A sinkhole is a depression or hole in the ground caused by the collapse of the surface layer when the underlying material can no longer support the weight of the land above it. Sinkholes are a common and integral feature of Florida’s natural landscape. Florida’s unique subterranean environment makes it one of the most sinkhole-prone areas in the world.
Florida’s Karst Geology
The reason Florida is prone to sinkholes is its unique subsurface composition, known as Karst topography. The entire state rests upon a massive platform of carbonate rock, primarily porous limestone formed from compressed marine organisms. This soft, water-soluble bedrock is easily eroded by mildly acidic water.
Rainwater absorbs carbon dioxide from the atmosphere and decaying vegetation, forming a weak carbonic acid as it seeps through the soil. This slightly acidic groundwater moves through the pores and fissures within the limestone bedrock, slowly dissolving the rock over millennia. This process creates an ever-growing network of underground voids, caves, and conduits.
The dissolution process is self-accelerating; as the water carves larger pathways, it dissolves the rock faster. This subterranean erosion creates the underlying condition for sinkhole formation, turning the solid bedrock into a honeycombed structure. This extensive limestone layer is also the foundation for the Floridan Aquifer, which supplies over 90% of the state’s drinking water.
How Different Sinkhole Types Form
Sinkholes in Florida are categorized into three types, defined by how the surface material fails once the underground cavity is formed.
Solution Sinkholes
The first type is the solution sinkhole, which occurs where the limestone is exposed or covered by very thin layers of soil. The limestone is dissolved at or near the surface, creating a bowl-shaped depression that is wide and shallow.
Cover-Subsidence Sinkholes
The second type is the cover-subsidence sinkhole, which forms where the limestone is covered by a thick layer of permeable, sandy sediments. The overlying sand slowly ravels downward, grain by grain, into the void space in the bedrock below. This gradual movement creates a gentle, localized depression on the surface that develops over a long period.
Cover-Collapse Sinkholes
The third and most dramatic type is the cover-collapse sinkhole, which forms where cohesive, less permeable sediments like clay overlie the limestone. The clay layer is strong enough to bridge the growing subterranean cavity, masking the void beneath. When the cavity becomes too large, the bridging material suddenly fails, leading to a rapid and abrupt collapse of the ground surface. This mechanism produces a steep-sided hole in minutes or hours, often causing significant property damage.
Visual Indicators of Sinkhole Activity
The mechanical failure of the ground is often preceded by observable surface changes that act as warning signs.
Ground and Soil Changes
One common indicator is the appearance of circular patterns of ground cracks or a localized depression or slumping of the soil. These subtle dips may start small but expand slowly over time, sometimes forming a circular shape.
Structural Damage
Structural issues in buildings signal subterranean movement, such as cracks appearing in a home’s foundation, walls, or floors. These cracks are often vertical, diagonal, or stair-step shaped and may widen as the ground shifts. Uneven settling can cause doors and windows to begin sticking or failing to close properly because the frame is shifting out of alignment.
Water and Vegetation Clues
Changes in water patterns and vegetation are important clues to watch for on a property. Unexplained pooling of water, or the sudden disappearance of water from a pond or stream, can indicate a change in underground drainage. Additionally, localized wilting or death of vegetation may signal that the underlying soil structure or water table has been disrupted.