Sandstone Arches Get New Explanation

The national park signs may need updating.

Arches National Park

Natural sandstone arches occur in many parts of the world, most notably in Arches National Park in eastern Utah.  Visitors, upon reading the confident-sounding interpretive signs describing arch formation, might be surprised to learn that the origin of these structures is not fully understood.  In a recent paper in Nature Geoscience, researchers from the Czech Republic who ran some new lab experiments imply that prior theories are incomplete, if not wrong:

Weathering and erosion of sandstone produces unique landforms such as arches, alcoves, pedestal rocks and pillars. Gravity-induced stresses have been assumed to not play a role in landform preservation and to instead increase weathering rates. Here we show that increased stress within a landform as a result of vertical loading reduces weathering and erosion rates, using laboratory experiments and numerical modelling. We find that when a cube of locked sand exposed to weathering and erosion processes is experimentally subjected to a sufficiently low vertical stress, the vertical sides of the cube progressively disintegrate into individual grains. As the cross-sectional area under the loading decreases, the vertical stress increases until a critical value is reached. 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.

The new theory is summarized in the BBC News.  In addition, Smithsonian Magazine has an embedded video clip of the experiments.

In the authors’ view, gravity is the arch-hero, not the arch-villain.  The gravitational load causes a kind of self-organizing system: the sand grains lock together in the pillars, resisting erosion.  Eventually, of course, erosion wins, and the structure falls. It’s not clear if they can generalize this process to all arches.  They experimented with a particular sand in their home country.  “Critically, the sandstone from Strelec doesn’t contain cementing minerals that help bind the sand particles together,” the Smithsonian article says.  “Instead, the authors found that the stress put on the sandstone causes minerals to interlock and hold the rock together.”  It sounds like this is the best-case scenario to test the idea, because cementing materials would presumably increase the resistance to erosion in the pillars.  Whether this applies to the Entrada Sandstone in Arches National Park is not clear; it also would not seem to apply to granite arches, as pictured below.

Mobius Arch (granite) by David Coppedge

Whatever its merits, this new theory turns the old one on its head: “We should not say erosion or weathering carved the forms, as it was the stress field which give the forms the shape,” lead author Jiri Bruthans asserts in the Smithsonian article. “Erosion processes are mere tools controlled by stress.”  The BBC quotes Bruthans comparing the stress field to Michelangelo.  “The stress field is the master sculptor – it tells the weather where to pick.”

Not much is said in any of the articles about how long it takes for arches to form.  The Smithsonian says:

The Czech team wanted to take a different approach. While doing fieldwork in Strelec quarry in the Bohemian Cretaceous Basin, they noticed that small arches and pillars—only about 2 feet high at most—formed out of the sandstone over mere months or years, rather than the millennial time scales associated with large geologic architecture.

The BBC News, though, without a reference, says “The process had proved difficult to study, because natural slabs of sandstone erode over millions of years.”

Landscape Arch, by David Coppedge

Creationists like Michael Oard have shown how just a few centuries since the Flood are sufficient (Creation.com, 2010) to form arches.  He quotes secular authors who estimate only tens of thousands of years, not millions, for their formation; in fact, Oard argues, too much time is problematic: the arches should be long gone after even tens of thousands of years.  Dr. Andrew Snelling, creation geologist, agrees, discussing sandstone arches briefly in the latest Answers Research Journal from AiG (July 2014).  “What the park rangers won’t tell you,” Snelling says, is that 43 sandstone arches have collapsed since 1970.  “Their loss is a sober reminder how delicate—and recent—these formations are,” he ends, after providing a Flood model for their formation.  “Rapid processes created them and are now destroying them.”

The Nature Geoscience paper is not clear on timeframes, so it is not clear the new theory can speak to the age issue.  For the huge arches that were not observed forming, one can only estimate, considering variables and unknowns.  Did arches form under steady-state processes vs one-time processes (like the Flood)?  What were historical rainfall and wind patterns?  You can’t look at erosion in a desert today and simply assume it’s always been so slow.  It’s possible, in fact, to calculate reasonable upper limits on age of arches by taking conservative erosion rates today, and seeing how far back they can be extrapolated.  If those rule out the “millions of years” interpretation, they show old ages to be a philosophical bias, not a conclusion from the evidence.

Two other conclusions can be drawn from this story, though.  One is the use of arches for design inference.  What’s the difference between Landscape Arch pictured above, and the St. Louis Arch?  Clearly the former is natural, and the latter intelligently designed.  Beyond intuition, how can we tell?  Use of such pithy examples can help teachers convey the principles of intelligent design.  (Note: Comparing stress fields to Michelangelo is a personification fallacy.)

The other conclusion is that scientific theories are at best tentative, especially when they try to speak of the unobservable past.  The new theory  is partly empirical, because these scientists were able to reproduce some features in the lab.  But can one really scale up a small lab model by several orders of magnitude?  Other factors might intrude at that level.  For what it’s worth, the new theory overturns decades of thought and assumption about how these beautiful structures form, reminding us that science is, at best, a fallible human enterprise.  Remember this story when you read the authoritative-sounding National Park signs.