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Sinkhole Hazard Map description. See the map HERE

Sinkhole Hazard

Sinkhole opens up in the middle of soccer field in Illinois (it is a pseudo-sinkhole)

What is sinkhole hazard?

There are many types of sinkholes. Some sinkholes develop slowly, over thousands of years, and are not a risk. But those that open up suddenly, in hours or even minutes, pose a serious threat. Sinkholes are usually circular, closed depressions, with very variable sizes in terms of diameter and depth. They swallow water, hence their English name “sinkhole”, but they also swallow everything else that was on the surface before the sinkhole formed. They are primarily dangerous for buildings and infrastructure, but in rare cases they can also cause fatalities.

Bimah sinkhole in Oman: a collapse sinkhole in limestone. Photo: Tamás Telbisz

Christmas sinkhole (2024) in Wales. Source: LINK

True sinkholes and pseudo-sinkholes

Real sinkholes are formed as a result of dissolution processes. This is called karstification. A rock becomes karstified when water or other naturally occurring acids, most often carbonic acid, dissolves the rock. The water then transports away the rock material in dissolved form. A depression or a subsurface cavity forms at the site of the transported material.
Among the different rock types, rock salt dissolves very quickly, and gypsum dissolves a little more slowly, but still quite quickly. These rocks are quickly transformed when water reaches them. Globally, the surface area of rock salt and gypsum is relatively small, but there are fantastic rock salt and gypsum karsts on Earth. Limestone and dolomite are dissolved much more slowly, but these rocks extend over vast areas on Earth, and the thickness of these rocks can be up to several kilometers. In the case of these rocks, the dissolution process is a slow, geological-scale process, however, the collapse of slowly formed cavities is a sudden, rapid event, and therefore carries risks.
Pseudo-sinkholes are not karst formations, so according to the established vocabulary of karst scientists, they are “not true” sinkholes (not-true = pseudo). However, it is just these pseudo-sinkholes that are responsible for a significant part of the damage caused by sinkholes, so it is very important to consider them in the context of “sinkhole hazard”. Pseudo-sinkholes can be formed, for example, by the collapse of mine shafts. Or they often form when water washes out the relatively loose material under the surface (for example, under a road), and thus the surface (road) is left without support and collapses. This process is called piping.

True sinkholes are of karst (=solution) origin, while pseudo-sinkholes are formed by the collapse of other types of cavities (e.g. mines) or by a lack of material caused by water erosion (piping).

Dangerous and harmless sinkholes

There are several variations within true sinkholes. The vast majority are actually harmless. These are “solution sinkholes”. In these, the surface gradually and very slowly lowers. Rarely, in the case of extreme precipitation, these can also have “visible” changes within a few hours or days, but this is not typical.

Formation of solution sinkholes – these are generally harmless (figure: Waltham et al. 2005, photo: Tamás Telbisz)

Collapse sinkholes are among the most spectacular karst formations, especially when they are large. For example, we can mention the large collapse sinkholes at the Škocjan Cave in Slovenia, the Crveno Jezero in Croatia, which is the largest in Europe with a total depth of 500 meters, or the Xiaozhai Tiankeng in China, which is the deepest collapse sinkhole on Earth. These extreme forms are created by the collapse of large caves, usually in uninhabited areas and very rarely, so from the point of view of practical life they are not a significant source of danger.

Formation of collapse sinkholes, and the largest such object in Europe: Crveno Jezero in Croatia, where a lake is found in the collapse sinkhole (figure: Waltham et al. 2005, photo: Tamás Telbisz)

Xiaozhai Tiankeng – the world record holder 662 m deep collapse sinkhole created by multiple collapse

Other forms of karst sinkholes are characterized by the fact that the karstifying rock is covered by some material. This can be a loose sedimentary cover (or even a thick soil) or some type of clastic rock (e.g. loess) or a hard rock (e.g. basalt). Due to the collapse of a cave in the karstifying rock or the lack of material carried away by the infiltrating water, the caprock can also collapse or subside. The shape of the depression that forms in the cover material, depends on how stable the cover material is. In the case of loose sediment, a “suffosion sinkhole” is formed, in the case of stronger cohesion, a “dropout sinkhole” is formed, while in the case of hard caprock, a “caprock collapse sinkhole” is formed. These sinkholes are generally smaller in size than the features formed by the collapse of large limestone caves, but in some areas there are quite a lot of such forms increasing the hazard. In addition, because of the non-karstic cover, people are less aware of the possibility of sinkhole hazard, when they start building houses or roads on these risky areas.

Three subtypes of covered karst sinkholes: suffosion, dropout and caprock collapse sinkholes (Waltham et al. 2005). These types often contribute to sinkhole hazard

The role of humans in sinkhole hazard

Sinkholes, especially “true sinkholes”, are primarily formed by natural processes. The presence of suitable rocks and precipitation are important. Sudden, extreme precipitation events can accelerate these processes. Earthquakes can also be the trigger for specific collapses. But natural processes are often disrupted by humans. For example, karst water is often lowered because of mining or intensive agricultural water consumption. This can contribute to the increase of sinkhole hazard in many cases.
In most cases, the formation of pseudo-sinkholes occurs specifically under conditions that are created by humans. For example, during road construction, the accumulated material often remains loose and can be easily washed away by water. In addition, the collapse of mine tunnels or any other man-made cavities is related to human influences. In case of salt mines, freshwater can dissolve a significant amount of rock salt in very short time.

Man-induced cover collapse sinkhole at Solotvyno (UA). Freshwater entered the rock salt mine, and collapses occurred due to the dissolution of salt layers (Photo: Tamás Telbisz)

The purpose of this sinkhole hazard map

It is a geoeducation map. Its purpose is to introduce the different types of sinkhole hazard through unique examples. Map users may be students or just interested people, or even scientists. It is worth clicking on the icons on the map to read a short description of how and why sinkholes formed in a given area. To see what types are characteristic for a given area.
Of course, it is also worth looking at the images. There are several pictures associated with each location. By browsing the map, one can discover which geographical areas are more prone to sinkhole hazard. (Though not all sinkhole prone areas are presented in this map, yet.)
If you are curious, you can also use this map to plan your trip to visit these locations. However, it is important to note in this regard, especially in relation to pseudo-sinkholes, that in most cases, local people are trying to remove and fill in sinkholes, because these features are an obstacle for them. In some places, this effort is successful, while in others it is not, meaning that the sinkhole is reborn again and again in the same place or a little further away.
These sinkholes are usually relatively small features, nevertheless, in some cases, using high-resolution satellite images (e.g. Google Earth), it is possible to find out how the surface looked like before the sinkhole formed and how it changed.
We have tried to collect the information mostly from professional sources (scientific journals, professional websites) and publish it with simplified and abbreviated descriptions. Anyone who would like to get acquainted with the details has the opportunity to do so, because we have provided the source in all cases.
At present, there are still relatively few locations marked in the map, but we are continuously uploading new sites as our time permits.

Extract from the Sinkhole Hazard Map