Earth Science

How Sandstone Arches Form

Stress and pressure. Those phenomena are generally accepted as destructive. For sandstone arches, however, stress and pressure are at the root of their very existence.

“Delicate Arch” at Arches National Park, Utah. Source: http://goo.gl/dv0OOk

Until now, the spectacular forms exhibited by these structures have been attributed entirely to the erosive effects of wind, rain, and temperature variations. Research led by hydrogeologists Jiri Bruthans and Alan Mayo, however, points to a different formative element.  

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.

Landscape Arch in Utah. Source: http://goo.gl/RRwOd7

When Sand Gets Stressed

In a paper published 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. Miners there use explosives to blast away masses of sandstone in pursuit of “rock sand”.  Though much of that sandstone falls and disintegrates during the blasting process, substantial amounts are left behind in arch-like formations similar to 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.

How natural sandstone arches form

According to Mayo, as the sandstone block begins to erode, pressure builds on the remaining grains. This, in turn, causes the remaining sandstone grains to become progressively compressed and interlocked; strengthening their resistance to displacement.

As the process continues, the grains bear more and more weight until, eventually, they compress into impervious interlocked networks able to support spectacular pillars and arches.

Subsequent simulations in the lab confirmed that the shapes follow stress fields. They also demonstrate that negative feedback between stress and erosion is “the primary control of the shape evolution of sandstone landforms.”

The “Double Arch” in Arches National Park, Utah.
Source: http://goo.gl/DoWbDT

To be doubly sure that they were on the right track, the researchers visited recent rockfalls in Arches National Park, Utah.  There they found that while the rockfall remnants were once parts of a larger structure, once the formation collapsed, the pieces simply disintegrated. Without the gravity-induced stress compressing them, the grains disassociated and 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.

Further Reading