What are sinkholes, what causes them and what are the chances of sinkholes in the UK?

The ground giving way beneath your feet, the city street collapsing into a pit… Look at pictures of the 2010 sinkhole that appeared in Guatemala and you see humanity pitched as pizza topping, precariously layered on a brittle crust.

Sinkholes in the UK have appeared beneath everything from Primark outlets to train lines, although these are small fry compared to those recorded recently in China – including this one from last year in Guangxi.

What causes sinkholes and what is the likelihood of one appearing under your feet? Here’s our brief guide to the wonderful world of ground erosion.    

What are sinkholes?

Broadly speaking, a sinkhole is a depression in the ground, caused by some type of subterranean collapse or dissolution. Basically, it’s a hole in the ground that’s made by erosion, normally due to water drainage. The can encompass everything from holes a few metres across, to holes big enough to swallow streets and buildings.

What causes sinkholes?

Sinkholes happen naturally, but can also be the result of human activity. In both cases, the cause is typically a combination of underground erosion due to water drainage, and ‘karst’ land – soluble rocks such as limestone, dolomite and gypsum.

The action of the water wears the rock away over time, leading to caves that collapse and cause sinkholes. Phenomena such as changing ground water levels can also cause sinkholes. If there is a drought, for example, a subterranean cavern that was previously supported by water can become weaker, meaning a sudden downpour could cause its rock to crumble.

Human activity precipitates this process, either because of old, leaky water pipes or because of concrete and tarmac on the ground concentrating rainwater to thin patches of soil. Poor water drainage in a city can, over time, cause sinkholes. Fracking is another practice that’s been linked to sinkholes, if the massive abstraction of water from the ground that’s needed for the process is done incorrectly.

READ NEXT: What is fracking?

“The abstraction of water from the shallow ground surface can cause a collapse if the ground already has the propensity to collapse,” David Shilston, president of the Geological Society of London, told The Guardian, adding that “fracking will have to take water from safe sources”.

What are the different types of sinkholes?

Geologists divide sinkholes into three categories:

Dissolution sinkholes

This type of sinkhole happens when you have karst rock, such as limestone, with very little soil or vegetation covering it. Rainwater trickles into the bedrock, dissolving it over time. These generally aren’t that dangerous, and the most dramatic thing they’re likely to produce is a pond.

Cover-subsidence sinkholes

These sinkholes form when you have sand covering the bedrock. Water filters through the sand and wears away the karst rock, so that over time the surface of the ground sinks. Because the sand sinks alongside the bedrock, these sinkholes happen gradually over time.

Cover-collapse sinkhole

These are the dangerous type of sinkhole, where a layer of clay covers the bedrock. Like the cover-subsistence sinkhole, water wears away the subterranean bedrock. In this case, however, the surface remains intact. As time passes, an underground cavern forms. As more time passes, the cavern expands, and the bottom of the surface also begins to crumble. This goes on until only a thin layer remains between the surface and a cavern. If that layer collapses, then the sinkhole dramatically opens up.

(Above: The Bimmah Sinkhole in the Sultanate of Oman)

How likely is a sinkhole to swallow me whole in the UK?

The answer to this very much depends where you live: 10% of the world’s surface is made up of karst stone, and certain places have more of it than others.

Florida, for example, is entirely made of karst bedrock. In the UK, karst hotspots include the limestone-heavy Peak District and Yorkshire Dales, the chalk-heavy south-east, and the gypsum-heavy areas around Ripon and Darlington in the north-east. This gypsum is particularly liable to erode, which is why a number of sizeable holes have previously appeared in Ripon, such as one in 2016 that took out a set of gardens.

The good news is that fatalities because of sinkholes are pretty low. Only a handful of deaths have been caused by sinkholes over the past decade.

Thankfully, stories like that of Jeffrey Bush are rare: in 2013, a sinkhole opened up beneath the home of the 36-year-old Florida man as he slept. His body was never found.

Lead image credit: Flickr/Creative Commons, Lee Craven