We rely on the ground. It is, pardon the wordplay, the bedrock of our civilization. We walk on it. We build on it. Without it, we'd have to figure out how to make floating cities either on the ocean or in the air.
So until we figure that out, we rely on the ground. Sometimes, however, the ground isn't reliable — like when sinkholes happen.
Sinkholes can swallow streets, cars, houses and even whole buildings in urban areas. If they form in the wilderness, they can result in lakes, pits or even tourist attractions, like Belize's Great Blue Hole.
So how do these gaping maws in the Earth form? Is it possible to anticipate them?
What causes sinkholes?
More specifically, sinkholes are the result of water collecting underground and lacking external drainage of some sort, according to the U.S. Geological Survey (USGS). As the water collects and circulates, it slowly erodes the bedrock and creates caverns and underground spaces.
Water can erode just about anything given enough time, but soluble minerals and rocks, like evaporaites (salt, gypsum) and carbonates (limestone, dolomite), are particularly vulnerable and can be worn away more easily than some other types of rocks and minerals.
Over time — often a very long time — these caverns grow and grow until the very top layer of the ground is no longer supported. Then the ground opens up and swallows anything sitting there, and you have a sinkhole.
Adding additional weight on the surface, be it buildings or just heavy rainfall, can break the top of such a cavity and create a sinkhole.
What are the types of sinkholes?
Not all sinkholes are the same, however, even if the overall process — water eroding bedrock — is the same. There are three types of natural sinkholes.
1. Dissolution sinkholes. These sinkholes are the result of there not being much groundcover, like vegetation, over the bedrock. Water slips through pre-existing holes in the bedrock and begins to circulate through the bedrock. A depression in the ground can form, and if the bedrock layers beneath are sturdy enough or there's enough debris blocking the flow of water, the sinkhole may stop deepening. This could result in the formation of a pond-like areas and even wetlands, according to the USGS.
2. Cover-subsidence sinkholes. These sinkholes start with something permeable covering the sinkhole while also containing a good deal of sand. This sediment begins to spill — or spall as the proper nomenclature refers to it — into those empty caverns among the bedrock. Over time, a depression in the surface may occur. This sediment can block the caverns and prevent the flow of water. These sorts of sinkholes are never very large, according to the Southwest Florida Water Management District, since the sediment prevents the water from further eroding the surrounding bedrock.
3. Cover-collapse sinkholes. Perhaps the most well-known of sinkholes, cover-collapse sinkholes are also the most dramatic. The surface area above the bedrock in this instance is mostly clay, spalls into the cavities. But since the clay is sturdy, arches form as its slowly spalls. This arch continues to support the surface ground until it becomes so thin that it collapses into the cavern below, swallowing up everything above it.
There is one final type of sinkhole, and that's man-made sinkholes. These sinkholes are the result of a variety of practices, from drilling to mining to changes in water diversion systems to broken pipes.
Can we predict when sinkholes will happen?
While sinkholes have a reputation for being sudden occurrences, they happen over long periods of time. This means there are sometimes signs that a sinkhole could be forming under your feet.
If you're looking for signs of a sinkhole below a building, the University of Florida recommends being aware of structural cracks in walls and floors, cloudy well water and doors and windows that won't close properly.
On the ground, there are likely to be more signs, including wilting or dying vegetation, previously buried things — like fence posts, roots or structural foundations — becoming visible, the formation of new and small ponds and slumping trees and fences.
Should a sinkhole occur near you, the Southwest Florida Water Management District recommends evacuating the premises and then notifying your insurance agency and the city.