Blue hole
Encyclopedia
Cave
A cave or cavern is a natural underground space large enough for a human to enter. The term applies to natural cavities some part of which is in total darkness. The word cave also includes smaller spaces like rock shelters, sea caves, and grottos.Speleology is the science of exploration and study...
(inland) or underwater sinkhole
Sinkhole
A sinkhole, also known as a sink, shake 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 or suffosion processes for example in sandstone...
. They are also called vertical caves. There are many different blue holes located around the world, typically in low-lying coastal regions. The best known examples can be found in Belize
Belize
Belize is a constitutional monarchy and the northernmost country in Central America. Belize has a diverse society, comprising many cultures and languages. Even though Kriol and Spanish are spoken among the population, Belize is the only country in Central America where English is the official...
, the Bahamas, Guam
Guam
Guam is an organized, unincorporated territory of the United States located in the western Pacific Ocean. It is one of five U.S. territories with an established civilian government. Guam is listed as one of 16 Non-Self-Governing Territories by the Special Committee on Decolonization of the United...
, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
(in the Great Barrier Reef
Great Barrier Reef
The Great Barrier Reef is the world'slargest reef system composed of over 2,900 individual reefs and 900 islands stretching for over 2,600 kilometres over an area of approximately...
), and Egypt
Egypt
Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world...
(in the Red Sea).
Blue holes are roughly circular, steep-walled depressions, and so named for the dramatic contrast between the dark blue, deep waters of their depths and the lighter blue of the shallows around them. Their water circulation is poor, and they are commonly anoxic
Anoxic sea water
Anoxic waters are areas of sea water or fresh water that are depleted of dissolved oxygen. This condition is generally found in areas that have restricted water exchange....
below a certain depth; this environment is unfavorable for most sea life, but nonetheless can support large numbers of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
. The deep blue color is caused by the high transparency of water and bright white carbonate
Carbonate minerals
Carbonate minerals are those minerals containing the carbonate ion: CO32-.-Anhydrous carbonates:*Calcite group: Trigonal**Calcite CaCO3**Gaspeite CO3**Magnesite MgCO3**Otavite CdCO3**Rhodochrosite MnCO3**Siderite FeCO3**Smithsonite ZnCO3...
sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
. Blue light is the most enduring part of the spectrum; where other parts of the spectrum—red, yellow, and finally green—are absorbed during their path through water, blue light manages to reach the white sand and return back upon reflection.
The deepest blue hole in the world—at 202 metres (662.7 ft)—is Dean's Blue Hole
Dean's Blue Hole
Dean's Blue Hole is the world's deepest known blue hole with seawater. It plunges in a bay west of Clarence Town on Long Island, Bahamas.-History:The full depth of the cave was reached by Jim King in 1992....
, located in a bay west of Clarence Town
Clarence Town
Clarence Town is a town in the Bahamas. It is located on Long Island.Clarence Town is the capital of Long Island and has a population of approximately 350 people. It has a marina, two restaurants as well as the government dock where the mail boat docks on a weekly basis...
on Long Island
Long Island, Bahamas
Long Island is an island in the Bahamas that is split by the Tropic of Cancer. Its capital is Clarence Town. Long Island is one of the Districts of the Bahamas and is known as the most scenic island in the Bahamas. The population is roughly 4,000 inhabitants.-Geography:Long Island is about 130...
, Bahamas. Other blue holes are about half that depth at around 100–120 m (328.1–393.7 ). The diameter of the top entrance ranges typically from 25–35 m (82–114.8 ) (Dean's Blue Hole
Dean's Blue Hole
Dean's Blue Hole is the world's deepest known blue hole with seawater. It plunges in a bay west of Clarence Town on Long Island, Bahamas.-History:The full depth of the cave was reached by Jim King in 1992....
) to 300 metres (984.3 ft) (Great Blue Hole
Great Blue Hole
The Great Blue Hole is a large submarine sinkhole off the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll from the mainland and Belize City. The hole is circular in shape, over across and deep...
in Belize).
Formation
Blue holes formed during past ice ageIce age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
s, when sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...
was as much as 100–120 m (328.1–393.7 ) lower than at present. At those times, these formations were subjected to the same erosion from rain and chemical weathering common in all limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
-rich terrains; this ended once they were submerged at the end of the ice age.
Most blue holes contain freshwater and saltwater. The halocline
Halocline
In oceanography, a halocline is a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water. Because salinity affects the density of seawater, it can play a role in its vertical stratification...
is the point in these blue holes where the freshwater meets the saltwater and where a corrosive reaction takes place that eats away at the rock. Over time this can create side passages, or horizontal "arms", that extend from the vertical cave. These side passages can be quite long; e.g., over 600 metres (1,968.5 ft) in the case of the Sawmill Sink in the Bahamas.
Occurrence
Blue holes are typically found on shallow carbonate platformCarbonate platform
A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonous calcareous deposits . Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms which induce carbonate precipitation through their metabolism...
s, exemplified by the Bahama Banks
Bahama Banks
The Bahama Banks are the submerged carbonate platforms that make up much of the Bahama Archipelago. The term is usually applied in referring to either the Great Bahama Bank around Andros Island, or the Little Bahama Bank of Grand Bahama Island and Great Abaco, which are the largest of the...
, as well as on and around the Yucatán Peninsula
Yucatán Peninsula
The Yucatán Peninsula, in southeastern Mexico, separates the Caribbean Sea from the Gulf of Mexico, with the northern coastline on the Yucatán Channel...
, such as at the Great Blue Hole
Great Blue Hole
The Great Blue Hole is a large submarine sinkhole off the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll from the mainland and Belize City. The hole is circular in shape, over across and deep...
at Lighthouse Reef Atoll, Belize
Belize
Belize is a constitutional monarchy and the northernmost country in Central America. Belize has a diverse society, comprising many cultures and languages. Even though Kriol and Spanish are spoken among the population, Belize is the only country in Central America where English is the official...
.
Many deep spring basins formed by karst processes and located inland also are called blue holes, for example, Blue Hole (Castalia)
Blue Hole (Castalia)
The Blue Hole is a fresh water pond located in Castalia, Erie County, Ohio, in the United States. From the 1920s to 1990 the Blue Hole was a tourist site, attracting 165,000 visitors annually at the height of its popularity, partly because of its location on State Route 269, about seven miles...
in Ohio
Ohio
Ohio is a Midwestern state in the United States. The 34th largest state by area in the U.S.,it is the 7th‑most populous with over 11.5 million residents, containing several major American cities and seven metropolitan areas with populations of 500,000 or more.The state's capital is Columbus...
.
See also
- Karst topographyKarst topographyKarst topography is a geologic formation shaped by the dissolution of a layer or layers of soluble bedrock, usually carbonate rock such as limestone or dolomite, but has also been documented for weathering resistant rocks like quartzite given the right conditions.Due to subterranean drainage, there...
- CenoteCenoteA cenote is a deep natural pit, or sinkhole, characteristic of Mexico and Central America, resulting from the collapse of limestone bedrock that exposes groundwater underneath...
- Blue Hole (Red Sea), a famous diving spot at the coast of the Sinai peninsula
- Great Blue HoleGreat Blue HoleThe Great Blue Hole is a large submarine sinkhole off the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll from the mainland and Belize City. The hole is circular in shape, over across and deep...
- Dean's Blue HoleDean's Blue HoleDean's Blue Hole is the world's deepest known blue hole with seawater. It plunges in a bay west of Clarence Town on Long Island, Bahamas.-History:The full depth of the cave was reached by Jim King in 1992....
, deepest known blue hole with seawater
External links
- PBS TV program "Extreme Cave Diving"
- Bahamas Blue Holes Guide
- Bahamas Introduction
- The Blue Holes Foundation
- Belize Audubon Society
- What's a Blue Hole? Explanation at the Bahamas Caves Research Foundation