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Subsidence

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.

There are three main types of subsidence, listed below in order of increasing scale:

  1. Subsidence caused by collapse into an underlying space.
  2. Subsidence caused by motion along geological faults.
  3. Subsidence caused by thermal contraction of the lithosphere.

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 accomodated 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.



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