Sinkholes and Blue Holes


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.

Sinkholes may vary in size from less than 1 to 300 meters (3.3 to 980 ft) both in diameter and depth, and vary in form from soil-lined bowls to bedrock-edged chasms. They may be formed gradually or suddenly, and are found worldwide. These terms are often used interchangeably, though many distinguish between features a surface stream flows into and features with no such input. Only the former are described as sinks, swallow holes or swallets. A sinkhole on a glacier is called a moulin or a glacier mill.


Mechanism

The mechanisms of formation involves natural process of erosion 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. Occasionally a sinkhole may exhibit a visible opening into a cave below. In the case of exceptionally large sinkholes, such as 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.

The sinkholes are common where the rock below the land surface is limestone, 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 can be human-induced and new sinkholes have been correlated to land-use practices, especially, from ground-water pumping, construction, and development practices. 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

Sinkholes may capture surface drainage for running or standing water, but may also form in currently high and dry locations. 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.

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.

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 Yucat‡n Peninsula (known as cenotes) as places to deposit precious items and sacrifices.

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.

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. A majority of sinkholes in Alpena are also found underwater. Many divers explore these on a regular basis.

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 Zacaton cenote in Mexico (the world's deepest water-filled sinkhole), the Boesmansgat sinkhole in South Africa, Sarisari–ama 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.

In the United States, the most damage from sinkholes tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania. Palmyra, Pennsylvania is the sinkhole capital of the United States.

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.




Louisiana 2013



Quite a Jolt: Earthquakes Heralded Opening of Sinkhole   Live Science - April 18, 2013
Earthquakes signaled the opening of a giant toxic sinkhole in southeastern Louisiana last year, researchers reported here today (April 17) at the Seismological Society of America's annual meeting. Strong shaking first rattled residents of Bayou Corne on June 8 and July 3 in 2012, prompting officials to install earthquake monitors near the small town. After July 14, seismometers detected 10 to 12 sharp tremors of about magnitude 2.5 jolting the region daily, said Steve Horton, a seismologist at the University of Memphis and lead study author.




Guatemala 2010

A sinkhole was formed suddenly in Guatemala in May 2010, where torrential rains from Tropical Storm Agatha and a bad drainage system were blamed for creating a sinkhole that swallowed a three story building and a house. This sinkhole measured approximately 66 feet (20 m) wide and 100 feet (30 m) deep. A similar hole had formed nearby in February 2007.

However, geologist Sam Bonis says that actually it is not a sinkhole but a "piping feature". Guatemala City is built mainly on pumice fill and, according to Bonis, "When you have water flowing from storm water runoff, a sewage pipe, or any kind of strong flow, it eats away at the loose material. We don't know how long it has to go on before it collapses. But once it starts collapsing, God help us."


   Guatemala Sinkhole 2010   YouTube


Sinkhole  Wikipedia


  Sinkholes   YouTube




Sinkholes in Florida



Devil's Millhopper near Hawthorne, Florida, USA.



Mulberry, Florida, Sinkhole




Blue Holes


A blue hole is a submarine cave or sinkhole. They are also called vertical caves. There are many different Blue Holes located around the world, such as in the Bahamas and in Belize.

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 below a certain depth; this environment is unfavorable for most sea life, but nonetheless can support large numbers of bacteria.

The deepest blue hole in the world - at 335 metres (1,100 ft) - is Tamaulipas, Mexico's Zacaton. The next deepest blue hole is Dean's Blue Hole at 202 metres (663 ft), located in a bay west of Clarence Town on Long Island, Bahamas. Other blue holes are about half that deep at around 100Š120 metres (330Š390 ft).

Blue holes formed during past ice ages, when sea level was as much as 100Š120 metres (330-390 ft) lower than at present. At those times, these formations were subjected to the same chemical weathering common in all limestone-rich terrains; this ended once they were submerged at the end of the ice age.

Blue holes are typically found on shallow carbonate platforms, exemplified by the Bahama Banks, as well as on and around the Yucat‡n Peninsula, such as at the Great Blue Hole at Lighthouse Reef Atoll, Belize





Blue Hole in Belize

First discovered by Jacques Cousteau, one of the worldÕs most famous divers and marine conservationists, the Great Blue Hole in Belize is the best of the bunch, and every diverÕs dream. Measuring 305m (1,000 ft) across and 123 m (400 ft) deep, the Blue Hole is almost perfectly circular and can be found in Lighthouse Reef Atoll, about 60 miles from Belize City.

The Great Blue Hole in Belize is a Mecca for those wanting to dive among its giant stalactites and stalagmites that were formed during the last Ice Age. Starting at around 30-33 meters, the limestone formations become more intricate with depth, but few get to see what they look like as most divers are only qualified to go to a depth of around 30 meters, although, when it comes to diving in blue holes, many flout the rules.


  Blue Hole Belize   YouTube





Dean's Blue Hole

Dean's Blue Hole is the world's second deepest blue hole (underwater sinkhole), which plunges 202 metres (663 ft) to the ocean floor, in a bay west of Clarence Town on Long Island, Bahamas. The maximum depth of most other known blue holes and sinkholes is 110 metres (360 ft), which makes the 202 metres (663 ft) depth of Dean's Blue Hole quite exceptional, though Zacat—n in Mexico is even deeper at 335 metres (1,099 ft).

Dean's Blue Hole is roughly circular at the surface, with a diameter ranging from 25 to 35 metres (82Š115 ft). After descending 20 metres (66 ft), the hole widens considerably into a cavern with a diameter of 100 metres (330 ft).


  Dean's Blue Hole   YouTube





Blue Hole in the Red Sea

Blue Hole is a diving location on east Sinai, a few kilometres north of Dahab, Egypt on the coast of the Red Sea. It is a submarine pothole (a kind of cave), around 130 m deep. There is a shallow opening around 6 m deep, known as 'the saddle', opening out to the sea, and a 26 m long tunnel, known as the arch, the top of which lies at a depth of 52 m. The hole itself and the surrounding area has an abundance of coral and reef fish.

Dahab was a tiny Bedouin fishing village on the Sinai coast, about 50 miles from Sharm el-Sheikh to the south. Over the past 25 years it has become known worldwide for having great windsurfing conditions and some of the best shore diving anywhere.


Technical diver passing under the Arch.

The Blue Hole, a short drive north of the main centre of Dahab, is notorious for the number of diving fatalities which have occurred there earning it the sobriquet, "World's Most Dangerous Dive Site" and the nickname "Diver's Cemetery". The site is signposted by a sign that says "Blue hole: Easy entry". Accidents are frequently caused when divers attempt to find the tunnel through the reef (known as "The Arch") connecting the Blue Hole and open water at about 52 m depth. This is beyond the PADI recreational diving limit (40 m), and the effect of nitrogen narcosis will be significant at this depth. Divers who miss the tunnel sometimes continue descending hoping to find the tunnel farther down and become increasingly narced.


  Dahab Blue Hole   YouTube




The Blue Hole of Gozo

Gozo is a small island of the Maltese archipelago in the Mediterranean Sea. The island is part of the Southern European country of Malta; after the island of Malta itself, it is the second-largest island in the archipelago.

The Blue Hole of Gozo is not clearly as visible from the surface, the mouth to MaltaÕs Blue Hole starts at 7 m deep, and is the most popular dive site on the islands.

Located within the blue hole is a cave at 15 m below where shoals of tuna, groupers and barracuda are often found hovering by the large boulders and rocks. Divers can swim through a short tunnel, or chimney, to get to a different area within the hole which has great coral gardens and reefs teeming with marine life, which is quite amazing considering how fished out many parts of the Mediterranean are.


   The Blue Hole & Azure Window, Gozo   YouTube




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