How Sandstone Arches Form

Stress and pressure.  We all feel it from time to time and for humans, it’s typically perceived as destructive.  For sandstone arches, however, stress and pressure are indispensable formative elements.

Until now, the spectacular forms exhibited by landmarks like the Utah’s “Delicate Arch” have been attributed entirely to the erosive effects of wind, rain, and temperature variation, but research led by hydrogeologists Jiri Bruthans and Alan Mayo points to a different formative element.  According to this discerning duo, while erosive processes certainly play their part in clearing away much of the material, it is gravity-induced stress that really makes things take shape.

According to their paper in Nature Geoscience, the idea that gravity-induced stress fields may be the defining factor in the morphology of sandstone formations came from a visit to the Stralec Quarry in the Czech Republic where miners use explosives to blast away masses of sandstone in pursuit of “rock sand”.  While much of the sandstone falls and disintegrates during the blasting process, substantial amounts are left behind in arch-like formations not unlike those found in nature.  The researchers deduced that the arches might be forming in response to a redistribution of weight as the surrounding rock was blasted away.

To test their hypothesis, Bruthans and Mayo took blocks of sandstone from the quarry and subjected them to gravity-induced stress in the form of metal weights.  With the pressure of the weights bearing down on them, the blocks were then exposed to simulated erosive processes like varied temperature cycles, submersion in water, and even leaving one block vulnerable to rain over a 15 month period.  The end result of their experiments was a collection of miniature pillars and arches.

According to Mayo, as the sandstone block begins to erode, pressure builds on the remaining grains.  This, in turn, causes these remaining sandstone grains to become progressively compressed and interlocked; strengthening their resistance to displacement.  As the process continues, the grains must bear more and more weight until eventually they compress into impervious interlocked networks able to support spectacular pillars and arches .  Subsequent numerical modeling confirms that the shapes follow stress fields and demonstrates that negative feedback between stress and erosion is “the primary control of the shape evolution of sandstone landforms.”

To be doubly-sure that they were on the right track, the researchers visited recent rock falls in Arches National Park, Utah.  There they found that while the rock fall remnants were once parts of a larger structure, once the formation collapsed, the pieces simply disintegrated.  Without the gravity-induced stress creating compression, the grains disassociated and gradually succumbed to erosion.

So there you have it!  While wind, rain, and temperature all have their parts to play, it is gravity that really ensures that these magnificent structures don’t crack under pressure.

Want to learn more?  Check out the Further Reading section below!

Further Reading:

• Geologists reveal the ‘master sculptor’ behind the world’s dramatic sandstone formations – The Independent
• Sandstone Arches Form Under Their Own Stress – Nature News
• Sandstone landforms shaped by negative feedback between stress and erosion. Nature Geoscience 7.  597–601(2014)
• Vast Rock Arches Sculpt Themselves Out of Sand – New Scientist