Why did the fissure cross the road? New and old earth fissure activity in Cochise County, Arizona
Joseph P. Cook, ARIZONA GEOLOGICAL SURVEY, Tucson
Giant Desiccation Cracks
Not all large surface cracks in Cochise County are earth fissures. Giant desiccation cracks (GDCs) are abundant around the present and former margins of Willcox Playa, on the distal toes of alluvial fans, and along the valley axis of fine grained alluvial basins. GDCs form in fine grained, clay-rich soils in response to prolonged drought or repeated wetting and drying (Neal et al., 1968, Harris, 2004). GDCs form polygonal networks reminiscent of mudcracks except GDC polygons can be hundreds of feet across (figure 6). Much like earth fissures, surface cracks forming GDC networks may start out as hairline cracks but evolve into large collapse features due to subsurface piping and erosion. Unlike earth fissures, GDCs are strictly surface features extending mere feet into the subsurface. Older GDC networks may become nearly or partially filled due to wall collapse. Vegetation may preferentially grow in or along GDCs because of loose soil fill and a greater affinity for capturing and storing water. Over time vegetation may help completely fill the surface crack through the capture of windblown sediment resulting in a vegetated mound along the former GDC trace.
Figure 6. InSAR interferogram spanning May 2008 through May 2010. Each repetition of the color ramp represents 2.8 centimeters (just over an inch) of subsidence. In the past two year, more than 3 inches of ground subsidence occurred. Note close agreement between areas of pronounced differential compaction and earth fissure formation. And note, too, areas of strong differential compaction that lack earth fissures. Are these areas where we can expect fissures to occur in the future? Continuous fissures are indicated in black and discontinuous fissures are indicated in red.
While earth fissures form as a result of the overpumping of groundwater, GDCs can form naturally in response to soil characteristics and climate patterns. In some cases, earth fissures have formed across preexisting GDC networks, as observed in aerial photos as far back as 1935 (Harris, 2004). Where GDCs and earth fissures form in close proximity, it can be difficult to discriminate between them. It is possible the introduction of a deep earth fissure in the vicinity of preexisting GDCs may enhance or renew the formation of GDCs through increased desiccation of near surface soils, base level lowering and subsequent downcutting, or a combination of both. In the Three Sisters Buttes earth fissure study area, some features previously identified as combination earth fissure-giant desiccation cracks were mapped as earth fissures based on similar appearance, morphology, and crack geometry to nearby fissures. Other more shallow polygonal features in the study area were interpreted as GDCs and were not depicted on the map.
Unfortunately, it appears inevitable that additional earth fissures will appear in southern Arizona in response to historical and ongoing groundwater mining. New fissures are opening up in areas with older, supposedly dormant, generations of earth fissures. Although earth fissures are widespread throughout a number of deep alluvial basins in Arizona, there are numerous areas where subsidence occurs unaccompanied by earth fissure formation. Are the latter areas unique in that their subsurface bedrock or consolidated basin sediment topography discourages substantial differential compaction, or is fissure formation awaiting an extreme runoff event? In any case, we must remain alert to the potential for earth fissure formation in basins with significant groundwater lowering. Where earth fissures now exist, we need to monitor changes in fissure length and, using InSAR data, local subsidence patterns to better understand the relationship between differential subsidence and fissure formation and development over time.
Both GDCs and earth fissures can be hazardous to livestock, buildings, and roads. Both can disrupt the natural drainage patterns in the area. Based on our current groundwater usage, earth fissures are a human-induced hazard that we will be dealing with for some time.
Arizona Department of Water Resources (ADWR), 2010, Interferometric Synthetic Aperture Radar, Hydrology, Geophysics / Surveying Unit.
Arizona Department of Water Resources Groundwater Site Inventory (GWSI) database. https://gisweb.azwater.gov/waterresourcedata/GWSI.aspx
Arizona Geological Survey, 2011, Earth Fissure Map of the Three Sisters Buttes Study Area: Cochise County, Arizona, Digital Map Series – Earth Fissure Map 22 (DM-EF-22), Arizona Geological Survey, Tucson, Arizona.
Arizona Land Subsidence Group, 2007, Land Subsidence and Earth Fissures in Arizona, Contributed Report 07-C, Arizona Geological Survey, Tucson, Arizona.
Harris, R.C., 2004, Giant Desiccation Cracks in Arizona, Open File Report 04-01, Arizona Geological Survey, Tucson, Arizona.
Neal, J.T., Langer, A.M., and Kerr, P.F., 1968, Giant desiccation polygons of Great Basin playas, Geological Society of America Bulletin, Vol. 79, pp.69-90.
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