Thursday, March 27, 2008
Geology as Art
Last weekend we were walking past this alcove where you can see some pretty fresh looking Wingate sandstone. The exterior of the alcove has varying degrees of desert varnish while the inside looks virtually untouched. There is a nice large dark coating of varnish river right of the water drop off. When we climbed up to the alcove you could see lots of sandstone blocks with different degrees of varnish lying on the ground. As a rule of thumb I have always heard that a good thick layer of varnish takes about 2,000 years to develop. This is handy when trying to date petroglyphs or in this case rock fall.
I was always taught that the varnish is a result of biological action. Looking through my old notes I found thepaper by Dorn and Oberlander (Science Volume 213, 1981) where they state that desert varnish is formed by colonies of bacteria absorbing trace amounts of manganese and iron from the atmosphere and wind blown dust and metabolically precipitate it out as a black layer of manganese oxide or reddish iron oxide on the rock surfaces. So for all of these years I have been teaching my students about the biologically mediated mechanism of desert varnish formation.
Well it appears that there has been some additional research done since my MA in 1989. There seems to be total agreement (as total as one could expect) with the material not coming from the parent rock and that there must be a mechanism to deliver the material to the rock surface.
Just by walking through the canyons you can see that the presence of water is integral to the process of desert varnish. I like the idea of atmospheric water using desert dust as condensation nuclei instead of bacterial action. Iron and manganese are found in the reservoir of desert dust which acts as condensation nuclei for the water vapor. Upon contact with the sandstone walls, the minerals precipitate out onto the rock. The water then evaporates and the excess dust blows away leaving the patina of varnish. I am concerned though as I have quite a reservoir of desert dust in the back of the jeep and I really don't need and minerals precipitating out on the window
This of course still leaves the question about the presence of bacteria, DNA and organic compounds in the varnish. Instead of being a part of the formation mechanism organisms might be coexisting in the developing varnish as it is being developed.
I don't pretend to be a chemist, geo or otherwise I just enjoy the puzzle of the origins of the patterns on the rock. But really, I usually just enjoy the desert varnish as geology as art!