Florida’s Sinkholes

Billed as a “Geological Park”, Devils Millhopper, in Gainesville, contains a very large sinkhole which houses a profusion of humidity-loving tropical and semi-tropical native plants and wildlife.

Devils Millhopper sinkhole

Sinkholes are a common feature in Florida. Most are just a few yards wide. Some are large enough to swallow a house. Big sinkholes often fill in with water to become lakes.

About 225 million years ago, tectonic forces began to break Pangaea apart and create the Atlantic Ocean, with North and South America moving to the west and Europe and Africa to the east. But as they moved further apart, a small part of the African plate remained behind, stuck to the North American plate. This chunk of bedrock, consisting of black volcanic basalt, became Florida.

During the time of the dinosaurs, Florida was at the bottom of a warm shallow tropical sea. Over a period of millions of years, ocean sediments, mostly from the calcium carbonate shells of tiny marine creatures, built up at the bottom of this ocean and, as the layer got thicker and heavier, became compressed over time into limestone. Compressed even further, it became harder and formed a rock known as “dolomite”. Layered geological formations which are composed mainly of limestone and dolomite are known to geologists as “karst”.

As rainwater percolates through the sand and down to this limestone layer, it picks up dissolved organic minerals from these rocks to form carbon dioxide. This solution is slightly acidic, and this very weak acid eats away slowly over time at the limestone to form cavities.

On the surface, these cavities can form small tunnels which drain away rainwater to form underground streams. Occasionally, a river or stream will form a sinkhole in its path which then leads underground, resulting in a “lost river” or “disappearing stream”.

If a surface cavity reaches far enough to hit the groundwater, it can form a surface spring. There are numerous such springs in Florida, and they provide important refuges for aquatic wildlife. Florida springs often lead into extensive underground cave systems that have become filled with water, and exploring these caverns is a popular, though dangerous, activity for cave divers. Quite often, these caves hold archaeological artifacts or fossilized bones that have been washed inside by rainwater and preserved, making them a favored spot for scientists.

If a cavity is deep enough underground to avoid reaching the surface at all, it can expand into an underground cavern, which may be of any size. As mineral-rich water drips into the cavern and evaporates, it leaves behind tiny deposits of calcium which, over time, produce formations of stalactites (which hang down from the ceilings) and stalagmites (which poke up from the floor). These underground caverns can grow to enormous size, while leaving no visible trace on the surface and no indication that they are there. Usually they are not discovered at all until something accidentally breaks the surface and reaches the interior space. Many such caverns then become tourist attractions. One of the largest of these is in Florida Caverns State Park, in the Panhandle.

On occasion, however, the roofs of these underground cavities can become so thin from erosion that they break, causing the ground above them to collapse and producing a gaping crater known as a “sinkhole”. This often happens with a sudden change in the water table, either when water withdraws from the chamber during a drought, or when water floods into it from heavy rains.

Although sinkholes can form any place where there are karst formations, Florida, being almost entirely composed of ancient marine deposits, is particularly vulnerable. (West-central Florida between Tampa Bay and Orlando has earned the nickname “Sinkhole Alley”.) And since sinkholes can open up at any place and time without warning, they can be dangerous. Small sinkholes in yards, parking lots or sidewalks are fairly commonplace and can be a nuisance for property-owners, and it’s not unusual to see news stories about larger sinkholes that swallow trees or cars or mailboxes. In 1981, a sinkhole in Winter Park FL grew to 350 feet wide and 75 feet deep, swallowing portions of two streets and one house. It was stabilized by the city and became Lake Rose.

Occasionally, such large sinkholes can be deadly. There are at least six known instances of people being killed. In one incident in Seffner FL in 2013, a man was sleeping in his home when a sinkhole opened up underneath and dropped the entire structure into a 20-feet deep chasm. His body was never found.

And the problem is increasing. As the population of Florida has exploded, humans have been taking more and more of the water and diverting it for human uses. This is leading to a steady depletion of the underground aquifer that supplies most of Florida’s groundwater. As water levels in the aquifer decrease, it has two effects. The first is the intrusion of saltwater from the Gulf of Mexico and the Atlantic Ocean into the aquifer, which will cause tremendous ecological damage if the fresh groundwater becomes salty. And the second effect is the withdrawal of water from underground cavities, leading to weakened support for the limestone ceilings of these chambers and an increased risk of sinkholes.

Despite their danger to humans and human structures, though, sinkholes have long been a part of Florida’s natural ecosystem, and they play a role by creating new ponds, lakes and springs. These in turn provide new habitat for fish, turtles, alligators and other aquatic wildlife. If they are able to gain an outlet to a river or to the seashore, these springs, with their constant year-round water temperatures, provide important winter refuges for Manatees.

The Devil’s Millhopper State Park, near Gainesville, centers around the ecological role played by Florida’s sinkholes.

It is not really known when the Devil’s Millhopper sinkhole formed, but it was at least several thousand years ago. Archaeological finds inside the sinkhole demonstrate that the Native Americans knew about the place. The limestone deposits here are around 120 feet deep and range from 5 to 34 million years old, and a myriad of marine fossils can be found embedded in the sides of the collapse. The sinkhole itself is around 500 feet wide. The lake that is currently inside the collapsed crater is fed by four freshwater springs and streams which keep it full even during the Florida dry season.

The site has been a popular tourist destination since at least the 1930s, when the Civilian Conservation Corps constructed an entrance gateway from limestone blocks. In December 1974, the state purchased 74 acres of land around the sinkhole and designated it as a State Geological Park. (The Federal Government, meanwhile, has declared the sinkhole a National Natural Landmark and lists it on the National Register of Historic Places.) There is a hiking path that runs around the rim of the sinkhole, and a boardwalk with a set of 236 steps leads down to an observation platform near the bottom of the hole. The surrounding area is a botanical preserve featuring the wide variety of native Florida plants that are attracted by the constant warmth and humidity inside the sinkhole.


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