithosphere (i.e. ~100 km), the effects of isostasy must be considered, as the hot asthenosphere tends to act like a fluid over geological time.
Subsidence caused by extraction of natural gas
If natural gas is extracted from a natural gas field the 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] (http://www.waddenzee.nl/asp/getecomare.asp?hrec=4836528&srcdoc=ecomare.asp&language=english). See also this subsidence lecture (http://www.uwsp.edu/geo/faculty/ozsvath/lectures/Deep%20Subsidence.htm).
This type of subsidence can similarly be caused by extraction of other resources, e.g. ground water, mineral oil or rock salt.
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 air 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.nl:bodemdaling