<HTML>Hi Claire,
Coxhill made the following statement:
"Secondly, these weathering features require intense weathering to form their present profile, and condensation/evaporation is a relatively mild and insignificant form of mechanical weathering, in this arid climate."
In science, we call this an unsupported assumption. How does he know that condensation/evaporation is a relatively mild and insignificant form of mechanical weathering at Giza? What authority is he basing this assumption on? Who's collected and analyzed that data?
More importantly, he also mistates Gauri et al.'s premise. It's not just condensation/evaporation, its this process coupled with salt crystal stress induction (which causes salt crystal stressed induced exfoliation [ScrySIE]). A bit of science 101 . . .
When salts are present at the surface of a rock (or a piece of pottery, or a temple column at Luxor), and they are exposed to water (in the form of condensation in this case), the salts dissolve in the water, becoming mobilized (they move with the water). The water then fills pores or micro-cracks in the surface. As the water evaporates, the salts cannot evaporate with it and they start forming crystals. As these crystals grow, they put stress on the rock surrounding the pore or micro-crack. The pressure is released by fracturing the rock. What Gauri et al. found at Giza is that this process was dismantling the rock from the surface inward - creating the classically spherically weathered facies on the Sphinx and its enclosure. According to Hawass, you can actually see this happening at Giza. It's happening at Luxor as I write, except there the water enters the stone work through capillary action from the ground. I have seen pottery in a museum in Arizona that has fallen apart because of this process. Just slight variations in the relative humidity in the museum have destroyed hundreds of pieces of Anasazi art.
As I wrote in my article at Ian's site and is now reprinted here, I think the west wall of the Sphinx enclosure could be particularly susceptible to this process because that wall gets warmed up very quickly when the sun rises. The water, which condensed on the wall during the night, evaporates quickly - the salt crystals form quickly - the surrounding rock cannot adjust and it fractures, faster than anywhere else in the enclosure.
Gauri et al.'s idea is elegant. It explains all of Schoch's and Coxhill's observations (not to mention Colin's). Until Schoch can demonstrate this did not happen at Giza, it stands as the best working hypothesis.</HTML>
