Solution sinkholes occur in areas where limestone is exposed at land surface or is covered by thin layers of soil and permeable sand. Solution is most active at the limestone surface and along joints, fractures or other openings in the rock that permit water to move easily into the subsurface. Dissolved limestone and some insoluble residue are carried downward by percolating water along enlarged openings as solution of the limestone progresses. Large voids commonly do not form because subsidence of the soil layer occurs as the limestone surface dissolves. The result is a gradual downward movement of the land surface and development of a depression that collects increasing amounts of surface runoff as its perimeter expands.
This type of sinkhole usually forms as a bowl-shaped depression with the slope of its sides determined by the rate of subsidence relative to the rate of erosion of the walls of the depression from surface runoff. Surface runoff may also carry sand and clay particles into the depression, which may form a relatively impermeable seal in the bottom. A marsh of lake forms when water is ponded because infiltration is restricted by the clayey seal. The gently rolling hills and shallow depressions typical of solution-subsidence topography are common over large parts of Florida.
Cover Collapse Sinkholes
Cover-collapse sinkholes occur where a solution cavity develops in the limestone to a size such that the overlying cover material can no longer support its own weight. Collapse is generally abrupt when this occurs and is sometimes catastrophic. Collapse sinkholes provide dramatic local changes in topography. They may occur in any areas of soluble rock; however, they are less likely to occur in areas of deeply buried rocks.
Collapse sinkholes generally occur in areas where the limestone is near land surface and the limestone aquifer is under water-table conditions. Ground-water circulation is most vigorous at and just below the water-table where solution of the limestone is accelerated. Accelerated solution also may occur at certain depths where bedding planes in the limestone or changes in rock composition concentrate the flow of groundwater.
Limestone is commonly exposed in the vertical or overhanging walls of collapse sinkholes shortly after they form. The sinkholes generally are circular in shape and the walls may be round and smooth, but mostly they are irregular in shape because of the influence of joints and fractures in the rock. Surface drainage, erosion, and deposition of sediment into collapse sinkholes will eventually smooth the sides and reduce their slopes until they may become indistinguishable from other types of sinkholes.
Cover Subsidence Sinkholes
Limestone, like most bedrock, generally lies beneath a cover of soil and other unconsolidated material, such as sand and clay. Coastal sediments and beach deposits left during periods of high sea level occur throughout Florida at various altitudes. The variable thickness and composition of this cover is important in sinkhole development.
Cover-subsidence sinkholes occur where the cover material is relatively incohesive and permeable, and individual grains of sand move downward in sequence to replace grains that have themselves moved downward to occupy space formerly held by dissolved limestone. In areas where the sand cover is 50 to 100 feet thick, subsidence sinkholes generally are only are few feet in diameter and depth.
Where limestone is buried beneath a sufficient thickness of unconsolidated material, few sinkholes generally occur. Spalling of sand into solution cavities that have developed along joints in the limestone may cause subsidence due to upward migration of the cavities (a process known as piping) to cylindrical holes at the land surface. If the overburden is incohesive sand, the upward-migrating cavity is dissipated by a general lessening of density over a large area, and the result will be a relatively broad and extensive subsidence of the land surface that occurs over a period of time. Generally, subsidence of the type may go unnoticed for several years. If the overburden is a dense plastic clay, its low permeability may impede downward movement of ground water and slow the development of solution cavities in the underlying limestone.