The USGS estimates that around 20% of the U.S. and 10% of the world is “karst terrain” and susceptible to sinkhole events.
The word ‘karst’ is Slavic for barren, stony ground. Wilfred George Moore defines a Karstland in a Dictionary of Geography as “a limestone region in which most or all of the drainage is by underground channels, the surface being dry and barren: named after the Karst limestone district of the Dinaric Alps.”
Croatia, for instance, with its many impressive landforms is over 50% Karstland.
Although it is beautiful, living on Karstland can be treacherous. It can be disconcerting, for instance, to buy a piece of land, build a house and then, without warning, see hole open up under it. If one isn’t diligent in an area such as Florida, that’s exactly what can happen.
As reported by the Tampa Bay Times, such a hole, technically termed a sinkhole, opened up under a home in Seffner, Florida, the hole “swallowed an entire bedroom, where Jeffrey Bush had been sleeping in his bed. His body was never recovered.”
What is a Sinkhole?
W.G. Moore defines a sinkhole- what he also terms a ‘swallow’ hole- as a “saucer-shaped depression in the earth’s surface, usually found in limestone (karst) regions, through which water may enter the ground and pass along an underground course.”
Jeremy Berlin of the National Geographic has a broader definition: “A sinkhole is basically any collapsed or bowl-shaped feature that’s formed when a void under the ground creates a depression into which everything around it drains.” This latter definition goes beyond sinkholes formed by the dissolution of a water-soluble rock to include almost any type of void underground.
Although sinkholes most often form from capturing surface drainage, they can also form in dry places such as around the Dead Sea in Israel. It is also worth noting that ‘piping pseudokarst,’ essentially sub-surface drainage in non-limestone-type rocks can certainly create devastating collapses. Such an event occurred in Guatemala City in 2010 as fluid from a sewer eroded the uncemented volcanic ash.
Types of Sink Holes
There are basically two types of sinkholes.
One is slow in developing – we call it a solution or cover-subsidence sinkhole. Essentially this type of sinkhole plugs itself with sand, silt and clay from the increased surface-water runoff, preventing the water from flowing down into the aquifier. This action sometimes forms lakes.
The other type, often sudden and potentially catastrophic, like the Seffner Florida disaster, is the cover-collapse sinkhole. This usually occurs when the surface land is unable to stay intact from lack of underground support.