These
giant desiccation cracks are a short distance from a dirt road
southwest of Wintersburg. These cracks may look just like gullies
formed by runoff, but if you follow them they usually don’t
drain anywhere.
Like the smaller versions, these cracks form in clay rich
sediments. Cracks form in clays because clays shrink and swell
based on their moisture content. Certain pure clays, when wet, are
up to 1000% larger than when they are dry. Since pure clay is not
typically found in nature, the shrinking and swelling is usually
less than that. As clay dries out, it contracts unevenly and form
cracks. These cracks almost always meet in 3s at about 120 degree
angles forming polygons. This is a characteristic feature of the
mud cracks.
Layers of clay have formed in the playas common in the
internally drained basins of the Basin and Range Province found in
southern Arizona. These clay layers are usually buried under other
sediments that make up the desert surface. As these buried clay
layers begin to dry out, large cracks form in the subsurface but do
not typically reach to the surface.
Counter intuitively, it is usually a heavy rainfall event that
causes the formation of the surface crack. The rainfall reduces the
cohesion of the surface material, usually sand, causing a sudden
collapse of the surface material into the buried cracks.
After collapse, rainwater collects in the cracks making them an
area where plants are more likely to thrive.
Logging requirements:
Send me a note with :
- The text "GC17NQ7 Giant Desiccation Cracks on the Harquahala
Plain" on the first line
- The number of people in your group.
- Find a point where three cracks meet and send me the
coordinates.
- How deep is the deepest portion of the crack
- Take a guess at to the age of the crack based on the vegetation
growing in it.
The above information was compiled from the
following sources:
- Harris, Raymond C. 2004 Giant Desiccation
Cracks in Arizona, Arizona Geological Survey, Open File Report
04-01, January 2004.
- Arizona Geological Survey, Earth fissures and
subsidence complicate development of desert water resources.
On-line PDF Document.