Sinkholes are often saucer-shaped hollows that are the result of some kind of collapse, or removal, of an underlying layer of rocks that used to support the layer of material at the surface. However, sinkholes can appear in a variety of different shapes and settings depending on the composition of the rocks and the different processes that interact.
Research in BGS extends beyond the distribution and processes associated with sinkhole formation to the broader subject of karst, which is a geomorphological term applied to the landscape that results from the dissolution of soluble rocks, e.g. the characteristic limestone pavements of the Pennines.
In this section:
- Sinkhole appearance
- What triggers sinkholes?
- Types of sinkholes
- Where does karst occur in the UK
- Where do sinkholes occur in the UK
- sides: vary from gently sloping to vertical
- shape: saucer-shaped hollows, conical, cylindrical potholes or shafts
Some sinkholes result from the removal of a soluble rock such as chalk, gypsum or limestone. These are called solution sinkholes. For example limestone rocks dissolve when attacked by rainfall or groundwater that is acidic.
Streams or rivers may enter a sinkhole and disappear underground. This type of sinkhole is often called a swallow hole.
Although a natural process, the formation of sinkholes is often accelerated or triggered by human influences, such as:
- broken drains, water mains and sewerage pipes
- modified drainage and diverted surface water that washes sediment into the underlying materials, causing subsidence
What triggers sinkholes?
Sinkholes can occur rapidly and apparently without warning. It is therefore important to consider the potential for sinkholes during planning (hazard susceptibility studies), and characterise them so that people can mitigate associated risks. Several processes can trigger sinkholes. Whilst the process of gradual dissolution can cause a sinkhole to form at the surface, other factors, including humans, can induce sinkholes to form, such as:
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 are also potential triggers. Modifying surface drainage or altering the loads imposed on the ground without adequate support can caused sinkholes to develop.
Drought or 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.
Types of sinkhole
Here, we describe three types of sinkholes that occur in limestone rocks.
Solution sinkholes are formed by local chemical weathering of the limestone where water accumulates around a fissure or joint in the rock. This may be underneath the soil or on the ground surface. The hollow that is formed is drained of water through the fissure or joint, but not before it has dissolved some of the limestone. The depression gradually gets enlarged.
(Note: solution sinkholes also occur in other rock types.)
Collapse sinkholes occur where the gradual collapse of a cave passage occurs and eventually causes subsidence at the surface level. Sinkholes formed exclusively this way are quite rare, although many sinkholes are in part formed by collapse: chemical weathering in a solution sinkhole may cause a part of the wall to become unsupported and unstable, resulting in collapse.
Collapse sinkholes are not rare where limestone is overlain by sandstone.
Collapse sinkhole formation: as the water filters down through the rocks above, the limestone starts to dissolve and the rocks above drop down to form a sinkhole. Source: BGS ©UKRI. All rights reserved.
Suffosion sinkholes form where solution of the limestone has created a depression on the surface of the limestone, but under a covering of soil. The unsupported soil subsides into the cavity and leaves a depression in the landscape. These are sometimes referred to as subsidence sinkholes and in Yorkshire, where they are particularly common, they are often known as ‘shake holes’.
The glacial tills of Malham Moor are pockmarked by suffosion sinkholes, between about one and 15 m across. Some still have soil and grass on the floor of the sinkhole, but in others the fissured limestone is exposed, the soil having been eroded away by the rain and washed down into the underground river system.
Where does karst occur in the UK?
There are a number of soluble rocks across the UK. These include Devonian and Carboniferous limestones, Permian and Triassic halite and gypsum, as well as Jurassic and Cretaceous limestones and chalk. Hydrogeologists think of karst systems in terms of the recharge via fractures or dolines (diffuse or focused), through-flow via conduits that may reach the size of caves and discharge from springs (or boreholes).
Dolines or sinkholes are classified in accordance with their formational processes. The rate at which they form is guided by the size of openings though which fluids flow, the head that is driving the flow system and the geochemistry. 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. Others may be buried by more recent deposits. Sinkholes may also form where dissolution of soluble rocks take place beneath a thin covering of loose superficial material such as sand, clay or soil. In this setting, the soil can be washed into widened fissures or cavities below. If the cover material is sandy, it will tend to gradually slump into the fissures, slowly creating a sinkhole over time (suffosion sinkhole). However, if the material is more cohesive, like clay, then the cavity can grow quite large before suddenly collapsing; a process termed a ‘drop out’ sinkhole. It is these more spectacular collapses (dropout) that sometimes hit the headlines, for example in the area of Ripon.
Where do sinkholes occur in the UK?
Although most sinkholes are relatively small or are in upland rural locations. They include areas underlain by Carboniferous Limestone, notably the Mendips, parts of Wales, the Peak District, and the northern Pennines including the Yorkshire Dales.
The most susceptible area in the UK is the Permian gypsum deposits of north-east England, particularly around Ripon and areas underlain by a similar geology. In Ripon many large sinkholes have developed, some of which have affected property and infrastructure. This is because gypsum is far more soluble than limestone, and thus dissolves more rapidly.
Sinkholes also occur over salt deposits, commonly in areas such as Cheshire where brine has been extracted making it difficult to separate naturally formed sinkholes from those created by man.
The Chalk is also susceptible, and surface lowering of the Chalk is especially evident where it is covered by younger clay and sand deposits (the ‘Clay-with Flints’ and Palaeogene strata), notably in parts of Dorset, Hampshire and the Chilterns. In Scotland, sinkholes are generally rare except in parts of Assynt underlain by the Cambrian Durness Limestone.