What is a sinkhole? A sinkhole is a place where the ground has collapsed to leave a hole, caused by material below the ground being removed. Sinkholes can range from just a few meters deep to enormously deep, and they can be caused naturally or by human activity. The largest sinkhole in the world is in China. It is called Xiaoxhai Tiankeng, which means “heavenly pit”. It is 660 m deep, and it has a volume of 130 million cubic meters. Both natural and human caused sinkholes happen because material underneath the ground has been removed. It is usually removed by water, but it can also be removed by mining or pumping.
A lot of sinkholes form in areas where the rock is limestone. If there is a lot of rainfall or an underground river flowing into the limestone, it can start to dissolve. Water dissolves limestone very easily because, as it flows through the soil, the water absorbs carbon dioxide, which makes it slightly acidic. The carbon dioxide comes from all of the microbes and bacteria that are breaking down the dead matter in the soil. When carbon dioxide dissolves in water, it is called carbonic acid and, interestingly, is the reason why carbonated water tastes slightly sour. Carbonic acid is not strong enough to dissolve most types of rock, but it can dissolve limestone. The carbonated water flows into cracks in the limestone. It takes a very long time, but it slowly dissolves the limestone and makes the cracks bigger. Over thousands, sometimes tens of thousands of years, the carbonated water can dissolve enough of the limestone that a huge hole develops. This will be a sinkhole.
Sinkholes can emerge in three different ways. They all begin with carbonated rainwater dissolving the limestone under the surface. In the first case, solution sinkholes, the limestone starts to dissolve at the surface, and a depression is formed. Over time, more rainfall falls, and there is a gradual increase in the depression until it fills with water. At this point, the water can flow down into the limestone and continue the erosion, or it can get blocked with soil and just become a lake. The second type is called cover-subsidence sinkholes. Rainwater comes down through the soil and dissolves the limestone. Soil and sand from the surface slowly trickle down into the expanding hole, and the surface gradually drops. This is a slow process and not that damaging. It might slow down as the soil fills the gaps. The third type is called cover-collapse sinkholes, and these are the most dangerous and the most damaging type. They begin in the same way as the second type of sinkhole. Carbonated water dissolves the limestone, and the sand and soil from the surface start to trickle down. The difference here is that the top layer of the ground stays in place, forming an arch. The limestone hole gets bigger, more soil falls down, and the top of the arch gets thinner and thinner until it can’t support itself anymore. At that point, it collapses into the very deep hole that has formed underneath it. This can cause a lot of damage and even fatalities.
Sinkholes can also form in different types of ground. Salt is another common mineral that sinkholes form in because it can be dissolved very easily. There are some areas where sinkholes have formed because the salt has been mined or pumped out of the ground, and this is where human caused sinkholes form. There are many ways of making salt, but one of the methods is to mine it from under the ground. If a large area of salt is found, it can be mined by pumping in water, which dissolves the salt, and then pumping out the water. The salt can then be extracted from the water. The problem with this is that the salt was supporting the ground, and if you remove it, the ground can collapse. The land around the Dead Sea has a lot of sinkholes because it is an extremely salty area with lots of rock salt. Humans can cause sinkholes in other ways as well. They could come from underground water systems that begin to leak. Lots of water leaks into the ground, which dissolves the bedrock, and sinkholes form. Mining can also cause sinkholes because it removes large amounts of earth, which can weaken the ground. They also alter the water table sometimes. Sinkholes can appear in cities sometimes as well because buildings can funnel rainwater and concentrate it in certain places, where it starts to sink into the ground. Sinkholes in cities or built-up areas can be disastrous and result in loss of life. And this is what I learned today.
Sources
https://www.usgs.gov/faqs/what-difference-between-a-sinkhole-and-land-subsidence
https://www.usgs.gov/faqs/what-difference-between-a-sinkhole-and-a-pothole
https://en.wikipedia.org/wiki/Sinkhole
https://education.nationalgeographic.org/resource/sinkhole
https://www.bbc.com/travel/article/20221117-xiaoxhai-tiankeng-the-worlds-biggest-sinkhole
https://igws.iu.edu/outreach/lessonplans/dissolving
https://en.wikipedia.org/wiki/Carbonated_water
Photo by Robert Senz: https://www.pexels.com/photo/view-of-a-cave-15319512/