The stunning natural sandstone arches, like those in Utah’s Arches National Park, draw awe-inspired crowds (and sometimes daredevils), but just what formed these landmarks is not what many might think, a new study suggests.
The study published in Nature Geoscience from Charles University in Prague debunks the idea that the stone took shape as a result of abuse from wind and rain and says that the shape is actually inherent in the rock itself.
“Erosion gets [excess] material out, but doesn’t make the shape,” Jiri Bruthans, a hydrogeologist the university, told Nature, calling erosion a “tool” in the process.
The study found that “gravity induced stresses,” which previously were assumed to not play a role, “instead increase weathering rates.”
“Here we show that increased stress within a landform as a result of vertical loading reduces weathering and erosion rates, using laboratory experiments and numerical modeling,” the study abstract stated. “We find that when a cube of locked sand exposed to weathering and erosion processes is experimentally subjected to a sufficiently low vertical stress, the vertical sides of the cube progressively disintegrate into individual grains. As the cross-sectional area under the loading decreases, the vertical stress increases until a critical value is reached. At this threshold, fabric interlocking of sand grains causes the granular sediment to behave like a strong, rock-like material, and the remaining load-bearing pillar or pedestal landform is resistant to further erosion.”
Bruthans told Smithsonian Magazine that the idea is similar to that of a brick wall.
“It is easy to pull out brick from the top of the wall but hard to pull brick from the bottom, as it is loaded,” Bruthans said.
Watch this video that shows a couple of the experiments conducted in the lab about these sandstone formations:
With these findings, the scientists concluded that the “stress field is the primary control of the shape evolution of sandstone landforms,” not erosion.
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