Geologic Hazards: What You Need to Know About Earth Fissures
Earth fissures are large cracks in the ground that are formed as a result of soil surface tension due to land subsidence (lowering of the ground surface elevation). Land subsidence is mainly caused by groundwater pumping. Fissure zones normally occur parallel to the edge of subsurface ridges where softer soils abut bedrock. The differences in subsurface characteristics causes the soil to experience differential settling as water is pumped out of the aquifer. This causes tension at the soil’s surface which results in the formation of a crack.
These surface fractures initially open up slowly as the land surface lowers with time. However, during large rainfall events, the cracks tend to open up much more quickly because the water erodes the soil in and around the opening. An earth fissure can potentially be hundreds of feet deep and several miles long; it is a geologic hazard that poses a significant risk to humans, animals, and our infrastructure.
Earth Fissure Problems
Earth fissures can occur at almost any place where groundwater is being pumped to the surface. There aren’t many places that are completely safe from this geologic hazard. These hazardous cracks have caused a number of problems for Americans over the past several decades. They have destroyed pipelines, roads, canals, and even homes. There was in interesting story in 2007, where a horse was actually killed when it fell into a fissure that had opened up during the previous night’s heavy rain. That article can be found here. It’s also interesting to note that Arizona is the earth fissure capital of the country.
Another issue with earth fissures is that it creates an easy place for groundwater to become contaminated. Polluted , pesticides, and other chemicals can simply make their way deep into the earth and enter an aquifer by flowing into the crack. The infiltration process that normally would purify the water is essentially skipped where a fissure exists.
The problem of earth fissures was almost non-existent 200 years ago. Today, we are using up our groundwater reserves more than 100 times faster than nature can replenish it. This imbalance has resulted in a net drop of the groundwater table and thus a drop the ground surface elevation. Unless we can solve our water problem, we won’t be able to solve the fissure problem.
What Can You Do to Protect Yourself?
On of the first things you should do before buying property is to learn if there are any fissures in or around the place you are interested in. First contact your state geological survey for information. Many states, such as Arizona, have fissures maps that are freely available to the public. You can also contact a geologist or geotechnical engineer to research and investigate the property for you. They can provide you with a detailed assessment of the geologic hazards impacting the land. Another place to look for information is in the seller’s disclosure report. And finally, you can take a look at the property you are interested in on Google Earth. Sometimes evidence of earth fissures can be easily seen on aerial photographs.
Avoid buying or building a structure that is on or near a fissure. Don’t be fooled by even the smallest of cracks. It only takes one good rain to open them up and there is no way to predict when that will happen. There is also no sure fire way to completely mitigate a fissure. Even if the fissure is covered up with new soil or concrete, it still has the potential to cause problems. The best tactic to prevent problems in the future is to avoid fissures entirely. Scientists and engineers are still studying ways to identify and mitigate fissure problems.
What if Your Property Already has Fissures on It?
Living with an existing fissure is possible, but there are some precautions you will need to take. Avoid disturbing the opening for any reason if you can. Every effort should be made to prevent water from getting into it. Keeping the fissure dry will help ensure that it doesn’t turn into a huge crevice. If you have plants, trees, or lawns near the crack, avoid watering them; this could also exacerbate the problem.
Flying a Drone Through an Earth Fissure
Video Courtesy of the Arizona Geological Survey
References and Resources
Arizona Division of Emergency Management: Hazards and Prevention: Earth Fissures. 2011 < http://www.dem.azdema.gov/operations/mitigation/hazprevent/fissures.html>
Arizona Geological Survey. Earth Fissure Center. 2011. <http://www.azgs.az.gov/EFC.shtml>
Arizona Geological Survey. Online Earth Fissure Map Viewer. 2011. <http://services.azgs.az.gov/OnlineMaps/fissures.html>
Gelt, Joe. Land Subsidence, Earth Fissures Change Arizona’s Landscape. Water Resources Research Center, The University of Arizona. 2011. <http://ag.arizona.edu/azwater/arroyo/062land.html>
Hansen, Jamie. Earth Fissures indicate Big Problem. Cedar City News. May 24, 2010. <http://ironcountytoday.com/view/full_story/6827273/article-Earth-fissures-indicate-big-problem?instance=home_news_1st_left>
Jones, L.L., and J. P. Warren. Land Subsidence Costs in the Houston-Baytown Area of Texas. American Water Works Association. Journal. V. 68 pg. 597-599. 1976.
L.D. Fellows, Ground-Water Pumping Causes Arizona to Sink. Arizona Geological Survey. Newsletter: Arizona Geology. v. 29, no. 3, pp.1-4. 1999.
Utah Geological Survey. Earth Fissure Reports. 2011. <http://geology.utah.gov/utahgeo/hazards/ground_cracks/fissures.htm>
Questions & Answers
Is there any relationship between earth fissures and earthquakes?
No. Earth fissures are a result of land subsidence and earthquakes are caused by tectonic plate movement.Helpful 4
How do earth fissures affect the water table?
Earth fissures extend vertically from the ground surface a few hundred or even a few thousand feet towards the groundwater table. In many cases, the open crack can reach the groundwater table. This can potentially lead to groundwater contamination issues since water that enters the fissure at the surface isn't being filtered by layers of soil before it reaches the aquifer. Many fissures have the potential to intercept significant volumes of surface runoff exacerbating the issue. In addition to this, earth fissures are often used by criminals to illegally dump trash and chemicals since the fissure crack help hide the dumped refuse. When fissures are used for dump sites, this can significantly increase the potential for groundwater contamination.
What are the effects of earth fissures on engineering projects?
The effects of earth fissures on engineering projects can be significant. For example, many types of flood control or water projects cannot be built in areas where fissures exist. Canals can crack resulting in loss of water or even flooding of adjacent lands. Dams and levees can be compromised by fissure activity as well. People that live downstream of these structures may be at risk for significant flooding. Roadways can also be impacted because an open fissure can cause the pavement to collapse and even result in accidents. When building roadways in earth fissure zones, it's imperative that a geotechnical engineer is involved to ensure that a safe and reliable structure can be built.
Earth fissures can also present failure risks to various types of utility and energy infrastructure, especially pipelines. In the end, pretty much anything built in an area with earth fissures can be damaged or compromised.
What kind of plate boundary would cause a geological fault?
Faults occur along all plates where the boundaries run into each other or slide past each other. Earth fissures and faults are not the same things. They are caused by different processes. Faults are a result of tectonic plate movement and fissures are a result of land subsidence.
Can we classify earth fissures depending on their length and depth?
Yes, this is certainly one way to classify them. Other ways can include looking at the rate that they grow over a period of time and setting up categories such as: Stable Fissures, Slow Growth, and Fast Growth. However, the way that I am used to seeing them classified is based on there their surface characteristics such as: Continuous Open Crack, Discontinuous Cracks, Hairline Crack, and Unconfirmed. Perhaps yet a fourth way to classify them is based on the distance from the fissure to bedrock or the water table below or the distance to the epicenter of ground subsidence which is causing them. As you can see, there are many possible ways to classify and characterize earth fissures.Helpful 1