Eolian erosion exposé
Posted on homepage: 15 October 2010 (GMT+10)
Wind (eolian) erosion is usually mentioned in the scientific literature as wind picking up sand particles (deflation) and “sandblasting” the bedrock (corrasion).1,2 The most visible results are sand deposits (dunes and associated forms) and strange “mushroom” rocks.
Little is however said or taught about the possibility of wind excavating large hollows in massive rocks. The most obvious features that come to mind are tafoni (singular tafone) which is defined as
“A hollow, produced by localized weathering on a steep face. Rock breakdown typically takes place by granular disintegration or by flaking, and the hollow shows a tendency to grow upwards and backwards.”3
Although most authors seem to emphasize localized weathering, mineral constituents inside the host rocks and local fracture concentration as cause for their formation, some have at least considered the role of wind in the overall excavation process.4
Wind “eats” rock
I believe that wind plays a more significant role in the formation of tafoni. During microclimate research in a salt mine near the city of Turda, Transylvania, Romania, I witnessed the formation of many “megascallops”—large (up do a meter long and 30 cm diameter at the wider end) scallop or spoon-like excavations—in rocksalt resulting from the opening new air shafts. The cause was the sudden increase in fresh air flow from the surface through the shaft. The fresh humid air being unsaturated in salt aerosols, unlike the normal, near-stagnant mine atmosphere would be able to dissolve the rocksalt in areas where turbulence ensured a longer air-rocksalt contact. These were generally in the upper corners of the mining galleries and on one location dozens of parallel megascallops formed in less than 10 years. There were no significant chemical inhomogeneities in the rocksalt to explain such features by selective dissolution. Microclimate measurements as well as smoke experiments have confirmed the major role of air turbulence in creating these features.
The presence of tafoni on Mars5 further emphasizes the role of wind in “tafonisation”.
Water can too
Similar scalloping morphologies caused by air currents also occur in ice as exemplified by the superbly scalloped walls of Arches Cave in the Khumbu Glacier (Mt. Everest, Nepal; figure 1).6 They also occur when wind blows over compacted snow.7 In these cases the mechanism is sublimation of homogenous crystalline matter.
Dissolutional scalloping and fluting is common in limestone, gypsum and salt caves. Subcritical turbulent flow of unsaturated water has been proven to be the mechanism responsible for scalloping in these situations.7 (Figure 2).
A common mechanism
There are many morphological similarities between tafoni (figure 3) and these rocksalt excavations and I suggest that air turbulence plays a significant role in the formation of tafoni. The origin of tafoni is undoubtedly polygenic, with variable porosity and matrix mineralogy acting as initiators of tafonisation. A more porous area in sandstone for example will tend to accumulate more pore water and if freezing occurs, cryoclasty will tend to cause granular disintegration. Once a tiny excavation forms, air flow will become more turbulent and deflation will occur, with a tendency for the excavation to deepen along turbulence (eddy) pathways; the larger the resulting excavation, the more airborne particles become available for corrasion. Studies have shown that tafoni tend to deepen rapidly as they expand (positive feedback)5. If the wind is consistent and persistent, large tafoni can form in a matter of years.
Some authors5 have pointed out that case hardening occurs on the outer roof areas of tafoni the result of persistent eddies that would further the specific tafoni excavation.
From tafoni to arch
What would happen if tafoni formed in narrow inhomogeneous sandstone ridges (“rock
fins”)? If the polygenetic conditions are consistent and persistent enough,
they may very well perforate the ridge from one side to the other creating arches
like the ones in the Arches National Park in Utah. Once a perforation like that
occurred, the turbulent airflow would increase, as well as preferential, contour
cryoclasty as now the open rim will provide more porosity for water retention. Collapse
will significantly enlarge arches until certain, temporary equilibrium is reached.
Cave (karst) scalloping and pothole distribution reveals that these features almost exclusively form on the walls and bedrock of conduits, never in the concavity of sharp bends where water flow impact is frontal. The reason is that coherent and persistant turbulence (vital for scalloping and potholing) occurs where parallel flow is affected by rockwall rugosity. Similarly, one would expect that tafoni preferentially form on the sides of narrow canyons, and statistically they do. In the Castle Rocks State Park, Idaho, tafoni form mostly in the main cluster of spires, rather than in isolated inselbergs.5
If winds were consistent for longer periods in the past, such an origin for many of the arches might be possible. I acknowledge that I have not visited the site and am relying entirely on photographic and video sources as well as field data elsewhere. Very strong and persistent seasonal winds were present during the deglaciation period in the Late Pleistocene even at mid-latitudes.8
Oard9 attributes the arches and natural bridges in Utah to Late Flood mechanisms. Natural bridges are different from arches in that they span an existing or dry stream which is believed to have undercut them. They are often excavated in relatively homogenous rocks unlike arches which always have a harder rock forming the roof and a softer one below. According to Oard, the arches would have formed during either the Sheet-flow or Channelize-flow Phase of the Retreating Stage of the Flood, while natural bridges probably formed during the Channelized Phase.10 Oard’s assumption is that somehow, as incised valleys formed during the Retreating Phase, undercutting of less resistant rock under a more resistant layer. However, this does not really answer the objection Oard himself quotes from the literature:
“Arch formation cannot be due solely to weathering and erosion, however, because these processes are not restricted to the sides of arches in rock fins. There must be some factor that locally enhances the effects of erosion within a rather small part of the rock fin to produce an arch. How erosion is localized within the rock fin to form an arch is enigmatic.”11
I suspect that there is nothing enigmatic about such localization once persistent seasonal air turbulence is inferred. Oard’s explanation is most likely valid in the case of natural bridges and probably for the formation of rock fins during rapid downcutting of incised valleys. In fact most rock fins represent meander necks, narrow ridges of rock separating the two sides of a hairpin meander in an incised valley. I think that these arches formed towards the end of the Ice Age; during rapid deglaciation. Those were times when winds were very strong and persistent, especially since most mid-latitude areas were completely deforested, allowing for extensive eolian erosion as is current on Mars.12 In fact climate patterns were most likely very different from the present ones as the thermohaline circulation system was massively disrupted by the sudden input of huge amounts of fresh meltwater coming from the rapidly-melting ice sheets.13
The role of early post-Flood climate in geomorphology has not been investigated to any extent by young-earth creation scientists, although it may in fact have played a significant role in sculpting at least some of the landmarks of every continent. Under such circumstances, any present attempt—including this one—cannot but be speculative in nature. I believe that post-Flood paleoclimate reconstructions are needed and they could be assisted if not complemented by geomorphic investigation. Furthermore, such reconstructions may prove useful for future faunal and floral distribution creationist studies, including anthropology.
- Selley, R.C., Applied Sedimentology, 2nd ed., Academic Press, San Diego, CA, p. 23, 2000. Return to text.
- Reading, H.G. (Ed.), Sedimentary Environments, Blackwell Science, Oxford, p. 125, 2002. Return to text.
- www.highbeam.com/doc/1O15-tafone.html Return to text.
- Rodriguez-Navarro, C., Doehne, E. and Sebastian, E., Origins of honeycomb weathering: the role of salts and wind, GSA Bulletin 111(8):1250–1255, August 1999; http://bulletin.geoscienceworld.org/cgi/content/abstract/111/8/1250. Return to text.
- Hacker, A., Controls of Tafoni Development in Castle Rocks, Idaho; http://keckgeology.org/files/pdf/symvol/16th/idaho/hackernew.pdf, 28 April 2009. Return to text.
- Benn, D. and Gulley, J., Under Thin Air: the 2005 Ngozumpa Glacier Cave research Project, NSS News 64(10):16, 2006. Return to text.
- Ford, D.D. and Williams, P., Karst Geomorphology and Hydrology, Chapman Hall, London, p. 305, 1992. Return to text.
- Oard, M., An Ice Age Caused by the Genesis Flood, Institute for Creation Research, El Cajon, CA, pp. 109–119, 1990. Return to text.
- Oard, M., Many arches and natural bridges likely from the Flood, Journal of Creation 23(1):115–118, 2009. Return to text.
- Oard, M., Flood by Design: Receding Water Shapes the Earth’s Surface, Master Books, Green Forest, AR, 2008. Return to text.
- Cruickshank, K.M. and Aydin, A., Role of fracture localization in arch formation, Arches National Park, Utah, GSA Bulletin 106: 879, 1996. Return to text.
- Reading, ref. 2, p. 125. Return to text.
- Broecker, W.S. et al., Routing of meltwater from Laurentide Ice Sheet during the Younger Dryas cold episode, Nature 341:318–321, 28 Sept. 1989. Return to text.