Sandstone arches stabilized by stress

Scientists have figured out how nature sculpts elegant arches, pillars and alcoves from seemingly soft and weak sandstone - and how those natural marvels manage to stay in tact.

Weight on top turns load-bearing sand grains into rock-like material

A composite image shows the Double O Arch before a storm at Devil’s Garden, Arches National Park, in Utah. A new paper is the first to explain the mechanisms that stabilize sandstone landscapes into natural sculptures, according to the study's authors. (Michael Atman)

Elegant arches, pillars and alcoves carved out of sandstone through erosion rely on the weight of the rock above them to give them strength and help them resist weathering, a new study finds.

The research, reported in the journal Nature Geoscience, is the first to explain the mechanisms that stabilize sandstone landscapes into natural sculptures, according to the study's authors.

The researchers, led by Jiri Bruthans of Charles University in Prague, conducted erosion experiments using blocks of sandstone exposed to a range of environmental conditions.

They found that as the sides of the sandstone cubes eroded away, the weight of the overlying material was carried by fewer and fewer sand grains, increasing the stress on these remaining grains.

Hikers cross the ocean floor of the Bay of Fundy as they explore Hopewell Rocks Provincial Park during low tide in Hopewell Cape, N.B., a Canadian site famous for its sandstone sculptures. (Sean Kilpatrcik/Canadian Press)

"As the cross-sectional area under the loading decreases, the vertical stress increases until a critical value is reached," they write.

"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 and colleagues found those areas of sandstone carrying less weight remained susceptible to erosion.

The researchers also found fractures or weaknesses in the sandstone changed the stresses to evolve the range of different shapes seen in nature, such as arches and alcoves.

Feedback mechanism

To show how erosion is stress regulated, Bruthens and colleagues developed numerical models based on a range of scenarios.

In one experiment, the researchers placed sandstone block samples into water.

The researchers found the blocks disintegrated quickly as air trapped in the rock's pores was compressed by the water, generating enough pressure to compromise and dislodge sand grains on the sides of the blocks.

However, when a weight was placed on top of a block, the rock stopped disintegrating once an hourglass shape was reached and the remaining grains in the neck achieved a critical stress level.

This critical stress level occurred when the neck of the hourglass was down to about 20 per cent of the size of the original block.

In another experiment, Bruthans and colleagues exposed weighted and unweighted sandstone blocks to simulated rain and flowing water.

The authors found blocks not carrying additional load disintegrated completely within five to seven minutes, while those carrying a weight, disintegrated more slowly and ceased eroding all together after an hour while still retaining about 70 per cent of their original volume.

Further tests subjected weighted and unweighted blocks to salt and frost weathering.

These test found weighted blocks disintegrated up to four times more slowly than blocks not carrying a load.

The weighted blocks also weathered to thin hour glass columns, similar to sandstone formations found in nature.

"Low stress allows the disintegration of the material into individual grains, whereas high stress activates fabric interlocking and the material resists erosion," the authors conclude.