An Illinois golfer emerged alive after the green gave way beneath him on the 14th hole last week, but the Florida man whose bed-room was swallowed whole is believed dead. A Pennsylvania driveway fell into a 12-metre-wide hole on Sunday, a chunk of the U.S. capital's sidewalk sunk three metres on Tuesday.
With the earth sinking beneath our feet, The Globe and Mail's Kathryn Blaze Carlson digs into the mystery of sinkholes with Sam Panno, a senior geochemist at the Illinois State Geological Survey.
Starting with the basics, how do sinkholes happen?
The kind we see in southwestern Illinois, where the golf-course incident occurred, are referred to as cover-collapse sinkholes. There's about 30 feet of soil and then there's fine silt below that. That stuff sits on limestone. The reason there are so many sinkholes in that area, which is called Illinois' Sinkhole Plain …
Is that the colloquial name?
No, that's actually the geological term for the area. There are over 15,000 sinkholes, so if you're walking, you're basically walking the ridges of sinkholes no matter where you go.
Frightening. Okay, back to what you were saying …
When there's a rain event, the water seeps through the soil and picks up a lot of carbon dioxide, forming carbonic acid that dissolves part of the limestone and enlarges [existing fractures]. The overlying soil becomes unstable and can collapse into the crevasse, and then groundwater washes it away and then more soil falls in. What starts to form is a dome-shaped structure.
And then the dome sort of implodes?
It actually kind of works its way to the surface and just peels off. In the case of the golf course, the crest of the dome was basically the grass, the roots of the grass and a little bit of soil. When the gentleman walked over to see what the [bathtub-shaped] depression was, the only thing between him and the hole was probably an inch or so of grass.
Other than watching out for bathtub-shaped divots, how can we know where these precarious domes are forming?
You can actually sometimes hear [small collapses]. Rain will cause groundwater to flow rapidly and wash away some of the material that has already collapsed. Homeowners will say they heard splashing in their yard and then there's a sinkhole the next morning.
How big do these sinkholes get?
In southwestern Illinois, the soil is 10-metres thick, so that's the deepest it's going to be. [Elsewhere], you can get really huge holes, but those are more related to salt mining. You can also have [a rock] cave collapse – that'll make an enormous hole. And you can have collapses of old coal mines after a while, too. If it happens near your house, you can have foundation problems.
Does anyone inspect for sinkholes before building or buying a house?
Not usually, no. Usually somebody sells the property with sinkholes on it. If [workers] dig and hit a hole, sometimes they'll fill it in. But that creates problems because the material dumped in will wash away and the sinkhole will come back after you've built the house.
How do you know if part of your house is about to collapse into a sinkhole?
You don't. And we don't know how frequent that is because people don't advertise it, knowing they'll never be able to sell.
What's the sinkhole landscape in Canada? Should we be nervous?
I don't think there's much in the way of sinkholes because you have crystalline, granitic bedrock. Rarely does that [fracture] enough to form sinkholes. It doesn't dissolve like limestone does and you really don't see much in the way of crevasses in that kind of rock.
What are the chances someone falls into a sinkhole and comes out alive?
[The golfer] was incredibly lucky. You fall into something like that and it's likely that other materials are going to fall on top of you.
Can we do anything to prevent sinkholes?
It's one of those things where they happen, and then you deal with it.