Earth Fissures of Apache Junction
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by TerryDad2
Hidden:
11/28/2007
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Ground water mining in Arizona has drastically lowered the water
table causing land subsidence. Different amount of subsidence has
resulted in Earth Fissures.
 
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
Additional Hints (
Decrypt
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Decryption Key
A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z
(letter above equals below,
and vice versa)
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Ernq gur abg gerffcnffvat fvta
(Decrypted Hints)
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 C:\Documents and Settings\Terry\My Documents\Earth
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