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Desert varnish or rock varnish is an orange-yellow to black coating found on exposed rock surfaces in arid environments. It is usually around one micron thick and represents extremely small scale layering. Rock rust and desert patina are other terms which are also used for the condition, but less often.
The varnish forms only on physically stable rock surfaces that are no longer subject to frequent precipitation, fracturing or wind abrasion. Primarily composed of particles of clay along with iron and manganese oxides, there is also a host of trace elements and almost always some organic matter. The color of the varnish itself varies from shades of brown to black.
Originally scientists thought that desert varnish was made from substances drawn out of the rocks it coats. However, microscopic and observations show that a major part of varnish is clay, which could only arrive by wind. Clay acts as an under layer to catch substances that chemically react when the rock reaches high temperatures in the desert sun. Wetting of the rock surface by dew is also important in the process.
Even though it contains high concentrations of iron and manganese, there are no significant modern uses of desert varnish. Some Native American tribes however, have created their petroglyphs by scraping or chipping away the dark varnish to expose the lighter rock beneath.
The varnish often obscures the identity of the underlying rock, and different rocks have varying abilities to accept and retain the deserts own varnish.
Photograph Copyright 2013 Jon Burch Photography
March 8th, 2013
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