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The Xiaozhai Tiankeng sinkhole in China, is the world's deepest sinkhole. (Source: Brookqi/via Wikimedia Commons)

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The science of sinkholes

Tuesday 6th October 2015 8:26 am

They can easily swallow up your car or your house. One tenth of the Earth’s surface is susceptible to them. And they’re called sinkholes.

In April, 2014, in Florida, a resident, Jeremy Bush, lost his brother. He was awakened in the night by a loud bang. He said: “I ran into Jeff’s room and just saw a massive hole.” The body of his brother was never recovered.

In May, 2010, a giant sinkhole opened up in a suburb of Guatemala City. The deep hole, which formed an almost perfect cylinder 20 metres wide and 30 metres deep, swallowed a three-story building.

In 2015, a small sinkhole formed in the backyard of a family home in Illawong, a suburb of Sydney. At 4 o’clock in the afternoon of June 15, a father and his baby were sitting on the couch when the ground outside suddenly fell away. It knocked over a five-metre tree and a washing line. Everyone was okay – but they did have to be evacuated from their home.

Three different sinkholes – with three different causes.

Speaking generally, a sinkhole can range anywhere between a slight depression in the ground, right up to an enormous cylindrical hole reaching down over half a kilometre. The 2015 Sydney sinkhole was about 15 metres by 15 metres across, with a three-metre depression. At the other extreme, the world’s deepest sinkhole is in Chongquig, China – reaching down 662 metres.

Sinkholes have no natural surface drainage. Any water that gets into a sinkhole, can’t get out via the surface – and so usually drains downward, into the subsurface layers.

So what causes sinkholes is fairly straightforward. It’s just the stuff immediately below the surface shifting to somewhere else. Depending on how much stuff shifts, you get anything from a small depression in the surface right up to a circular hole vanishing out of sight into the depths.

It’s water that does the shifting of material under the surface. There are three main situations.

First, the rocks below the surface are made of stuff that will dissolve in water – such as limestone or gypsum. This is the case in 20 per cent of the USA, or 10 per cent of the whole world.

Over hundreds or thousands of years, natural underground currents will slowly dissolve the limestone or gypsum – creating a void. The top of the void gradually migrates towards the surface. Slowly, the surface layer that supports what’s above it gets thinner and thinner. Solid rock turns into a bridge, which at some critical stage becomes too weak to support what is above it.

This was the situation in Florida, when part of the house fell into a sinkhole. In fact, nearly all of Florida is liable to sinkholes – and until 2007, it was compulsory to have sinkhole insurance.

The second cause of sinkholes occurs when the rock under the surface doesn’t dissolve in water – but instead is made of grains that are small enough to be carried away by underground water currents.

In Guatemala City, where a three-story building just vanished into the sinkhole, the underlying rock was predominantly weak crumbly volcanic rock, fine ash and other debris that had erupted from a volcano.

The third cause of sinkholes is not underground water currents at all, but instead, changes in the short-term water movements above ground.

This can be set off by intense rainstorms or floods or rather surprisingly, even drought – but most commonly, it happens because we humans changed the previous water-drainage systems. This can range from a burst water pipe, to long-term leakage from a broken sewer or storm-water pipe – and it also includes covering vast tracts of land with solid concrete. In all these cases, the water can’t go where it used to, so it finds a new path.

The suburban backyard sinkhole in Sydney in 2015 happened after weeks of intense rain with lots of flooding. And once the sub-surface stuff was washed away, the last stage happened quite abruptly. One afternoon, without any warning, the surface just slumped down to form a three-metre-deep basin.

But in each of the three main situations, the sinkhole didn’t form suddenly. It built up slowly over many months or years. It was just the last stage – the formation of the hole or basin at the very surface – that was sudden.

So be on the lookout! If you start to get that sinking feeling, and feel the earth move under your feet, you could be seconds away from being swallowed whole.

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This blog first appeared on Dr Karl's Great Moments in Science

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