What causes sinkholes?
Thursday, July 19, 2018
The rock layer under the entire state of Florida is a limestone, called “karst,” which was deposited by the plants and animals of an ancient sea that covered our peninsula for an estimated 60-million years. In more recent geologic time, the sea receded to its current level and the karst was overlaid with the sand and clay soil that sits under our houses today.
Unfortunately, karst is susceptible to being slowly dissolved by slightly acidic water. As rainwater filters through organic debris on the ground it gains acidity which, in turn, slowly dissolves the limestone as it drains through it—especially along any fractures in the rock. The chemical erosion can eventually create cavities that slowly enlarge, and the overlying soil eventually falls into the void, causing the a sinking or collapse of an area of soil that is a sinkhole.
There are three primary types of sinkholes:
- Limestone solution sinkholes - In some areas of Florida, the limestone is exposed or covered by only a thin layer of soil, so it is continuously exposed to the chemical breakdown process. These voids develop slowly.
- Cover subsidence sinkholes - Where there is a thick layer of sand over an area of dissolving karst limestone, the granules of sand continuously fill the void as it develops. Cover subsidence sinkholes are usually just a few feet in diameter and depth because they cannot develop to any significant size before being filled in with sand.
- Cover collapse sinkholes - These are the bad boys that fall suddenly. Where the soil above the void is both deep and composed of a high percentage of clay, the cohesiveness of the clay holds the soil together to form a natural bridge over the pocket. But, eventually the width of the sinkhole reaches a span that is unsustainable and it collapses.
Natural sinkholes have been occurring for thousands of years, and many lakes in our area were created by sinkholes. But man-made changes like the creation of retention ponds, excessive withdrawal of water from the aquifer, heavy traffic, and building construction can speed up the process—and these are labeled as “induced” sinkholes.
Also, see our blog post Where are sinkholes most likely to occur in Florida?
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