What keeps stone arches from falling down?

Jaroslav Soukup

What keeps stone arches from falling down?

Sid is a freelance science journalist.

People often wonder how delicate arches and finely balanced pillars of stone stand up to the stress of holding up their own immense weight. Actually, new research suggests, it’s that stress that helps pack individual grains of sand together and slows erosion of the formations. In lab experiments, scientists dropped small blocks of loosely consolidated sandstone into water—and watched them completely fall apart as the water dissolved minerals holding the grains together. But when the scientists placed weights on top of the sandstone samples before submersing them, disintegration ceased once stress in the eroding column rose to a certain threshold that packed the sand grains into a strong, rocklike material, the researchers report online today in Nature Geoscience. In other tests, weight-induced stress similarly protected samples against complete erosion from simulated rainfall. At large scale in the real world, stress transmitted through arches and pillars to their bases (in landforms such as Delicate Arch in Utah’s Arches National Park, shown) slows down—but doesn’t stop—natural sculpting due to wind and water, the researchers say. Bits of the landform that don’t bear weight are among the first to wear away, which helps explain why arches are often unusually smooth. Cracks, fissures, and soft layers in rock formations influence the shapes these natural sculptures take as they evolve.

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