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The essential factor of all sinkhole development is the dissolving of
the underlying limestone by slightly acidic water. As rain falls through the
atmosphere, it absorbs carbon dioxide and forms a weak carbonic acid. As
this water moves through the soil zone, it reacts with living and decaying
plant matter and becomes more acidic. The acidic water slowly dissolves
limestone, especially along the fractures and weak layers. This chemical
erosion eventually causes voids or cavities into which overlying sediments
may collapse or subside. The end result of chemical erosion of limestone,
followed by physical collapse or subsidence, is a sinkhole.
Sinkholes occur as a natural process of erosion of the limestone
by water. Ancient cavities dissolved in the limestone need a
triggering mechanism to cause the collapse. In predevelopment times,
sinkholes were usually triggered by heavy rains or a flood which made
the soil “roof” over the cavity very heavy, so that it eventually collapsed.
Droughts can also lower the ground-water levels, reducing the buoyant
support of a cavity roof and prompting a collapse. Natural sinkholes still
occur in Florida.
Increased numbers of sinkholes can generally be attributed to
changing or loading of the earth’s surface with development such as
retention ponds, buildings, changes in drainage patterns, heavy traffic,
drilling vibrations or declining ground-water levels. In urban areas, all
these impacts may occur at the same time, accelerating any sinkhole
tendencies. Urban construction, coupled with limestone depths of less
than 200 feet, contributes to many of the modern sinkholes.