What are sinkholes anyway?

by Rebecca Priestley / 31 December, 2015
In St Albans, UK, a 20m sinkhole forced the evacuation of five homes.

Most of us have seen horror videos of sinkholes swallowing houses, but few know how they actually work.

QUESTION: Last September, a huge sinkhole emerged beneath a beachfront campsite in Queensland, swallowing a car and a caravan. In October, in the UK town of St Albans, the ground caved in, creating what was described as “an apocalyptic sinkhole”. Closer to home, a 10m-deep sinkhole appeared next to a popular Cromwell walking tracking. And then there’s the mystery of the Siberian sinkholes, one 80m wide. What are the chances of encountering a summer holiday sinkhole in New Zealand? And what is a sinkhole, anyway?

ANSWER: Sinkholes can form naturally or be ­triggered by human activities, says Jon Carey, an engineering geologist at GNS Science. “Natural sinkholes generally form in karst landscapes – terrain underlain by soluble rocks such as dolomites, limestones and chalks,” he says.

“The natural process of water moving through these rocks slowly dissolves them, creating underground cavities.”

He says over time these cavities can expand upwards and outwards, creating bigger openings, until eventually the ground above them, which has been hiding what’s going on, might begin to subside.

“Sometimes this happens slowly or sometimes a sudden collapse might create a large sinkhole.”

The sudden collapses are known as cover-collapse sinkholes and are the ones that make the headlines.

Human-induced sinkholes can form in any rock that’s been subject to underground excavations, such during the creation of tunnels and mine shafts. “If you make a mistake in the way you design the excavation, it can collapse into some kind of hole,” says Carey.

Changes in groundwater conditions, such as leaking water mains and even swimming pools, can also cause land subsidence, as can groundwater extraction.

“By removing the water from a subterranean environment,” he says, “you can create a cavity and the ground above can slowly fall into it.”

In New Zealand, you’re most likely to find natural karst features – underground caves with sinkholes at the surface – in limestone country in parts of northwest Nelson, in the southern part of the Waikato region and in parts of Canterbury.

But the Cromwell walking track in which a sinkhole appeared – the 45th Parallel Walking Track – passes over an alluvial terrace, which is not the sort of geology usually associated with natural sinkhole development, says Carey. “My first impression would be that this is a human-induced sinkhole related to historical mining practices.”

In some parts of the world, such as a region of Florida known as “sinkhole alley”, people have died after being swallowed by the sudden emergence of the holes.

Carey says sinkholes are not something we should be too worried about in New Zealand, where volcanoes, earthquakes and landslides are more of a concern. But sinkhole-related accidents are not unheard of.

In 2014, a Waihi cyclist was seriously hurt after riding into a 1m-wide sinkhole. This hole was believed to have formed naturally, in contrast to other major sinkholes that started emerging in the town in 1999. Caused by the collapse of old mine workings, these large sinkholes, up to 50m in diameter, have destroyed houses and led to the permanent evacuation of nearby homes.

When heavy rain destabilises hillsides, most people are aware of the risk of landslide. In fact, that’s what last year’s Queensland “sinkhole” turned out to be – it was reinterpreted as a type of landslide or slope failure common on the sandy shores of beaches and rivers.

But we’re not at risk of the type of sinkholes that have appeared on Siberia’s Yamal Peninsula, says Carey. “Russian scientists are indicating that the repeated freezing and thawing of the rock may have caused a build-up of gas pressure – of methane or a gas hydrate – below the rock in some kind of cavity. Eventually, you get an eruption of the gas, and that release of pressure causes the creation of the sinkholes.”

If this theory is correct, a ­warming climate, with associated thawing of permafrost land, may worsen the problem.

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