A close look at the mystery of desert varnish

 



By Jeff Mitton

The canyon walls, monuments and delicate spires of the Colorado Plateau evoke awe and wonder.

The canyon walls have many hues, ranging from deep red through orange, tan and white. But in addition to all the variation in color, some walls in many places are also decorated with desert varnish.

Desert varnish comes in several patterns. In some places, entire canyon walls are colored deep reddish brown or black. In other places, desert varnish forms vertical stripes alternating between black and red and tan in displays far grander than those produced by Christo. On more complex surfaces, such as the edge of a hole in the roof of an alcove, desert varnish follows the intricate pattern of water streaming over rock.

Desert varnish does not form on all surfaces. It rarely forms on granite and is most commonly seen on sandstone. But within a sandstone canyon, one wall may have desert varnish while other walls are naked, unadorned rock.

It seems that desert varnish is most common on walls facing north or east and less common on walls facing south and west, suggesting that temperature has an influence. Patterns of desert varnish clearly suggest that water seeping down walls and into alcoves is a major determinant for the formation of varnish. Perhaps varnish forms more commonly on north-facing walls because water quickly evaporates from walls in direct sunlight.

Desert varnish is a thin veneer of clay, minerals and microbes that forms on the surface of rock. It accumulates slowly, at a rate of 1 to 40 micrometers or less than the width of a human hair in 1,000 years. The minerals are oxidized forms of manganese and iron, while the microbes include a substantial diversity of bacteria and some fungi.

The relative proportions of manganese and iron produce the range of colors. A predominance of manganese oxide produces black varnish while a predominance of iron oxide makes the varnish red. Similar concentrations of manganese and iron oxides produce tans and browns.

Desert varnish forms when water, clay, microbes and minerals interact over long periods of time. Water seeping over rock encourages microbes and fungi to grow and allows windblown clay dust to adhere. Manganese and iron arrive in water-soluble form, but manganese and iron oxidizing bacteria and fungi change the minerals to insoluble forms, cementing the clay and minerals to the rock surface and concentrating the minerals. Black varnish has 50 times the concentration of manganese in nearby soils.

The formation of desert varnish is controversial, for although the thin veneer of clay and mineral oxides always contains microbes and fungi, it is not certain that the micro-organisms oxidize the manganese and iron under all circumstances and in all localities. Some microbial studies have not found the familiar species known to oxidize minerals, and some geologists assert that soluble forms of manganese and iron can be oxidized by clay. It is possible that desert varnish is deposited by different mechanisms in contrasting environments.

Early Native Americans sought desert varnish as a substrate for their art. Across the Colorado Plateau, panels of petroglyphs have been carved into varnished walls. The relatively light color of the underlying stone contrasts with the varnished surface, making the art more conspicuous.

Photos of Martian landscapes show something that looks like desert varnish on rocks, inspiring some geologists to think about extraterrestrial life, but reminding others that varnish formation may not always require the activity of bacteria and fungi.

Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

June 2011