swallow up entire buildings, houses, trees, and even people. While such a description sounds like a scene from a disaster or science fiction film, incidents like this actually can and do occur around the world as sinkhole events. Essentially, a sinkhole is a depressed area in the ground that contains no natural external drainage. Because of the lack of drainage, all of the water that collects within the sinkhole remains and then drains down to the subsurface. Sinkholes vary in size from 1m to 600m wide, and are found throughout the world. In addition to their other destructive impacts (such as the capacity to swallow entire buildings), they can also lead to the contamination of local water resources¹.
So What Causes Them?
Sinkholes occur most commonly in areas known as “karst terrain,” where there are dissolvable types of rocks below the ground’s surface that can be eroded by both groundwater and rainwater. Such soluble rocks include salt beds and domes, gypsum, and limestone.
When it rains, the rainwater (which is naturally acidic) dissolves the soluble rock and forms spaces and caverns below ground. Over time, the underground spaces continue to grow larger and larger while the land above remains intact. Eventually (such a process can take hundreds or thousands of years under natural conditions as the water slowly dissolves the rock), the spaces get so large that the land above them can no longer be supported. Without this necessary support from below, a collapse of the land surface can occur, sometimes quite suddenly. Thus, a sinkhole is “born.”
Sinkhole collapses occur more frequently after intensive rain events, but they can also occur during times of drought when groundwater levels have significantly decreased, leading to a greater risk of a collapse of the ground above. In a world with a greater variability in rainfall and drought events due to climate change, sinkhole events may increase in number around the world².
In addition to naturally-produced sinkholes, there can also be man-induced sinkholes. Such sinkholes can be caused by old mines, leaky faucets, when sewers and waterlines crack and collapse, during groundwater pumping, and during construction. Industrial and runoff storage ponds may place quite a deal of weight on the subsurface below and can also lead to a collapse of the ground beneath.
When groundwater is pumped in order to provide urban water supplies and for use in irrigation, the delicate balance of the groundwater fluid pressure can be disrupted and the groundwater level itself can be lowered. Both of these processes can lead to a collapse of the soil above, forming a sinkhole.
There are two types of sinkholes²:
- Cover-collapse sinkholes, which develop over a period of a few hours, leading to large amounts of damage quickly.
- Cover-subsidence sinkholes, which develop much more gradually as the ground subsides or is depleted. Such events can go undetected for long periods of time.
The Detection of Sinkholes:
To determine if your property has a potential vulnerability for the development of sinkholes, it is important to regularly monitor your landscape for any slumping trees or fence posts, water collecting in areas where it has not previously collected, structural cracks in walls, and the wilting of small circular areas of vegetation. These warning signs can potentially indicate if there is a sinkhole that is currently forming on your property.
You can also find out if you live in a region that contains underlying soluble rock by checking with local, territorial, or national government offices and geological surveys such as the United States Geological Survey (USGS). However, even with these indicators, it is still not always easy to predict where a sinkhole event will occur.
Scientists are currently working on developing effective methods to find the underground cavities that might develop into sinkholes, including the use of radar, seismography, and electrical resistance. Since 10% of the world’s land area is estimated to contain karst topography, the development of such detection methods could go a long way in preventing the catastrophic damages that are associated with sinkholes².