Look Out Below! 4 Causes for Sinkholes

It’s difficult to wrap your head around the idea that the Earth is a globe. The ground you walk on isn’t really flat – it’s part of a huge sphere that spins through space. As if that wasn’t unbelievable enough, the ground making up the outer layer of Earth isn’t always as solid as it seems. 

Earth is made of many layers, including soil, subsoil and bedrock. With the right conditions, these layers can wear away and form depressions or even deep pits. Collapses are known as sinkholes and often open up unexpectedly. Here are four causes of sinkholes and what you can do to predict them. 

How Sinkholes Form

Think about the last time you stood in the ocean and let the water pull sand out from under your feet. Every wave eroded more soil, sinking you deeper and deeper into the ground. Eventually, the sand became so unstable that you moved to a new spot and started the game again. 

In a similar way, sinkholes are formed by water interacting with soil. When water isn’t able to drain properly from land, it will sink deep into the soil. Some materials underground are soluble, like limestone and other minerals. Water can dissolve these materials and create empty pockets underground. 

As water continues to flow down through the earth, it pulls sediment into these empty pockets. With enough time, this process can form a depression in the soil. If underground pockets are very deep, water can completely displace the soil and create a gaping sinkhole. Some sinkholes open up suddenly, while others develop slowly over a period of years. 

Empty underground pockets can exist for thousands or years before they cause a problem. Often, sinkholes open up after heavy rain or when water is displaced through human activities like drilling or mining. Periods of intense drought can also trigger sinkholes by changing the water table. 

Types of Sinkholes

Not every sinkhole is created equal. Some form a slight depression in the earth and then stop progressing, while others open up shafts that are over a hundred meters deep. Each type of sinkhole is caused by a combination of several different factors. 

Dissolution sinkholes happen when there’s not a lot of groundcover to soak up and divert water. As water collects in the area, the ground depresses and marshes can form. Cover-subsidence sinkholes happen in areas with sandy soil. Water fills underground caverns with sand, causing a slight depression. 

Cover-collapse sinkholes are the most serious type. They’re formed when water takes soil away in areas with heavy clay. The stronger clay takes longer to erode but will eventually collapse. This means the ground can look flat until it suddenly gives way and falls into the empty space underground. 

In addition to natural causes, sinkholes can also be triggered by human activity. For example, many urban sinkholes form after a broken sewer pipe carries soil away from its original location underground. Mining activities also contribute to sinkholes by creating caverns underground and disrupting the water table.  

Signs of a Sinkhole

Because sinkholes begin deep underground, they’re difficult to predict. Subsoil can erode for years before there’s any evidence of a problem. That’s why sinkholes are so scary – they can collapse suddenly and cause massive damage. 

Some areas of the world are more likely to develop sinkholes than others. These areas have “karst terrain,” or land influenced by water and underground caverns. Some areas that are prone to sinkholes in the United States include Florida, Texas and Kentucky. 

Start by researching where you live. If your state has soluble mineral deposits and underground cave systems, there are more likely to be sinkholes in your area. When a sinkhole is developing in an urban area, it can cause structural cracks as the ground shifts. This can help you identify the risk before a hole completely opens up. 

As the ground underneath loses soil and collects water, there will also be signs in the landscape. These include wilting vegetation, pooled water on the property and slumping trees. Sometimes, buried items will resubmerge from underground as the land falls into a cavern below. In extreme cases, entire trees can sink underground.

Can You Fix a Sinkhole?

In some cases, yes. If homeowners start noticing signs of a sinkhole on their property, they can use several tests to determine the depth and size of the cavern causing the sink. If it’s small enough, it can be filled with concrete or gravel to stop the ground from sinking further. 

It can be difficult to know conclusively what’s going on underneath the ground. However, you can use ground penetrating radar and electrical conductivity to learn more about the area underneath your property. These tests can help you determine whether you’re at risk of experiencing a massive sinkhole. 

The only way to completely avoid sinkholes is to move somewhere with geological conditions that make them impossible. You can check maps to learn more about the bedrock and soil consistency in your area. Rain, drought and industries that use water also make a difference. 

Most major sinkholes are perfectly round. This can be disconcerting, but its cause is simple. Think about the way water goes down the drain in your bathtub. The pull of the drain causes water to pour down in one place, smoothing the edges as it goes. Sinkholes are round because there’s a center point pulling water underground. 

Sinkholes and Wildlife Diversity

Although sinkholes can be very destructive, they’re also responsible for unique ecosystems around the world. For example, consider the Great Blue Hole in Belize. This ancient cave was flooded as water levels rose and now attracts scuba divers from around the world to explore its depths. 

The Sarisariñama Sinkholes in the forests of Venezuela are another good example. Several of these sinkholes have forest ecosystems down at the bottom, with unique flora and fauna. Because they’re difficult to access, there’s still much to discover in this part of the world. 

China is full of sinkholes because of its unique geology. In May of 2022, explorers in the Guangxi region of China discovered a prehistoric forest at the bottom of a sinkhole. The area contained rare plant species and ancient trees that had grown to a massive 130 feet tall. 

The largest sinkhole in the world is the Qattara Depression, located in Egypt. It covers roughly 7,500 miles and was formed by a combination of salt, wind and water. Several animal species live in this area, but people generally avoid it. The area is impassable to vehicles as it’s full of quicksand. 

Cause for Concern? 

Cover-collapse sinkholes get the most attention because they’re dramatic, sudden and can cause massive damage. However, there are many other types of sinkholes that shape the natural terrain in positive ways. The trouble is that even these sinkholes can negatively affect urban areas, damaging buildings by destabilizing the ground. 

If you live in an area that’s known for sinkholes, it’s important to pay attention to any changes on your property. Most sinkholes show signs above ground before they fully collapse. Although sinkholes can be a cause for concern, they’re not magical holes in the ground. By learning more about the causes for sinkholes, you can predict whether they are likely in your area.