Courtesy photo by N.H. Department of Transportation The stretch of I-93 where a sinkhole opened up Aug. 19 was built atop an older road, portions of which can be seen here. Below that is a century-old water line made of brick which failed, leading to the sinkhole.

Yes, there was a hole, and yes, you could sink into it. But from a geological perspective, the traffic-snarling chasm that opened up on Interstate 93 in Concord last week wasn’t really a sinkhole.

Texas may have sinkholes and Florida may have sinkholes – sometimes the Sunshine State seems to be nothing but a giant sinkhole in waiting, punctuated by the occasional invasive anaconda – but not us. We’re the Granite State and true sinkholes don’t happen in granite.

Natural sinkholes occur in what geologists call karst terrain, in which the underlying rock can be dissolved by groundwater. Limestone is the usual culprit, but the U.S. Geological Survey identifies a number of problematic soils and rock types, including gypsum and salt domes.

None of those is found in New Hampshire.

As you’ve no doubt heard, the pseudo-sinkhole that surprised drivers near the Exit 14 interchange Wednesday was man-made, albeit indirectly. A century-old pipe, built of brick and mortar by Concord for its sewer or water system prior to World War I, leaked water that ate away sand and gravel under an old roadbed that had supported this stretch of I-93 since it was built about 1956.

When enough of the supporting sand and gravel was gone, a 15-by-15-foot section of road became a 25-foot-deep hole with little warning.

Despite the suddenness of the collapse, perhaps prodded by heavy rains the day before, this process had been going on for many years.

“I call it a gestation period,” said Ernst Kastning of Concord, a retired professor of geology at Radford University. “The time from when sand starts being washed way to the final collapse can be as much as decades.”

Kastning knows whereof he speaks. He has been studying sinkholes for almost 50 years, and his other home in Virginia is actually built inside one – although he noted that it’s in a solution subsidence sinkhole that has existed for thousands of years, rather than a collapse sinkhole.

In some ways, he said, the I-93 hole appeared on schedule.

“From my observation, these start to happen after about 30 to 50 years,” he said. I-93 is among the oldest stretches of our interstate system, nearing the ripe old age of 60, and even the youngest bit of the system in Franconia Notch is entering its third decade.

These roads, like the state turnpike system, cross over plenty of pipes – thousands of them, from 12-inch culverts to 10-foot-wide crossings that might as well be bridges – said Dave Rodrigue, director of operations for the state Department of Transportation.

Dave Smith, assistant administrator of the Bureau of Turnpikes, said the turnpike system has an inspection program to identify problem sites and is working with the technology transfer program at UNH to create an interactive database that can be updated by workers in the field from handheld devices.

And while underlying pipes that are old enough to be built of brick and mortar are unusual, they’re not unusual enough.

“This type of a construction of culverts is likely in older urban areas. Manchester I would expect would have them, and Concord; you may find them in Dover, Portsmouth,” Smith said. Old mill buildings, which tended to create lots of outflow pipes to the nearest waterway, are particularly suspect.

What this means, I fear, is that we can’t be as snooty toward our limestone-underpinned brethren as our state nickname would lead us to believe. The granite beneath our feet, it seems, keeps us from worrying about house-swallowing sinkholes but not necessarily car-swallowing ones.

(David Brooks can be reached at 369-3313, dbrooks@cmonitor.com or on Twitter @GraniteGeek.)