A burst sewer pipe or storm drain probably hollowed out the underground cavity that allowed the chasm to form, according to Sam Bonis, a geologist at Dartmouth College in New Hampshire, who is currently living in Guatemala City (map).
The Guatemala City sinkhole, estimated to be 60 feet (18 meters) wide and 300 feet (100 meters) deep, appears to have been triggered by the deluge from tropical storm Agatha.
But the cavity formed in the first place because the city—and its underground infrastructure—were built in a region where the first few hundred meters of ground are mostly made up of a material called pumice fill, deposited during past volcanic eruptions.
"Lots of times, volcanic pumice originates as a flow [of loose, gravel-like particles], and because of the heat and the weight, it becomes welded into solid rock," Bonis said.
"In Guatemala City [the pumice is] unconsolidated, it’s loose," he said. "It hasn’t been hardened into a rock yet, so it’s easily eroded, especially by swift running water."
In general, the zoning regulations and building codes in Guatemala City are poor, Bonis said, and the few regulations that exist are often ignored. That means leaking pipes could have gone unfixed long enough to create the right conditions for the sinkhole. (Related pictures: “How Humans Can Trigger Earthquakes.”)
In fact, Bonis thinks calling the Guatemala City chasm a sinkhole is a misnomer—a true sinkhole is an entirely natural phenomenon. There is no scientific term for what happened in Guatemala, he said, adding that he recommends the pit be dubbed a piping feature.
Guatemala Sinkhole Not a Sinkhole
Natural sinkholes generally form when heavy, water-saturated soil causes the roof of an underground limestone cavity to collapse, or when water widens a natural fracture in limestone bedrock.
But there is no limestone beneath the section of Guatemala City where the new sinkhole appeared, at least not at the depth at which the hole formed, Bonis said.
"There may be limestone thousands of meters beneath the city, but not hundreds of meters," he said.
Instead, nature likely sped up a process set in motion by human actions. (Related: “‘Mud Volcano’ in Indonesia Caused by Gas Exploration, Study Says.”)
Recent eruptions of several volcanoes in Guatemala covered the city in a fresh layer of volcanic ash. If this material got into the city’s pipes and drains, it may have clogged the passageways, making ruptures more likely, Bonis said.
Heavy rains from tropical storm Agatha may also have overloaded underground sewage or drainage pipes, leading to a growing cavity that eventually collapsed, Bonis speculated.
The geologist added that the new sinkhole shares remarkable similarities with a sinkhole that formed in Guatemala City in 2007.
"Both of these things occurred in the same general part of town. They look the same," he said. "It’s more than a coincidence, especially if they trace" any faulty pipes associated with the 2010 sinkhole to pipes near the 2007 sinkhole.
Guatemala City Sewer Inspections a Must
The danger should not have been news to officials in Guatemala City, noted Bonis, who used to work for the Guatemalan government’s national geology institute.
As part of a volunteer team that investigated the 2007 sinkhole, Bonis co-authored a report warning the Guatemalan government that similar holes will very likely keep appearing unless action is taken to inspect the city’s sewer system for weaknesses. (Watch video of sewer divers in Mexico City.)
The government never replied, Bonis said—possibly due to a lack of funds.
"There’s a minimum of regulation, because that’s money that the government doesn’t have," he said.
But, he added, “there’s got to be ways of inspecting the sewer system. These are things that have to be done.”