It’s an unnerving sight. A hole opens in the ground in front of a home, swallowing part of the yard and driveway. The phenomenon isn’t an everyday occurrence, but some of Pennsylvania’s land is vulnerable to sinkholes.
A sinkhole develops when water dissolves rock like limestone and dolomite, according to the Pennsylvania Department of Environmental Protection. The easily dissolved rock is eaten away when the water and carbon dioxide mix to form an acid. The landscape where this type of rock exists is called karst.
Franklin Institute geologist and chief astronomer Derrick Pitts says over time, a cavern forms undermining the ground’s stability.
“As the cavern is growing, the support for the ground above decreases and eventually what happens is that land eventually subsides, settles or falls into this cavity,” he said.
The Pennsylvania Department of Conservation and Natural Resources considers sinkholes one of the state’s serious geologic hazards. The PA DCNR says 7-percent of the state’s 44,000 square miles sits on the sinkhole-vulnerable karst.
State surveys found swaths of karst in sections of Chester, Montgomery, Lehigh and Berks counties and the state has recorded many sinkholes and surface depressions in these areas.
Pitts says there’s no way to know how many sinkholes have formed in the state, but he says they’re not nearly as common as in another sinkhole prone state – Florida.
“Florida has what can be referred to as a very heavy limestone bearing region and the limestone that’s found…is one where the limestone is very easily dissolved and can become sinkholes,” he said.
In February, a Florida man was killed after a sinkhole suddenly formed under his bedroom. Jeff Bush, 36, and his bedroom furniture were swallowed by the 30-foot-deep hole that spanned 20-feet-wide. The home was demolished and Bush's body was never recovered.
Pitts says the chances of a sinkhole swallowing your home are slim, but he says it is important to know what type of rock you’re standing on.
“These things don’t form overnight,” he said. “They take hundreds, if not thousands of years to form, especially to the point that the surface is going to suddenly collapse and something the size of the sinkhole in Florida appear.”
Closer to home, a sinkhole opened in Sunday in the yard of a Bethlehem Township home -- an area where many sinkholes have been recorded. The home was evacuated for some time, but the family was allowed to return to the home Monday.
The Bethlehem Fire Department is investigating whether the sinkhole naturally occurred or was caused by nearby sewer work.
Living in a karst region isn’t the only place sinkholes can form. Pitts says underground plumbing can cause a sinkhole to form pretty much anywhere.
“Sinkholes can easily develop in an urban area like Philadelphia because of the extraordinary underground network of piping and conduits and things like that where subsurface water can carry away the underground support and you can have a sinkhole develop because of that.”
As for what happens after a sinkhole opens, Pitts says a survey of the area is needed to decide whether the area can be filled in and used again or if the hole needs to be left open.
“The question is how unstable is the surrounding area," he says. "Because if the surrounding area is just as unstable as the one place where the sinkhole developed, it could be that the sinkhole is going to continue to enlarge…”
The Pennsylvania Department of Conservation and Natural Resources has plotted all known sinkholes and surface depressions on an interactive map. See if there are any in your area, by clicking here.