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April 8th, 2013

Sinkhole Science is Actually Quite Simple

The drama of sinkholes, in which the earth can suddenly collapse and swallow up houses, has captured the popular imagination recently, but the science is actually as simple as water and rock.

The Bimmah sinkhole in Oman is approximately 30 meters deep, and is a popular spot for tourists.

Since a Florida man disappeared into a 30-foot sinkhole that opened beneath his bedroom, sinkholes around the world have been grabbing headlines. One video caught a girl walking down the street in China when a small sinkhole opened up beneath her, causing her to fall in. Another caused a portion of a Washington, D.C., sidewalk to disappear into the ground, injuring no one but teeing up a series of easy metaphors about politics and the “D.C. sinkhole”.

So, what forces cause seemingly stable formations of stone and earth to vanish at a moment’s notice, and why are they so difficult to fix?

What causes a sinkhole?

Similar to other geological phenomenon like earthquakes, sinkholes tend to occur in specific geographic locations, but are almost impossible to predict. Sinkholes almost always form in a type of geological formation called “karst terrain”, which the United States Geological Survey says covers about 20 percent of the country.


Karst formations in the United States (via USGS)


Karst terrain occurs when “carbonate stone,” such as limestone, is dissolved by acid rain or changing ground water levels.

Rocks may seem hard and dry, impenetrable and unchanging, but porous stone is actually an important part of the Earth’s drainage system. When acid water passes through these stones, or when waterways change their course or volume of water, it can cause the stone to weaken from within.

Once the stone is sufficiently weak, it collapses to fill the area below, leaving a gaping surface hole. Because it is nearly impossible to detect weakened stone without high-tech geological monitoring equipment, sinkholes often occur spontaneously.



Diagram of a sinkhole (via USGS)


While sinkholes are natural formations, and some famous sinkholes predate the earliest human beings, human actions like irrigation and drilling impact the top-crust layer of the Earth that is most responsible for sinkholes.

“Excessive groundwater withdrawal for irrigation purposes, for example, can sometimes trigger a sinkhole because it depressurizes the aquifer,” Lewis Land, a hydrologist at the New Mexico Bureau of Geology and Mineral Resources who studies sinkholes, recently told NPR.

How do you close a sinkhole?

Sinkholes can also be tricky to fix. The natural reaction is to fill the hole with a non-porous substance like concrete, but this can often exacerbate the problem by concentrating water drainage in other places, therefore causing more sinkholes.

A common technique for filling large sinkholes is a rock fill plug, which still allows water to drain through the ground. Large rocks are first pushed into the hole to plug the bottom, then progressively smaller rocks are piled on top to create a graded filter.

While sensational sinkholes like the one in Florida grab headlines, they are relatively rare.  Tiny sinkholes occur all the time, according to Land, but most aren’t not large enough to cause injury.

– Compiled by James Hercher for NewsHour Extra

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