How the heck is this still standing?

That’s probably the question heard most often at Arches National Park. The second? How did it form?

Last month, Jiri Bruthans and colleagues, publishing in Nature Geoscience, answered these questions—and, like the wind that may have contributed to the arch formation, it may just blow your mind.

The team started in the Stralec Quarry in the Czech Republic, where the sand is so soft, according to BBC News, “for many years it was mined in the quarry using hoses—but when it dried out, explosives had to be used.” How could it be hard and soft at the same time? Is this also the case in nature?

Bruthans took some of the sand from the quarry and hardened it by drying it out a bit. Then the scientists went to the lab to experiment with the stuff. They placed a weight on top of a cube of sand and went to work, exposing it to water.

They observed that as the sides of the sandstone cubes eroded away, the weight of the stone above was carried by fewer and fewer sand grains, increasing the vertical stress on the grains. Once a critical level of stress was reached, the weight-bearing grains became tightly locked together and were more resistant to erosion. In contrast, the parts of the sandstone bearing less weight were left susceptible to further erosion. Their model showed that the interlocking of sand grains in areas of high stress leads to the emergence of a stable, pillar-like landform.

(A video of the process is here, at the bottom of the page.)

So, the stress of wind and weather not only forms these natural wonders—it also strengthens them enough to last millions of years.

Image: Michael Atman

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