Sink holes sometimes fill with water, forming ponds.
A sinkhole is a naturally occurring geological phenomenon that causes a large depression to form in the ground, often hundreds of feet wide and dozens of feet deep. Prevalent in areas where layers of water soluble subterranean rock are covered with soil, sinkholes can open unexpectedly, sometimes creating a serious risk to both persons and property.
Cause
A typical sink hole forms when acidic ground water filtering down through overbearing layers of soil dissolve an underlying layer of soluble rock, such as limestone or gypsum. This leads to the formation of a void or space in rock, which eventually collapses, forming a depression in the surface.
Types
Three major types of sinkholes exist. The solution sinkhole typically forms in a joint, fracture or other line of weakness in limestone rock at the water table where limestone is either exposed or close to the surface. As the limestone dissolves, soil and other residue are gradually swept by wind, rain and gravity into the depression, often forming a gradual dip in the soil. In cover collapse sinkholes the ceiling of a void may thin to the point where it can no longer support the weight of the overbearing soil and rock. Eventually the ceiling collapses suddenly, producing a large hole. Cover subsidence sinkholes are caused by deep rock dissolving from under soil and allowing the soil to slump downward.
Risk
If a sinkhole is located under a structure, such as a home, the collapse of the sinkhole inevitably leads to either severe damage or the complete destruction of the property. The newly created edge of the hole may also continue to erode, sometimes taking days or even weeks to fall, making the perimeter of the hole unstable and dangerous. Voids are often irregularly shaped and may travel for long distances, resulting in the risk of more sinkholes developing in the area.
Influences
Several external influences can cause a sink hole to occur. When a cave is filled with water, the pressure the water exerts may help support the ceiling. If the water table drops due to drought or to pumping of excess water from the aquifer, the levels of water in the cave may drop, resulting in a collapse. Vibration from new construction or roads, increased pressure from rising water levels above the cave in ponds and lakes due to rain, broken water mains or a shift in the direction of flow of underground streams within the aquifer can also trigger a sinkhole.
Warning Signs
Several warning signs can help you spot a sinkhole. If you noticed exposed soil around foundations, the base of trees or fence posts, these may be the result of a sinkhole beginning to form. Soil sagging or slumping and the ponding of water in depressed areas can be caused by a sinkhole. Floors losing level, foundations, walls or pavements cracking, or doors or windows failing to close properly can also indicate a possible sinkhole.
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