Thursday, June 3, 2010

The "Piping Feature" at Guatemala City: Don't Call it a Sinkhole

Thank you to the always reliable Andrew Alden at About Geology for this note. I looked at these pictures about the giant hole that opened up under Guatemala City following the tropical storm. Most news reports called the gaping pit a sinkhole, but something didn't quite seem right about the appearance of the feature. Sinkholes usually form in areas where limestone has been eaten away from beneath by the slow dissolving action of acidic groundwater. Eventually a cavern opening becomes too big and the ceiling collapses, forming the sinkhole. This one just looked wrong, with the more or less uniform looking tan colored material and the sheer vertical walls.

Michael Reilly provides a much more satisfying explanation at Discovery News. Much of the valley where Guatemala City is located is covered by up to 600 feet of loose pumice and volcanic ash, materials that can be easily eroded. When sewer pipes or storm drains break or leak, they can start eroding the loose materials deep below the surface, eventually causing the surface to collapse inwards. In the Discovery News article, geologist Sam Bonis refers to the feature as a piping feature. This isn't the first time such pits have caused problems in the city; in 2007, the pipe below destroyed twelve houses and killed three people (thanks to Discovery News for the link). The stench of sewage and the sounds of flowing water in the 330-foot-deep hole confirmed the origin of the problem.

Unfortunately, the holes are apparently caused by human activity, not geologic processes...

The top picture from the Guatemala government Flicker Site, the bottom photo is from MSNBC News.

UPDATE: Oops, geologist Sam Bonis called a piping 'feature', not a 'structure'. I've made corrections.
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