Posts Tagged ‘cenote’

Occasionally the newspaper mentions a sinkhole swallowing a car, a building, or a person. I usually think “how bizarre”, and move on (the stories often come from Florida, where man and nature already seem too precariously balanced). But last week’s story somehow caught my attention, maybe because the poor man sank while sleeping – an insomnia generating vision! I wondered why I hadn’t I investigated sinkholes before. After all, they combine geography and disasters – my obsessions.

What is a sinkhole?

Obviously, sinkhole isn’t a scientific term (not nearly obscure or Latin enough), and it’s used to label lots of different events with lots of different causes. For this post, let’s just say a sinkhole is a sunken area or hole, caused by the ground compressing or collapsing– nicely vague!  Vague is good because, unlike earthquakes or hurricanes, no one tracks sinkholes. We don’t know how many or how often they happen, although big ones that destroy things usually make it into the news.

Cenote (sinkhole) near Chichen Itza, Mexico. Photo by Nikonian Novice.

Cenote (sinkhole) near Chichen Itza, Mexico. Photo by Nikonian Novice.

Sinkholes can grow slowly over years, or open rapidly in minutes. Width and depth don’t go together. Some are only a few feet wide and deep, noticeable only when they show up in your yard, field, or road (like really big potholes).  On the other hand, 2000 feet wide sinkholes aren’t unheard of – enough to swallow an entire block! Most newsworthy ones are only a few hundred feet wide (1 or 2 buildings worth).  The Xiaozhai Tiankeng sinkhole in China is the deepest at 2164 feet deep – it’s a favorite spot for base jumpers around the world. The widest is the Qattara Depression in Egypt – 75 x 50 miles of sludgy quicksand, 436 feet deep. My favorite sinkhole is the story of a woman from Guatemala who heard a bang in the night, and couldn’t figure out what it was. It was only when she moved her bed that she discovered a 3 foot wide, 40 foot sinkhole under it! Some sinkholes are even quite beautiful – the famous pools or cenotes at Chichen Itza in Mexico are sinkholes connected by underground rivers.

What causes sinkholes?

True sinkholes happen in areas with limestone or other dissolvable bedrock.  Groundwater seeps down, and gradually dissolves holes in the rock (known as karst formations for any geologists out there). If the soil above is soft and sandy, it slowly slips into the hole, while the ground above gradually sags – making what the geologists call a “cover subsidence” sinkhole (the slow-growing ones). Picture an egg timer, with sand slowly sinking into the hole while a sunken spot appears on top. When all the soil falls into the hole, you’re left with an opening into the cavern – very Tom Sawyerish, but not exactly newsworthy.

“Cover-collapse” sinkholes are the ones that make news. The difference is a layer of stickier clay over the softer soil and limestone hole.  As softer soil seeps into the hole, an empty space grows under the clay roof, spreading up and out. You don’t even know it’s there until – surprise! The clay suddenly collapses and everything sinks.

Formation of a sink hole. Diagram courtesy of Department of the Interior/USGS

Formation of a sink hole. Diagram courtesy of Department of the Interior/USGS

Where do sinkholes occur?

Florida is a bad place for sinkholes. One source suggests that water filled sinkholes compose 95% of the open water in Florida, including the 2000 acre Kingsley Lake. But don’t feel too sorry for Florida – sinkhole misery gets spread around. Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania also have large areas with similar geology. (Interestingly, karst formations are great for agriculture, but not so great for neighborhoods.) The United States has a large number of sinkholes, but the really big ones happen elsewhere, particularly in China and New Guinea.

United States map of karst formations (possible sink hole risk). Courtesy of USGS.

United States map of karst formations (possible sink hole risk). Courtesy of USGS.

Classic geography doesn’t explain all sinkholes. If you add large amounts of water to dissolvable or cavernous soil, it quickly accomplishes what groundwater takes years to do. Heavy rainfall or even leaky water pipes and sewage systems cause giant sinkholes to suddenly appear – a problem plaguing both Pennsylvania and Guatemala recently. Roads are also particularly vulnerable to this sudden loss of soil and support – probably why you hear so often about people driving into sinkholes.

Salt mines can create some astonishing sinkhole type incidents. In 1980, at Lake Peigneur in Louisiana, an oil drilling rig in the lake and a salt mine didn’t play well together. A misdirected oil bit punctured the salt mine. Even though the drill bit was only 14 inches, the lake water rapidly enlarged the hole  (remember what happens to salt in water?). A giant whirlpool formed in the lake over the hole, sucking in the drilling platform, eleven barges, and 65 acres of surrounding land, as well as much of the lake water. So much water drained into the mine that the lake outlet of the lake reversed and the tallest waterfall ever in the state of Louisiana appeared at the lake’s entrance!

How much harm do sinkholes cause?

It depends what you mean. Amazingly, the number of people killed in sinkholes is quite low – usually only when cars drive into them or if one opens right under your feet. On the other hand, sinkholes cause millions of dollars of damage every year. A 2004 study in Florida showed the number of sinkhole claims growing from 348 in 1999 to 1,018 in 2003, with sinkhole insurance payments increasing from $22 million to over $65 million for the same time period. As you can imagine, home insurance costs are skyrocketing in Florida. More importantly, how do you measure the emotional price of losing your home to a sinkhole, or just knowing the ground might open underneath you at any time? It shakes our basic faith in the way the world works.

"Sinkhole" in Guatamala following heavy rains - a three story building disappeared in the hole. Photo by Catherine Todd.

“Sinkhole” in Guatamala following heavy rains – a three story building disappeared in the hole. Photo by Catherine Todd.

Man is making sinkholes worse!

Too little water = more sinkholes. Even though groundwater dissolves the bedrock, it also helps hold up and support caverns and soil.  If you take too much water from wells for irrigation, lower ground water levels lead to more sinkholes. In fact, experts believe over-irrigation is a big part of the worsening problem in Florida. Another too little water problem is drought (climate change, anyone?). Soil dries out, and when the rains finally arrive, the soil can’t absorb it. Instead, the water just runs straight down the limestone holes, carrying the dried soil with it – again, more sinkholes.

Too much water = more sinkholes. With increasing parking lots and buildings, water runs off instead of soaking into the ground. This means more water in fewer places (perhaps at low spots from early sink hole depressions) – again beyond what the soil can absorb. Just like after a drought, the excess water runs straight down the “tubes”, speeding up soil loss under the clay top layer. Bang – collapse! Climate change experts also predict increased flooding, including areas unused to much rain. If these areas have underground caverns and holes, sinkholes may start showing up in new locations. Finally, all our water and sewage systems are aging. Fixing them will cost a bundle and will probably be low priority. I expect increasing sinkholes in cities –today’s potholes may seem a blessing in comparison.

What to do?

Don’t live in Florida (sorry – I’m only partly kidding). It requires a massive amount of technology and dollars to check for gaps under your soil that might turn into sinkholes – probably not an option for most. However, you can check out what type of soil you live on, learn about local groundwater levels (that’s what state geology offices are for), and ask about your city’s water and sewage systems. At least you’d know your risk – hazard analysis is the first step to disaster planning (and you might consider these things if you consider moving).

Photo of a house being taken by a sinkhole, Willow Sink, Florida. Photo by Richard Elzey.

Photo of a house being taken by a sinkhole, Willow Sink, Florida. Photo by Richard Elzey.

Most pending sinkholes don’t give warnings, but if you live in sinkhole territory and notice fresh cracks in the foundations or walls, or a door frame suddenly becomes skewed, get it checked out pronto! Similar outdoor signs might include cracks or little depressions in the ground, formation of new small ponds, wilting of small circular patches of vegetation, and trees or posts that suddenly start leaning. These could signal the beginning of a collapse. I’d get out of there!

One final warning. Ground water in karst formation/sinkhole areas is often polluted. Water travels freely underground in these areas, carrying such great things as leaking sewage, trash dumped in sink holes, and drainage from near-by cow pastures. Don’t drink well or surface water from these areas, unless massively filtered and treated for all sizes of microbes, as well as toxins and poisons.

Do you agree with sinkholes being a scary fact? I didn’t when I started this research, but I’ve added them to my list, now that I know Oregon is not immune. In fact, my second favorite sinkhole story comes from Oregon – in 2010, firemen rescued a 1500 pound camel named Moses from an 8 foot sinkhole about an hour from my house. I wish I could have seen that!

Stay safe,

Sheila Sund, M.D.