A sinkhole, also called a doline, swallow hole, shake hole, swallet, or cenote, is a depressed area on the land surface that is shaped like a bowl or cone. Sinkholes can range in size from several square yards to hundreds of acres. They are common in regions of karst, a type of bedrock where mildly acidic groundwater has dissolved rock such as limestone, dolostone, marble, or gypsum.
Karstlands are characterized by sinkholes, sinking streams, caves, springs, and solution valleys. They form in areas where surface water enters the ground and migrates downward through solutionally enlarged openings to conduits, such as caves.
Because sinkholes are places where there is rapid replenishing of groundwater from the surface and, therefore, are areas of potential groundwater contamination, managing surface water and waste disposal in sinkhole-prone areas is important to maintaining good groundwater quality.
It is possible for sinkholes to be human-induced. New sinkholes have been associated with certain land-use practices, especially ground-water pumping, construction, and development practices. Sinkholes may also form when natural water-drainage patterns are altered and new water-diversion systems are developed. Some sinkholes form when the land surface is changed, such as when industrial and runoff-storage ponds are created. The substantial weight of the new material can trigger an underground collapse of supporting material, thus causing a sinkhole.
Sinkhole Development Stages
Stage 1- For a sinkhole to form there must be an opening in the bedrock surface which allows overlying soil to descend into a cave passage.
Stage 2- Soil that collected in the cave passage in Stage 1 has been washed away by flowing water.
Stage 3- Soil that collected in the fracture or bedrock opening collapses into the cave or is washed into the cave by water movement from the soil into the cave.
Stage 4- Additional soil movement or collapse forms a void at the bedrock surface.
Stage 5- The void enlarges and moves upward in the soil profile, a process known as stoping.
Stage 6- Sooner or later the void enlarges until only a thin layer of soil remains at the surface.
Stage 7- The thinned soil roof can finally no longer support itself and invokes a surface collapse that may or may not choke the hole in the bedrock. Typically the initial appearance is a steep-sided hole at the surface several feet deep with a floor of soil that used to be at the surface. The collapse can be dramatic because the surface land usually stays intact until there is not enough support.
Stage 8- If the bedrock throat of the sinkhole remains plugged with the collapsed soil, the surface hole may fill with other eroded soil. In some examples the unsound, steep-sided surface hole may broaden into a conical depression, like the upper part of an hour glass. If the sinkhole throat stays clear, surface water will drain readily, but if the throat becomes blocked up with soil, water may pond temporarily or permanently in the depression, forming a sinkhole lake.
Cross-section sketch courtesy of Maryland Geological Survey, 2300 St. Paul Street, Baltimore, MD