Earth Fissures, Subsidence & Karst in Arizona
Fresh earth fissure that formed in the 2018 monsoon season and prior to 24 August 2018.
Earth Fissures in South-Central Arizona
Some arid valleys of central and southeastern Arizona are home to earth fissures: pervasive cracks that occur on valley floors that result from basin subsidence associated with extensive groundwater withdrawal. Earth fissures are an anthropogenic geologic hazard that threaten people, property, infrastructure (e.g., roads, gas lines, canals), and livestock.
Earth Fissure Facts
- Fissures crop out in Cochise, La Paz, Maricopa, Pima, and Pinal Counties.
- Twenty-six earth fissure study areas encompass ~ 1,400 sq. miles
- Aggregate of mapped earth fissures is 169 miles
- Aggregate of unconfirmed fissures is 180 miles
- First fissure appears near Eloy in 1929
Fissures range from discontinuous hairline fractures to open ground cracks up to two miles long, as much as 15- to 25-feet wide, and up to 90 feet deep (see the photo gallery images below). The observed fissure floor, however, does not reflect its true depth; geoscientists believe that fissures extend to the top of the groundwater table, which can be several hundred feet below the ground surface.
Torrential monsoon rains can rapidly widen and deepen fissures resulting in hazardous conditions to people, livestock and infrastructure. Fissures can provide a ready and open conduit to basin aquifers, which could facilitate delivery of runoff and contaminated waters to area groundwater. Rapid population growth in southern Arizona has increasingly juxtaposed population centers and fissures.
2-mile Long Earth Fissure
Bird's-eye view of 2-mile long earth fissure, Pinal County, Arizona. Drone video captured January 2017 by AZGS's Brian Gootee and Joe Cook.
Impacts on People, Property, and Infrastructure
Ground subsidence and resulting earth fissures impact more than 3,000 square miles in Arizona, including expanding areas of Phoenix and Tucson. The cost to the Arizona economy is undetermined, but probably reaches the millions of dollars annually. Repairs to an irrigation canal near Scottsdale Airpark in 2007 were estimated at $820,000, and that's a single incident involving one canal. During construction of the Red Mountain Highway (Loop 202) in Phoenix, the cost of mitigating an earth fissure that impinged on the road bed was $200,000 (Arizona Land Subsidence Group, 2007).
Some of the more common damage associated with earth fissures.
- Cracked or collapsing roads
- Broken pipes & utility lines
- Damaged or breached canals
- Cracked foundation/separated walls
- Loss of agricultural land
- Livestock & wildlife injury or death
- Damaged well casing or wellhead
- Disrupted drainage
- Contaminated groundwater aquifer
- Sudden discharge of ponded water
- Human injury or death
What should you do if you find an earth fissure on or near your property?
- Keep children, pets and livestock away from the fissure.
- E-mail AZGS (azgs-info@email.arizona.edu); alert your municipal or county emergency managers office.
- Prevent runoff or flood irrigation waters from entering the fissure. Erosion can rapidly increase the fissure width, length, and depth.
- Consult a geotechnician regarding the extent of fissuring and how to minimize damage to your home and property.
Geoscience at Work for Arizona - Monitoring Earth Fissures
To reduce the societal risk of earth fissures, the AZGS couples earth fissure mapping with an aggressive educational outreach and fissure map dissemination program.
Since the 1930s, the number and distribution of earth fissures in Arizona has increased dramatically. In 2006, the Arizona Legislature charged AZGS with establishing an earth fissure mapping and monitoring program (Earth Fissure Mapping program). Over the past decade, our fissure mapping team has mapped hundreds of earth fissures in 28 study areas in five counties.
AZGS Earth Fissure Mapping Procedures.
In each earth fissure study area, we begin by reviewing existing maps and technical reports, examining new and historical aerial photographs, and consulting with geoscientists and engineers in agencies (e.g. Salt River Project, AZ Department of Water Resources) or consulting firms with expertise or information on fissures.
Once we identify known or suspected fissures, we head to the field to examine, characterize, and map each fissure using a high-precision global positioning system (GPS) receiver. Along the fissure, we collect data (fissure width & depth) every 5-15 feet (about 2- to 4-meters), yielding 300 to1,000 data points per mile (250 to 500 data points per kilometer) of fissure!
Back in the office, the data go through post-processing and uploaded into a geographic information system (GIS) map environment. Since 2007, this process has been repeated 100s of times until all known or suspected earth fissures were mapped. The fissure map data are then added to study area field sheets, which are reviewed before release at the AZGS Online Document Repository.
Each study area map is constructed on an aerial photographic base map and includes a map legend denoting the status of individual fissures – continuous, discontinuous, or reported but unconfirmed.
Following review, new fissure data are compiled with existing data and published at the Natural Hazards in Arizona Viewer.
From the AZGS Photo Library
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From the introduction to Joe Cook's 2017 report on this fissure.
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To the left: Pinal County earth fissure southern tract as it appeared on 19 January 2017. Note the steep sidewalls and variable fissure width that ranges from ~10 feet to less than one foot. Just beyond the drone, the fissure necks down into a deep V-shaped geometry characteristic of young or active earth fissures. The fissure bottom is an uneven and irregular surface that includes a large collapse block, lower center, evincing early sidewall collapse. Brian Gootee (AZGS) is the drone operator. (Photo by B. Conway of the Arizona Dept. of Water Resources.)
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Overview of polygonal cracks and AZGS site observations. Polygonal GDC networks are present along both sides of U.S. 191 and some new cracks have formed since this aerial photo was taken (2016). Areas 1 and 2 are zones of wetted soil where rainwater infiltrates. Yellow dots represent observed surface cracks on July 8, 2021. Not all photo-interpreted cracks were visited. From J.P. Cook blog post (7/19/2021), : U.S.
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Overview of polygonal cracks and AZGS site observations. Polygonal GDC networks are present along both sides of U.S. 191 and some new cracks have formed since this aerial photo was taken (2016). Areas 1 and 2 are zones of wetted soil where rainwater infiltrates. Yellow dots represent observed surface cracks on July 8, 2021. Not all photo-interpreted cracks were visited. From J.P. Cook blog post (7/19/2021), : U.S.
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A fresh earth fissure ripping through a fallow farm field south of Apache Generating Station, Cochise County (pnoto from 17 Aug. 2021). White arrows demarcate the path of the fissure. Heavy monsoon rains in summer 2021 reactivated a number of earth fissure in the Willcox Playa area. Fissures encroaching on roads caused the temporary closure of several roads in the area.
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Over the years, the AZGS has published a series of guides to assist home owners, realtors and contractors in understanding the nature and distribution of geologic hazards that can impact human society.
Here are three such booklets:
Harris, R.C. and Pearthree, P.A., 2002, A Home Buyer's Guide to Geologic Hazards in Arizona. Arizona Geological Survey Down-to-Earth #13, 48 p. http://repository.azgs.az.gov/uri_gin/azgs/dlio/1875
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Groundwater harvesting in the Willcox Basin is driving ground subsidence. This picture from the corner of Old Fort Grant Rd and Ranch House Rd shows 9.7 feet - as marked on the telephone pole - of subsidence between 1969 and 2018. Looking west the Winchester Mountains are on the horizon. For more on subsidence in Arizona see the Arizona Dept. of Water Resources InSAR subsidence monitoring program: https://new.azwater.gov/hydrology/field-services/land-subsidence-arizona.
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Image on left: A suspended fence extends across a water-filled earth fissure just south of Parker Ranch Road.
Image on right: Collapse of Parker Ranch Road into a void caused by erosion along an earth fissure first observed in 2010. This is the second time this portion of the roadway has suffered major damage due to collapse at this location.
For more details and observations: https://blog.azgs.arizona.edu/blog/2021-08/2021-monsoon-rains-reactivate...
