Oh no! Why is there a sinkhole in your backyard?

The worst sinkhole I’ve ever seen developed at an 84-year-old man’s Brooksville, Florida home following tropical storm Debby last year.  At first, the house and property became inundated with water, but then sinkholes began opening up in the front yard.  You couldn’t help but stare when driving by, as the size of the sinkholes directly in front of the house seemed ominous. Luckily, he was saved by his neighbors.    If you’d like to read the news article related to this story, see: http://www.baynews9.com/content/news/baynews9/news/article.html/content/news/articles/bn9/2012/7/18/sinkholes_swallow_fr.html

So, why are there sinkholes prevalent in some areas of the world and not others?  The key is the type of rock below the land’s surface.  In some areas of the world, the land is underlain by carbonate limestone which is prime for sinkhole development.  Why?  Because this type of limestone erodes easily when exposed to rainwater that has become more acidic while percolating down through the surface layers.  Once the water reaches the limestone, it finds preferential pathways through the rock, dissolving it away and leaving behind voids.  As time passes, and I’m talking geologic time here, so thousands of years, these voids grow larger and eventually can collapse, causing the layers above to sink into the voids. Other factors including groundwater pumping, drought followed by large rainfall events and other changes to the pressure within the aquifer can also contribute to sinkhole formation.  Once a landscape is riddled with sinkholes and caves, it is typically referred to as a karst landscape. If you’d like to know more about the how and why of sinkhole formation, the St. John’s River Water Management District in Florida has a nice summary at http://www.sjrwmd.com/watersupply/howsinkholesform.html. Also, the Southwest Florida Water Management District has a helpful illustration on sinkhole formation as provided below.

The Making of a Sinkhole

Source: www.watermatters.org

The figure shows an example sinkhole caused by numerous factors including vibrations from construction, local groundwater pumping, pressure from standing water and rainfall percolation through acidic layers.

There are a couple of sinkhole types I’d like to mention.  First is the subsidence or solution sinkhole and these gradually form depressions in the landscape, sometimes forming ponds or lakes as in the picture below.


Source: www.watermatters.org

These sinkholes can also be seen within water bodies such as lakes and oceans.

The second type is a collapse sinkhole and this type can be quite catastrophic such as the one in Guatamala in 2010. Here’s a great shot of that sinkhole: http://www.boston.com/bigpicture/2010/06/a_rough_week_for_guatemala.html. This sinkhole event followed a volcanic eruption and tropical storm, and the sinkhole swallowed a factory and intersection.

The cause of these types of sinkholes are illustrated below:


Source: www.usgs.gov; http://pubs.usgs.gov/ha/ha730/ch_g/G-Floridan7.html

As you can see, the roof of the cavity gives way and forms a tubular-type void, causing the land surface to collapse and a large, deep sinkhole forms.

Unfortunately, there’s no way to predict when a sinkhole will occur.  For Florida, you can find additional information on sinkholes at the following link to the Florida Geological Survey through the Department of Environmental Protection: http://www.dep.state.fl.us/geology/feedback/faq.htm

If you have any examples of sinkholes in your area, please feel free to leave information in the comment box below!