What Is A Sinkhole?

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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.

Carbonates are especially vulnerable to acidic groundwater the same way that oyster and other seashells are vulnerable to ocean acidification. In this case, we can blame carbon pollution.

Increasing carbon dioxide in the atmosphere leads to rainwater containing higher levels of carbon dioxide, otherwise known as acid rain. When carbon dioxide in water is given a carbonate ion, it prefers to make bicarbonate ions rather than staying happily in the water minding its own business. Any rock or shell made of carbonate then becomes more at risk of dissolution as the ocean and rainwater become more acidic, much like tooth enamel dissolves away when soaked in a glass of soda pop.

Once the bedrock starts to dissolve and erode away, the type of surface sediment makes the difference as to whether the sinkhole will be catastrophic or not.

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If little to no surface sediment covers the bedrock, for example, then the consequence is likely to be noticed immediately as the hole first begins forming; usually because drainage patterns in the area change. Rainwater that used to wash away will begin to collect, forming ponds and wetlands. This type of sinkhole is a called a dissolution sinkhole.

In cases where the bedrock is overlaid with sand or another type of permeable layer, the surface will collapse at the same time as the hole is forming. Like sands in an hourglass, the surface will fall in to fill the opening as it is made in the bedrock. This type of sinkhole is called a cover-subsidence sinkhole.

The most frightening and dangerous cover-collapse sinkholes, however, form when the surface sediment is mostly clay and sitting on either evaporate or carbonate bedrock. Rather than collapsing in on the bedrock holes as they are made, the surface remains deceptively intact while just beneath it a growing hole is eating away at its integrity. Often, the top surface layer acts as a bridge over the hole as the lower levels of clay beneath it wash away until the entire structure collapses.

According to the USGS, the states most at risk to sinkholes are: Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania.

IMAGE: Aerial picture of a sinkhole at 240 Faithway Drive in Seffner, that opened up, killing Jeff Bush while he was in bed. The sinkhole is exposed as demolition of the house continues. (Dirk Shadd/Tampa Bay Times/Corbis)