Rock Arches, Geology and the Wonders of Abduction

During the Pennsylvanian period (300 to 320 million years ago), this area was part of the Paradox Basin, a giant inland sea that dried up intermittently, leaving behind a thick accumulation (5000 feet, 1525 meters) of layered marine salt. Loading by deposition of subsequent Permian through Triassic layers caused the ductile salt to flow to areas of lower pressure–principally along fault lines. Gradually, salt accumulation or salt walls up to 10,000 feet (3050m) thick, three miles (5 km) wide, and 70 miles (110 km) long developed beneath the northwest-trending Moab and Salt Valley faults. The area between sagged in response to withdrawal of the subsurface salt. Approximately 60 million years ago, and long after the salt walls had been emplaced, compressive forces associated with the  ongoing Late Cretaceous Laramide Orogeny warped the region into anticlinal folds….

And so goes the answer to the question ‘Why are there so many arches in Arches National Park, Utah’? (From Geology Unfolded: An Illustrated Guide to the Geology of Utah’s National Parks, Thomas H. Morris, Scott M. Ritter, Dallin P. Laycock, Brigham Young University Press, 2010)

No matter how many times I see it in effect, I remain in awe of the marvels of scientific abduction (post-facto inference to the best explanation).  A response like the one above sometimes resembles nothing so much as magic, a peering back into an  unimaginably distant time, revealing detail with unerring precision. And if it feels like there is some sleight of hand involved, then that is as it should be, for didn’t I just say it felt like magic? (As should be clear, I am convinced by this answer and would accept it had I asked the question. Come to think of it, I did. )

I’ve taught scientific explanation in several classes now: for instance, Philosophy of Science, Scientific Revolutions, and the Philosophy of Biology. Teaching it–sometimes in the context of cosmology, sometimes the theory of evolution–always lets me point out how such inferences, while seemingly pulled out of a hat, and thus subject to the polar extreme of creationist-style queries of ‘How do you know? Did you see it happen?’,  are in fact, eminently sensible in light of our previously acquired knowledge. An explanation works because it makes sense, and it will only make sense in light of how well it fits in with our previously held beliefs. Those beliefs, in turn, have been held and retained by us because of their accordance and sympathy with yet others.  And so on.  (It also helps to point out that such explanation is of the same variety as the kind that we indulge in on a daily basis. Very little of what we claim to believe meets the ‘seeing is believing’  test.)

Underlying the seemingly-magical, reverse-crystal-ball gazing nature of the explanation of the profusion of rock arches in a particular part of the southwestern United States then, is a whole body of scientific knowledge, acquired slowly, painstakingly, and carefully interlocked with other scientific disciplines: carbon dating, physical chemistry, materials science etc. It seems speculative to the uninitiated, but a closer look reveals that this particular house of cards has a very strong foundation indeed.  You could bring it crumbling down, but you’d have to work very hard.

Oh, and by the way, this is what one of those arches looks like (click for a larger image):

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