Sinkholes are one of the many terrifying disasters of nature. Unpredictable and sometimes deadly, they seem to swallow homes and roads without discretion. You see devastating stories about them on the news all the time, but how much do you know about sinkholes? What exactly are they? Where do sinkholes occur? Is there anything you can do to protect yourself from them? Let’s find out!
What Are Sinkholes?
A sinkhole is a large depression in the ground that occurs as a result of underground dissolution and erosion of limestone and other carbonate rocks. Most of the time they actually form slowly, but some sinkholes appear suddenly and unexpectedly after a long process of erosion underground. Sinkholes have several alternate names, including cenote, swallet, and doline.
Where Do Sinkholes Occur?
So, where do sinkholes occur? The answer is that they can and do appear all over the world; there are sinkholes on every continent and even under the ocean (the Great Blue Hole).
What Factors Make an Area Prone to Sinkholes?
Where do sinkholes occur within a region? There are a few factors that make a sinkhole more likely to develop. Sinkholes are most likely to occur in what is known as a karst landscape. A karst landscape is a stretch of terrain composed of large amounts of soluble minerals such as limestone. The land in Florida is mostly composed of limestone, which is why that state has many problems with sinkholes.
Where do sinkholes occur in the ocean? Just like on land, ocean sinkholes tend to occur in areas with a lot of soluble sedimentary rock. Some notable underwater karst landscapes are found off the coast of Belize, the Bahamas and the Yucatan Peninsula.
The deepest underwater sinkhole in the world, which is named Dragon Hole, is found in the South China Sea. In 2016, it was estimated to be 987 feet deep.
The second-deepest underwater sinkhole is Dean’s Blue Hole in the Bahamas, which is estimated to be 663 feet deep.
Oceanic sinkholes are often referred to as blue holes because they appear as a dark blue circle that contrasts with the lighter-colored water around them. Blue holes fascinate biologists because of the rare bacterial life forms that survive in their oxygenless depths.
How Do Sinkholes Form?
Sinkholes can form as a result of either natural or artificial processes or a combination of both. There are three types of nature-derived sinkholes: dissolution sinkholes, cover-subsidence sinkholes, and cover-collapse sinkholes.
The formation of dissolution sinkholes begins when acidic rainwater seeps through the ground and dissolves soluble rock (usually limestone) beneath the surface. This creates a gap in the sediment layers under the earth, weakening the foundation. The ground eventually collapses into the gap, resulting in a sinkhole. Though dissolution sinkholes are considered to be nature-derived, human pollution indirectly contributes to their formation by causing acid rain.
Cover-subsidence sinkholes form much more slowly than other types of sinkholes. Like dissolution sinkholes, they mostly occur in limestone-rich karst landscapes. The difference is that cover-subsidence sinkholes involve sand sinking into the ground instead of acid rain. The sand siphons into cracks and gaps in the rock layers below. Over time, this forms a sinkhole in the ground. Where do these sinkholes occur? They mostly form in karst landscapes with sandy, granular soil on the surface.
Cover-collapse sinkholes appear rapidly. Where do these sinkholes occur? Whereas cover-subsidence sinkholes occur in sandy areas, the cover-collapse variety tends to show up in areas where there is copious clay in the topsoil. This type of sinkhole, like the others, begins with the dissolution of rock deep underground. In this case, the clay-rich topsoil holds out for a long time before collapsing. A huge gap appears beneath the surface, forming a sort of arch underground. When the arch collapses, the results are devastating.
Sinkholes with man-made causes are most often the result of groundwater pumping, which can affect water drainage patterns deep beneath the soil. In 2013, a small sinkhole opened up in San Francisco because an aging sewer system exploded. A similar incident occurred in Florence, Italy in 2016 when an underground water pipe exploded and created a sinkhole. Above-ground construction can also pose a sinkhole hazard if the building materials place too much pressure on vulnerable tracts of land.
How Can You Protect Yourself from Sinkholes?
In areas that are known to be high-risk for sinkholes, professional geologists should be consulted before buildings are constructed. Buildings can sometimes be reinforced with special piers that are screwed into the ground to stabilize them. Another reinforcement technique is to use chemical grouting. This involves injecting certain chemicals into the sediment. The grouting fills gaps in the soil and strengthens it.
Sinkholes can be very difficult to predict. Signs tend to be subtle: plants dying due to loss of groundwater, development of new ponds or puddles, failure of windows to shut correctly, etc. If the land around your house starts to sink, it is recommended that you evacuate as quickly as possible and notify the building inspection department of your town. Insurance may cover damages caused by the sinkhole, especially in areas where sinkholes are a common occurrence.
To recap, a sinkhole is a depression that forms in the ground as a result of the dissolution of rocks under the earth’s surface. There are several different types of sinkholes, and they can appear both on land and under the ocean. They occur all over the world, but they tend to happen most often in karst landscapes.
Sinkholes can be slow-forming or sudden and come in a variety of sizes. Damages can be offset by consulting with geologists, reinforcing building foundations, and injecting grouting chemicals into the soil.