The unfolding tragedy in southwest Florida demonstrates the unpredictable nature of sinkholes, which can swallow roads and buildings as they suddenly form and expand. These geophysical landforms develop in karst regions, where a layer of soluble bedrock (limestone or dolomite) lies near the surface.
Channelized by underground streams, the sheets of porous bedrock are penetrated by a network of tunnels and caverns which surface as springs. Eventually, as a cave enlarges and its ceiling thins, it may collapse from the weight of the overlying soil, vegetation and/or structure, producing a sinkhole. Depending upon the depth and width of the collapsing cavern or tunnel, the sinkhole may be small or massive and stress on the adjacent ceiling might produce an unstable collapse, prone to further expansion.
The Florida Platform, composed of limestone (deposited during periods of high sea level) overlying older basement rock, makes the Sunshine State especially prone to sinkhole development. Other active karst regions (where a wet climate supports continued erosion of the limestone or dolomite bedrock) are concentrated in Kentucky, Tennessee and Missouri. In fact, as I write this blog, I am sitting atop a thick slab of Mississippian Limestone which, this week, supports the additional weight of a foot or more of heavy, wet snow, some of which will feed a web of channels that run beneath our city; indeed, just south of town, Rock Bridge Memorial State Park harbors a fascinating collection of sinkholes, caverns, springs and underground streams.