Under natural conditions, sinkholes form slowly and expand gradually. The dissolution of carbonate rocks by acidic water is the cause of the subsidence that creates all sinkholes. The water enlarges pre-existing openings ranging from pore spaces between limestone particles to fractures in the rock. The enlarged spaces eventually form a network of caves, pipes, and other types of conduits, all of which collect and channel large volumes of ground water. In the Floridan aquifer system, the greatest dissolution occurs where the upper confining unit of the system is thin or absent.
Sinkholes have important effects on both surface and ground water.
Lakes commonly occupy the depressions created by sinkhole collapse. Streams, such as the Withlacoochee River near Valdosta, Ga., lose their entire flow at low-flow stages to the Upper Floridan aquifer through swallow holes in the streambed. Where they are not plugged, sinkholes form a direct connection from the land surface to the Upper Floridan aquifer, allowing surface runoff to move directly and quickly into the aquifer. In the latter case, infihling of the sinkhole keeps pace with dissolution of the limestone. Gradually subsiding sinkholes commonly form where slow dissolution takes place, mostly along joints in the limestone.