A sinkhole, also known as a sink, snake hole, swallow hole, swallet, or doline, is a natural depression or hole in the Earth's surface caused by karst processes—for example, the chemical dissolution of carbonate rocks or suffosion processes in sandstone. Sinkholes may vary in size from 1 to 600 metres (3.3 to 2,000 ft) both in diameter and depth, and vary in form from soil-lined bowls to bedrock-edged chasms. Sinkholes may be formed gradually or suddenly, and are found worldwide. The different terms for sinkholes are often used interchangeably.
Formation:-
Sinkholes may capture surface drainage from running or standing water, but may also form in high and dry places in a certain location.
The mechanisms of formation involve natural processes of erosion[4] or gradual removal of slightly soluble bedrock (such as limestone) by percolating water, the collapse of a cave roof, or a lowering of the water table. Sinkholes often form through the process of suffosion. Thus, for example, groundwater may dissolve the carbonate cement holding the sandstone particles together and then carry away the lax particles, gradually forming a void.
Occasionally a sinkhole may exhibit a visible opening into a cave below. In the case of exceptionally large sinkholes, such as the Minyé sinkhole in Papua New Guinea or Cedar Sink at Mammoth Cave National Park in Kentucky, a stream or river may be visible across its bottom flowing from one side to the other.
Sinkholes are common where the rock below the land surface is limestone or other carbonate rock, salt beds, or rocks that can naturally be dissolved by circulating ground water. As the rock dissolves, spaces and caverns develop underground. These sinkholes can be dramatic, because the surface land usually stays intact until there is not enough support. Then, a sudden collapse of the land surface can occur.
Sinkholes also form from human activity, such as the rare but still occasional collapse of abandoned mines in places like Louisiana. More commonly, sinkholes occur in urban areas due to water main breaks or sewer collapses when old pipes give way. They can also occur from the overpumping and extraction of groundwater and subsurface fluids. They can also form when natural water-drainage patterns are changed and new water-diversion systems are developed. Some sinkholes form when the land surface is changed, such as when industrial and runoff-storage ponds are created; the substantial weight of the new material can trigger an underground collapse of supporting material, thus causing a sinkhole.
Local names of sinkholes:-
1.Black holes – This term refers to a group of unique, round, water-filled pits in the Bahamas. These formations seem to be dissolved in carbonate mud from above, by the sea water. The dark color of the water is caused by a layer of phototropic microorganisms concentrated in a dense, purple colored layer at 15 to 20 metres depth; this layer "swallows" the light. Metabolism in the layer of microorganisms causes heating of the water, the only known case in the natural world where microorganisms create significant thermal effects. Most impressive is the Black Hole of Andros.[12]
2.Blue holes – This name was initially given to the deep underwater sinkholes of the Bahamas but is often used for any deep water-filled pits formed in carbonate rocks. The name originates from the deep blue color of water in these sinkholes, which in turn is created by the high lucidity of water and the large depth of sinkholes – only the deep blue color of the visible spectrum can penetrate such depth and return back after reflection. The deepest known undersea sinkhole is Dean's Blue Hole in the Bahamas.
3.Cenotes – This refers to the characteristic water-filled sinkholes in the Yucatán Peninsula, Belize and some other regions. Many cenotes have formed in limestone deposited in shallow seas created by the Chicxulub meteorite's impact.
4.Sótanos – This name is given to several giant pits in several states of Mexico. The best known is the 372 m deep Sótano de las Golondrinas (Cave of Swallows).
5.Tiankengs – These are extremely large sinkholes which are deeper and wider than 100 m, with mostly vertical walls, most often created by the collapse of underground caverns. This term is proposed by Chinese geologists as many of the largest sinkholes are located in China. The largest tiankeng is the 662 m deep Xiaozhai tiankeng, which is also the largest known sinkhole of the world.
6.Tomo – This term is used in New Zealand karst country to describe pot holes.
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