Dips are really scary. In addition to the dangers that lie in wait for us on the surface of the earth or above it, the earth itself can suddenly go out from under our feet, engulfing us along with a house or a vehicle. What are sinkholes and how are they formed?
These are literally holes created by subsidence of the earth due to a combination of three factors: water, erosion and time. Flowing groundwater seeps through granular soil, stones, sand in the topsoil and even through grains of the main bedrock. If the bedrock is composed of something like gypsum, limestone, or dolomite, the water can simply wash it away, dissolving the fragile deposits. If these rocks are continuously or repeatedly exposed to water, large holes or even vast cave systems can form in them. High acidity of water, for example due to absorption of carbon dioxide, can significantly speed up the process.
Water-corroded bedrock fractures, eroding deeper sedimentary layers. In addition, it happens that water first destroys the internal bedrock and then, making a hole, rises to the overlying (uppermost) layer of the earth. After the overburden is eroded from the bedrock area, the continuation of the process depends on the type of soil.
If the overburden is loose sand, the bottom particles will wash away or sink through depressions or cracks in the bedrock. The loose layers above will follow, causing the entire overburden to plummet downward to fill the void. This can happen quickly or slowly depending on the rate of erosion of the bedrock. As a result, a sinkhole can work its way up to them. Such dips are called sinking.
Much more dramatic are the cases when the covering layer is made of clay. Since clay sticks together much more easily than sandy soil, it does not all drain into the hole in the bedrock. Instead, the flow of water constantly erodes the bottom of the clay layer, creating a large karst void that moves closer and closer to the surface. When the remaining layer can no longer withstand the force of gravity, it falls into the abyss underneath. If at this place there is a residential building or a salon full of valuable cars, then the owners will have to say goodbye to property, and maybe to life. Such dangerous craters are called sinking craters.
A video from the US state of Louisiana shows how coastal trees fall due to a hole that formed underwater.
Scientists know quite well which areas are most prone to sinkholes. Based on geological studies, this is typical of karst landforms and "vulnerable" rocks (salt and gypsum). One of the signs of an area prone to failure is the extensive cave systems carved into the bedrock. NASA recently announced that, using radar data from satellites, they can track ground movement and detect sinkholes in a preliminary stage.
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