Natural or man-made; an expert explains the difference for sinkholes

Published: October 7, 2021 4:07 PM EDT
Updated: October 7, 2021 6:08 PM EDT

Two big holes have opened up in the ground in recent days, but neither were from natural causes. From a fireline break in Harbor Isle to a water main break in Collier County, it begs the question; how do we know if we’re dealing with a sinkhole or something man-made?

The short answer is any time there is construction or human activity, that is going to alter the layers of rock beneath the surface, which makes it a challenge for geologists to say for certain if it’s a sinkhole or a human-induced collapse.

However, a deeper understanding of what’s beneath our feet helps.

Two holes were left behind about one week apart after a fireline and water main break. But are they really sinkholes? Florida SouthWestern College’s Professor of Geology Doctor Joseph van Gaalen says no. “Not in the sense of a geologic perspective it isn’t because of the way we view it, but from the verbiage, yeah, it’s still a sinking hole.”

In simple terms, sinkholes result from rocks dissolving underground, and they can happen quickly.

“If you have water flowing through it, it will dissolve, and now you’re left with a void and so in the case of a cover collapse, this is the rapid one, if you have this void and that cover is very thin, then you’ll sink rapidly,” said Dr. van Gaalen. “You can also have a case where the material underneath is dissolving that you’re standing on and so that one is where you’re going to be slowly sinking.”

And once there’s construction in a given area, it makes it trickier for geologists to determine if a sinkhole is from natural or human causes. Dr. van Gaalen said, “any kind of construction anywhere, you’re going to have some changes that are occurring in the substrata, the layers beneath the ground.”

But the good news is, van Gaalen says sinkholes aren’t common in areas like Lee and Collier Counties because of the rocks’ thickness between where we stand, and where rocks would dissolve. “In most of Lee County and Collier County, those parts of areas, Cape Coral, Fort Myers and so on, most of those areas, that’s between 30 to 150, 200 feet, something like that, so it’s probably not too much of a concern.”

He went on to say that when you get further down beyond Marco Island and into the Everglades, that area thins out, but there’s little construction out there to cause a man-made sinkhole.