Precariously Balanced Rocks and earthquakes

Precariously balanced rocks such as spires, hoodoos, and stacked rocks make interesting scenery.  They also may provide a valuable tool for assessing the seismic stability of an area according to a study by geologists from Arizona State University. (Report referenced below.) They studied balanced rocks in the Granite Dells near Prescott and devised a method of calculating how much ground shaking it would take to destabilize the rocks.   In this post, we will take a look at several types of precariously balanced rocks and see how they form.

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The photo above was taken at the Texas Canyon rest stop on I-10 between Benson and Willcox, Arizona.  It shows the weathering pattern and pedestal rocks in the Texas Canyon quartz monzonite (a granite-like rock).  The development of this geomorphology begins underground with chemical and physical weathering along joints in the rock and removal of material to make the joints wider.  Erosion eventually exhumes the rocks.  The photo below from the Chiricahua Mountains shows the results of this process  in somewhat softer volcanic rocks.

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Hoodoos, such as these in the Chiricahua Mountains (photo below) contain a hard capstone over softer material.  The capstone prevents complete erosion of the underlying material.

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Perhaps the most spectacular rock spires in Arizona are those in Monument Valley, seen in the photo below on a misty day.  Here, hard sandstone occurs between softer siltstone and shale layers.  The spires are remnants of differential erosion by wind and water.

The Arizona State University study goes into great detail on methodology and technology about proposed analysis of precarious rocks for usefulness is accessing the seismic stability of a region.  One wonders, however, if these rock formations are really as precarious as they appear because the hoodoos and balanced rocks in Texas Canyon and in the Chiricahua Mountains survived the 1887 Sonoran earthquake.

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According to the Arizona Geological Survey (Fieldnotes, summer 1987): “On May 3, 1887 Arizona and the Southwest experienced a major earthquake that had an estimated magnitude of 7.2 on the Richter scale.  The epicenter was in Sonora, Mexico approximately 40 miles south of Douglas, Arizona.  The earthquake caused several dozen deaths, damaged buildings as far away as Phoenix, generated rock falls and fires triggered by rock falls in the mountains, and caused panic among the population.”

Reference:

Haddad, D.E., and Arrowsmith, J.R., 2011, Geologic and geomorphic characterization of precariously balanced rocks, Arizona Geological Survey Contributed Report CR-11-B.

See also:

Earthquake hazard near Flagstaff assessed, Video

Where the Next Big American Earthquake and Tsunami Might Occur

Spanish Scientists Find Technique to Predict Earthquakes Claiming 80% Accuracy

The Measure of an Earthquake

Local atmospheric changes may foretell large earthquakes

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9 comments

  1. Interesting information. I live in Pearce/Sunsites and am interested in things to do with the earth. Thank you.

  2. I’ve lived in this area for many years and never will forget the first time I drove through Texas Canyon.  I have been back that way often and am always awe-struck by the beauty.  I feel so lucky to live here.

  3. For more about local geology see my posts (get links in the ARTICLE INDEX):
    Saginaw Hill, another old mine in a Tucson area neighborhood
    Sierrita Mine is only U.S. source of Rhenium
    Surprising Structure of the Copper Deposits near Green Valley, Arizona
    The I-10 copper deposit
    The Pontatoc mine in a north Tucson neighborhood

  4. Testing Disqus:

    Apparently it will not allow pasting in embedded links.  You will have to, for the time being, paste in complete URL to links

      1. I’m still getting used to Disqus.  It seems more complicated than the previous system and apparently will not allow pasting embedded links.  I will hold my opinion until I’ve used it longer.

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