Swallowed by a sinkhole: panic strikes when the ground opens without warning. Can it happen anywhere?
Last May, Guatemala City was pelted with torrential rains from Tropical Storm Agatha. After the deluge, part of the soggy city's ground gave way, leaving a 20 meter (66 foot)-wide crater. An entire three-story building--along with most of a street intersection--plunged into the 100 m (330 ft)-deep chasm.
The collapse wasn't the city's first; a similar hole opened in 2007. Reporters called the crater a sinkhole, or a type of naturally forming hole that occurs all over the world. But experts warn that something more is behind Guatemala City's fast-forming pits. Read on to learn the causes of natural and human-made sinkholes and the areas at risk of falling into the earth.
Sinkholes usually form in karst terrain. These areas contain carved out features created when rocks such as limestone and gypsum dissolve away. About 20 percent of the continental United States sits on this type of bedrock, or solid rock layer beneath the soil (see map, p. 12). Sinkhole hot spots include Missouri, Texas, Florida, Alabama, Kentucky, Tennessee, and Pennsylvania.
Even though sinkholes form in the ground, the process begins in the sky. "Water, when it falls through the atmosphere, naturally picks up a little bit of carbon dioxide," says Doug Gouzie, a geologist at Missouri State University. This turns rainwater into a weak acid. The acidic water sinks through the surface layers of soil and sediment and then slowly dissolves pathways through the bedrock.
As these pathways grow, water flows through them, carrying away sediment. The result: The ground surface sinks as the particles of dirt and rock beneath it wash away. The process is similar to how an hour glass works. "As the sand trickles through the hourglass's neck, you see that the top of the sand will gradually go down and start forming a cone," explains James Kaufmann, a scientist at the U.S. Geological Survey. Such a cone- or saucer-shaped depression in the ground surface is a sinkhole.
Most saucer-shaped sinkholes don't make headlines. They usually form so slowly--over hundreds or thousands of years that no one notices the gradual sinking, or subsidence, of the land. It's much more dramatic when sinkholes form without leaving clues on the surface, and then suddenly cave in.
The sinkholes that catch people by surprise, called collapse sinkholes, start off the same way subsidence sinkholes do. Water dissolves pathways through bedrock and carries sediment away. But then a blockage occurs. Gouzie says this is similar to what happens when you cut the comer off of a bag of M&Ms and pour them. "Every so often, the M&Ms all jam in the comer of the bag and nothing comes out," he says. In the same way, sediment particles can become stuck.
Water continues to carve out a space beneath the jammed particles, which act like a bridge to hold up the land above. The void grows until the bridge can no longer hold the weight. Then suddenly, the bridge collapses, taking along anything that sits on the surface. That's what happened in Nixa, Missouri, in 2007 when a home's two-car garage disappeared into a 23 m (75 ft)-deep sinkhole!
Instead of forming naturally, Guatemala City's collapse was caused by human activity. The underlying ground isn't made of karst, but of loose, volcanic-rock particles called tephra. "This stuff is probably the most easily eroded sediment anywhere," says Kaufmann. Leaking sewer or drainage pipes beneath Guatemala City caused huge voids to form in the tephra. Eventually, the surface gave way (see Guatemala Sinkhole Explained, top left).
Some experts suggest collapses that don't occur through natural processes, like the one in Guatemala, should be called "piping features" instead of sinkholes. Since leaking pipes are usually the culprit, these holes are common in cities around the world. For instance, in 2003, a bus took a nosedive into a pit that opened up in Lisbon, Portugal.
Regardless of the cause, all types of collapses have something in common. "It all boils down to the same thing: water moving sediment," Kaufmann says. This means that any activity that changes water flow can create piping features or speed up sinkhole development. Pumping water out of the ground when digging a quarry or to supply water to a city can also cause the ground to cave in.
Although human-induced collapses can strike almost anywhere, natural sinkholes are unlikely to form outside of karst areas and are usually too small to cause a stir. "Statistically speaking, the chance of a sinkhole forming where you're standing is very, very small," says Kaufmann. The
chance of a sinkhole forming somewhere within an area, for instance, the size of a state, is much higher.
GUATEMALA SINKHOLE EXPLAINED
A damaged sewer pipe is blamed for Guatemala City's recent sinkhole. Water from the broken pipe washed away soil. Over time, an underground hole formed, and then the earth above collapsed. Tropical Storm Agatha's heavy rains likely sped up the process.
U.S. SINKHOLE HOT SPOTS
Areas that sit on top of karat bedrock are vulnerable to sinkholes. This type of terrain contains evaporite or carbonate rocks that have begun to dissolve away. Regions where evaporite rocks have yet to become karst are also prone to sinkholes.
* What is a sinkhole?
* How do sinkholes form?
* Do you think that a sinkhole is a natural or human-made feature?
DID YOU KNOW?
* The largest sinkhole on record occurred outside of Cairo, Egypt. It measures 120 kilometers (75 miles) long by 80 km (50 ml) wide and is 133 meters (436 feet) deep. The Egyptian government is considering digging a channel from the sinkhole to the sea so that the sinkhole will till with water and form a lake.
* A series of sinkholes formed many years ago in Venezuela's Jaua-Sarisarinama National Park. Inside these sinkholes, unique ecosystems have formed. These ecosystems contain species of plants and animals that can be found nowhere else on Earth.
* Blue hole is the name given to any sinkhole that forms underwater.
* The article discusses that some sinkholes are created by human activity. What are sonic other human activities that damage the Earth's geology? Do you think that there are ways to prevent these types of human-caused problems from occurring?
LANGUAGE ARTS/TECHNOLOGY: Write a news story about a family that came home from vacation to find that their home had fallen into a sinkhole. Make sure your report includes facts to inform your viewers what a sinkhole is anti where they can be found in tile U.S. Include "interviews" with tile fictional family members. Take it further: Film the news story to show to the class.
You can access these Web links at www.scholastic.com/scienceworld
* VIDEO EXTRA: Watch a video about the Guatemala sinkhole at: www.youtube.com/watch?v=muvVDvm9rRI.
* The U.S. Geological Survey has a fact sheet on sinkholes and how they form: http:llga.water.usgs.gov/edu/sinkholes.html.
* Check out this National Geographic slide show about sinkholes around the world: http://news.nationalgeographic.com/news/2010/06/photogalleries/ 100604-sinkhole-pictures-around-the-world-guatemala-city.
TESTING TYPES OF BEDROCK
PREDICT [right arrow] What type of bedrock will form a sinkhole?
MATERIALS [right arrow] 7 saltine crackers * 18 sugar cubes * 2 paper plates * measuring cup * water * eyedropper
PROCEDURE [right arrow] 1 Place a cracker on the first paper plate, Arrange 9 sugar cubes in a 3-by-3 square on top of the cracker. Arrange a second layer of 9 sugar cubes on top of the first layer. Cover the sugar cubes with another cracker. This is your "karst bedrock" (see diagram, below).
2 Place a stack of 5 crackers on the second plate. This is your =non-karst bedrock."
3 Fill the measuring cup with 60 milliliters (1/4 cup) of water. Use the eyedropper to slowly drip water on the center of the top cracker of your "non-karst bedrock" (see diagram, right). Continue refilling the eyedropper and dripping water onto the top cracker until the cup is empty. Observe any changes.
4 Repeat step 3 with the "karst bedrock."
CONCLUSIONS [right arrow] Did either type of "bedrock" form a sinkhole? If you were building a house, what type of bedrock would you look for? Explain your reasoning.
Answers will vary but should include: The "karst bedrock" formed a sinkhole. If I were building a new home. I would choose non karst bedrock because the potential for a sinkhole to spontaneously form there is much rower than on karst bedrock.
[VIDEO EXTRA] Watch a video about the Guatemala sinkhole at: www.scholastic.com/ scienceworld
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|Title Annotation:||EARTH: EROSION|
|Date:||Dec 6, 2010|
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