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Gravity provides secret to shape of natural arches, study finds

By Natalie Crofts | Posted - Jul 22nd, 2014 @ 1:35pm

SALT LAKE CITY — Gravity has been the secret factor that gives sandstone arches its shape, according to a new study.

The formation of natural arches has long been attributed to the power of wind and water, but gravity-induced stress actually has primary control of the shape, according to a study published in Nature Geoscience Sunday.

"The stress field is the master sculptor — it tells the weather where to pick," lead author Jiri Bruthans told BBC News.

The reason gravity plays a positive role in the formation of arches, instead of contributing to increased weathering, is because the stress from the weight of the rock at the top causes the sand grains below to interlock. The rock that is left is more resistant to erosion, according to the study.

In other words, as parts of a sandstone block erode and are carried away, the remaining sandstone grains carry more weight and become stronger. Bruthans likened it to a dry brick wall.

A lot of textbooks need to have some adjustments made to them now.

–Alan Mayo

“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,” he told Smithsonian Magazine.

For the study, researchers simulated the formation of arches in a lab. Even crumbly materials were able to form arches when there was a weighted material on top, according to the study.

When the weight was removed, researchers said the structures collapsed.

The relationship between gravity and arch formation is a simple explanation that holds up to scientific rigor, according to Alan Mayo, a geologist at Brigham Young University who is a member of the research team.

"A lot of textbooks need to have some adjustments made to them now," he told the Christian Science Monitor.

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