Sinkholes

Florida is known for having more sinkholes than any other state in the nation and there are obvious features that make Florida so susceptible to these natural disasters. Florida’s natural karst topography is an immense contribution to this problem. This karst topography can best be described as landscape pockmarked by sinkholes or highly irregular landscape of high standing rock towers and intervening depressions and valleys.  Karst terrains mostly develop in areas underlain by carbonate rocks such as limestone. They often have drainage systems that are shown on the surface as sinkholes, springs, disappearing streams or even caves. The term karst, therefore, refers to the terrain and the term sinkhole is one of the types of drainage features reflected by that type of terrain. Sinkholes form in karst topography primarily from the collapse of surface sediments into underground cavities in a limestone (abundant in Florida). This collapsing is due to the slightly acidic groundwater slowly dissolving away at the limestone cavities and caves over many years. From here, the cavity eventually expands to a point that the limestone ceiling can no longer support the weight of what is above it. Once this overlying sediment is heavy enough, the earth literally collapses into the cavity itself.

There are three broad types of sinkholes. Each type generally corresponds to the thickness of the sediments overlying the limestone of the Floridan aquifer system. The three types of sinkholes are considered to be subsidence, solution, and collapse. Many are familiar with the less catastrophic type of sinkhole that is bowl-shaped and forms at the surface of the Earth over a long period of time. Collapse sinkholes are most common in areas where the overburden is thick, but the confining layer is breached or absent. Collapse sinkholes occur where sediments that overlie the void in the rock suddenly collapse due to triggering mechanisms such as heavy rainfall, drought, or mechanical loading.

Subsidence sinkholes form where the overburden is thin and only a veneer of sediments is present overlying the limestone. Subsidence sinkholes are considered as a void in the rock that is filled by sediments slumping downward from above and eventually will illustrate a gentle circular depression.

Solution sinkholes form where the overburden is absent and the limestone is exposed at land surface. In this case, the rock is readily dissolved away at the ground surface or along joints or other openings. From research and well-drilling, we know that much of the underlying bedrock in Florida is dispersed with many cavities of different sizes and depths. From identifying at all the cavities there are in Florida, few ever collapse and directly effect urban areas.

Many ponds and lakes in Florida are a result of sinkhole formation. The characteristics of a sinkhole lake can give clues as to how it was formed. A shallow circular lake results from impermeable sediments washing into a subsidence sinkhole. A circular lake indicates that the lake evolved from a collapse sinkhole. If a lake rests above groundwater level, then it is considered to be above a confining bed.

Many people consider sinkholes to have no purpose for the environment and should only be thought of as a negative impact against humanity. But the truth of the matter is, sinkholes actually provide a primary pathway for rainwater to replenish subsurface groundwater. When water fills a cavity, it supports the walls and roof, but when the water-table drops, the limestone cavity/aquifer is especially susceptible to further erosion that eventually results in the collapsing of the cavity. Thus, causing a sinkhole to form where is becomes a primary site of water recharge (where surface water can enter the aquifer and replenish the groundwater supply. In conclusion, they are an important part of the aquifer system that supplies 95% of Florida’s drinking water. From learning about this, you can conclude that these groundwater aquifers can be especially vulnerable to water pollution via run-off. Without the natural leaching of water from the surface to the groundwater, an aquifer can easily be contaminated. You can compare a sinkhole to a cut and a corresponding aquifer to your bloodstream, in a sense that the sinkhole provides a direct source for infection to enter your body. This means that Florida has to pay extra attention to prevent these pollutants from enter the site of a sinkhole.

 

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One Response to Sinkholes

  1. thefinch26 says:

    I did not know that Florida had the most sinkholes in the United States. It was interesting to find that out, and also that there are benefits to them. I have always thought of them as a problem but to find out that there are benefits to them is interesting. It was also interesting to find out how the sink holes can form and why they form.

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