The sinkhole that swallowed a man sleeping in his Seffner home has spawned concern over why these natural occurrences take place. Here's a few reasons provided by the Florida Department of Environmental Protection’s Florida Geological Survey.
Facts about sinkholes in Florida:
-The entire state of Florida sits on top of several thousand feet of limestone. Limestone is a rock that can form with natural void spaces called porosity. In limestone where the void spaces are connected, the rock is permeable. Porous and permeable limestone makes great aquifers and provide millions of gallons of fresh drinking water for residents and agriculture. The most significant factor in the development of sinkholes is the dissolution of the limestone underlying Florida by naturally acidic groundwater.
-Sinkholes are a natural and common feature of Florida's landscape. They are only one of many kinds of karst landforms, which include depressions, caves (both air and water filled), disappearing streams, springs and underground aquifer systems, all of which occur in Florida. Thousands of naturally occurring sinkholes can be seen throughout the state of Florida including many that connect underground to springs, rivers and lakes.
-Sinkholes form in karst terrain from the collapse of surface sediments into underground voids. In Florida one may see solution sinkholes, cover-subsidence sinkholes or cover-collapse sinkholes. The first two types will show very little topographical disturbance to the naked eye, while the third is the type which shows a abrupt change in topography and is most associated with the thought of sinkholes.
Questions about sinkholes in urban and suburban environments:
-My yard is settling... Do I have a sinkhole? Maybe. But a number of other factors can cause holes, depressions or subsidence of the ground surface. Expansive clay layers in the earth may shrink upon drying, buried organic material, poorly-compacted soil after excavation work, buried trash or logs and broken pipes all may cause depressions to form at the ground surface. These settling events, when not verified as true sinkholes by professionals, are collectively called "subsidence incidents." If the settling is affecting a dwelling, further testing by a licensed engineer with a licensed geologist on staff or a licensed geology firm may be in order. Property insurance may pay for testing, but in many cases insurance may not cover damage from settling due to causes other than sinkholes.
-A sinkhole opened in my neigborhood... should I be concerned? Although sinkholes in Florida sometimes occur in sets, most are isolated events. The bedrock underlying the state is honeycombed with cavities of varying size, most of which will not collapse in our lifetimes. A quick inspection of your property for any sinking or soft areas might be prudent. Unless the sinkhole is very large, and extends to your property, there’s likely to be little reason for concern.
Should a sinkhole open in an area near you the hole should be immediately cordoned off and clearly marked to protect traffic. Contact local law enforcement to report the hazard and call your city or county road department to initiate repair work. If the road is private, repair of the hole is usually the responsibility of the landowner or property owners’ association.
-Is there a safe area of Florida where there is no chance of sinkholes? Technically, no. Since the entire state is underlain by carbonate rocks, sinkholes could theoretically form anywhere. However, there are definite regions where sinkhole risk is considerably higher. In general, areas of the state where limestone is close to surface, or areas with deeper limestone but with a conducive configuration of water table elevation, stratigraphy, and aquifer characteristics have increased sinkhole activity.
In an effort to assist the state residents are encouraged to report sinkhole information by filling out a subsidence incident report form on the Department’s website and submit the completed copy to the FGS. This website also contains information on sinkholes and how they form.