Gravity Plays A Key Role In Shaping Sandstone Arches
Brett Smith for redOrbit.com – Your Universe Online
Every artist has their secret and it turns out Mother Nature is no different.
Through the study of small cubes of sand, study researchers discovered that the pressure of gravity guards parts of a sandstone rock against erosion, leaving the remainder of a sandstone rock vulnerable to being washed away
“The stress field is the master sculptor – it tells the weather where to pick,” study author Jiri Bruthans, from Charles University in Prague, told BBC News.
Using mathematical models as their basis, study researchers placed compact cubes of sand under various weight loads and immersed them in water. The researchers watched as the edges of the cube began to slip away within a few minutes, leaving less grains of sand to carry the extra weight. As that course of action continued, the stress on the leftover column gradually caused the grains to secure together and withstand further erosion.
When the researchers added supplemental faults or other disruptions to the cube and changed the way pressure was applied, they were able to reproduce a range of shapes seen in natural formations.
The study team said their results were caused by erosion undercutting the cubes in ways that would typically cause them to fail, with stress mounting along the leftover stone where the greatest amount of material has been eliminated. Ultimately, a fundamental pressure is achieved at which the sand grains secure together become highly stable.
Bruthans said the team was able to shape the blocks in their study by applying pressure in certain locations.
“You can control it completely,” he told BBC’s Jonathan Webb. “You select the pillar direction, by choosing the points where you apply the compression.”
“It’s just the stress which controls the shape – nothing else,” he added.
The study used a special kind of ‘sticky’ sand found in a quarry, in the north of the Czech Republic. The sand in the quarry was so soft that it used to be mined using hoses. The team extracted this special sand from the quarry and then dried it out in the laboratory.
“It was very clever to find this rock out of a quarry that would behave in an accelerated way, compared to those famous sandstone arches,” said Simon Mudd, a lecturer in landscape dynamics at the University of Edinburgh who was not directly involved in the research.
“We looked at the blocks on the ground, and they were completely disintegrated,” he says. “(They) no longer had that critical stress.”
Writing in an accompanying editorial, Chris Paola of the University of Minnesota said the new research finally answers the long-standing question of how these “lovely and elegant” arches formed.
“These natural sculptures have delighted countless visitors, some of whom must have paused to wonder where they come from,” Paola wrote. “Here is an answer.”