A sink hole in Oman

The Devil's Hole sinkhole near Hawthorne, Florida, USA.

A sinkhole, also known as a sink, snake hole, swallow hole, swallet, doline, or cenote, is a natural depression or hole in the Earth's surface caused by karst processes — the chemical dissolution of carbonate rocks^[1] or suffosion processes^[2] for example in sandstone. Sinkholes may vary in size from 1 to 600 meters (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.^[3]

Formation mechanisms[link]

Sinkholes near the Dead Sea, formed by dissolution of underground salt by incoming freshwater, as a result of a continuing sea level drop.

A special type of sinkhole – formed by rainwater leaking through the pavement and carrying dirt into a ruptured sewer pipe.

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 Minyé sinkhole in Papua New Guinea or Cedar Sink at Mammoth Cave National Park, USA, 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 West Virginia, USA. 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.

Occurrence[link]

Sinkholes are usually but not always linked with karst landscapes. In such regions, there may be hundreds or even thousands of sinkholes in a small area so that the surface as seen from the air looks pock-marked, and there are no surface streams because all drainage occurs sub-surface. Examples of karst landscapes dotted with numerous enormous sinkholes are Khammouan Mountains (Laos) and Mamo Plateau (Papua New Guinea).^[5] The largest known sinkholes formed in sandstone are Sima Humboldt and Sima Martel in Venezuela.^[5]

The most impressive sinkholes form in thick layers of homogenous limestone. Their formation is facilitated by high groundwater flow, often caused by high rainfall – such high rainfall causes formation of the giant sinkholes in Nakanaï Mountains, New Britain island in Papua New Guinea.^[6] On the contact of limestone and insoluble rock below it there form powerful underground rivers which may create large underground voids.

In such conditions have formed the largest known sinkholes of the world, like the 662-metre (2,172 ft) deep Xiaozhai tiankeng (Chongqing, China), giant sótanos in Querétaro and San Luis Potosí states in Mexico and others.^[5][7][8]

Unusual processes have formed the enormous sinkholes of Sistema Zacatón in Tamaulipas (Mexico) – here more than 20 sinkholes and other karst formations have been shaped by volcanically heated, acidic groundwater.^[9][10] This has secured not only the formation of the deepest water-filled sinkhole in the world – Zacatón, but also unique processes of travertine sedimentation in upper parts of sinkholes, leading to sealing of these sinkholes with travertine lids.^{[citation needed]}

The state of Florida in the USA is known for having frequent sinkholes, especially in the central part of the state. The Murge area in southern Italy also has numerous sinkholes. Sinkholes can be formed in retention ponds from large amounts of rain.^{[citation needed]}

The Great Blue Hole, located near Ambergris Caye, Belize.

Sinkholes have been used for centuries as disposal sites for various forms of waste. A consequence of this is the pollution of groundwater resources, with serious health implications in such areas. In contrast, the Maya civilization sometimes used sinkholes in the Yucatán Peninsula (known as cenotes) as places to deposit precious items and sacrifices.^{[citation needed]}

Many sinkholes are found in Northern Michigan. These are prominent in Alpena County in Northeast Michigan. In Lachine, Michigan there are five sinkholes that are found to be very deep and within 2 miles (3.2 km) from each other. Alpena's visitor information cites their sinkholes as an attraction for visitors to the area. In August 1998 a 16 year old Alpena boy survived a 200 feet (61 m)+ fall in an open sinkhole .75 miles (1.2 km) from Leer road in Lachine, Michigan.^[11] A majority of sinkholes in Alpena are also found underwater. Many divers explore these on a regular basis.^{[citation needed]}

When sinkholes are very deep or connected to caves, they may offer challenges for experienced cavers or, when water-filled, divers. Some of the most spectacular are the Zacatón cenote in Mexico (the world's deepest water-filled sinkhole), the Boesmansgat sinkhole in South Africa, Sarisariñama tepuy in Venezuela, and in the town of Mount Gambier, South Australia. Sinkholes that form in coral reefs and islands that collapse to enormous depths are known as Blue Holes, and often become popular diving spots.^{[citation needed]}

Image of the entire surface water flow of the Alapaha River near Jennings, Florida going into a sinkhole leading to the Floridan Aquifer groundwater.

The overburden sediments that cover buried cavities in the aquifer systems are delicately balanced by groundwater fluid pressure. The water below ground is actually helping to keep the surface soil in place. Groundwater pumping for urban water supply and for irrigation can produce new sinkholes in sinkhole-prone areas. If pumping results in a lowering of groundwater levels, then underground structural failure, and thus sinkholes, can occur.^{[citation needed]}

Local names of sinkholes[link]

Large and visually unusual sinkholes have been well known to local people since ancient times. Also nowadays sinkholes are grouped and named in site specific or generic names. Some examples of such names are:^[12]

• cenotes. Characteristic water filled sinkholes in Yucatán Peninsula, Belize and some other regions. Many cenotes have formed in limestone which deposited in shallow seas created by Chicxulub meteorite impact.
• 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.
• 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.
• blue holes. This name initially has been given to the deep underwater sinkholes of Bahamas but often is used for any deep water-filled pits formed in carbonate rocks. 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 after reflection return back. The deepest known undersea sinkhole is Dean's Blue Hole in Bahamas.
• black holes. Group of unique round, water filled pits in Bahamas. These formations seem to be dissolved in carbonate mud from above, by the sea water. Dark color of water is caused by the layer of phototropic microorganisms concentrated in dense, purple colored layer in 15 – 20 metre depth – this layer "swallows" the light. Metabolism in the layer of microorganisms causes heating of water – the only known case in natural world where microorganisms create significant thermal effects. Most impressive is Black Hole of Andros.^[13]
• tomo used in New Zealand karst country to describe pot holes.

Piping pseudokarst[link]

What has been called a "sinkhole" by the popular press formed suddenly in Guatemala in May 2010. Torrential rains from Tropical Storm Agatha and a bad drainage system were blamed for creating the 2010 "sinkhole" that swallowed a three story building and a house.^[14] This large vertical hole measured approximately 66 feet (20 m) wide and 100 feet (30 m) deep. A similar hole had formed nearby in February 2007.^[15][16]

This large vertical hole, called a "sinkhole" in the popular press, is not a true sinkhole as it did not form via the dissolution either of limestone, dolomite, marble, or any other carbonate rock.^[17][18] Guatemala City is not underlain by any carbonate rock; instead, thick deposits of volcanic ash, unwelded ash flow tuffs, and other pyroclastic debris underlie all of Guatemala City. Thus, it is impossible for the dissolution of carbonate rock to have formed the large vertical holes that swallowed up parts of Guatemala City in 2007 and 2010.^[17]

The large holes that swallowed up parts of Guatemala City in 2007 and 2010 are a spectacular example of "piping pseudokarst", created by the collapse of large cavities that had developed in the weak, crumbly Quaternary volcanic deposits underlying the city. Although weak and crumbly, these volcanic deposits have enough cohesion to allow them to stand in vertical faces and develop large subterranean voids within them. A process called "soil piping" first created large underground voids as water from leaking water mains flowed through these volcanic deposits and washed fine volcanic materials out of them, then progressively eroded and removed coarser materials. Eventually, these underground voids became large enough that their roofs collapsed to create large holes.^[17]

Notable sinkholes[link]

Some of the largest and most impressive sinkholes in the world are:^[5]

• Xiaozhai tiankeng – Chongqing Municipality, China. Double nested sinkhole with vertical walls, 662 m deep.
• Dashiwei tiankeng – Guangxi, China. 613 m deep, with vertical walls, bottom contains isolated patch of forest with rare species.
• Red Lake – Croatia. Approximately 530 m deep pit with nearly vertical walls, contains approximately 280 – 290 m deep lake.
• Minyé sinkhole – East New Britain, Papua New Guinea. 510 m deep, with vertical walls, crossed by powerful stream.
• Sótano del Barro – Querétaro, Mexico. 410 m deep, with nearly vertical walls.
• Cave of Swallows – San Luis Potosí, Mexico. 372 m deep, round sinkhole with overhanging walls.
• Sima de las Cotorras - Chiapas, Mexico. 160 m across, 140 m deep, with thousands of green parakeets and ancient rock paintings.
• Zacatón – Tamaulipas, Mexico. Deepest water-filled sinkhole in world, 339 m deep. Floating travertine islands.
• Sima Humboldt – Venezuela. Largest sinkhole in sandstone, 314 m deep, with vertical walls. Unique, isolated forest on bottom.
• Teiq sinkhole – Oman. One of the largest sinkholes in the world by volume – 90 million cubic metres. Several perennial wadis fall with spectacular waterfalls into this 250 m deep sinkhole.
• Dean's Blue Hole – Bahamas. Deepest known sinkhole under the sea, depth 203 m. Popular location for world championships of free diving.
• Blue Hole – Dahab, Egypt. A round sinkhole or blue hole, 130m deep. Includes an extraordinary archway leading out to the Red Sea at 60m, renowned for freediving and scuba attempts, the latter often fatal. Also see Bell's to Blue Hole drift dive.
• Great Blue Hole – Belize. Spectacular, round sinkhole, 124 m deep. Unusual features are tilted stalactites in great depth, which mark the former orientation of limestone layers when this sinkhole was above the water level.
• Kingsley Lake – Florida, USA. 2,000 acres (8.1 km²) in area, 90 ft (27 m) deep and almost perfectly round.
• Gypsum Sinkhole – Utah, USA. Nearly 50 ft (15 m) in diameter and approximately 200 ft (61 m) deep.
• Harwood Hole - Abel Tasman National Park, New Zealand, 183 m deep
• Bimmah Sinkhole - Wadi Shab and Wadi Tiwi, Oman, 80 m deep

See also[link]

References[link]

External links[link]

Wikimedia Commons has media related to: Sinkholes

• Daily Telegraph slide show of 31 sinkholes
• Florida Sinkhole Guide
• Kapsia Cave: Exploration of a Sinkhole in Arkadia, Greece
• Three people died and a dozen homes were swallowed up Friday by this sinkhole in Guatemala City. ( February 24, 2007)
• Video of Sinkhole forming in Texas (May 8, 2008)
• Spectacular sinkhole appeared in Guatemala City, Guatemala (May 31, 2010)
• Google map of deepest "hole" for each state (Andy Martin)
• Tennessee sinkholes
• Largest and most impressive sinkholes of the world by Wondermondo
• Sinkholes by Wondermondo