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Science Spotlight: How do sinkholes form?

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Scott Sutherland
Meteorologist, theweathernetwork.com

Tuesday, May 20, 2014, 5:58 PM - Sinkholes are scary. As if we don't already have enough to worry about in life from things on the ground or above it, the ground itself can suddenly open up under our home, our feet, or even our vehicle when we're travelling along at highway speeds, with little to no warning. So, what's happening here? Exactly what are sinkholes, and how do they occur?

Sinkholes have one of the most straight-forward, self-explanatory names we've ever given to something, because they are - quite literally - holes that form from sinking earth. 

The process behind how they form is fairly straight-forward as well. It's just a combination of water, erosion and time. As water flows through the ground, it filters its way through the grains of dirt, rock and sand in the top soil and even through the grains of the underlying bedrock. If that bedrock is made of something like gypsum, limestone or dolomite, the water can wear it away, simply by dissolving away some of the sediments that make up the rock. This can take a long time with normal water, but with constant or repeated exposure, this can carve large holes and even extensive cave systems through the bedrock. However, if the water happens to be acidic, such as from absorbing carbon dioxide before it trickles down through the top-soil, this can dramatically speed up the process. In either case, the water will eat away at the bedrock, possibly cracking it apart and exposing deeper layers of the sedimentary rock to the erosion as well. It's also possible that the water could eat away at the interior of the bedrock first, with the hole eventually working its way up to the soil resting on top - which is called 'overburden' for very obvious reasons.

Once the overburden is exposed to the eroded area of bedrock, exactly what happens will depend on what kind of soil it is.

If the overburden is a loose, sandy soil, the bottom grains will wash or sink down through the depression or cracks in the bedrock, and the loosely-packed layers above will follow, causing the entire overburden to slump down to fill the void. This will continue, sometimes fast or slow depending on how quickly the bedrock erodes away, and the sinkhole can eventually extend all the way down to the bedrock. This is called a cover-subsidence sinkhole.

However, a far more dramatic case is when the overburden is made of clay. Since the clay sticks together much easier than sandy soil, it won't all slump down into the hole in the bedrock. Instead, the water flow constantly picks away at the bottom of the layer of clay, forming a larger and larger cavern that comes closer and closer to the surface. When the strength of the remaining layer can't hold up to the gravity pulling it down, it will suddenly give-way into the abyss below. In fortunate cases, there won't be anything but open ground above the abyss, but in other cases, this can instantly swallow a showroom full of valuable cars, or open up underneath a car on the road, under someone's home, or even under their feet. Appropriately, these are known as cover-collapse sinkholes.

As the above video from Louisiana shows, we don't necessarily see all of these holes form, as this one opened up underneath the water, dragging several trees down into it. 

Where these sinkholes happen is something that's not as straight-forward as their name and how they form. Scientists have a good idea of the areas that are most prone, simply based on geological surveys and knowing that they form most often in Karst landforms and 'evaporate' rocks (salt and gypsum). One way to tell if you live in one of these 'prone' areas is if the region is known for having extensive cave systems carved into the bedrock. Geological surveys can have a wealth of information on the subject, but another more readily available source is from groups that seek these areas out as a hobby (ie: cavers). 

However, actually knowing when and where they're going to form is another matter. There is some hope, though. Recently, NASA announced that by using radar data from airborne missions and satellites, they can monitor the movement of soil to catch some sinkholes before they open up. There's no telling when such a 'sinkhole forecast system' might actually become a reality, though, so until then it seems we're just going to have to stay informed and take our chances. 

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