WHAT IS A SINKHOLE?
A sinkhole is an area of ground (typically a visible surface depression) that has no natural external surface drainage–when it rains, all of the water stays inside the sinkhole and typically drains into the subsurface. Sinkholes are a natural and common geologic feature in areas underlain with rock types that are soluble in natural water. In Florida, the most common of these soluble rock types are limestone and dolomite. Other type of soluble rocks include evaporates like gypsum and anhydrite and salt beds and domes. The term sinkhole is used for closed depressions in the land surface that are formed by surficial solution or by subsidence or collapse of surficial materials owing to the solution of near-surface limestone or other soluble rocks. Sinkholes can vary from a few feet to hundreds of acres and from less than 1 to more than 100 feet deep.
HOW DO SINKHOLES FORM?
Sinkholes occur in a variety of shapes from steep-walled “natural wells” to funnel-shaped or bowl-shaped depressions. Sinkholes are a product of solution-erosion processes and are analogous to valleys carved by rivers in areas underlain by insoluble clastic rocks. Just as rivers constantly erode the land surface by carrying away particles of rocks a grain at a time, limestone is carried slowly away in chemical solution an ion at a time. The ions are carried by water that percolates through cavities and conduits that were developed by solution of the limestone along fracture systems, bedding places, and other permeable zones in the rock. Surface erosion by rivers is well understood and subject to some control by man, but the detection of cavities at depth and predication of potential sinkhole collapse is inherently difficult.
Sinkholes are of interest in Florida because they are one of the predominant landform features of the State; because they may cause flooding during storms when the drainage capacity of natural subsurface conduits is exceeded; and because they may provide an avenue for pollutants on the land surface to move rapidly and possibly contaminate the groundwater. An important goal of geologic investigations is to describe the geologic and hydrologic features controlling the development of sinkholes so that sinkholes and their related problems may be better understood.
Sinkholes are also of interest in Florida because they have the potential to be a contributing cause of distress to a home or structure. Voids and cavities within the underlying limestone may result in surface depressions that can directly affect a structure’s integrity. These voids and cavities within limestone may also result in a condition referred to as raveling. Raveling is the vertical and/or lateral migration of sediments to deeper voids or cavities within limestone. It is a mechanism for sinkhole formation.
Sinkholes are common in Florida because the bedrock underlying most of the state is either limestone or dolostone, which is naturally soluble and is easily dissolved by rainwater and groundwater. But sinkholes are also common in other areas of the United States and all over the world. Essentially, wherever carbonate (limestone and dolostone) and evaporates (gypsum, salt domes) are the dominant bedrock.
To the left is a map of where sinkholes have been confirmed in Florida since 1954 (source: Florida GeologicalSurvey)
Sinkholes are also common in many places all across the country. Again, sinkholes are common where carbonate and evaporate rocks are either exposed at the surface or buried within 200 feet of the land surface. Sinkholes can also form in areas where porous volcanic rock and poorly consolidated soils are the dominant surface materials (Pseudokarst). Here’s a map of where sinkholes can form in the United States.