Sinkholes: Where will they strike next?

But how common are sinkholes? And should we be worried that others might suffer the same fate as Bush?

What are they?

Sinkholes are voids or disruptions beneath the earth that, for various reasons, lead to a collapse of the surface material. The more intense and unstable the cavity or hole underground, the greater the chance there will be a sinkhole.

They can range from shallow to deep, from coin-sized holes to gaping chasms that are larger than several football fields.

And they happen all the time.

Daniel Doctor, a research geologist at the U.S. Geological Survey, describes sinkholes as common, chronic hazards that occur throughout the world.

Why do they happen?

The formation of sinkholes is often closely tied to a type of terrain called karst — areas with limestone or other soluble rock like gypsum or salt. Because water flows through these rocks over long periods of time, it eventually dissolves them and creates voids, Doctor said.

A sinkhole’s development is strongly related to changes in the water underground.

“A drought, for example, can drop the level of (underground) water down deeper and cause a collapse because the sediments lose some of the support the water provides,” Doctor said.

On the flip side, a hurricane or heavy storm can pump a lot of water into the ground, causing the sediment to erode from within. That can also cause the creation of voids and thus the spread of sinkholes.

“Normally what you have is the sediment over and above the rock that’s actually collapsing and expressing itself as a ground hole in the surface,” he said.

In some cases, sinkholes can occur due to man-made errors.

Such was the case with the sinkholes in Guatemala City, which were caused by poor infrastructure systems, Doctor said.

“(It was a) failure of the drainage network for stormwater runoff or sewer water runoff,” he said.

Guatemala City is essentially built on volcanic ash and city workers tunnelled into the ash to create the infrastructure systems. Voids, and therefore massive sinkholes, were formed when underground pipes broke, causing water erosion and instability underground.

Where do they occur?

Sinkholes can occur anywhere, anytime. Some countries are more susceptible to them than others. In the U.S., Florida, Missouri, Iowa, Virginia, Texas, Alabama, Tennessee, Pennsylvania and Kentucky all have sinkholes.

Canada is not immune, as we saw March 29 when a sinkhole opened up in a parking lot near Montreal’s Pierre Elliott Trudeau International Airport and swallowed two cars.

Roughly 20 per cent of the U.S. is situated in areas prone to sinkholes, according to USGS data.

“Florida is like Swiss cheese,” Doctor said, explaining how Florida is essentially limestone.

What are costs?

Deaths from sinkholes are rare, Doctor said, but there can be enormous monetary costs.

Massive sinkholes cost millions to fix, with the tab being picked up by whoever owns the property, be it the government or a private homeowner.

Some Florida insurance companies even offer homeowners sinkhole coverage.

While the USGS has several online maps showing areas in the U.S. where sinkholes can occur, “to know whether or not your house is sitting on a sinkhole is a much tougher question to answer,” Doctor said.