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desert varnish

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Desert varnish forms on the upper surface of rocks that are stable on the desert surface. The color varies from orange brown to black, and is formed primarily of manganese and iron oxides and hydroxides. It was previously thought that clay was an important constituent, carried in by the wind, however a study published in 2008 found that silica was present, but not clay. The issue is unsettled.

Bacteria have a part in the process of forming this coating, since they can oxidize manganese and can absorb manganese and iron from the atmosphere and precipitate it as a thin layer on rock surfaces. Bacteria of the genera Metallogenium and Pedomicrobium, among others, form desert varnish.

All living systems require adenosine triphosphate in order to function. Electrons, formed from the oxidation of glucose, flow along the membranes of the mitochondria in cells to form ATP. Varnish-forming bacteria do not have mitochondria, instead having a similar inner membrane structure through which electrons flow to produce ATP. The electrons come from the oxidation of manganese and iron instead of glucose.

Manganese-rich varnishes are black. Varnishes with similar amounts of iron and manganese are brown. Varnish forms on rocks with a smooth surface. A complete coat of varnish may take up to 50,000 years to form.

Native Americans overturned rocks with a desert varnish to form large geoglyphs in the Yuha desert of Imperial County, California. The aerial photo shows geoglyphs near Blythe. This varnish example was collected in San Bernardino County, California, from a stable desert pavement northeast of Barstow.

Both sides are shown. The top is dark, the bottom light.




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