Australia’s Great Barrier Reef is one of the planet’s most biodiverse and intriguing areas in the world, but it’s also relatively inaccessible. It shouldn’t surprise us then that we’re still discovering more and more about it.
The blue hole. Image credits: Johnny Gaskell/Instagram.
Johnny Gaskell, an Australian who loves marine wildlife and underwater photography, was looking around Google Maps when he discovered something unexpected: a big blue hole. Gaskell, who also works as a researcher for Sharks and Rays Australia, took a boat to the site and explored the hole. Although it is much further out than he usually goes, the trip was well worth it.
“What we found inside was hard to believe, considering five months ago a Category 4 cyclone went straight over the top of it,” Gaskell told Mashable. “At around 15 to 20 metres (16 to 21 yards) deep, there were huge Birdsnest Corals (Seriatopora) and super elongated Staghorn Corals (Acropora), both of which were among the biggest and most delicate colonies I’ve ever seen,” Gaskell said.
This hole, while nowhere near as large as other giants, still measures a respectable 50 to 65 feet deep (15-20 meters), and as Gaskell notes, it hosts some large and healthy coral colonies — something unexpected in the Great Barrier Reef, which is slowly being killed off, in large part due to anthropic pressure.
Research is underway at the blue hole.
“Yesterday’s Blue Hole mission in the Great Barrier Reef,” Gaskell wrote on his Instagram. “This Blue Hole has previously been described and documented by geologists who suggested it could be even older than the famous Great Blue Hole, in Belize. Its location is in one of the least explored parts Great Barrier Reef, over 200km from Daydream Island. To get there we had to travel overnight for 10 hours and time the tides perfectly… Was well worth it!”
Gaskell has located two other blue holes, roughly 200 kilometers away from the first one. Logistical preparations are currently being made to explore the other ones. However, their location is still kept secret for conservation fears. Corals are greatly suffering from bleaching caused by rising temperatures, and tourism can add even more stress with potentially devastating consequences. Half of the reef has vanished in the past 27 years, and it shows no sign of slowing down — this is what makes the healthy corals in the hole even more surprising.
Blue holes are basically marine sinkholes — natural depressions (or holes) in the surface of the Earth’s surface, (usually) caused by karst processes. Karst processes occur when the bedrocks are soluble – in other words, in 99% of all cases, in carbonate rocks (like limestone or dolomite) or evaporitic rocks (like gypsum or anhydrite). Marine sinkholes are also called blue holes. Blue holes typically contain tidally-influenced water of fresh, marine, or mixed chemistry, and can differ greatly from the surrounding environment.
A huge sinkhole which emerged in Fukuoka, Japan, was covered and repaired in less than a week – out of which health and environment checks took three days.
Before and after. Image credits: Youtube
Sinkholes are natural depressions (or holes) in the surface of the Earth’s surface, (usually) caused by karst processes. Karst processes occur when the bedrocks are soluble – in other words, in 99% of all cases, in carbonate rocks (like limestone or dolomite) or evaporitic rocks (like gypsum or anhydrite). They’re relatively common in areas such as Florida and southern UK and often occur fast, without warning, causing massive damage.
This was exactly the case in Fukuoka. According to The Guardian, the sinkhole caused power cuts and disrupted phone signals, gas and water supplies, but there were no reports of injuries. It completely trashed a five-lane road, however.
The sinkhole itself was huge: 30 by 27 meters, and 15 meters deep – a monster of nothingness in the middle of the city. But Japan demonstrated its proverbial efficiency once more, repairing it completely in six days, three of which were spent on testing only. In total, 6,200 cubic metres of sand and cement were poured into the hole, astonishingly fast even by Japanese standard. Even so, the city’s Mayor Soichiro Takashima released a statement apologising for the “great trouble.”
I hate to fall into stereotypes, but when it comes to infrastructure, few do it like the Japanese.
A massive sinkhole formed in Ottawa, Canada on Wednesday 8th of June, leading to the collapse of one of the city’s busiest streets and damaging gas and water lines in the area. Gas, electrical and water services in downtown Ottawa are temporarily cut off and roadblocks set in areas of the city while authorities scramble to stabilize the area.
The sinkhole damaged the street and buildings in the area. Image via reddit
First signs of the sinkhole forming were reported at around 10:30 local time on Wednesday near the Canadian Parliament building in downtown Ottawa, Canada. The area, which grew to a large section of the Rideau Street, eventually collapsed later in the day. The street has been closed off to most traffic for some time now — except for a few taxis, buses and pedestrians — due to ongoing construction works. Several nearby buildings had to be evacuated but there thankfully were no immediate reports of injury or deaths caused by the collapse.
Some suggest that the ongoing work on the underground railway system below Rideau Street may have lead to the collapse, but it is still unclear whether the ongoing project had something to do with the appearance of the sinkhole, said Ottawa Mayor Jim Watson. The city is largely built on a type of soil known as Leda clay or quick clay, known for its tendency to collapse. It’s not the first time such a collapse happened in the city; in 2014, a smaller sinkhole formed in an area east of the city, believed to have been caused by a failure in a water line. In 2010 a massive sinkhole suddenly collapsed in north-east Montreal, destroying an entire house and killing four people.
But no matter how it formed, authorities are now looking for solutions, trying to figure out the best way of patching up the massive sinkhole that is now causing a major disruption in one of Ottawa’s primary streets. City officials were forced to temporarily cut off some water, gas and electrical services in downtown Ottawa and roadblocks had been set up as well in different parts of the city for public safety.
“All hands are on deck to make sure the site is secured and no harm is done to any individual,” Watson told reporters on Wednesday.
Watson said they are planning to use a special type of concrete to help stabilize the sinkhole. This might take some time, however, and people should be prepared for some delays.
Sinkholes always seem to be in the news. Unfortunately, they’re often the cause of some tragedy and they always seem to surprise authorities. But while they are unpredictable, sinkholes are less of an oddity than you might think. Let’s see what sinkholes are, how they form, and what dangers they pose.
What is a sinkhole
They go by called many names (snake hole, swallow hole, or doline). They’re big, they appear seemingly out of nowhere, and they can “devour” houses in the blink of an eye. But in reality, they develop over many years and require very specific conditions and processes to form (although we may not always see them with the naked eye).
They are basically natural depressions or holes in the Earth’s surface, typically caused by karst processes. Karst processes occur when bedrocks are soluble. In practical terms, this means that in 99% of all cases, sinkholes pop up in carbonate rocks (like limestone or dolomite) or evaporitic rocks (like gypsum or anhydrite). While they can occur in other environments, this is rarely a reason for concern.
A common mechanism for sinkhole formation
The problem with the above-mentioned rocks is that they can be dissolved by water. A sinkhole has no natural external surface drainage – when it rains, all the water stays inside the sinkhole and typically drains into the subsurface, dissolving away at the surrounding rock.
This is the dominant phenomenon for sinkhole formation regardless of their size. Sinkholes can be smaller than a meter, or over 100 meters in depth (in Venezuela for instance, multiple sinkholes have reached about 1,000 feet wide and 1,150 feet deep). They can also have very different shapes; some are just like a shallow saucer or bowl, while others are much more vertical. Some actually form water, and you’ve probably seen this in the form of little ponds in limestone.
Unfortunately, even though sinkholes form so slowly, their formation is not always visible on the surface.
What really makes them dangerous is the fact that they’re really unpredictable. They form so slowly, that without thorough geological or geophysical research, you can’t really tell if something’s changing — this is why collapses have dramatic, unexpected effects, especially in urban settings.
But we can know which areas are more prone to sinkhole formation, and this can help us be more prepared.
Areas prone to sinkhole collapses
If you want to see whether your area is prone to sinkhole formation, the first thing to do is check out a geological map.
As mentioned previously, sinkholes are almost always prone to karst areas. In such areas, it’s sometimes possible to see hundreds or even thousands of sinkholes in a small area, and no surface streams, because all the drainage occurs subsurface. Evaporite rocks underlie about 35 to 40 percent of the United States, but in most cases, they are buried very deep, so there are only a few threatened areas.
The most impressive sinkholes form in thick layers of homogenous limestone. Their formation is facilitated by high groundwater flow, often caused by high rainfall. When these thick, relatively soluble layers sit on top of insoluble rocks, significant underground streams (or even rivers) may form, dissolving a significant quantity of rock, creating large underground voids.
So don’t be fooled by what’s on the surface: the surface can be completely fine, but if the subsurface is karst, it’s worth a more careful look.
Sinkholes can also be underwater. They’re typically called blue holes in this scenario and have been described especially in the Bahamas area.
The name originates from the deep blue color of the water in these sinkholes, which in turn is created by the high lucidity of water and the great depth of sinkholes; only the deep blue color of the visible spectrum can penetrate such depth and return back after reflection. The deepest known sinkhole is the Dean’s Blue Hole in the Bahamas.
Types of sinkholes
Strictly speaking, there are three types of sinkholes, but from a practical perspective, it’s also worth mentioning a fourth (which is the first one on this list).
Wait, artificial sinkholes?
Yes, sinkholes can be created, or at least exacerbated by human activity.
This photo shows a sinkhole under a house in the village of Mececani, central Croatia, Thursday, March 4, 2021. Image credits: Darko Bandic.
This type of sinkhole is actually quite common (not as common as natural ones, but still relevant, especially in urban areas). They can be caused by a number of human activities, most notably groundwater pumping and construction and development practices.
The most common activities which lead to this type of phenomena are drilling, mining, significant changes in weight (think lots of construction in a relatively pristine area), and a heavy increase in water flow (like say, a formation of an artificial pond, pipe leakage, etc.
In one sense, urban sinkholes can be seen as a way for the environment to fight back. Although sinkholes are fairly common in nature, but humanity’s interference can make them much more common if the conditions are right
Basically, either more water appears in the system, which dissolves and creates an underground void, or the underground is already present, and there is an increase on the pressure exerted on the surface – or a combination of these. Whenever the structural and chemical balance is disturbed – sinkholes can occur.
As the name describes, dissolution (the process of dissolving a solid substance into a solvent to make a solution) is the driving factor here. Dissolution of the limestone or dolomite is most intensive where the water first contacts the rock surface.
Cover-subsidence sinkholes tend to develop gradually where the covering sediments are permeable and contain sand. Not really your typical sinkhole, this type is not common, and can often go undetected for long periods of time – which is pretty much the only thing that can make them dangerous.
These are pretty much the most dangerous type of sinkhole. Cover-collapse sinkholes develop very fast (sometimes even in a matter of hours), and can have catastrophic damage. They occur where the covering sediments contain a significant amount of clay; over time, surface drainage, erosion, and deposition of sinkhole into a shallower bowl-shaped depression. To put it in simple terms, the ground simply can’t support the load on top of it.
Climate change is making sinkholes worse
You can add sinkholes to the long list of things that climate change is making worse. Yes, really.
“The occurrence frequency and intensity of many natural geohazards, such as landslides, debris flows and earthquakes, have increased in response to global warming,” one recent study found. How much worse? For every 0.1℃ rise in temperature, the number of sinkholes increases by 1%-3%.
So far, the average global temperature on Earth has increased by a little more than 1° Celsius (2° Fahrenheit) since 1880, but projections show that we are on track for at least double that unless drastic change happens soon.
So how common are sinkholes, really?
The thing is, unlike earthquakes or hurricanes, sinkholes aren’t really tracked. No government agency keeps track of sinkholes from man-made causes. Since they appear in so many areas, it’s practically impossible to predict where they will appear — we can only talk about areas prone to them. Any karst-dominated area has some, be they big or small, as do areas with surface evaporites. This is why big portions of the Middle East, for example, are just riddled with sinkholes. In the US, Florida is almost certainly the most threatened state. From what I can dig up, from 2006 to 2010 in Florida alone, there were 24,671 insurance claims for sinkholes. In parts of England or Italy, sinkholes are also quite common.
If you live in an area that doesn’t have sinkhole-favorable geology, there’s no need to worry too much about it. If you do, however, it could be worth getting in contact with your recent geological survey organization and ask if they know anything about potential risk.
To some extent, you can also look for certain tells, such as sagging trees or fence posts, doors or windows that won’t close properly, and rainwater collecting in unusual spots. If you’re worried, then you should always consult a professional geologist or geotechnical engineering firm that can explain the situation. If indeed there is a possibility of a sinkhole near your home, they can even inject a specific concoction to fill up the cracks and strengthen the foundation.
Recent research on sinkholes
Sinkholes are an area of active research, both in terms of local surveys and general science. Here are some of the recent research findings in the field.
Earthquakes can trigger sinkholes. The appearance of sinkholes is not unusual following strong seismic activity, but researchers in Croatia were recently shocked to see over 100 previously unknown sinkholes after one earthquake. Sinkholes also seem to be appearing in the wrong places. Previous maps don’t seem to account for recent risks, and a number of areas where sinkholes hadn’t been previously reported are starting to have problems.
Thankfully, researchers are also finding new ways to study sinkholes. While geophysical surveys already offer a way to map the subsurface and identify nascent sinkhole areas, one of the more innovative ideas is to use internet fiber optics.
“We discovered the fibers could pick up a wide variety of signal vibrations, from thunderstorms to human walking steps to music concerts,” said Tieyuan Zhu, assistant professor of geophysics at Penn State and principal investigator on the project. “We can even distinguish the specific song at a concert by the patterns of the high and low tones. That’s a great demonstration of the sensitivity of these sensors.”
Using this approach, researchers can have access to a passive monitoring system of an urban area’s subsurface and look for sinkholes. While this is an emerging technology and there are current limitations, it’s a promising way to identify and monitor sinkhole formation.
The Dead Sea is draining at an alarming rate, and its coastline is being littered with sinkholes small and large – much like the world’s sandiest swiss cheese.
The sea, which is actually a lake, is well known for being 10 times as salty as the ocean and for boasting the lowest exposed strip of land on Earth (its shoreline, at -413 meters). The climate and unique water and swimming conditions made the Dead Sea a popular tourism spot. In the last few decades, however, it’s shores have become better known for their sinkholes rather than tanning spots.
Not prime tanning environment. Image via israel-tourguide
ABC News reports that more than 3,000 sinkholes exist along the banks of the Dead Sea, some of the craters diving more than 80 feet into the ground – about as high as an eight-story building.
“These sinkholes are the direct result of inappropriate mismanagement of water resources in the region” said Gidon Bromberg, the Israeli director at EcoPeace Middle East for ABC news.
The main source of water for the Dead Sea is the Jordan river. Significant quantities of water -some 2 billion gallons a year- have been diverted for human consumption starting in the 1960s, according to the American Associate Ben-Gurion University of the Negev. Since then, the Sea’a water line has dropped by one meter per year on average, or a total of 30 meters since 1970, according to research conducted by Duke University.
Image via dqhall59
And while many different hypotheses have been suggested, not many disagree that the declining water levels are the cause of these sinkholes.
“With the Dead Sea level dropping so rapidly [a meter a year, on average], these sinkholes are inevitable,” said Mark Wilson, a geology professor at the College of Wooster.
David Ozsvath, a professor of Geology at the University of Wisconsin, said beneath the clay-like surface layer are cavernous spaces that are filled with water. However, as these subterranean spaces dry up with the receding water levels, the surface layer can collapse into the emptied space creating chasms along the banks.
Image via Foeme
Image credits to Mark Wilson
In Mark Wilson’s blog, he explained that the canyon in the image above may have formed in less than three months. “This little canyon is cutting through Dead Sea sediments that are exposed by the rapid fall of water level,” Wilson said in his post. According to his blog, sinkholes often appear as small, round holes. But, he said, “looking inside, we could see that there is a significant room-sized cavern underneath. Soon the roof will collapse and a new mature sinkhole will appear.”
Ozsvath said some sinkholes form over time, while others appear overnight. An earthquake or even heavy rain can cause a sinkhole to collapse into the drained voids in the subsurface.
He added that the number of developing sinkholes could be reduced by diverting less water from the Jordan River and allowing water levels in the Dead Sea to rise.
It has been suggested that mining of minerals from the Dead Sea has also contributed to the disappearance of the lake’s dense and salty water.
The probe usually orbits 67P at a distance of a few hundred kilometers. Footage received from Rosetta over the last year showed a number of dust jets coming from the comet, which we expected to see. But, after analyzing high-fidelity images from the lander’s OSIRIS instruments, taken just ten to 30 km from the comet’s center, scientists saw that at least some of the dust jets come from specific locations on the comet’s surface, being projected from huge sinkholes.
The scientists have picked out 18 quasi-circular pits in the northern hemisphere of the comet, some of which are still active now. Each sinkhole is anywhere from a few tens of metres to hundreds of metres in diameter and go below the surface by up to 210m to a smooth dust-covered floor.
A catalogue of sinkholes spotted by Rosetta on comet 67P/Churyumov-Gerasimenko. Image via: forbes.com
“We see jets arising from the fractured areas of the walls inside the pits. These fractures mean that volatiles trapped under the surface can be warmed more easily and subsequently escape into space,” says Jean-Baptiste Vincent from the Max Planck Institute for Solar System Research, lead author of the study.
Similar to the ones on Earth, these sinkholes form when a cavity opens up under the surface. As it widens and deepens, the loss of material makes the ceiling too thin to support its own weight, and collapses. After the collapse, the volatile materials can evaporate or be eroded more easily, and the sinkhole enlarges over time.
“Although we think the collapse that produces a pit is sudden, the cavity in the porous subsurface could have growing over much longer timescales,” says co-author Sebastien Besse, of ESA’s ESTEC technical centre in the Netherlands.
So, what caused these cavities to form in the first place? The team has three theories that they are pursuing.
The first one is that they are artifacts of the comet’s weak gravitational field. When it formed, material accreted by means of low-velocity impacts, leaving behind void areas due to the imperfect fit between primordial building blocks. Over time, seismic events or space impacts cause the surface to weaken enough to cause it to collapse.
Another possibility is that the pits are full of volatile ices like carbon dioxide and carbon monoxide, sitting just beneath a layer of dust. These ices could be melted by the warmth of the Sun as the comet draws closer in its orbit every year.
Or it could be that the ice manages to melt itself away by transforming from amorphous ice made up of irregularly packed molecules to crystallised ice, a process that would release heat which could be sufficient to cause evaporation.
Close-up photo of sinkholes on 67P. Image via: esa.int
“Regardless of the processes creating the cavities, these features show us that there are large structural and/or compositional differences within the first few hundred metres of the comet’s surface and the cavities are revealing relatively unprocessed materials that might not otherwise be visible,” says Besse.
Researchers analyzing the interior structure of the sinkholes found that their interiors differ quite significantly, with some showing fractured material and terraces, others showing horizontal layers and vertical striations and others also showing globular structures nicknamed “goosebumps”.
“We think that we might be able to use the pits to characterise the relative ages of the comet’s surface: the more pits there are in a region, the younger and less processed the surface there is,” explains Vincent. “This is confirmed by recent observations of the southern hemisphere: this is more highly processed because it receives significantly more energy than the northern hemisphere, and does not seem to display similar pit structures.”
Active pits on Churyumov-Gerasimenko.
Rosetta scientists are hopeful that the spacecraft might yet get to see the formation of a sinkhole in action. The probe did see one outburst during its approach to the comet back in April 2014, which generated between 1,000kg and 100,000kg of material. But although a pit collapse could have been responsible for this, it was much smaller than the researchers expect.
With the collapse of a typical large pit of 140m wide and 140m deep, the team would expect to see the release of around a billion kilograms of material.
“Being able to observe changes in the comet, in particular linking activity to features on the surface, is a key capability of Rosetta and will help us to understand how the comet’s interior and surface have evolved since its formation. And with the extension of the mission until September 2016, we can do the best job possible at unravelling how comets work” says Matt Taylor.
New analyses of NASA airborne radar data collected in 2012 reveal the radar detected indications of a huge sinkhole before it collapsed and forced evacuations near Bayou Corne, La. that year. Researchers believe that they can use this type of data, usually routine gathered, to foresee at least some of the sinkholes.
The sinkhole at Bayou Corne, Louisiana.
I’ve written a comprehensive article about sinkholes (what they are, how they form, and how you can stay safe) which I really recommend you read if you live in a sinkhole-threatened area, or if this is of interest to you. If you don’t want to read it however, I’ll sum it up for you: sinkholes are are natural depressions (or holes) in the surface of the Earth’s surface, (usually) caused by karst processes. Karst processes occur when the bedrocks are soluble – in other words, in 99% of all cases, in carbonate rocks (like limestone or dolomite) or evaporitic rocks (like gypsum or anhydrite). They usually form without apparent warning, and can be extremely dangerous.
NASA collected their data as part of an ongoing campaign to monitor sinking of the ground along the Louisiana Gulf Coast. Researchers Cathleen Jones and Ron Blom of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, analyzed interferometric synthetic aperture radar (InSAR) imagery of the area acquired during flights of the agency’s Uninhabited Airborne Vehicle Synthetic Aperture Radar (UAVSAR). InSAR is a radar technique used in geodesy and remote sensing, detecting and measuring very subtle deformations in Earth’s surface.
Analyses by NASA’s UAVSAR after the Bayou Corne, La., sinkhole formed show it detected precursory ground movement of up to 10.2 inches (260 millimeters) more than a month before the sinkhole collapsed. Colors represent surface displacement (one full color wrap equals 4.7 inches (120 millimeters). Image Credit: NASA/JPL-Caltech
Their analysis showed the ground surface layer deformed significantly at least a month before the collapse moving mostly horizontally up to 10.2 inches (260 millimeters) toward where the sinkhole would later form. This may not seem like much, but the ground surface doesn’t just move a quarter of a meter for no reason – and the deformations were observed over a much larger area than the actual sinkhole – about 1,640 by 1,640 feet, (500 by 500 meters). The sinkhole measured about 10×0 meters.
The results of this study are published in the journal Geology.
“While horizontal surface deformations had not previously been considered a signature of sinkholes, the new study shows they can precede sinkhole formation well in advance,” said Jones. “This kind of movement may be more common than previously thought, particularly in areas with loose soil near the surface.”
The problem with sinkholes is that they hit hard – and fast. The Bayou Corne sinkhole formed on Aug. 3, 2012, after weeks of minor earthquakes and bubbling natural gas that provoked community concern. It was clear that something’s going on, but it’s hard to estimate just how urgent the situation really is.
“Our work shows radar remote sensing could offer a monitoring technique for identifying at least some sinkholes before their surface collapse, and could be of particular use to the petroleum industry for monitoring operations in salt domes,” said Blom. “Salt domes are dome-shaped structures in sedimentary rocks that form where large masses of salt are forced upward. By measuring strain on Earth’s surface, this capability can reduce risks and provide quantitative information that can be used to predict a sinkhole’s size and growth rate.”
The good thing is that at least in the US, NASA gathers this type of information relatively frequently – and not only do they gather it, but they also share this unique knowledge with the global community, working with institutions in the United States and around the world, so hopefully, we’ll be getting a “heads up” when it comes to sinkholes.