Fallen arches

Wall Arch, at Arches National Monument, near Moab, took a tumble Monday night, Aug. 4-5, 2008.
This arch has been sitting around for half a million years or so without falling. Of course the long term cause is gradual erosion, but I suspect the immediate causes were weather and tide related:a drier cllimate than normal, coupled with shear stresses caused by a temperature gradient. The problem is, August 1 had a 40 degree differential compared to a 25 degree drop Monday night, and the new moon occurred 2 days earlier as well. The winds were stronger Friday than Monday night. Did it fall two days after it cracked? I doubt it, but there must have been some reason it fell when it did. We're a month past aphelion, with its weaker tides, but the arch has gone through the worst of a summer full of expansion and contraction. Winter freezing and thawing does the long term damage, but summer probably provides better triggering mechanisms. Ice ages may set the arches up to tumble during interstadials. I think natural global warming could be the culprit.
Here's a tenuously proposed triggering mechanism: the humidity was rising that night, and sandstone constitutes a semipermeable membrane. H2O molecules are lighter than N2 and O2, vibrate more rapidly, and find their way through the rock faster than dry air. This enables the rock to build up a partial pressure of water vapor faster than the total pressure of dry air in the rock can be dissipated, so that the gasseous pressure in the rock may build up a few mmHg of pressure--maybe enough to trigger a collapse. I suppose the mechanics and magnitude of such a phenomenon could be studied experimentally. --AGF