The particular susceptibility of the Sunshine State's porous ground to spontaneous gaping was made tragically clear last week when Jeremy Bush was sucked out of his bedroom and disappeared into the ground as he was preparing to go to sleep on an evening that his brother, Jeff, described as having been much like any other.
That is until a variation of the biblical story of Korah's rebellion -- see Numbers 16:31-32 -- played out in the Tampa suburb of Seffner:
"He had hardly finished speaking the words when the ground suddenly split open beneath them. The earth opened its mouth and swallowed the men, along with their households and all their followers who were standing with them, and everything they owned."
It was a freak occurrence that shocked the nation. Americans have been faced for years with photos of massive holes opening in the middle of a Guatemalan intersection, below Chinese buildings, and in a pastoral Russian town. They're not used to seeing their fellow citizens stolen by the abyss.
And when news broke Monday that a second sinkhole had opened up in Seffner, Floridians were left feeling as if the ground beneath their feet may give way at any moment. Quite literally.
Like earthquakes, no one can predict exactly when and where the next sinkhole might strike. But experts know what causes them and what areas are most likely to experience them, which can provide some modicum of comfort to people worried that they or their house may be the next to be consumed by the void.
Hydrogeologist Joe Yelderman, Ph.D., a professor of geology at Baylor University in Texas, explained some of the most common concerns and questions about sinkholes.
Here are some excerpts from a phone conversation, during which Yelderman addressed the reasons sinkholes form, what to do to ensure you don't fall victim to one, and more:
What exactly is a sinkhole, and what causes them to form?
Yelderman: A sinkhole is basically a depression that is formed as a result of collapse. Usually what that means is that there's a void underground, sort of like a hole or cavern, that at one time was probably filled with water, and as that water level drops, the roof can collapse. The water will sort of hold it up until it collapses. Sometimes also the roof gets thinner and it can't hold up the weight on top of it any longer.
If you add weight to the top or you thin the roof through dissolution so it has less support, it can collapse. All those things can cause a limestone roof to collapse, and that’s what we call a sinkhole.
What are the underlying conditions that cause sinkholes to form?
Yelderman: It doesn’t have to be limestone, but limestone is the rock where sinkholes are most common. In many cases there can be other rocks, maybe a gypsum formation, or an evaporite salt; those can cause a sinkhole as well. There are different kinds of rock formations that can be soluble like that, but the most common rock that can be dissolved like that is limestone.
What are the two types of sinkholes and how are they different?
Yelderman: Some sinkholes develop very slowly over time. It's a process where you get a depression on the surface that builds over time, but that doesn’t have the danger of a collapse. This is called a subsidence sinkhole.
The other sinkhole type [a cover-collapse sinkhole, described above] is caused when the roof over an underground void collapses suddenly.
Can you describe what makes Florida particularly sinkhole-prone?
Yelderman: Florida is sinkhole-prone mainly because it has these limestones that have a lot of cavernous development beneath them -- we call them dissolution features -- where the rocks have been dissolved. Those rocks have usually been dissolved down where the water table is, so it's near -- yet below -- the surface.
And the other thing that makes Florida susceptible is that Florida has a lot of groundwater usage via pumping that takes place. When you pump out groundwater you lower the water table, and that can induce collapse, especially if it's done very rapidly. It doesn’t always do it, but it can.
Sinkhole development can be caused by extreme groundwater usage over time, or they can also develop naturally over time and have nothing to do with pumping.
What other areas are most likely to be hit by sinkholes?
Yelderman: We have sinkholes in Texas where the Edwards Aquifer is, and they also take place in other limestone areas in states like Missouri and Arkansas. There are a number of different places around the United States that have sinkhole development, but probably Florida is the most famous for it, though they certainly do occur in other limestone areas. There is high potential for sinkholes wherever you find extensive limestone development and shallow groundwater.
Is there a reason why a particular area (for instance Seffner) would experience a number of sinkholes within a set period of time?
Yelderman: When you look at a cavern underground it usually follows a fracture and it's kind of a linear feature, though it may bend or curve or head upward or downward. So often you will get sinkholes that form as dots along a line where a fracture or a cavern is developed. Those kinds of things can occur. Some sinkholes are more random in nature, but it's common for them to occur in a pattern or a line.
Does the Seffner situation suggest a rising number of sinkholes either in Florida, nationally or worldwide?
Yelderman: I don’t know, I don’t have the data to determine that, but certainly it's not a world wide phenomenon, because it's unique to individual locations. Even within Florida, not all of Florida has the same potential for these kinds of problems.
But I think that if you have more than one sinkhole form in one area, such as [Seffner], around the same time, you should be careful because it suggests that more may form along that line. You would need to have somebody study the area to compare the potential risk from one area to another.
My guess is that the water table has been lowered [in Seffner]. Whether it's recently or over a period of years, that area has become susceptible to sinkholes.
If you're going to do anything in that area right now you have to take into consideration that several sinkholes have occurred and maybe more may form, and they're the collapse type so they can be dangerous.
Does heavy development really have an impact on the likelihood of sinkholes forming?
Yelderman: Development contributes in two ways: it adds weight to the top a lot of times, and it also requires water, so if it's getting it from the groundwater, that's pumping, which causes a decreased water table.
What can people do – if anything – to protect themselves from a sinkhole or ward them off beforehand?
Yelderman: The first thing that can be done is to have someone come in that understands the problems that cause them to do an analysis. And depending on how much money you want to spend, there are some things that can be done to investigate and learn more and get some data, whether that’s drilling or geophysical methods, but you need a geologist probably to do that kind of work.
And there are geological agencies, state agencies and even federal agencies that may have readily available information that may be helpful that you wouldn’t have to pay for.
In your opinion, what drives the fascination with sinkholes felt by many in the wake of the Seffner tragedy?
Yelderman: I think anytime there's a sudden earth movement that causes a disaster even if there's not death involved, whether it’s a a volcano or a sinkhole, it gets our attention.
I think people are interested in sinkholes, just like anything, any disaster, because they worry that they might affect them or someone they know.
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