The incredible photos of the massive sinkhole in Guatemala City have the world talking.
"Are those photos real?"
"It looks like something out of a movie!"
"How did this happen?"
Sinkholes are a common geo-morphic occurrence that range from small puddles to the incredible hole in Guatemala City. A sinkhole is a depression in the ground that has no natural drainage out; therefore any water entering the depression stays inside the sinkhole and typically drains into the subsurface.
In most situations, water draining into the subsurface simply recharges the aquifer beneath it and the result is, in fact, beneficial. The changing of the landscape due to this process sometimes slow and only perceived over a lifetime.
On the other hand, when the sinkhole is under-lain by rocks that can be dissolved by water drama can ensue, especially in an urban area like Guatemala City or Miami.
Sinkholes are most commonly associated with karst type geology - or rocks made from carbonates, like limestone or dolomites. The chemistry of these rocks makes them susceptible to dissolution, especially into slightly acidic water like rain water and runoff, making dramatic land forms like caves, springs or water falls and, of course, sinkholes.
Geologists classify sinkholes into various types, depending on what type of dirt and sediments over lay the dissolving rock. As the rocks dissolve into the groundwater, mass is removed as the water flows away; what happens at the surface, or what we can see, depends upon what is on top of the dissolving rocks.
The most basic is called a "dissolution sinkhole" and is when the karst rocks are overlain by only a thin layer of soil. When the rocks are dissolved, a depression forms and sometimes fills with water.
"Cover subsidence sinkholes" form when the dissolving rocks are overlain by thick, sandy soils. In this case, the sand falls into and fills the spaces of the karst system. In this case the depression at the surface forms slowly and are less dramatic than other types.
Although it has not yet been fully studied by geologists, the sinkhole in Guatemala City is most likely the last type of sink hole, or what is called a "cover collapse sinkhole." This is when the dissolution of the rocks occurs under a thick layer of clayey material.
Because the over-burden soils are more sturdy (sticking together because of the clay), they support themselves for some time while the dissolution continues to grow beneath. Of course, the clayey material does eventually erode, and slowly the cavity becomes larger with a progressive collapsing of the clayey roof.
Eventually, the roof can not support anything and a dramatic collapse occurs. These can be shallow bowl shaped or "well shaped" like a cylinder.
Sinkholes occur naturally, but anthropogenic (human induced) effects often exaggerate or accelerate natural processes. For example, periodic pumping from a karst aquifer can raise and lower the water table, which causes changes in the geo-chemistry and can cause aggressive dissolution. Additionally, water under ground provides support to the system above it, as you drain an aquifer by pumping, sinkholes can form.
There are a lot of hypothesis and questions floating around about what happened in Guatemala City.
"Did a UFO make it?"
"Was it human induced?"
"Was it caused by leaking damaged pipes?"
"Residents had complained to the city since 2005 about groaning sounds in the ground."
"Was it because of tropical storm Agatha's increased rainfall and runoff?"
Geologists will surely study this fascinating and unbelievable example of a sinkhole, perhaps coming up with new hypothesis as to its particular mode of formation. Today we can indeed make an educated guess: This sinkhole was probably not caused by UFOs; and most likely several factors coming together at the right moment after a history of development made a spectacular example of geologic processes in action.
United States Geological Survey, Water Science for Schools: http://ga.water.usgs.gov/edu/sinkholes.html