subsidence

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subsidence lowering of a portion of the earth's crust. The subsidence of land areas over time has resulted in submergence by shallow seas (see oceans ). Land subsidence can occur naturally or through human activity. Natural subsidence may occur when limestone, which is easily carved by underground water, collapses, leaving sink holes on the surface, such as in Florida. Earthquakes can also cause subsidence of the land because of the movement of faults . Permafrost, or the permanently frozen ground in tundra regions, can subside during local warming trends, a phenomena called thermokarst. Oceanic crust produced at spreading ridges (see seafloor spreading ) subsides after cooling, as do calderas, the craterlike features at a volcano's peak. An atoll is a coral reef that forms a ring with no apparent central peak and may form when volcanic islands subside—an explanation first proposed by Charles Darwin. Human activity has contributed greatly to subsidence over the last few centuries. For example, withdrawal of oil from the field at Long Beach, California, beginning in 1936 resulted in subsidence at rates ranging from 0.5 to 2.0 ft (0.15–0.61 m) per yr in the center of the field. By 1962 the center of the oil field had subsided slightly over 27 ft (8.5 m), caused by the removal of fluid from the pore spaces in the underground rock, allowing the grains to compact. Similarly, withdrawal of groundwater through well pumping has resulted in subsidence in such cities as Mexico City, Houston, Tex., and Venice, Italy. Subsidence is also caused by the collapse of underground salt, ore, and coal mines.

subsidence
1. A progressive depression of the Earth's crust, which allows sediment to accumulate and be preserved. Subsidence is caused by mantle convection and by sediment loading. The subsidence rate will control the proportion of deposited sediment which will be preserved in the subsiding area. Subsidence rates in sedimentary basins typically vary from 0.3 to 2.5 mm per year.

2. Sinking or settling of the ground surface due to natural or anthropogenic causes. Surface material with no free side is displaced vertically downwards with little or no horizontal movement.

3. Local sinking, due to underground mine workings.

4. Downward movement of air, characteristically gentle (1–10 cm/s) and often in large anticyclones, which is related to the divergence in lower layers near the ground surface. Subsidence results from radiation cooling or from convergence of air horizontally in the upper troposphere. It typically brings settled weather, with evaporation of cloud drops by adiabatic warming in the subsiding air mass, causing cloudless skies above the friction layer near the surface, so that in winter fog and low cloud may prevail when moisture is sufficient.

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