Watch for barbed wire along the road near the coordinates. Parking is along a public roadway with a wide dirt shoulder. The fissures can be seen from behind the fence on both the north and south side of the road. The area is owned by the State of Arizona and marked with No Trespassing signs. However permits for non-motorized entry can be obtained from the State Land Commissioner by calling 602-364-ASLD.
Phoenix is in what is called the Basin and Range Province. This area is characterized by mountain ranges separated by deep valleys. Typically these valleys have been largely filled with sediment washed down from the surrounding mountains. These basins are sometimes filled with over thousands feet of sediment. This sediment is typically fine to coarse grained unconsolidated sand with some layers of clay and silt. Over the millions of years, ground water has also accumulated in the sediments of these basins.
In the mid-1930s, agriculture based on irrigation from ground water became widespread in southern Arizona. In the current desert climate, little water percolates down to replenish the water that was pumped out. So ground water extraction far exceeded the recharge and water levels began declining. Ground water was being mined since it was being taken out faster than it was put back. In places ground water levels have fallen over 100 feet.
Ground water actually helps keep individual grains of the unconsolidated sediment apart. Once the water is removed, the grains are compacted closer together by the weight of the sediment above. A comparison is a sponge that has a weight on it. As the water in the sponge is removed, the thickness of the sponge decreases. The result is that the land sinks. This is also a permanent, so even if the ground water level comes up again, the ground surface does not move. The compaction also reduces the volume of water that can be stored in the aquifer.
In the middle of the basin, where the sediment is thickest, the sediment compacts more than at the edge of the basin. Using the example above, there is a thicker sponge in the middle of the basin than at the edge. The effect is the same if there is a bedrock ridge or fault buried in the middle of the basin.
These cracks actually begin deep in the ground near the water table and work their way upward. Then after a heavy rainfall event, the roof of the crack collapses in to the crack creating a sudden appearance of a deep fissure or series of fissures. Typically these fissures are long and roughly straight. Over the years these fissures are gradually filled in unless land subsidence continues.
Logging requirements:
Send me a note with :
- The text "GC17NQ9 Earth Fissures of Apache Junction" on the first line
- The number of people in your group.
- What is the state doing to the land surrounding the fissure?
- How deep is the visible bottom of the fissures?
- Based on the surrounding landscape, do you think they were caused by a buried ridge or fault or because they are at the edge of the basin?
The above information was compiled from the following sources:
- Arizona Geological Survey, Earth fissures and subsidence complicate development of desert water resources. On-line PDF Document.
- Harris, Raymond C. November 13,1999, Field Guide to Earth Fissures and Other Land Subsidence Features in Pacacho Basin, Arizona Geological Survey Open-File Report 99-26, Arizona Geological Society Fall 1999 Field Trip Arizona Department of Water Resources
- Phoenix AMA Fissure Zone 6 Baseline and Meridian area http://www.azwater.gov/dwr/Content/Hot_Topics/Earth_Fissures_in_Arizona/fissures_phx_z6.pdf