What causes Sinkholes: Geological Phenomena With Human Cost

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Sinkhole in Florida. Image credit: USGS

This sinkhole in Florida opened suddenly. Image credit: USGS

It made for dramatic news coverage when a suburban house fell into a sinkhole in Florida… but the fact that the death of Jeffrey Bush, who couldn’t be found after the collapse of his home, was followed by the opening of a second sinkhole nearby has focused attention upon what is actually a relatively common occurrence.

What is a Sinkhole?

The United States Geological Survey identifies a sinkhole as, “an area of ground that has no natural external surface drainage – when it rains, all of the water stays inside the sinkhole and typically drains into the subsurface.” It’s not a dramatic description, and in most cases sinkholes are themselves not dramatic: They generally form over a period of years, evolving slowly into bowl-shaped depressions which barely merit a glance from passers-by, rather than suddenly collapsing as has happened twice recently in Florida.

You can divide sinkholes into three types.

One type, typical features of limestone scenery, are the result of dissolution of carbonate rocks by rain and surface water. These, typified by such stunning examples as Belize’s Great Blue Hole (an underwater sinkhole) are known as dissolution sinkholes.

The gently-evolving holes which best fit the USGS’s description above are known as ‘cover subsidence‘ sinkholes, and their morphology is accounted for by an underground void (usually in limestone) overlain by a typically sandy soil through which water percolates downwards. Over time the overlying soil trickles into the hole: hence the subsidence.

Differences in soil type, however, are influential in the evolution of sinkholes.

Where the overlying soil has a high clay content, the development of the sinkhole is different. Although groundwater still leaches soil downwards, the cohesion of the clay soil holds a ‘bridge’ of substrata above the void like a roof. The net result is that the hole migrates upwards, the covering layer becomes gradually thinner – and eventually collapses. It was one of these ‘cover collapse‘ sinkholes that took the life of Jeffrey Bush.

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