Hiking the labyrinthine canyons and corridors of the desert southwest invokes an air of awe and mystery. Along the course of your canyon adventure, you will likely discover a puzzle that has baffled naturalists since the time of Charles Darwin. Along the walls and atop the boulders along your route, you might notice a dark coating ranging from a dark brown rust to a polished gunmetal blue. This coating is broadly known as desert varnish or desert patina.
The ingredients for desert varnish.
When a rock surface is exposed to the air, it comes into contact with aeolian (wind-blown) dust. 50-90% of desert varnish is composed of wind-blown clay particles. The broken and decayed fragments of clay minerals provide the template for one of the most intriguing ingredients of desert varnish; Manganese oxide.is a relatively rare element found on Earth, making up only about 0.12% of the weight of the crust. The mystery that has plagued scientists is that the elemental concentrations of Manganese found in desert varnish may be anywhere from 50-300 times higher than the surrounding soil. The leading hypothesis for these high concentrations are tiny manganese-oxidizing microbes that live on the rock face. These microbes provide concentrated amounts of manganese oxide which, paired with oxidized iron particles found in the aeolian soil, cement the clay particles to the rock face forming desert varnish.
How is desert varnish useful?
Even though the major components of desert varnish are manganese and iron, the coating is generally too thin to be mined in any useful fashion. But, there are other trace elements that become trapped in the clay particles that scientists can study to investigate changes in Earth’s climate, and the layering formation of desert varnish provides a reliable timestamp to date these changes. For example, wetter environmental conditions form more manganese-rich layers and drier periods, more iron-rich layers. Some scientists also speculate that there may be desert varnish on some Martian rocks, suggesting that there may be active colonies of these microbes or at least fossilized evidence of this extreme form of alien life.
Does all desert varnish look the same?
Some desert varnish may appear as a dull black or rusty brown coating. Some may have a glassy blue sheen. Generally, the blacker the varnish coating, the higher the concentration of manganese. When desert varnish appears glossy or shiny, it has been coated with a hardened gel-like coating of silica that leaches out of the rock.
How long does desert varnish take to form?
Although it has been extensively studied, there are still a lot of uncertainties surrounding the formation of desert varnish. The underlying cause lies in the fact that the average rate of formation for desert varnish is, get this, one MICROMETER per MILLENNIA! That means that, on average, it could take as much as 50,000 years for a coating of desert varnish the reach the thickness of a piece of paper. The length of this process and the extreme hardness of desert varnish (almost as hard a quartz) make it a little difficult to reproduce in a lab setting. Ancient peoples, however, took advantage of these qualities of desert varnish. A nice, flat rock surface coated with a thin sheen of dark varnish made a perfect canvas for ancient Native American rock art. The native peoples would chip away the thin, dark coating of varnish to expose the lighter rock underneath, leaving behind images of spirals, bighorn sheep, people, and other petroglyph (petro=rock, glyph=carving) forms we can see today some hundreds and even thousands of years later.
So keep your eyes out for this natural wonder the next time you find yourself hiking the desert. You may even find a message from the Ancestors.