Sinkholes may be more common than you might think, but
they cannot occur everywhere. Steff Gaulter explains why
You probably heard the terrifying story of Jeffrey Bush, the man in Florida who was swallowed up by the earth as he slept in his bed. It sounds like something that should only happen in a horror film; one minute the man is soundly sleeping, the next he is screaming for his life as the ground gives way.
His brother, Jeremy, gave a moving account of how he desperately tried to save his brother. He rushed into Jeffrey’s room, to find much of the furniture had disappeared into the earth. He jumped in the hole and clawed away at the ground with his bare hands, but was eventually dragged away by the emergency services who feared the entire house might also collapse into the earth below them.
Most people who heard the story were horrified, but consoled themselves by thinking it was just a one off. However, it turns out that sinkholes aren’t as rare as you might think. Within a week of that tragedy, another sinkhole appeared in the same region of Florida and just one more week after that, a 9m sinkhole opened up in Pennsylvania. Fortunately there were no injuries in either of these two incidents, but last year there were at least two cases of buses being swallowed by the earth.
One was in California, and the other in Guilin, southern China. Neither buses had passengers in at the time, but you can imagine how shocked the drivers were. You may also remember the incredible sinkhole which suddenly appeared in Guatemala in 2010. The hole was a hundred metres deep and so perfectly formed that many people I know initially thought the image had been altered.
Sinkholes may be more common that you might think, but they cannot occur everywhere. They can only form in a landscape that’s of a particular type of ground, and the actual collapse of the earth is usually triggered by the weather.
Underneath the soil on the surface of the ground (or sand in Qatar’s case!) is hard consolidated rock. This rock, known as bedrock, can be at the surface of the earth, or can be several hundreds of metres deep, and can be made of a variety of substances.
Sometimes it will be made of a glassy rock, formed when the molten earth cooled, other times it may be made of tiny pieces of rock or shells which have been compacted together into a solid substance. These are called sedimentary rocks and it’s these which cause the problems, because some of them have the rather fundamental flaw in that they can dissolve in water.
When it rains, water soaks into the soil, and then filters down through the bedrock. Tiny cracks in the rock allow the water to pass through, but as the years pass, if the bedrock is soluble, the water will slowly dissolve the rock. The cracks will gradually widen, creating empty cavities beneath the surface. As the years continue to tick by, the top soil above the bedrock will gradually be washed down into the spaces below.
Eventually the soil above the cavity will cave in, but how long that will take varies from place to place. It depends on how thick the soil is above the void, and of what the soil is made. Sand will not be able to support its own strength for long, clay, on the other hand, can hold out for hundreds of years.
The actual collapse is then triggered by a change in the weather. This could be a rapid downpour, which will add weight to the layer of the overlying soil, or it could be a long period of drought. Although the water washes away the rock, it also fills the voids and supports the layer above. In a long period of drought, the ground can become weaker, and can no longer support its own weight.
Although it usually takes thousands of years for caverns to develop under the ground, it’s worth noting that some rocks dissolve far faster than others. Gypsum is the most soluble; if you were to place a block of gypsum the size of a Toyota Landcruiser in a river, it would dissolve completely within about 18 months. Worrying, much of the bedrock in Qatar is made of this readily dissolving rock, and just to concern you a little more, there are also plenty of sinkholes in Qatar too.
There are estimated to be as many as 9,700 large and small depressions in Qatar, including a number of large sinkholes. The majority are found in the north and central parts of the country. The largest is Musfer Sinkhole, which is found just off Salwa Road, and is at least 100m deep.
However, there is no need to panic, it’s very unlikely that you will be swallowed up by the ground on your way to the office. The majority of the sinkholes in Qatar are thought to have been formed hundreds of thousands of years ago, when the climate was very different.
Changes in the Indian Summer Monsoon are believed to have periodically made Qatar far wetter and more humid. The present dry climate isn’t nearly as conducive to the formation of sinkholes as a wet one because the rainwater does not trickle through the sand into the rocks. Having said that, I think the next time there’s a decent rainstorm, I might keep my eyes peeled for any changes to the ground.

(The author is Senior Weather Presenter at Al Jazeera English channel. She can be contacted on [email protected]
or on Twitter at @WeatherSteff)