Sinkholes: When the Earth Opens Up

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On occasion, the ground opens up and creates an opening deep into the Earth, generally in a fairly sudden manner. We know these as sinkholes, and they’re responsible for both terrible disasters and some of the most beautiful places in the world depending on where and when they happen.

So, let’s take a closer look at sinkholes, their causes, and a couple of famous examples of this natural phenomenon.

What Are Sinkholes?

Sinkholes occur when a void is created beneath the ground, causing the surface to collapse.

Sinkholes vary widely in size, from small depressions a couple of meters across and a meter or so deep to gigantic holes that can swallow buildings. The latter are what most people picture, although they’re quite uncommon.

Basically, the ground seems to open up. The depth of sinkholes also varies, from a few feet to hundreds. It’s a widely classed phenomenon, but they all occur through the same basic action: a void is created underneath the ground and the weight of the ground itself causes the collapse.

Due to the different ways they form, they can happen suddenly or over time.

How Do They Happen?

There are a number of actions that can lead to sinkholes occurring, both natural and man-made. These are broadly classified into a few categories, but each situation is a bit different.

The “classic” sinkhole is usually caused by underground erosion of the bedrock. Rocks like limestone, which is water soluble, are particularly prone to this type of sinkhole. Essentially, the action of groundwater eventually dissolves a pocket underneath the surface. These are generally known as dissolution sinkholes.

Eventually, the ground has thinned enough it can no longer support its own weight and collapses to the depth of the pocket underneath. These sinkholes occur naturally in many cases, and they’re responsible for some interesting geography in some areas.

Perhaps the most famous are the cenote caverns found in Central and South America, particularly on the Yucatan Peninsula. They’re found as deep shafts breaking the surface of the jungles, extending downwards into the groundwater.

Ik-Kil Cenote near Chichen Itza in Mexico

The water found in cenotes can be brackish or marine in nature. Essentially, these holes go down into the exposed groundwater, creating a sheltered spot. They’re often connected to underwater cave systems in the limestone. While some are popular spots for tourists, diving in the caverns and connected caves is a highly specialized task.

Not all sinkholes are caused by complete erosion of the bedrock, of course.

In other cases, known as cover-collapse sinkholes, a void can remain undisturbed in sediment for some time. Think of a void in something like an alluvial gravel deposit, it’s formed because the rocks are simply locked into place around the pocket. 

It’s a simplistic example, but for various reasons infilling sediments may not actually fill an entire area, thus creating a natural hazard.

As time goes on and the surface becomes heavier due to more sediment, it will eventually collapse.

Cover-collapse sinkholes tend to create a more dramatic, sudden effect. While dissolution generally occurs over fairly long periods of time, a cover-collapse sinkhole will… well, collapse. They’re generally the cause behind the more dramatic events that make the news cycle.

Human activity can cause sinkholes in a few way, although they follow the same general rules. Some examples include:

  • Mining- Improper support or a lack of backfilling can create sinkhole risks when a shaft or tunnel collapses under its own weight. This will naturally have surface effects.
  • Poorly Planned Construction- If natural drainage and topography isn’t taken into account, then construction can actually change these features and create the conditions for a sinkhole to form.
  • Leaking Water- Busted pipes, whether water main or sewer, are a common cause of human-created sinkholes. The leaking water changes the composition of the local sediment and may allow it to run off, dissolve, or otherwise create the void responsible for the sinkhole.

In essence, no sinkhole happens without space for the surface to collapse. The effects can be quite dramatic, and our modern times have seen some real disasters caused by the effects of sinkholes.

It’s All in Karst Topography

Karst topography is the biggest single identifier of areas where sinkholes occur. The entire basis of this kind of system is due to water-soluble rock forming and being moved by water. It can form complex cave systems, including those which can cause sinkholes to occur.

Essentially, the rocks involved are almost universally carbonate-based. Even slightly acidic water, such as that which has picked up carbon dioxide, can dissolve these stones over time. The effect becomes pronounced over time.

Sinkholes are only one feature of karst systems. Also occurring are sinking streams, which may go underground at various points, springs, and caves are common features of this type of geology.

It’s also been observed in a few other stone types, but they’re most common in areas where dolomite or limestone is the bedrock.

Karst topography isn’t always apparent on the surface of an area. If a layer of harder rock forms over the carbonate-rich minerals beneath, for instance, it can result in an area with few surface water features and none of the features associated with karst systems.

Mitigating Sinkholes

When they happen, sinkholes can be an expensive problem to fix.

On private and commercial properties, minor sinkholes may emerge. These can devalue the property and create a future hazard as the sinkhole continues its downward journey.

The recommended course of action for even a minor sinkhole can be pretty extreme. Essentially, you’ll have to dig out the hole to the bedrock and then backfill it with sand or another sediment.

In the event of larger, sudden sinkholes the costs to the local infrastructure can be incredible. Sinkholes that occur in the city may require millions of dollars just to fill them completely back in, and that’s not getting into the cost of damage and toll of human life that larger sinkholes can make.

For the most part, it’s much easier to design infrastructure in a way that minimizes sinkhole formation since treating even minor examples is often ridiculously expensive and requires specialized knowledge to prevent it from happening again. 

Famous Sinkhole Examples

In a strange way, sinkholes tend to either be beloved natural landmarks or absolute disasters for nearby humans. While the occasional “annoying” sinkhole may show up, such as a small depression over a shallow but busted water main, most people associate them with much larger holes.

Some great examples, from both arenas, are the following.

The Cave of Swallows (Sótano de las Golondrinas)

One of the most dramatic examples of a sinkhole can be found near the Mexican town of Aquismón. The Cave of Swallows opens up into the jungle with a hole nearly 200 feet across.

More astonishing is the depth. The distance from the lip of the cave to the floor is over 1200 feet. Most visitors will only take a look over the edge, but it’s also been the site of people engaging in base jumping and rappelling to reach the floor.

Oddly, the Cave of Swallows is primarily inhabited by non-swallow species. The floor is also covered in guano, so it’s probably not worth the trip to the bottom unless you’re engaged in extreme activity.

It remains an example of one of the largest natural sinkholes in the world.

The 2007 Guatemala City Sinkhole

image: (image: horslips5/cc)

In 2007, a disaster struck Guatemala City. A sewage line rupture created a large void underneath the city, removing the uncemented sediments underneath the surface of the earth.

The end result was the loss of five human lives, twelve homes, and a hole that went 330 feet straight down with vertical walls. The area under the city is largely comprised of pyroclastic ash and limestone that isn’t cemented, making large water leaks under the surface a serious problem.

It was filled in with a local concrete mixture.

The massive sinkhole of 2010 in Guatemala City (image: horslips5/cc)

In 2010 a similarly sized hole emerged, although this particular example “only” killed one person. This one ended up swallowing a three-story factory.

Due to the area’s geology, it’s likely that entire systems will be need to reworked to prevent these sinkholes from happening again.

Zacatón

Zacatón is a 1000-foot deep, water-filled sinkhole located in the Mexican state of Tamaulipas. It’s named for the grass which forms into large mats that float around the surface of the hole’s water.

It’s unique in that it’s filled with hydrothermal waters, and it’s the only one of the five major sinkholes located in the region that seems to have water moving in and out of it.

Interestingly, the sinkholes in this region seem to be closing on their own. Travertine, a calcium carbonate rock, appears to be forming as a sort of “skin” on the surface of the water. This process will take thousands of years, but considered on its own it’s definitely a geological wonder.

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