14 DAY TRIAL // A paper published in the journal Nature Geoscience this July 20 explains how sandstone arches like the one pictured above come into being. This study contradicts previous claims that environmental factors alone birth these structures.
Study leader Jiri Bruthans with the Charles University in the city of Prague in the Czech Republic says that, over the years, many have argued that wind and rain sculpt such structures from slightly bigger ones.
However, recent evidence indicates that erosion due to wind and rain alone does not explain their formation. It is the properties of the rock that these structures are made of that give sandstone arches their shape, Nature informs.
In their paper in the journal Nature Geoscience, Jiri Bruthans and fellow researchers argue that, when squashed from above, sandstone, which is chiefly made up of teeny tiny mineral fragments or rock grains, strengthens.
Otherwise put, the sheer weight of overlying sandstone births stress fields which in turn make the minerals or rock grains lock together. The resulting structure is more resistant to further erosion, and therefore, sticks around for longer periods of time.
This strengthening of the rock that eventually comes to form sandstone arches goes hand in hand with a withering away of the sides of the original structure. Experiments have shown that erosion of the sides of a sandstone cube ups stress at the core and encourages the strengthening of the remaining material.
“As the cross-sectional area under the loading decreases, the vertical stress increases until a critical value is reached,” Charles University hydrologist Jiri Bruthans and fellow researchers detail this process in the Abstract to their study.
“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,” they add.
This theory is backed up by the fact that, based on this data, Jiri Bruthans and colleagues have managed to recreate sandstone arches, pillars, and other similar structures simply by exposing cubes of sandstone to environmental conditions that chipped away at their sides and birthed stress fields at their core.
Hence, the researchers argue that, contrary to popular opinion, sandstone arches come into being not just due to erosion caused by wind and rain, but also thanks to the properties of the material that they are made of. What's more, it is these properties that are of utmost importance for their formation.
“We show that increased stress within a land-form as a result of vertical loading reduces weathering and erosion rates, using laboratory experiments and numerical modeling. We conclude that the stress field is the primary control of the shape evolution of sandstone landforms,” the specialists argue.