Sinkholes

This category includes outstanding sinkholes – large natural depressions or holes, which for the most part represent collapsed caves. Often there is used the 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 a very distinct trend.

Many sinkholes, which have vertical walls, to some extent can be considered to be caves as well – although, strictly speaking, the cave is an 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 the natural solution of carbonate (f.e. limestone, dolomite), sulfide (f.e. gypsum), and salt rocks. The 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. A 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 a 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, and sometimes also with silica. Water dissolves this cement (of course, dissolving silica is a 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 the whole layer of rock is soluble, water dissolves and brings away even the collapsed roof, and there forms a 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. An 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, volcanoes can 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 kilometers 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 ongoing above sea level – if there is a 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 the sea level was lower than now. Some 20 thousand years ago the sea was even more than 100 meters below the current level.

Numerous areas around the world were dry whereas today is the sea. In some of these areas took place intense karst processes forming also deep and impressive sinkholes. Now such sinkholes are covered with seawater, forming exciting natural attractions – blue holes. Some of the most impressive are Great Blue Hole (Belize, Belize District), Dragon Hole (South China Sea, Paracel Islands) 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 mudflats (not a common phenomenon either) – seawater 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 – a layer of phototrophic sulfur bacteria. This layer "swallows" the light and causes the black color of the water. Bacteria have heated up this layer up to 40°C – possibly the only case in the 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 the high transparency of water and bright white carbonate sand. Blue light is the most enduring part of the spectrum: where other parts of the 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 the 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 Yucatán, Mexico).

Formation of cenotes is linked to another unusual monument of natural history – the fall of the Chicxulub meteorite some 65 million years ago. After the fall of the meteorite formed a large basin where a 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 meters 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): an incredible 662 meters deep and up to 537 meters 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.

Wondermondo’s articles about sinkholes: