Why Massive Sinkholes Open In The Ground Without Warning - Swallowing Cars, Homes, And Sometimes People
Had she known what was about to happen that night, Rachel and her family would have moved out of the house years earlier, she explains in a new NOVA documentary , which is how we learned about her story. But she had no idea that their modest Florida home was sitting on top of a ticking time bomb - a sinkhole that would change their lives forever.
While Florida has a reputation for sinkholes, they happen all over the United States and all over the world , often suddenly and without warning. On Wednesday at 9pm, a special NOVA premiere ("Sinkholes-Buried Alive") will explore where, why, and how they occur.
The events that precede a sinkhole are subtle, which is why they often go unnoticed.
First, rainwater from the ground's surface passes through a tiny crack in the earth, wearing away at the sediment beneath. Eventually, more rainwater continues to flow inside until the water carves out a hollow opening underneath the surface.
But if the soil has clay in it, meaning it sticks together better than loose, dry soil, the surface will remain strong even as the ground beneath starts to open up. This is the dangerous part, since it's unlikely that anyone will notice anything's changed.
The void underneath begins to swell, growing larger and larger, and then suddenly, it gives way.
This is a cover collapse - and it's the most dangerous type of sinkhole. These kinds of sinkholes typically take place in areas where limestone or other types of water-soluble rock makeup a primary component of the underground sediment . Because liquid passes through limestone so easily, it is particularly vulnerable to getting worn away by rainwater.
If the causes of sinkholes seem entirely natural, it's important to keep in mind that there are several human activities that can turn a pending threat into disaster.
One of them is farming.
Using a model he built from clay and sand, University of Arizona hydrogeologist Ming Ye found that when farmers withdraw water from the ground to irrigate crops, they cause the entire water table to be pulled down. That means that in areas where potential sinkholes are forming - holes in the ground that haven't caved in completely - the water that may have been supporting them gives way to air.
This change alone isn't always enough to cause a sinkhole, but a storm or a bit of flooding can be enough to turn a potential sinkhole into a gaping hole in the ground.
"It would be a disaster if you pumped too much water from limestone layer and at the same time have a thunderstorm that adds too much water to the ground," Ye says in the NOVA special.
That's precisely what happened in Plant City, a town in East Tampa, Florida where massive underground pumping projects caused 60 sinkholes to open up in the ground within the span of a year in 2010.
Old salt mines, too, can cause disaster when they get too close to the site of a potential sinkhole. And although most American salt mines were built more than a century ago, we still use them for everything from table and rock salt to chorine gas, a key component of plastic.
The problems begin when mining companies send drilling pumps into ancient salty seabeds , pillar-like formations that form over millions of years as surrounding sediments propel them upwards. The pumps dissolve the salt into a salty brine, creating a big watery cavern around the pillar.
If the cavern gets too close to a sinkhole, soil from the sinkhole leaks into the saltwater cavity, causing the ground above to collapse.
In Bayou Corne, Louisiana, a massive sinkhole opened up when an underground gap in the earth collided with an expanding salt mine, swallowing trees and land. Residents were forced to relocate. As of Oct. 2014, the sinkhole continues to spread.
In this 2013 video, the Bayou Corne sinkhole swallows up several trees in seconds:
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