Some are shaped like shallow bowls or saucers whereas others have vertical walls; some hold water and form natural ponds. Typically, sinkholes form so slowly that little change is noticeable, but they can form suddenly when a collapse occurs. Such a collapse can have a dramatic effect if it occurs in an urban setting. The map below shows areas of the United States where certain rock types that are susceptible to dissolution in water occur.
In these areas the formation of underground cavities can form, and catastrophic sinkholes can happen. These rock types are evaporites salt, gypsum, and anhydrite and carbonates limestone and dolomite.
Evaporite rocks underlie about 35 to 40 percent of the United States, though in many areas they are buried at great depths. Since Florida is prone to sinkholes, it is a good place to use to discuss some different types of sinkholes and the geologic and hydrologic processes that form them. The processes of dissolution, where surface rock that are soluble to weak acids, are dissolved, and suffusion, where cavities form below the land surface, are responsible for virtually all sinkholes in Florida.
Dissolution of the limestone or dolomite is most intensive where the water first contacts the rock surface. Aggressive dissolution also occurs where flow is focused in preexisting openings in the rock, such as along joints, fractures, and bedding planes, and in the zone of water-table fluctuation where groundwater is in contact with the atmosphere.
Rainfall and surface water percolate through joints in the limestone. Dissolved carbonate rock is carried away from the surface and a small depression gradually forms. On exposed carbonate surfaces, a depression may focus surface drainage, accelerating the dissolution process.
Debris carried into the developing sinkhole may plug the outflow, ponding water and creating wetlands. Gently rolling hills and shallow depressions caused by solution sinkholes are common topographic features throughout much of Florida. Cover-subsidence sinkholes tend to develop gradually where the covering sediments are permeable and contain sand.
In areas where cover material is thicker, or sediments contain more clay, cover-subsidence sinkholes are relatively uncommon, are smaller, and may go undetected for long periods. Cover-collapse sinkholes may develop abruptly over a period of hours and cause catastrophic damages. They occur where the covering sediments contain a significant amount of clay. Over time, surface drainage, erosion, and deposition of sinkhole into a shallower bowl-shaped depression.
Over time, surface drainage, erosion, and deposition of sediment transform the steep-walled sinkhole into a shallower bowl-shaped depression. New sinkholes have been correlated to land-use practices, especially from groundwater pumping and from construction and development practices. Sinkholes can also form when natural water-drainage patterns are changed 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. The overburden sediments that cover buried cavities in the aquifer systems are delicately balanced by groundwater fluid pressure. The water below ground is actually helping to keep the surface soil in place.
Groundwater pumping for urban water supply and for irrigation can produce new sinkholes in sinkhole-prone areas. If pumping results in a lowering of groundwater levels , then underground structural failure, and thus, sinkholes, can occur. Do you think you know about groundwater? Quiz icon made by mynamepong from www. Follow me to the Land Subsidence website! Groundwater is one of our most valuable resources—even though you probably never see it or even realize it is there.
There is water somewhere beneath your feet no matter where on Earth you live. Groundwater starts as precipitation, just as surface water does, and once water penetrates the ground, it continues moving, sometimes quickly and sometimes very slowly. Eventually groundwater emerges A huge amount of water exists in the ground below your feet, and people all over the world make great use of it.
But it is only found in usable quantities in certain places underground — aquifers. Read on to understand the concepts of aquifers and how water exists in the ground.
You can't see it, but a large portion of the world's freshwater lies underground. It may all start as precipitation, but through infiltration and seepage, water soaks into the ground in vast amounts. Water in the ground keeps all plant life alive and serves peoples' needs, too.
Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human That water moves through spaces and cracks underground, slowly dissolving limestone and creating a network of cavities and voids. As the limestone dissolves, pores and cracks are enlarged and carry even more acidic water.
Sinkholes are formed when the land surface above collapses or sinks into the cavities or when surface material is carried downward into the voids. Drought, along with resulting high groundwater withdrawals, can make conditions favorable for sinkholes to form.
Also, heavy rains after droughts often cause enough pressure on the ground to create sinkholes. In urban or suburban areas, sinkholes are hazardous because they can destroy highways and buildings. Sinkholes also can cause water quality problems. During a collapse, surface waters may leak into the aquifer, our underground source of drinking water. Many natural sinkholes cannot be prevented. However, those caused by human activity may be avoided, especially those caused by over-pumping groundwater.
Changing groundwater gradients due to removing or introducing water to the system can cause loose material to flush out quicker from the voids and the surface to collapse in response. Any change to the hydrologic system putting more water in or taking it out causes the system to become at least temporarily unstable and can lead to sinkholes. Sinkholes can result from seasonal changes in the groundwater table, freeze and thaw of the ground, and extremes in precipitation drought vs heavy rain.
Water impoundments - basins, ponds, dams Heavy loads on the surface - structures, equipment Sometimes several factors combine to cause a sinkhole. A sinkhole is not a hole in the rock A common misunderstanding is to think that a sinkhole is the hole in the rock. For more information This DEP sinkholes website is designed to provide very basic information on sinkholes in Pennsylvania.
For a more advanced explanation about sinkhole formation and karst hydrology, consult the following sources: Kochanov, William. Pennsylvania Geological Survey, Educational Series Langer, William.
Geological Survey, Open-File Report Newton, J. US Geological Survey, Circular White, William B. Oxford University Press. What causes a sinkhole. Who can I call for help.
Info on the web. Government that Works Protect clean air, clean water, and public health and conserve working farms, forests, and natural lands.
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