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