Sinkhole formation is a natural process, but it can be catalyzed by human activities. It typically occurs in areas where soluble rock formations like limestone (calcium carbonate) or salt beds lie beneath the surface. Surface water is unusual in these regions and streams can sometimes sink into the ground; the resulting landscape is called karst terrain. The groundwater here can dissolve some of the bedrock and carry it away in places, leaving cavities that will later expand to form sinkholes. Sometimes sinkholes can also be created by caves or mines that leave holes and weaknesses in this type of bedrock.
In the spring of 2010, a sinkhole nearly 60 feet wide swallowed an entire three-story apartment building in Guatemala City. According to National Geographic, the sinkhole ran down an entire 300 feet and had probably been forming for weeks or years before storm waters triggered its collapse. Most sinkholes aren't this large, but they can still cause massive property damage -- one reason why it's important to understand their formation.
Once a cavity has formed in the bedrock, it's typically occupied at least initially by water. If the water table falls, however, it causes further stress since the ground above no longer has its support. The new stresses induced by this change widen the cavity, which may join together with other growing cavities or voids to form a larger one. Soil from above falls into the cavity, weakening the ground above it. Once the ground is weak enough, the cavity collapses to form a sinkhole.
Often heavy rainfall can act as a trigger to sinkhole collapse. Wet soil weighs more than dry soil, so the soggy ground after a rainfall is more likely to fall into the void beneath. Removing large quantities of water from the aquifer and thereby drawing down the water table can accelerate the whole process, sometimes with catastrophic results. Pumping water into shallow dry wells or running water from culverts into a drainage site can also lead to sinkhole formation.
Sinkhole formation is a natural process and it does create a conduit that allows rainwater to flow into the aquifer, recharging the groundwater over time. It can also create ponds or lakes; water bodies formed by large collapse sinkholes are typically circular in shape. Urbanization in karst terrain typically accelerates sinkhole formation, because it reroutes drainage pathways and increases water usage. The states that suffer the most damage from sinkholes are Florida, Texas, Alabama, Kentucky, Missouri, Pennsylvania and Tennessee; of these, Florida sees more sinkholes than any single other state.
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