What Are Sinkholes and How Are They Formed?

In broad terms, sinkholes are circular or elliptical depressions that form in the ground due to the dissolution or collapse of the ground surface. They usually occur in areas where soluble rocks predominate, such as limestone. They can vary in size from a few meters to several hundred meters in diameter and depth.

These structures are common in karst landscapes, i.e. those shaped by the action of water on soluble rocks. Their appearance can range from gentle depressions in the ground to huge craters with steep walls. Some sinkholes may contain water, forming small lakes. Others remain dry and act as natural sinkholes. They can also occur in urban areas, often as a result of the effects of human infrastructure.

Learn more about how the Earth's surface with our article on what is river erosion?

Sinkholes are formed due to dissolution and collapse processes in soils with soluble rocks, especially in karst regions. The action of water plays a fundamental role in their origin. When infiltrating the soil, water slowly dissolves the underlying rock, creating underground cavities that can cause the surface to collapse over time. They can also become periodically larger after the original sinkhole is formed.

There are two main types of sinkhole formation:

• Dissolution sinkhole: rain water is slightly acidic due to the carbon dioxide it absorbs from the atmosphere and the soil. Dissolution is the process by which soft rocks such as limestone or gypsum are progressively dissolved by this rainwater. As the rock dissolves, the ground slowly sinks, giving rise to a gentle and progressive depression.
• Collapse sinkhole: in this case, the process is more sudden. The dissolution of the rock in the subsoil can generate underground cavities that lose their capacity to support the weight of the soil above over time. When the structure weakens sufficiently, the roof of the cavity gives way and collapses. This leads to an abrupt subsidence on the surface and the formation of the sinkhole.

In addition to these natural mechanisms, factors such as human activity can accelerate or intensify the formation of sinkholes. For example, excessive extraction of groundwater or construction on karst terrain can destabilize the soil and cause unexpected subsidence.

We explain what is ground water and how does it form in our related guide.

Sinkholes can be classified according to their origin and structure. Although they all share the characteristic of being depressions in the terrain, their formation and structure can vary. We discover the main types of sinkholes and their main characteristics in the sections below:

Dissolution sinkholes

• They are formed when water progressively dissolves limestone or gypsum, creating a smooth and progressive depression on the surface.
• They are usually wide and shallow with rounded edges.
• They are common in areas where rainwater or water from surface rivers infiltrates the soil and acts for long periods.

Collapse sinkholes

• They are generated when the roof of an underground cavern collapses due to the dissolution of the rock.
• Its formation is sudden and can cause abrupt subsidence in the ground.
• They have more pronounced edges and vertical walls, with a depth that can reach tens or even hundreds of feet.

Subsidence sinkholes

• They occur when surface material, such as sediment or soft soil, slowly sinks into underground cavities without abrupt collapse.
• They are less dramatic than collapses and develop gradually.
• They usually appear in areas with a layer of clay soil on a base of dissolved limestone rock.

Erosion sinkholes

• They are formed when water drags materials from the ground into an underground cavern, increasing the size of the sinkhole over time.
• Its process is slow, but it can generate large depressions.
• They are associated with areas where there are active groundwater flows.

Learn about the different types of erosion in geography, some of which can result in sinkhole formation.

Catchment sinkholes

• They are depressions in which water accumulates, forming small lakes or ponds.
• They are usually the result of dissolution or collapse sinkholes that have filled with rainwater or underground aquifers.
• Some may become permanent if the water level remains stable.

We share our article on types and examples of volcanic caldera if you would like to learn about other depressions which can occur in the Earth's surface.

Let's look at some of the best-known examples of sinkholes which can be found in various places around the world:

Great Blue Hole (Belize)

Located on Lighthouse Reef in Belize, this underwater sinkhole is approximately 300 meters in diameter and 125 meters deep. It was formed during the last ice age when sea levels were lower and the area was a limestone cave that collapsed when it filled with water.

You can learn more about this type of sinkhole with our article on what is the Great Blue Hole?

Xiaozhai Tiankeng sinkhole (China)

Known as the Dragon Sinkhole in its English translation, it is one of the largest sinkholes in the world at 662 metres deep and 511 metres in diameter. It is located in Chongqing Province and houses a unique ecosystem within. Its formation is due to the collapse of a large underground cavern.

Shooting Star Crater (Oman)

This sinkhole is located on the coast of Oman and is famous for its crystal-clear turquoise water. It is approximately 50 meters in diameter and 20 meters deep. Although it is popularly believed to have been formed by the impact of a meteorite, it is actually the result of the dissolution of limestone rock and the collapse of the terrain.

Wolf's Hole (Spain)

Located in the Spanish province of Cuenca this sinkhole is part of the Serranía de Cuenca Natural Park. It is a clear example of karstic action on limestone rock, with steep walls and considerable depth. Other similar formations are also found in the area, such as the Torca de la Novia.

Cave of Swallows (Mexico)

This huge sinkhole in San Luis Potosí is one of the deepest in the world, at nearly 512 metres from its highest point. Its name comes from the birds that live on its walls, and it is a popular spot for rappelling and caving enthusiasts. It was formed by water erosion on limestone rock over thousands of years.

Torcas de los Palancares (Spain)

The Torcas de los Palancares are a group of sinkholes also located in the province of Cuenca, within the Serranía de Cuenca Natural Park. It is one of the most impressive karst formations in Spain, with more than 30 sinkholes of different sizes and depths spread across the area.

If you want to read similar articles to What Are Sinkholes and How Are They Formed?, we recommend you visit our Environment (other) category.