A sinkhole is a hole in the ground formed when the land collapses away leaving a big hole on the surface. It happens when the rock underneath is dissolved by water.
They can range in size enormously, as you can see from the videos above and below.
Sinkholes form when rainwater comes into contact with a certain type of soft rock – such as chalk or limestone – and dissolves it, leaving a gap deep in the ground.
The land on the surface then collapses into this gap.
The rainwater is able to dissolve soft rock like this, after seeping through the top layers of soil and having CO2 added to it, which makes it more acidic.
Additionally, sinkholes can result from both natural and human causes. Man-made sinkholes are created when a city development compromises the structural integrity of underlying rock. Roads, buildings and other infrastructure can cause water to collect in certain areas and wash away any supporting layer of rock, resulting in sinkholes.
Where do they happen?
Areas on top of land with limestone or chalk foundations are more at risk of sinkholes.
They are pretty common in the American state of Florida because virtually the entire state is built on a limestone platform, but they are rare in the UK.
They can also occur in areas where there has been mining activity in the past and so the composition of the land deep underground has been affected.
Even though they are formed over quite a long period of time, it can be rather difficult to predict when they are going to happen.
What causes sinkhole?
Some sinkholes result from the surface dissolution of soluble rock. For example, limestone rocks dissolve when attacked by rainfall or groundwater that is acidic.
Collapse dolines occur as consequence of the gradual collapse of a cave passage at depth. The collapse may gradually propagate up through the overlying strata to cause subsidence at the surface (a ‘collapse sinkhole’). These sometimes extend up into rocks that are not themselves prone to dissolution, creating a ‘caprock sinkhole’. Sinkholes of this type are common in parts of South Wales where sandstone rocks overlie cavernous limestone and in Ripon, Yorkshire, where sandstone and limestone overlie gypsum. Others may be buried by more recent deposits.
Some sinkholes are caused not by dissolution of limestone, but the erosion of weak, unconsolidated material by flowing water. Loose material can removed by a process called ‘soil piping’, creating large voids within the sediment. One of the most spectacular examples of this type of collapse is the event that occurred in May 2010 in Guatemala City. Here, cavities developed in weak, unconsolidated, volcanic deposits following a tropical storm. These then collapsed, creating a shaft approximately 100 m deep and 20 m wide.
What triggers sinkhole?
- Heavy rain or surface flooding can initiate the collapse of normally stable cavities, especially those developed within superficial deposits.
- Leaking drainage pipes, burst water mains, irrigation or even the act of emptying a swimming pool are all documented examples of sinkhole triggers.
- Construction and development modifying surface drainage or altering the loads imposed on the ground without adequate support can cause sinkholes to develop.
- Groundwater abstraction can cause sinkholes by changing the level of the water table. This removes the buoyant support water provides to a cavity. Draining these cavities can cause them to collapse.
- Mining can be a factor in causing sinkholes, either by dewatering and lowering of the water table or by intercepting clay-filled voids, which subsequently collapse. Several sinkholes in Norwich have been caused by old chalk mines intercepting otherwise stable, sediment-filled voids.