Tucson Chapter News
June Meeting
 Topic:
“Urban Recharge”
Speaker:
Jeff Kennedy
Schedule Details
Social Half Hour—6:00 PM
Speaker—6:30 PM
May Meeting Summary — Earth Fissures by Todd Shipman
On May 13th, the Tucson Chapter
hosted a meeting at the offices of Errol L.
Montgomery and Associates. Eleven people
attended “Crack in Arizona”, presented by Dr.
Todd Shipman, a Senior Research Scientist with
the Arizona Geological Survey. Dr. Shipman is
currently in charge of the earth fissure mapping
program for the state of Arizona.
Earth fissures are cracks or faults caused by
the compaction of basin fill sediment. Most
documented earth fissures form in response to
groundwater overdraft; however, natural processes,
such as earthquakes, can also cause the
formation of earth fissures. In Arizona, the vast
majority occur in agricultural areas, with the
first reported occurrence was near Picacho Peak in the 1920’s.
There are two main types of earth fissures
– shear and tension. Shear earth fissures
have a dip-slip component. Displacement can
be horizontal, vertical, or both, and the magnitude
of displacement can increase with time.
Earth fissures caused by tension typically show
a surface crack of only a few inches, but are
frequently enlarged by erosive forces. Dramatic
pictures of this phenomena were presented.
Often times, just a single intense
storm can open a tension fissure by a few feet
in just a couple of hours.
Earth fissures must be distinguished
from polygonal cracks. Polygonal cracks have a
similar surface expression to earth fissures, but
are caused by completely different phenomena.
Though the exact cause of their formation is
not understood, the current theory prevalent in
Arizona states that they form in response to
capillary induced tension caused by the drying
of a shallow clay body. As such, polygonal
cracks are near-surface phenomena that can be
safely filled in by land developers, while earth
fissures usually hinder development. An example
of what happens when a developer attempts
to fill in an earth fissure was presented.
During just one storm event west of Phoenix, a
rural road that was paved over a filled-in earth
fissure was washed away as the fissure rewidened.
The only safe way to construct roads
or canals over earth fissures is to bridge them,
as was done during construction of Highway
202 in Phoenix, AZ and the CAP canal.
Earth fissures are one of the few geologic
occurrences that happen on a human time
scale. As such, mapping earth fissures is challenging.
Their dynamic nature necessitates that
any map depicting earth fissures be continually
updated. Often times, earth fissures have minimal,
if any surface expression, and so they are
easily missed.
There is still much to be learned about
earth fissures. Interactions between the aquifer
and water that in transmitted into the
ground through earth fissures has yet to be
studied in detail. It is thought that earth fissures
can aide in recharge, though there is a
concern that they could also serve as a direct
conduit for pollutants. Earth fissures have been
known to close as well, and there is not an understanding
of what factors determine the
length of time an earth fissure will remain
open.
For more information on earth fissures,
visit the AZGS’s website at: http://
www.azgs.az.gov/. There you will also find the
most recent earth fissure map completed for
the State of Arizona, which was released on
April 23rd, 2008.
Looking Ahead...
Summer schedule — No Tucson Chapter meetings are planned for July
and August. In September, members are encouraged to attend the
Annual Symposium in Flagstaff Arizona. Regular monthly meetings will resume in October.
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