List of sinkholes described by Wondermondo

Map of described sinkholes

General description

Category includes outstanding sinkholes - large natural depressions or holes, which for most part represent collapsed caves. Often there is used equivalent term - doline - what in Southern Slavic languages means "depression". Some consider that "sinkhole" is more used in American literature and "doline" - in European, but there is not very distinct trend.

Many sinkholes, which have vertical walls, to some extent can be considered to be caves as well - although, strictly speaking, cave is underground void where can enter people: e.g. there is needed a layer of rock above these people. The opening of sinkholes is above the visitors.

Formation

Karst processes

Most sinkholes are formed by karst processes - by natural solution of carbonate (f.e. limestone, dolomite), sulphide (f.e. gypsum) and salt rocks. Solution process is initiated by water.

Especially efficient is rainwater (because it is slightly acidic from atmospheric carbon dioxide) and even more - hot water (because it speeds up the solution and bears more dissolved material) and acidic water. Unique example of sinkholes created by heated, acidic water, is Sistema Zacatón in Mexico, Tamaulipas. Here have formed more than 20 karst features including more than 300 m deep sinkholes filled with warm water and some even sealed with travertine lids!

Suffosion

Few large sinkholes have formed also in sandstone rock. Here takes place another geological process - suffosion. In many respects it is similar to karst processes. Sand particles in sandstone are cemented - often with carbonates, clay, sometimes also with silica. Water dissolves this cement (of course, dissolving silica is very very slow process) and then carries away the unfettered sand particles. The largest sinkholes in sandstone are Sima Humboldt and Sima Martel in Venezuela. These are unique, up to 314 m deep holes with vertical walls and their formation has required many millions of years.

Collapse sinkholes

Most sinkholes are collapsed cave rooms (collapse sinkholes). Streams flowing through cave passages gradually increase the size of cave rooms until the roof becomes thin and collapses. If whole layer of rock is soluble, water dissolves and brings away even the collapsed roof and there forms fine hole in the ground - often with the stream still flowing through it.

Erosion, vadose sinkholes

Sometimes sinkholes are formed by water entering through a vertical crevice in the rock. This crevice then gradually increases and also forms a deep hole in the ground (erosion sinkhole, might be called also vadose sinkhole). Sometimes these sinkholes still have beautiful waterfalls falling into them. Extreme example of such sinkholes is Qingkou Tiankeng in China, Chongqing Municipality - it is 295 m deep, up to 250 m wide.

Volcanic calderas

To some extent the process of the formation of volcanic calderas is similar to the formation of traditional sinkholes. Also in volcanoes there form large underground voids which collapse, leaving enormous holes in the surface of the Earth.

The size of some volcanic calderas exceeds the size of any sinkhole. La Garita Caldera (Colorado, United States) is 35 by 75 kilometres large.

Nonetheless the mechanism of formation of volcanic calderas is driven by completely different geological processes and Wondermondo reviews them as landmarks related to volcanoes.

Changes of sea level

Karst processes mostly are on-going above sea level - if there is layer of carbonate rock and enough rain, caves and sinkholes and sinkholes are formed by underground streams.

In the past major areas of Earth were taken by glaciers and sea level was lower than now. Some 20 thousand years ago the sea was even more than 100 metres below the current level.

Numerous areas around the world were dry where today is sea. In some of these areas took place intense karst processes forming also deep and impressive sinkholes. Now such sinkholes are covered with sea water, forming exciting natural attractions - blue holes. Some of the most impressive are Great Blue Hole (Belize, Belize District) and Dean's Blue Hole (Bahamas, Long Island).

Some types of sinkholes

Below are listed some popular kinds of sinkholes - black holes, blue holes, cenotes, sótanos and tiankengs.

Black holes

Black holes are unique geological and biological monuments found only in Andros and Grand Bahama islands (Bahamas) - in total some 30. The best known is Black Hole of Andros (Bahamas, South Andros).

Black holes are formed in carbonate mud flats (not a common phenomenon either) - sea water slowly dissolves this clayey substance, creating amazing, round structures filled with weird, black water. These sinkholes are not too deep - up to 47 m.

The menacing black color is caused by the most amazing feature of black holes - layer of phototrophic sulphur bacteria. This layer "swallows" the light and causes the black color of water. Bacteria have heated up this layer up to 40°C - possibly the only case in world where bacteria manage to heat up the water.

Blue holes

Blue hole is a term applied to some of those sinkholes which are filled with water - irrespective of their mechanism of formation. It seems that the decisive factor in the use of this term is the beautiful deep blue color of water in these formations.

This color is created by high transparency of water and bright white carbonate sand. Blue light is the most enduring part of spectrum: where other parts of spectrum - red, yellow and finally the green - are absorbed during their path through the water, blue light manages to reach the white sand and return back after the refraction.

The term has been first used in Bahamas, where are numerous blue holes.

Cenotes

Specific monuments of nature and often - culture are many cenotes - large sinkholes found mainly in Yucatán Peninsula, Mexico. These round, water filled sinkholes often had huge importance for nearby Mayan cities as a source of potable water and sometimes as sacrifice and ritual sites (Sacred Cenote in Quintana Roo, Mexico).

Formation of cenotes is linked to another unusual monument of natural history - fall of the Chicxulub meteorite some 65 million years ago. After the fall of meteorite formed large basin where thick layer of carbonate rocks was deposited. Present-day cenotes have formed along the rims of this ancient basin.

Sótanos

Some giant, deep sinkholes with nearly vertical walls in Mexico are called sótanos - the best known is the 372 m deep Sótano de las Golondrinas - Cave of the Swallows (Mexico, San Luis Potosi). several of these sinkholes correspond well to criteria of tiankengs.

Tiankengs

Extremely large and visually impressive natural sinkhole is called tiankeng - this term is introduced by Chinese geologists.

To earn this name the hole must be at least 100 metres deep and wide, with vertical walls and with a cave river below. There are some 75 tiankengs in the world, most of them in China. The largest one is Xiaozhai tiankeng (Chongqing, China): incredible 662 metres deep and up to 537 metres wide hole with vertical walls. Another region with tiankengs is Papua New Guinea, especially New Britain island, where these sinkholes are located in the middle of lush jungle and have jungle also on their bottoms. Some more tiankengs are in Mexico, Croatia, Venezuela and other countries.

Outstanding sinkholes

Here are selected some of the most surprising and interesting sinkholes of the the world, arranged by the part of the world and in an alphabetic order.

Africa

Often it is mentioned that Qattara Depression in Matruh Governorate, Egypt, is the largest sinkhole in the world. This feature is approximately 19,500 km² large, up to 360 m deep. Borders of this structure are not very well defined - in nothern part it has precipice, in southern there is gradual rise without cliffs. Mechanism of its formation is not fully clear. As a whole it has little in common with sinkholes.

Asia

Bala'a sinkhole, Lebanon
Bala'a sinkhole, Lebanon. Photo from Wikimedia Commons by user Elie plus, CC-BY-SA-3.0

Australia and Oceania

Europe

Crveno Jezero sinkhole
Crveno Jezero sinkhole, Croatia. User Tieum512, Wikimedia Commons, CC BY 3.0

North America

South America

Sima Humboldt and Sima Martel, sinkholes in Venezuela
Sima Humboldt and Sima Martel, sinkholes in Venezuela. Photo by Luis Ruiz Berti, public domain.

Creative Commons Non-Commercial Share Alike 3.0 6 August 2010 Gatis Pāvils

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