Watch for barbed wire along the road near the coordinates.
Parking is along a public highway 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 visable 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
