The geologic term “karst” is used to refer to a characteristic type of topography in regions underlain with carbonate (typically limestone) rocks, which chemically weather to produce hilly features or depressions at the surface and sometimes spectacular caves beneath, such as Carlsbad Caverns in New Mexico and the Mammoth Cave system in Kentucky. Beauty can turn to tragedy, however, if those chambers collapse to form sinkholes, as in the sad and surprising story of Jeffrey Bush, whose Florida home was partially swallowed by such a structure while he was sleeping on the evening of February 28th. Mr. Bush unfortunately did not survive the experience, which may lead some to wonder, could it happen to me? How dangerous is this phenomenon? Well, it depends on where you live.
The above map, developed by the United States Geological Survey (USGS), shows a generalized distribution of karst features within the United States. Why is Florida so riddled with cavities? The state’s land mass was underwater for much of its history, so that most of its basement rock is composed of limestone (chemical formula CaCO₃), formed by the deposition and lithification of corals and the hard parts of other marine organisms. Those limestone formations, hundreds or even thousands of feet thick, were then covered with much thinner layers of sand and clay thanks to the erosion and transportation of material from the Appalachian Mountains.
Rainwater can pick up carbon dioxide (CO₂) as it passes through the atmosphere and then the soil, forming a weak carbonic acid (H2CO3) that can act to dissolve the limestone bedrock. The process is exacerbated by man-made acid raid, which often comes in the form of the more reactive hydrochloric acid (HCl). Hydrochloric acid’s reaction with calcium carbonate is so violent that introductory geology students use it to help identify limestone and related rocks.
Of course the diagnostic solutions are more highly concentrated than what falls from the sky, but it can help you imagine what goes on beneath your feet. The effects of pollution from the industrialized mid-west are felt on the east coast thanks to the jet stream.
Sinkholes are then more common after rain events, but they can be produced by droughts as well, as in the Bush case. Florida State Geologist Jonathan Arthur notes that dry conditions can cause overlying soil to collapse. Human extraction of groundwater can have the same effect, as 65 sinkholes appeared in Florida after overuse by strawberry farmers in 2010.
WHAT DOES THIS MEAN?
About 20% of the United States is underlain by karst terrain, making it susceptible to subsidence and sinkholes. While they often appear quickly and without warning, basic geologic principles tell us that past is a good prediction of future. Just as Californians know to always expect earthquakes, spots currently plagued by sinkholes will likely continue to endure the uncertainty. Know your local geology and what potentially hazardous effects you might be able to expect. If you live in an area prone to sinkholes, learn to note the signs, such as muddied well water, new ponds, or slumping features.
More generally, we have to continue our staggeringly slow realization that what we do affects the world and environment around us. Acid rain doesn’t just kill a few fish; it can destroy property and even take human lives. Overuse of resources not only puts strain on the local ecology but could do irreparable damage to our neighbors. When couched in more practical terms like that, maybe stewardship isn’t quite so hard to swallow. And maybe something good can come from otherwise senseless deaths.