By Sid Perkins
From San Francisco, at a meeting of the American Geophysical Union
Scientists have developed a quick, easy, portable, and nondestructive way to determine the age of desert varnish, the mysterious dark coating that slowly develops on rocks in many arid regions of the world (see “Thin Skin,” in this week’s issue: Available to subscribers at Thin Skin).
Desert varnish is rich in iron oxides and manganese oxide–both possible products of biological processes–and contains varying amounts of clay, says Nicholas E. Pingitore Jr. of the University of Texas at El Paso. Researchers don’t fully understand how desert varnish forms on exposed rock surfaces, but they agree that it does so exceedingly slowly. Ancient artists created petroglyphs scraping away the dark coating to reveal the lighter rocks beneath.
Data from many sites suggest that the iron and manganese oxides in desert varnish accumulate at a slow and relatively steady rate but that clay in the coating builds up sporadically. Using varnish samples of known ages, Pingitore and a colleague have developed an X-ray–fluorescence technique that measures the iron and manganese in desert varnish and thus indicates its age. Each measurement takes just 2 minutes.
Besides its possible use in dating petroglyphs, the technique could determine the age of ancient rock slides or to assess when ground motions along fault zones exposed fresh rock, says Pingitore.
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