Why are there so many sinkholes in Florida?

Sinkhole 1

Why are there so many sinkholes in Florida? The question is a common one, and took on a new urgency this week after the re-opening of a Floridian sinkhole two years after it swallowed a man’s bedroom in the dead of night. Jeff Bush, of Tampa, was sucked into the sinkhole in February of 2013–while his brother Jeremy watched in helpless disbelief.

In the wake of that incident, the sinkhole was filled in, and Hillsborough County purchased the land so that no one could build property on it. However, yesterday, the sinkhole–which is 20 feet in diameter–re-sank, to the horror of nearby residents and the wonder of engineers. Those engineers say they don’t believe anyone else is at risk, and the county is at pains to reassure its citizens.

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However, Hillsborough County is part of what’s known in Florida as “Sinkhole Alley,” because of the prevalence of this very specific natural disaster to the area. According to a CNN study, Sinkhole Alley accounts for two-thirds of all sinkhole-related insurance claims in the entire state of Florida–which is itself the most sinkhole-prevalent (and notoriously sinkholey) state in the country.

So: Why are there so many sinkholes in Florida?

In order to understand sinkholes, we must first define sinkholes. According to the United States Geological Survey, a sinkhole is “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.” That drainage becomes a problem in areas where the underlying rock is easily dissolved: “the processes of dissolution, where surface rock that are soluble to weak acids, are dissolved, and suffosion, where cavities form below the land surface, are responsible for virtually all sinkholes in Florida.”

There are two categories of easily-dissolved rock: the first is made up of carbonites; such as dolomite and limestone; the second is comprised of evaporites (a name that’s easy enough to remember), which include in their ranks anhydrite, gypsum, and salt. Unfortunately for Florida, the state has a shallow limestone base that’s just ripe for the dissolving.

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There are three types of sinkholes; two are responsible for almost all sinkhole activity in Florida. The first is known as a dissolution sinkhole, which is exactly what it sounds like: rainfall and surface water attack existing joints in the surface rock, dissolve it over time, and carry it off, leaving a depression in its place. And, because gravity is a thing, depressions accelerate dissolution: more and more water accumulates in the void, eating away at whatever vulnerable rock is still around.

The second is called a cover-subsidence sinkhole, and is far more sinister. With this type of sinkhole, water sinks into the sediment (such as soil, or sand) above the surface rock, and eats away at the invisible rock beneath. It’s therefore easier to be surprised–say, in your bedroom in the middle of the night–by a cover-subsidence sinkhole, since simply looking at the ground or feeling the floor won’t necessarily give away that the surface is rotting away…from beneath.

The third and less-common type of Florida sinkhole is called a cover-collapse sinkhole. It’s similar to a cover-subsidence sinkhole, except that the underlying structure is not rock, but clay. Because clay is softer and more susceptible to decay than rock, cover-collapse sinkholes often form very quickly: “over a period of hours,” says the USGS. It’s also for that reason that their damage can be “catastrophic.”

Sinkhole 2

Most of the American states at high risk of sinkhole activity are in the south–Alabama, Kentucky, Missouri, Tennessee, and Texas are further prime candidates. In the north, Pennsylvanians are the most likely to suffer from sinkholes.

Florida remains the unfortunate king of sinkholes, though. The state’s Office of Insurance Regulation logged 6,700 sinkhole damage claims in 2010 alone, and cited a total sinkhole-related cost of $1.4 billion from 2006 to 2010–for an average of $280,000,000 in sinkhole damages per year.

Fortunately for Floridians, there is one new tool in the ongoing fight against sinkholes. It’s called InSAR, and it’s a NASA-based Interferometric Synthetic Aperture Radar, which drones and satellites can use “to show subtle movements of the ground surface.” InSAR was developed to study volcanoes and earthquakes primarily, but, in the words of GovExec: Think of it as an ultrasound for Mother Earth. Because not all sinkholes deform the surface of the earth before they collapse, though, InSAR “is not a magic bullet,” according to Jet Propulsion Laboratory geologist Ronald Blom. “But it could be a useful tool as part of a more complete observational scheme.”


(Photo credits: Sinkholes in Florida: Sinkholes one, two, three, four via Flickr)

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