NPS photo by Neal Herbert
Desert varnish is one name for the thin red-to-black coating found on exposed rock surfaces in arid regions. Varnish is composed of clay minerals, oxides and hydroxides of manganese and/or iron, as well as other particles such as sand grains and trace elements. The most distinctive elements are Manganese (Mn) and Iron (Fe).
The color of rock varnish depends on the relative amounts of manganese and iron in it: manganese-rich varnishes are black; manganese-poor, iron-rich varnishes are red or orange; intermediate compositions are some shade of brown.
Bacteria also play a role in the formation of desert varnish. Some bacteria's physiological activities take manganese out of the environment, then oxidize and emplace it onto rock surfaces, cementing clay and other particles in the process. These microorganisms live on most rock surfaces and may be able to use both organic and inorganic nutrition sources, allowing them to thrive in deserts by filling a particular environmental niche.
The sources for desert varnish components come from outside the rock, most likely from atmospheric dust and surface runoff. Streaks of black varnish often occur where water cascades over cliffs. No major varnish characteristics are caused by wind.
Thousands of years are required to form a complete coat of manganese-rich desert varnish so it is rarely found on easily eroded surfaces. A change to more acidic conditions (such as acid rain) can erode rock varnish. Lichens can also chemically erode rock varnish, as well as visitors who scratch graffiti into it.