A sinkhole is a depression or hole in the ground caused by the collapse of the surface layer. These geological formations occur when there is no natural external surface drainage, meaning rainwater remains within the depression and typically drains into the subsurface. Florida has the highest number of sinkholes in the United States, a consequence of its unique geological makeup.
Florida’s Sinkhole Prevalence
Much of central Florida has earned the designation “Sinkhole Alley” due to the frequent occurrences. This region encompasses counties such as Pasco, Hernando, and Hillsborough, stretching northward to areas like Maitland and Winter Park. The prevalence of sinkholes can lead to substantial property damage, with annual estimates reaching hundreds of millions of dollars.
How Sinkholes Form
Sinkholes primarily form in “karst terrain,” regions where soluble bedrock like limestone, dolomite, gypsum, or salt beds can be dissolved by circulating groundwater. As rainwater seeps through the soil, it absorbs carbon dioxide and interacts with decaying vegetation, becoming slightly acidic. This acidic water permeates cracks and spaces within the soluble rock, slowly dissolving it and creating an intricate network of underground cavities. The surface land often remains intact until these underground spaces become too large to support the overlying material, leading to a sudden collapse.
Sinkholes are classified by their formation mechanisms. Dissolution sinkholes develop where water directly dissolves exposed limestone or rock covered by thin soil, forming small depressions. Cover-subsidence sinkholes occur when permeable sediments, like sand, gradually settle into voids in the underlying limestone. Cover-collapse sinkholes form abruptly when a cohesive soil layer, often clay, suddenly collapses into a large underground cavity.
Why Florida is Prone
Florida’s susceptibility to sinkholes stems from its geology, climate, and hydrogeology. The state rests upon a thick bedrock of porous carbonate rock, predominantly limestone, which extends thousands of feet deep and is often near the surface. This geology creates the extensive karst terrain across Florida. The Floridan aquifer, composed of limestone and dolomite, contains numerous underground caves and conduits formed by dissolution.
The state’s climate contributes to sinkhole formation with abundant rainfall, tropical storms, and hurricanes. Rainfall becomes acidic as it moves through the soil, accelerating limestone dissolution. Periods of drought followed by intense rainfall can destabilize the ground, as cavities previously supported by groundwater become unstable during dry spells and then face sudden water influx.
Florida’s hydrogeology, characterized by a high water table and extensive aquifer systems, plays a role. Groundwater moves downward from the shallow surficial aquifer into the deeper Floridan aquifer, enhancing sinkhole formation by facilitating erosion of unconsolidated sediments into subterranean cavities. Human activities also exacerbate these conditions; excessive groundwater pumping for municipal or agricultural use can lower the water table, reducing buoyant support for underground cavities and accelerating collapse. Land development, including construction, rainwater diversion, and the weight of new structures, can stress the ground and trigger sinkhole development.