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You have probably seen pictures of sinkholes on the news, when a cavity dramatically opens up in a residential area and threatens houses. However, you might not know what a sinkhole actually is, and why they form. This blog gives some background information on sinkholes, why Florida is particularly prone to them, and some of the ways people can find out more about sinkholes in their area.
The United States Geological Survey defines a sinkhole as “a depression in the ground with no natural surface drainage”. That means that any water that collects above a sinkhole will drain into the rocks beneath the surface.
Sinkholes form when these rocks are made of minerals that are soluble in water, usually limestone (karst) or evaporites like salt. As the water from the surface seeps down into the rocks, it dissolves some of the minerals away, leaving cracks and later holes and caverns. Over time, the soil and sediments (called the overburden) above these holes start to slump downwards, and a sinkhole is formed.
There are three major types of sinkholes – solution, cover-subsidence, and cover-collapse. Which of these develops depends on the types of sediments in the soil and overburden.
Solution sinkholes – these occur where the rock is exposed at the ground surface, or has only a thin covering of soil. They are often visible as dips filled with water.
Cover-subsidence sinkholes – these happen when the overburden is sandy and doesn’t stick together well. As the hole in the underlying rocks forms, the sediments trickle down into it gradually, causing subsidence on the surface.
Cover-collapse sinkholes – if the overburden has a lot of clay in it, then the sediments will stick together more. Rather than subsiding down into the hole, the surface of the overburden forms a hollowed-out arch. Over time, the surface layers above the sinkhole become thinner, and eventually collapse when the weight of the ground becomes too much to support. Cover-collapse sinkholes are the most difficult types of sinkholes to detect and predict, as they can grow underground without any visible signs on the surface, and usually collapse suddenly with no warning.
What causes sinkholes in Florida?
Florida is said to have more sinkholes than any other state in the USA. This is because of the geology – most of the state is underlain by limestone or evaporites – and the climate. The plentiful rainfall means there is water to dissolve the rocks, while the presence of wet / dry seasonal changes can make sinkholes more likely to collapse.
Sinkholes are part of Florida’s natural environment – they are a normal part of how water flows from the surface to the groundwater stores, and how the land changes over time. In many cases, when they form out in forests or fields, people may not even notice them. However, when they occur in populated areas, they become a source of concern.
The risk of sinkholes, and the most likely type to occur, varies across the state, as the geology and thickness of the overburden changes. However, all parts of the state can be affected.
How are sinkholes detected?
Given sinkholes start out underground, and can be quite advanced before obvious signs are seen on the surface, how can people find out if sinkholes are present on or near their property?
Unfortunately, the answer is that there are no guarantees. This is partly because there is no 100% accurate way of detecting a sinkhole before it is visible on the surface, and partly because sinkholes are a constantly changing phenomenon, and may form or enlarge after an area has been investigated.
The good news is that methods for mapping sinkhole risks are improving all the time. One recent technique is the use of airborne or satellite radar (InSAR or Interferometric Synthetic Aperture Radar) to detect tiny changes in the ground surface, before they are visible by eye. Analyses by NASA suggest that these techniques can highlight areas at risk of sinkhole formation and possibly provide early warnings of collapse. The bad news is that these techniques are cost and time intensive and are still being researched, so widespread maps are not yet available.
While research into techniques like InSAR is ongoing, what resources are available to homeowners and prospective buyers who would like to know more about sinkholes in their area?
Firstly, it is good to ask questions and gather as much local information as possible. The Florida Department of Environmental Protection has a good sinkhole FAQ section, which provides lots of information and resources for residents and landowners. It includes a contact phone number for anyone who has a specific question not answered on the website.
To get an idea of how common sinkholes are in a particular area, people can consult online resources like Sinkhole Maps, which show where sinkholes have been reported in different counties. The Florida DEP keeps a downloadable database of subsidence incident reports, although they note that not all incidents recorded will be verified sinkholes.
People with concerns about sinkholes on their property, or on a property they are planning to buy have the option of employing a licensed professional to carry out an inspection and assessment – normally this is a firm of professional geologists or geotechnical engineers. These inspectors will look for surface signs of subsidence (cracks in walls, water pooling on the ground, etc), and undertake geophysical surveys of the land using ground-penetrating radar and electrical resistance to “map” the underlying rocks. This can often show if underground holes are present. However, because these techniques map anomalies (i.e. changes in the structure or make-up of the ground), rather than taking perfect pictures, the results can be complicated by the types of rock structures present, and by water levels.
Ultimately, living in Florida means living with the risk of a sinkhole developing, in the same way that people in Southern California live with the risk of earthquakes. Asking questions, researching online and in local resources, and talking to experts are all things that can help people decide what level of risk they are comfortable with.
Guest blogger Dr Alison Blyth is an earth scientist with a particular interest in karst.
The content in this blog is an overview provided for general information purposes only, and does not constitute professional advice. The author does not accept any liability arising from the use of the information provided here or from the omission of information.