Following the tragic death of a Florida man who was swallowed by the Earth in his sleep, the geologic phenomena of sinkholes has gained a great deal of national attention. But what are they, exactly?
The U.S. Geological Survey classifies three types of sinkholes. The scariest is the type that killed Jeff Bush as it opened up under his bedroom: a cover-collapse sinkhole, where the surface sediment is mostly clay and remains intact as the bedrock and lower levels of the clay are dissolved beneath it. In these situations, a sinkhole can form suddenly and catastrophically over the course of a few hours, with little to no earlier signs of danger.
Two aspects of the geology are important in identifying sinkhole risk: bedrock and surface sediment.
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If the bedrock is one of two types, or rock that can be easily dissolved by groundwater, then what type of surface sediment above it is critical in evaluating risk.
Bedrocks made of salt, gypsum and anhydrite are known as evaporates, whereas bedrock made of limestone and dolomite are known as carbonates. Evaporates dissolve easily because they were originally formed from minerals precipitating out of solution during periods of drought. Adding groundwater then puts the minerals back into solution, eroding the bedrock and leaving divots, cavities and even chasms of space for the surface sediment to fall into and fill.