Until now, scientists have not had an explanation for the seemingly gravity-defying stone arches, like the Rainbow Bridge in Utah. Now, after a successful study based on cubes of sand, geologists have the answer: pieces of stone squeezed by vertical stress are immune from erosion, but others are not. The forces of gravity act as a chiseling force over millennia, carving an arch out of solid rock.
Scientists were not sure how to make an effective study of these stone arches, because the process of erosion required to form one of the arches takes millions of years. A new study published in Geoscience finally gives an answer.
"Locked sand" dug from a quarry in the Czech Republic held the key to the mystery.
Dr. Jiri Bruthans from Charles University in Prague, the principal author of the study, said that gravity was the "Michelangelo" behind the world's stony arches. "The stress field is the master sculptor - it tells the weather where to pick," he told BBC News. The force of gravity kept the rock from eroding evenly under the pressure of water and wind.
The team created an accelerated model made of cubes of sand. Under vertical pressure, the cubes would erode until there was sufficient pressure to cause the grains to fuse together. This pressure halted erosion.
Without vertical stress, the cubes completely dissolved.
Want to see how sand reacts to the pressure of gravity? Watch this cool video that the scientists made as a supplement to their research.
Bruthans said, "It was clear that processes responsible for them were operating at present time in the quarry and thus the processes can be potentially isolated and explained."
Not all sandstone contains cementing agents, Bruthans says. These cementing agents are necessary for the formation of the arches. The researchers chose samples of sand from regions where stone arches had formed, like the United States and the Czech Republic.
Stone arches are very beautiful. In Utah, a national park called Arches contains over 2,000 of these natural arches. The website for the park boasts that "this red rock wonderland will amaze you with its formations, refresh you with its trails, and inspire you with its sunsets." But these amazing formations are the fruit of simple gravity. How can nature create such seemingly geographically perfect shapes just by chance?
"To create perfect shapes you do not need intelligence or planning," says Bruthans. "The opposite is true for nature. Most perfect things are made by simple mechanisms."
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