The science of sinkholes: How do they happen?

A man from suburban Tampa is missing and presumed dead after his entire bedroom collapsed into a sinkhole last Thursday night. How did this happen? Can sinkholes show up anywhere? Can you tell if one is going to open up underneath you?

According to the U.S. Geological Survey (USGS), sinkholes can happen anywhere where the rockbed underneath the soil is limestone or dolomite rock, salt beds, or any other kind of rock that can be dissolved by water flowing through them. It might take just a matter of hours, or it might take thousands, or hundreds of thousands of years, but the result is the same.

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If the rockbed underneath the soil erodes away, there is nothing to support the soil and it collapses into the ground. How deep the resulting hole is depends on how much of the rockbed was eroded, and how it eroded.

If the rockbed erodes slowly, from the top down, the sinkhole can form very slowly, as the soil gradually slips down to fill the gap. If the rockbed erodes from the bottom up, the underground cavern it opens up can be quite deep by the time the top erodes away, and the soil above it plummets to the bottom.

Sinkholes are common in Florida, and the USGS has identified several other places in the United States where they are likely to happen, but what about Canada?

Within the last few years there have been several instances of sinkholes. Back in May 2010, a family of four was killed when their house in Saint-Jude, Quebec slipped into a sinkhole/landslide. About eight months ago, a 200-metre sinkhole opened up underneath Highway 83 in southwestern Manitoba. In early September, a sinkhole swallowed a car in Ottawa, and more recently, on January 29th, a sinkhole opened up in a Quebec quarry.

Sinkholes are apparently common in eastern Ontario and western Quebec, where the soil lies over clay beds, which can 'liquefy' quite quickly and quite unexpectedly, and limestone beds that can erode due to acidic groundwater. There are also limestone beds all throughout the prairie provinces, deposited there millions of years ago, when a shallow inland sea covered much of Canada's interior plains.

If you're concerned about sinkholes on your property, you can watch out for the following signs (courtesy of Florida's Lake County Public Works Dept):

• Fresh exposures on fence posts, foundations or trees caused by the ground sinking
• Slumping, sagging or slanting fence posts, trees or other objects
• Doors and windows that stop closing properly
• Small ponds of rainfall where water hadn’t collected before
• Wilting of small, circular areas of vegetation
• Cloudy water instead of clear water pumped from nearby wells
• Cracks in walls, floors, pavement and the ground surface that’s different from a few hairline cracks normally seen between concrete blocks

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However, sinkholes are notoriously difficult to predict.

"You don't always get a warning sign," said Peter Bobrowsky, an engineering geologist for the Geological Survey of Canada, according to the CBC News story back in May 2010. "It can just suddenly happen and it can get quite extensive within a matter of minutes to tens of minutes."

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