layers of earth

A father and his two teenage children drowned when this cavernous pit swallowed up several buildings in the Guatemala City barrio of San Antonio. The hole, which appeared on February 22, is approximately 100 feet wide and 200 feet deep.

Reeking water, still swirling in the bowels of the hole, offers a telltale clue to what happened: Sewage flowing from an eight-foot-wide ruptured sewer main at the bottom of the hole eroded ash and pumice layers deposited by ancient volcanic eruptions. The leaking liquid created a shaft that grew upward through the soft ash by a process called “piping.”

Eventually the shaft became so large that it could no longer support the upper layers of earth, which abruptly collapsed into the empty space. Recent rains in Guatemala City probably contributed to the collapse by weakening the surface soil and adding storm-water runoff to the percolating sewage.
Ric Finch, a retired Tennessee Technical University geology professor who has done field studies in northern Central America, has not visited the site but has examined photos of the collapsed shaft. He says the shaft’s walls contain easily eroded volcanic materials, which are found throughout the valley where Guatemala City is located. The shaft may have developed very rapidly, Finch says.

Where did the eroded materials go? Mostly likely, they were washed downstream through the partly blocked sewer main, which is more than eight feet in diameter. The bodies of the two drowned teenagers were found in a nearby canyon where the sewer system discharges.

Many news accounts have referred to the collapsed shaft as a “sinkhole,” but that is not the correct term here. Sinkholes form in places where the underlying layer consists of limestone or other soluble rock, which dissolves in water rather than simply washing away like ash. Geologic maps for Guatemala City indicate that any limestone in the area of the cave-in would be located well underneath the volcanic deposits.

Limestone-associated sinkholes are common in other regions of Guatemala (and in Florida). It’s uncommon, however, for a sinkhole to be as large and deep as the Guatemala City pit. Holes in the ground sometimes open up without warning, but not in this case. Neighbors reportedly heard noises and felt tremors for weeks before the collapse.

Some 200 residents have been evacuated from the San Antonio neighborhood, and officials have cordoned off the area around the shaft. Tom Miller, a geologist at the University of Puerto Rico who has visited the hole, says that it is slowly enlarging. Officials have used a remotely controlled camera to examine the damage, and are currently attempting to re-route the sewage. “The neighborhood does not smell pleasant,” Miller says.—Dawn Stover