Florida’s sinkholes are all over the news: a new sinkhole opened up in Seffner, Fla., the same town where a sinkhole swallowed a man just days ago. This is the third (reported) sinkhole in the Florida town in a month. So let’s take a little time to answer a few questions, like what are sinkholes, how do they form, and how dangerous they really are.
What is a sinkhole
Sinkhole come in many names and shapes: snake hole, swallow hole, swallet, or doline are the typical names you’ll find them by. They are natural depressions (or holes) in the surface of the Earth’s surface, (usually) caused by karst processes. Karst processes occur when the bedrocks are soluble – in other words, in 99% of all cases, in carbonate rocks (like limestone or dolomite) or evaporitic rocks (like gypsum or anhydrite). So while they can occur in other environments, I wouldn’t really worry about it.
A sinkhole has no natural external surface drainage – when it rains, all the water stays inside the sinkhole and typically drains into the subsurface. This is the dominant phenomena regardless of size – it can be smaller than a meter, or over 100 meters in depth (In Sarisarinama, Venezuela, multiple sinkholes have reached about 1,000 feet wide and 1,150 feet deep). They can also have very different shapes; some are just like a shallow saucer or bowl, while others are much more vertical. Some actually form water, and you’ve probably seens this in the form of little ponds in limestone.
What really makes them dangerous is the fact that they’re really unpredictable. They form so slowly, that without thorough geological or geophysical research, you can’t really tell if something’s changing – this is why collapses have dramatic, unexpected effects, especially in urban settings.
Areas prone to sinkhole collapses
As I said, sinkholes are almost always prone to karst areas. In such areas, it’s possible to see hundreds or even thousands of sinkholes in a small area, and no surface streams, because all the drainage occurs subsurface. Evaporite rocks underlie about 35 to 40 percent of the United States, but in most cases, they are buried very deep, so there are only a few threatened areas.
The most impressive sinkholes form in thick layers of homogenous limestone. Their formation is facilitated by high groundwater flow, often caused by high rainfall. When these thick, relatively soluble layers sit ontop of insoluble rock, significant underground streams (or even rivers) may form, dissolving a significant quantity of rock, creating large underground voids.
Underwater sinkholes can form too. They’re typically called blue holes, and have been described especially in the Bahamas area. The name originates from the deep blue color of water in these sinkholes, which in turn is created by the high lucidity of water and the great depth of sinkholes; only the deep blue color of the visible spectrum can penetrate such depth and return back after reflection. The deepest known sinkhole is the Dean’s Blue Hole in the Bahamas.
Types of sinkholes
Sinkholes can be created by humans! I just wanted to get this clear right off the bat. This type of sinkholes is actually quite common (not as common as natural ones, but still relevant, especially in urban areas). They can be caused by a number of human activities, most notably due to groundwater pumping and from construction and development practices. The most common activities which lead to this type of phenomena are drilling, mining, significant changes in weight (lots of construction in a relatively pristine area), and heavy increase in water flow (like say a formation of an artificial pond, pipe leakage, etc.
Basically, either more water appears in the system, which dissolves and creates an underground void, or the underground is already present, and there is an increase on the pressure exerted on the surface – or a combination of these. Whenever the structural and chemical balance is disturbed – sinkholes can occur.
As the name describes, dissolution (the process of dissolving a solid substance into a solvent to make a solution) is the driving factor here. Dissolution of the limestone or dolomite is most intensive where the water first contacts the rock surface.
Cover subsidence sinkholes
Cover-subsidence sinkholes tend to develop gradually where the covering sediments are permeable and contain sand. Not really your typical sinkhole, this type is not common, and can often go undetected for long periods of time – which is pretty much the only thing that can make them dangerous.
These are pretty much the most dangerous type of sinkhole. Cover-collapse sinkholes develop very fast (sometimes even in a matter of hours), and can have catastrophic damage. They occur where the covering sediments contain a significant amount of clay; over time, surface drainage, erosion, and deposition of sinkhole into a shallower bowl-shaped depression. To put it simpler, the ground simply can’t support the load ontop of it.
So how common are sinkholes really?
The thing is, unlike earthquakes or hurricanes, sinkholes aren’t really tracked. Also, since they appear in so many areas, it’s practically impossible to discuss about where they appear – we can only talk about areas prone to them. As I said, any karst dominated area has some, be they bigger or smaller, as do areas with surface evaporites. This is why big portions of the Middle East, for example, are just riddled with sinkholes. In the US, Florida is almost certainly the most threatened state. From what I can dig up, from 2006 to 2010 in Florida alone, there were 24,671 insurance claims for sinkhole.
So there’s no need to worry if you don’t live in one of these areas. If you do, however, look for certain tells, such as sagging trees or fence posts, doors or windows that won’t close properly, and rainwater collecting in unusual spots. If you’re very very worried, then you should consult with a professional geologist or geotechnical engineering firm which can explain the situation, and if indeed there is such a possibility, they can even inject a specific concoction to fill up the cracks and strengthen the foundation.
Pictures via Wikipedia and USGS