DESERT VARNISH
Desert varnish is a dark coating on rocks found in arid regions. The coating is composed dominantly of fine-grained clay minerals. Within the clays are black manganese oxide and red iron oxide. A more general term is rock varnish which applies to dark coatings on rocks in general. Varnish can be a prominent feature in many landscapes. It often coats canyon walls, particularly in the areas where water flows down the sides of canyons.
Desert varnish plays an important role in archeology. Many petroglyphs are created by chipping through a dark coat of desert varnish to expose a lighter colored underlying rock.
Desert varnish is commonly seen coating rocks in deserts. On the east side of Death Valley, you can observe canyons with rock slides of different ages distinguished by the degree of varnish development. The older slides have a more mature coating of varnish.
FORMATION
Desert varnish forms only on physically stable rock surfaces that are no longer subject to frequent precipitation, fracturing or wind abrasion. The varnish is primarily composed of particles of clay along with iron and manganese oxides. There is also a host of trace elements and almost always some organic matter. The color of the varnish varies from shades of brown to black.
COMPOSITION
Originally scientists thought that the varnish was made from substances drawn out of the rocks it coats. Microscopic and microchemical observations, however, show that a major part of varnish is clay, which could only arrive by wind. Clay, then, acts as a substrate to catch additional substances that chemically react together when the rock reaches high temperatures in the desert sun. Wetting by dew is also important in the process.
An important characteristic of black desert varnish is that it has an unusually high concentration of manganese. Manganese is relatively rare in the Earth's crust, making up only 0.12% of its weight. In black desert varnish, however, manganese is 50 to 60 times more abundant. One proposal for a mechanism of desert varnish formation is that it is caused by manganese-oxidizing microbes (mixotrophs) which are common in environments poor in organic nutrients. A micro-environment pH above 7.5 is inhospitable for manganese-concentrating microbes. In such conditions, orange varnishes develop, poor in manganese (Mn) but rich in iron (Fe). An alternative hypothesis for Mn/Fe fluctuation has been proposed that considers Mn-rich and Fe-rich varnishes to be related to humid and arid climates, respectively.
LOGGING REQUIREMENTS
In order to log this EarthCache, send me your answers to the following questions either through email from my profile page.
If you look around this area, you will see several examples of desert varnish. Pick one area to make your observations.
1: Describe the color of the desert varnish in the area you chose.
2: Based on the color, do you think Manganese is present?
3: Describe the coverage of the desert varnish. Is it patchy or solid?
4: Give a reason for your answer to #3.
5: Pictures are not required but are appreciated. Please post a pic of you or your GPS with the desert varnish you selected in the background.