Initial reaction from geologists was that the hole is a sinkhole. Sink holes can appear suddenly but take thousands of years in the making. The gaping holes are usually caused by rainwater gradually eating away at porous rock such as limestone below the surface, weakening it, and creating a honeycomb of caverns and caves which can become packed with mud. Floodwater may have flushed away that mud - leading everything above it to collapse.
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But according to a new report on Discovery, the hole in Guatemala is not a sinkhole.
The hole in Guatemala City is not a sinkhole. When you inspect it, it doesn't even look like one. The sides are clean and featureless, and the shape is crisp and geometrical. It helps to know that Guatemala City is in volcanic country, not limestone country. In fact the city, like many in Central America, sits in a former river valley that is filled to the brim with loose volcanic tuff. When something compacts it at a deep level, or when groundwater flow carries it away, the tuff can settle downward. Reilly interviewed a geologist practicing in Guatemala who gave us the correct name for this structure and its formative process: a piping structure. Piping is also called tunnel erosion, and it's always a concern around large dams, for instance, or when a water main or sewer line breaks underground. [via About.com]Incidentally, a similar hole, 300 ft deep, opened up in Guatemala in 2007 after a sewage pipe broke pipe just a few blocks from this weekend's disaster.