CRACKS IN ARIZONA
NEW EARTH FISSURE MAPS
Earth fissures abound in basins of south-central Arizona. They manifest as surface cracks, seams, or gullies ranging from 100s of feet to miles long, inches to 10-15 feet wide, and up to 40 feet deep. They form when groundwater pumping collapses basin fill sediments leading to differential basin subsidence. Mapping by AZGS geoscientists reveals the distribution, locations, lengths and, by inference, the potential impact of earth fissures in Cochise, Maricopa, Pima and Pinal Counties.
On 24 February 2009, AZGS published earth fissure maps of Picacho Basin, Pinal County, the Heaton area of Maricopa and Pinal Counties, and Wintersburg, Maricopa County. The 1:24,000 scale maps are available online at AZGS’s EFC.
Picacho Basin, situated south of Eloy in the Phoenix-Tucson corridor, is the most prolific earth fissure site in Arizona; nearly two-thirds of the earth fissures mapped to date are here. We confirmed nearly 43 miles of earth fissures in Picacho Basin, including 27.8 miles of continuous fissures and 14.9 miles of discontinuous fissures (i.e., short fissure segments separated by unbroken ground). Because resurfacing by plowing, road building and construction or by natural infilling with sediments camouflages fissures, an additional 80 miles of previously reported fissures were classified as unconfirmed earth fissures on AZGS maps.
In Picacho, a single earth fissure network, comprising several dozen anastamosing fissures, extends southward for nine miles along the west front of the Picacho Mountains, before cutting across Interstate 10 and terminating west of Picacho Peak. Reactivation along the south end repeatedly damaged Interstate10 near mile marker 216, and potentially threatens the nearby Union Pacific Railroad track and El Paso Natural Gas pipeline.
The Wintersburg study area in Maricopa County hosts a single north-south trending, 1300 ft long fissure situated two miles southeast of the Palo Verde Nuclear Power Plant. The fissure poses no apparent threat to the plant. Plant operators have known of its existence since at least 2004.
Earth fissures in the Heaton study area are largely confined to a few square miles east of North Hidden Valley and North Rio Bravo Roads in the vicinity of US Highway 238, adjacent to the City of Maricopa.
The Luke study area map, originally issued in December 2008, was re-released to show earth fissures on the Luke AFB, Maricopa County.
To date, nine of 23 designated earth fissure study areas are mapped and published. Work continues in the Tator Hills and Signal Peak areas of Pinal County, and near Sulfur Springs in Cochise County. In basins torn by earth fissures, these maps are powerful tools for informing land-use planning.
AZGS’s earth fissure mapping team comprises Todd Shipman (program manager), Mimi Diaz, and Mike Mahan.