An arch is such a sturdy way to include a window or door in a stone wall with no beam-like lintel across its top, or to stack rocks in a way that they’ll stay up with nothing directly holding them from underneath. But putting one together is an art. It has to have a support system until it is done, a keystone wedged into the peak. This week some kinds of natural arches are in the news. A roundup of media coverage follows. But first an explanation what leads me to see how reporters handled one key aspect of nature’s self-organized production of bridges.
This morning the eyes fell on news of a research team, mostly of Czech researchers, that says in Nature Geoscience its experiments revealed a simple explanation for the magical-looking bridges and arches of Utah, Arizona, and many other places. It has to do, their paper says, with how “the stress field is modified by discontinuities to make a variety of shapes stable under fabric interlocking, owing to the negative feedback between stress and erosion.” Oh. That is, I think, while a block of sandstone that has lot of weight on it erodes due to wind or water the stress from overlying weight gets concentrated, natch, into parts not yet eroded. The grains where stress builds up first tend to get squeezed and recemented and made stronger – hence harder to erode. They form some kind of internal scaffolding, supporting the bulk. Less stressed hence softer places erode out first. Eventually the whole thing may go but in the meantime voila!, there emerges a weird unnatural looking configuration pre-hardened before erosion reached it and slowed down. Just check out that series of pics above, lifted from the paper.
This makes sense to the simple mind (mine) for formation of pillars – some vertically squeezed places may get a little more stress than others, harden up, and eventually we see towers with big blocks on top. Arches are so mechanically adept at redistributing vertical loads that it is beguiling to think that a dumb lump of sandstone does it automatically. But exactly how do the sand grains route stress that way while they are still inside a larger bulk? Again, seems from here that compressive forces couldn’t build up until the whole shape forms. Right? Why isn’t the shortest route, straight down, the vastly preferred configuration for this spontaneous process? And are domed rooms in natural caverns nothing but arches in three dimensions? Do such domes ever get eroded into the clear as free-standing arches are?
Many of the stories that ran on this off beat development in geomorphological evolution are charming enough. But did anybody push the researchers to explain how an arch can form this way, before it is a whole arch and while it is largely still inside the original rock mass? I did not pore through the paper. But its images imply that some arches start as holes in the sides of walls, with the stress routed along their upper parts. Most outlets seem content to run fabulous pics of natural bridges with a story saying smart people figured it out … somehow. Let’s see who leaned on the paper’s authors for more.
Stories:
- LIve Science – Charles Choi: How Mysterious Natural Arches Form ; Choi describes what happens, not much on the genesis of the arch shape. Also gets a nice quote on how manipulating the experiment set-up is revealing: “You know what it will carve – you have full control of erosion like a magician.”
- Scientific American/Nature – Richard A. Lovett: Sandstone Arches Form Under Their Own Stress ; Rick writes evocatively that the controlling factors are “stress fields created by the weight of overlying rock.” Also reveals how the lead author got started on the project, noticing formation of small arches as erosion attacked fairly loosely compacted sand in a Czech quarry. This is a story that also acts as press release, featured on Nature’s website.
- Smithsonian – Helen Thompson: How Does Nature Carve Sandstone Pillars and Arches? ;
- AAAS Science – Sid Perkins: What keeps stone arches from falling down? A shorty.
- ABC (Australia) Stuart Gray – Stress not erosion sculpts nature ; No real help explaining why arch shapes are so common.
- BBC – Jonathan Webb: Sandstone shapes ‘forged by gravity’ ; Says here “by studying cubes of sand in the lab, they showed that areas squeezed by vertical stress are protected from erosion, while others wash away.” True but one thinks that the point of an arch is that it distributes the squeezing stress well away from the vertical.
- Independent (UK) – James Vincent ; Geologists reveal the ‘master sculptor’ behind the world’s dramatic sandstone formations ;
- Christian Science Monitor – Pete Spotts: What keeps nature’s sandstone arches from falling/ Oddly, it’s gravity ; Pretty good way to put the ‘what’ of the process: “Wind has a much tougher time chipping away at this scrunched sandstone. Facing a vastly slower rate of erosion, what remains – arch, pillar, pedestal, or alcove – becomes a fixture of the landscope.” Yes – but why arches exactly?
Leave a Reply