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About 20 percent of the United States is more susceptible to sinkholes than the rest of the country. Sinkholes can be man-induced or natural, AccuWeather Meteorologist Jim Andrews said. Weather extremes such as heavy rains or drought can lead to sinkhole development.
"Man-induced sinkholes typically involve collapse of old mine workings, drainage infrastructure or other underground workings. Naturally, such can fail over time, and rainfall can be a major factor," Andrews said. "Extremes of rainfall, both above and below normal, can be a factor in activating existing sinkholes. These can disrupt the water table, up or down, while also dislodging the loose material covering openings in the limestone."
The most damage from sinkholes occurs in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania, according to the U.S. Geological Survey.
Natural sinkholes are almost always formed in limestone bedrock, Andrews said.
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"Limestone is slightly soluble in acidic groundwater [owing to organic matter as well as acidic rainfall]. Over vast spans of time, limestone bedrock can become riddled with openings dissolved out by ground water flowing to the water table," Andrews said. "Some openings become the caves, great and small, that we are all familiar with. The groundwater carries away the dissolved calcium carbonate that is the essential mineral in limestone."
An overburden of soil and other loose material normally covers the bedrock undergoing dissolution, Andrews explained.
"Inevitably, this loose material will find its way into the bedrock openings, hiding them from sight. Once the void exceeds certain limits, a significant collapse of material into the void can happen, resulting in the surface disturbance we know as a sinkhole," Andrews said. "Over geological time, landscapes on limestone bedrock develop distinctive character shaped by dissolution and collapse. This is known as Karst; there are often pits with permanent or temporary ponds/lakes."
Warm, wet climates cause the fastest dissolution of limestone bedrock and this is where Karst, with its natural sinkholes, is best developed, Andrews said. However, Karst can even be found in deserts, where it is likely be a leftover from a “fossil” wet climate, long-ago turned dry.
Active sinkholes can be relatively small, a few feet, or they can measure many tens of feet across/deep, depending upon how big the underground void was and how much material flowed into it.
"Anything of importance above the collapse can be drawn into it. The bigger the sinkhole, the greater is the risk of property loss," Andrews said. "New sinkhole collapses usually involve existing sinkholes; these may or may not have been known to exist before collapse."
A new sinkhole collapse involving an existing sinkhole occurred in August of 2015 when a sinkhole opened up in Seffner, Florida, two-and-a-half years after the initial sinkhole swallowed up part of a house, killing one man inside.
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