The Florida Geological Survey is the premier state government institution specializing in geoscience research and assessments to provide objective, quality data and interpretations. FGS uses applied field and laboratory investigations to address environmental, conservation and public-welfare issues. FGS is dedicated to facilitating a culture of environmental stewardship by proactively sharing knowledge through accessible geoscience data, active participation on advisory committees, research and assessment publications, websites, public and technical presentations, and educational activities.
Sinkholes are a common karst feature of Florida’s landscape. They are only one of many kinds of karst land forms, which include caves, disappearing streams, springs, and underground drainage systems, all of which occur in Florida. Karst is a generic term which refers to the characteristic terrain produced by erosion processes associated with the chemical weathering and dissolution of limestone or dolomite, the two most common carbonate rocks in Florida.
Sinkholes form from the collapse of surface sediments into underground voids and cavities in the limestone bedrock. Slightly acidic ground water slowly dissolves cavities and caves in the limestone over a period of many years. When the cavity enlarges to the point that its ceiling can no longer support the weight of overlying sediments, the earth collapses into the cavity.
In Florida there are three types of sinkholes – solution sinkholes, cover-subsidence sinkholes and cover-collapse sinkholes.
- Solution sinkholes usually occur where there is little or no sediment cover over the limestone. The rock is readily dissolved away at the ground surface or along joints or other openings.
- Cover-subsidence sinkholes are located where thick permeable sediments cover the limestone. In this case, the void in the rock is filled by sediments slumping downward from above. Eventually, the ground surface often shows a gentle circular depression. If a relatively thick layer of impermeable sediments covers the limestone there may not be a surface expression of a subsurface collapse.
- Cover-collapse sinkholes occur where sediments that overlie the void in the rock suddenly collapse due to triggering mechanisms such as heavy rainfall, drought or mechanical loading.
Other Subterranean Events
Other subterranean events can cause holes, depressions or subsidence of the land surface that may mimic sinkhole activity. These include:
- Subsurface expansive clay or organic layers that compress as water is removed;
- Collapsed or broken sewer and drain pipes or broken septic tanks;
- Improperly compacted soil after excavation work; and
- Buried trash, logs and other debris.
Commonly, a reported depression is not verified by a licensed professional geologist to be a true sinkhole, and the cause of subsidence is not known. Such an event is called a subsidence incident. The Florida Geological Survey maintains and provides a downloadable database of reported subsidence incidents statewide. While this data may include some true sinkholes, the majority of the incidents have not been field-checked and the cause of subsidence is not verified.