Great Blue Hole Of Belize cover

Great Blue Hole Of Belize

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Photographs taken from the air show an incredible, nearly perfectly round, deep blue circle in a shallow atoll some 70 km east from Belize City. This unique formation is known as the Great Blue Hole – an enormous sinkhole that has existed for at least 150,000 years.
The coast of Belize (and Mexico) is adorned with several elongated atolls. The Lighthouse Reef Atoll is located further away from the mainland. The Great Blue Hole is located in the central part of this atoll.


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Great Blue Hole Of Belize

The Great Blue Hole

Photographs taken from the air show an incredible, nearly perfectly round, deep blue circle in a shallow atoll some 70 km east from Belize City.

This unique formation is known as the Great Blue Hole – an enormous sinkhole that has existed for at least 150,000 years.

The coast of Belize (and Mexico) is adorned with several elongated atolls. The Lighthouse Reef Atoll is located further away from the mainland. The Great Blue Hole is located in the central part of this atoll.

Great Blue Hole

Great Blue Hole

Public domain image by US Geological Survey

History of Exploration

Jaques-Yves Cousteau

The first explorations of this unique formations were made by the legendary Jaques-Yves Cousteau. In 1971, he and his team from the ship Calypso charted the depth of the hole. They found that this is a karst formation that formed in at least four stages. They were excited to find stalactites deep below the sea level - dripstone forms only above the water level. Cousteau noticed that the stalactites were tilted, showing that at the time when they formed, the Blue Hole was at different angle than now.

These exciting discoveries were shown in the very popular TV series "The Undersea World of Jaques-Yves Cousteau", turning the Great Blue Hole into true celebrity. Cousteau announced that this is one of the premium diving sites in the world.

History of Exploration (Cont.)

Anthony Jones and Robert Dill

In 1997 there was an expedition organized by the Californian scientists Jones and Dill. They saw an opportunity to explore the past of the Earth.

Using the latest equipment in diving gear, members of the expedition managed to reach the bottom of the sinkhole, as well as being able to explore the side passages. Special attention was given to the exploration of stalactites. Isotope analysis of sample pieces of the stalactites provided valuable data about the past of Great Blue Hole.

Explorers reached and explored another exciting feature: An abyss in the northern part of Lighthouse Reef Atoll. This narrow fissure turned out to be at least 152 m deep

Lighthouse Reef

Lighthouse Reef

NASA image of Lighthouse Reef, Belize, which includes the Great Blue Hole roughly near its center.

Created by Jesse Allen, using EO-1 ALI ,data provided courtesy of the NASA EO-1 Team

Diving Site

Based on the popularity of Cousteau, the Great Blue Hole became a popular diving site.

As there is no dry land of significant size near the Great Blue Hole, a diving trip here requires a full-day trip using local operators.

Divers, however, often don’t have their expectations met. They expect to see countless weird sea organisms in such an extraordinary location, but they just see an incredibly large hole with the occasional larger fish, including numerous sharks. The true beauty here lies in this unique geological formation itself, and the numerous riddles and answers about the past of Earth that are hidden here.

Rather late, this was declared a marine protected area in 1996. Around this same time, the Great Blue Hole was included in UNESCO World Heritage Site list as a part of the "Belize Barrier Reef Reserve System".

Size And Shape

The Great Blue Hole is a nearly perfectly round sinkhole, flanked by a crest of coral reef rising above the sea in low tide.

The diameter of the sinkhole is approximately 318 m. This sinkhole is deep blue, which stands out against the light turquoise blue lagoon around it. Such formations are possible only in large carbonate platforms covered by the sea.

The depth as measured by Cousteau’s team was 125 m. Jones and Dill in 1997 could not find a location deeper than 124 m - whether due to the measurement imprecision of the previous team or due to the accumulated sedimentation over the 26 years long period.

There is reason to believe in the sedimentation theory: there are no waves or currents below the depth of 18 - 20 m in the Blue Hole and thus the sinkhole serves as a sediment trap.

The sinkhole also has an hourglass shape in the cross section.

Stand Out

Stand Out

Image by Eric Pheterson

(CC BY 2.0)

Hydrogen Sulphide Layer

An unusual feature of the GreatBlue Hole is the turbid, brownish layer of water at the depth of 90 - 101 m, enriched with hydrogen sulphide H2S.

The water above it and below it is lucid - with the difference that below this layer, the water does not contain oxygen. The bottom of the Great Blue Hole is lifeless, and covered with large pieces of limestone.

This unusual hydrogen sulphide layer has been created by the constant pouring of organic substances into Great Blue Hole. As these pieces of organic matter sink, they are decomposed by microorganisms. These microorganisms deplete the oxygen at the bottom layer of the water and form hydrogen sulphide. This in turn makes the water more acidic and it dissolves the limestone walls. Most likely this process has helped to create the hourglass shape of the sinkhole.

West Wall Cave, Stalactites & Stalagmites

In 1997, at the 50 m depth in the west wall of the Blue Hole, a cave was discovered: the West Wall Cave.

Past a deep entrance, the cave rises up to 37 - 41 m depth and then extends horizontally for more than 46 m. The cave contains many skeletons of marine turtles and other creatures. The depth of this cave coincides with the depth of the majority of the stalactites and stalagmites.

The most exciting find by Cousteau was the stalactites. It was these stalactites that also lured the expedition of 1997.

Most of the stalactites are located at the depth of 30 - 45 metres. They can reach impressive size - some are even 8 metres long. The divers found also several stalagmites.

Great Blue Hole Corals

Great Blue Hole Corals

Brain and tube corals of the Great Blue Hole in Belize, with trunkfish.

Image by jayhem

(CC BY-SA 2.0)

Record Of The Past

Dripstone formations only form in atmosphere - above the sea level.

This means that at some time in the past at least the upper part of Great Blue Hole was above the sea level. Further, the whole depth of the sinkhole needed to be above the water level, as formation stops at water level (there are exceptions in volcanic regions - see El Zacatón sinkhole in Mexico.).

The stalactites themselves can provide an answer about the age of their formation through the method of isotope analysis. The team of the expedition in 1997 took numerous samples (one of the rare cases when damage to stalactites was justified) and analyzed them.

The result was exciting and at the same time foreseeable - stalactites (and the Blue Hole itself) formed during at least four episodes of low sea level - 153,000, 66,000, 60,000 and 15,000 years ago. This corresponds well to other data of climate changes in the world, showing lower temperatures at these periods.

Tilt

Another exciting feature of the stalactites and stalagmites is their tilt.

Some are straight, some tilted, some are half tilted and half straight. These dripstone formations have fixed some tectonic change in the past, when the angle of Great Blue Hole and surrounding layers of carbonate changed.

The tilt of the dripstone formations is some 5° - less than 11 - 13° reported by the Cousteau expedition.

Reports of Cousteau’s team about the stalactites at the120 metre depth are not credible either – the acidic water at this depth would dissolve any dripstone formations.

Unclear Origins

The mechanism of the formation of the Great Blue Hole remains unclear.

First - it is too deep, if we remember that its full depth is a lot larger and that the true bottom is hidden below a thick layer of sediments. Sea level in the past did not fall THAT much.

Second - it is not clear how the sinkhole formed. Lighthouse Reef Atoll is small and separated from the mainland by at least 1.200 m deep sea. Sinkholes require a rather powerful flow of freshwater - and consequently a sizeable landmass is needed.

Main Characteristics

Coordinates: 17.3155 N -87.5345 W

No: 131 (list of all attractions)

Category: Sinkholes

Values: Geology, Visual

Rank: 4

Depth: 124 m

UNESCO World Heritage status: Part of "Belize Barrier Reef Reserve System", 1996, No.764