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The Science of Sinkholes

Dave Fairburn via Facebook

This week a sinkhole in Interstate 93 near Concord prompted an emergency road closure and a major traffic jam. Despite the disruption it caused, the cavity doesn’t actually meet the definition of a sinkhole, but it maybe the sort of thing that the Granite State could see more of.

To answer how can a gaping hole suddenly appear in the ground, you need to start somewhere in the sky.

As David Weary, geologist with the US Geological Survey explains, all rainwater is acidic.

“When the little drops fall down through the atmosphere they pick up a little bit of carbon dioxide and that forms a weak acid called carbonic acid.”

When Weary says this acid is weak, he means very, very weak. Think black coffee or a banana. But over time, if you’ve got the wrong kind of bedrock, a cup of coffee or rainwater is acidic enough to hollow out huge caverns beneath the surface.

“It’s a fairly slow process," Weary explains, "it takes probably tens of thousands of years to produce caves.”

A sinkhole happens when the ceiling of an underground cavern like this collapses. Sometimes it happens slowly and gently, other times suddenly and dramatically.

Credit US Geological Survey
This map shows the areas most susceptible to naturally occurring sinkholes.

The sinkholes you sometimes see on the news in places like Florida, were tens if not hundreds of thousands of years in the making. To get swallowed up by one is the geological version of being in the wrong place at the wrong time.

Which brings us back to this week’s incident on I-93. But in this case, the geology lesson we just had doesn’t apply. In fact, geologists wouldn’t even call it a sinkhole.

Rick Chormann, state geologist and director of the New Hampshire Geological Survey explains that “the phenomenon that occurred on I-93 was more an engineering, hydro-technical event.”

So it wasn’t the slow process of rainwater dissolving bedrock that caused the gaping hole in I-93. Instead, the culprit was a leaking brick culvert that runs under the highway.

“The phenomenon is called piping," says Chormann. "It’s essentially water under pressure following the path of least resistance and having sufficient force to move particles along its path.”

The cracked culvert meant fast moving flood water was carving out the road-bed under 93, instead of following the brick pathway to the Merrimack River.

For Chormann the event is a signal that the state’s transportation system needs attention.

“The issue that we’re facing is a lot of our infrastructure," says Chormann. "Whether it’s water supply distribution pipes or storm water conveyances, culverts and the like, is getting old.”

And according to Chormann, these man-made sinkholes are really the only ones you should be worried about. The chances of a naturally occurring sinkhole here?

“In New Hampshire, there’s essentially virtually no chance of that.”

That’s because the Granite State is well, granite.

Places with real sinkholes tend to be made of softer stuff like limestone. So there’s really no cause for concern when it comes to real sinkholes.

But the bad news is that if the state wants to avoid more holes like this one opening up in the future, there’s work to be done.

Jason Moon is a senior reporter and producer on the Document team. He has created longform narrative podcast series on topics ranging from unsolved murders, to presidential elections, to secret lists of police officers.
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