Two processes create three types of sinkholes

Three types of sinkholes are common in Florida: dissolution, cover-subsidence and cover-collapse sinkholes. They develop from dissolution and "suffosion." Dissolution is the ultimate cause of all sinkholes, but the type of sinkhole is also controlled by the thickness and type of overburden materials and the local hydrology. Although it is convenient to divide sinkholes into three distinct types, sink-holes can be a combination of types or may form in several phases.


Cover-collapse sinkhole near Ocala , Florida

 

Processes:
Dissolution of soluble car-bonate rocks by weakly acidic water is ultimately responsible for virtually all the sinkholes found in Florida.




Water (H2O) falling through the atmosphere and percolating the ground dissolves carbon dioxide (CO2 ) gas from the air and soil, forming a weak acid-carbonic acid (H2CO3 ).

As the carbonic acid infiltrates the ground and contacts the bedrock surfaces, it reacts readily with limestone (CaCO3 ) and/or dolomite [CaMg(CO3)3 ].

Cavities and voids develop as limestone or dolomite is dissolved into component ions of calcium (Ca++ ), magnesium (Mg++ ), and bicarbonate (HCO3-).


When the ground water becomes supersaturated with dissolved minerals, further dissolution is not possible, and carbonate salts of calcium and magnesium may precipitate from the water, often forming interesting shapes such as stalactites. The reactions are fully reversible, and when precipitates are exposed to undersat-urated ground water they may redissolve. The geochemical interactions are controlled partly by the rate of circulation of water.


Suffosion occurs when unconsolidated overburden sediments infill preexisting cavities below them. This downward ero-sion of unconsolidated material into a preexisting cavity is also called raveling and describes both the catastrophic cover-collapse sinkhole and the more gradual cover-subsidence sinkhole.

The erosion begins at the top of the carbonate bedrock and develops upward through the overlying sediments toward the land surface.

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