Desert varnish is a distinctive, thin layer that coats exposed rock surfaces in dry and semi-arid environments worldwide. This natural coating, often appearing as a dark brown to black film, can also exhibit reddish or orange hues depending on its specific chemical makeup. It forms a patina on rocks in landscapes where it is common, such as the Colorado Plateau. Its widespread presence and mysterious formation have long interested scientists.
Formation and Composition
Desert varnish develops through a complex, slow process involving physical, chemical, and biological interactions over thousands of years. Wind-blown dust, rich in clay minerals, settles on stable rock surfaces. Moisture, such as dew or rainfall, allows microorganisms, including bacteria and fungi, to thrive on these surfaces.
These microbes accumulate and oxidize manganese and iron from the dust and water. As these microorganisms carry out their metabolic processes, they precipitate insoluble forms of manganese and iron oxides, which then bind the clay particles to the rock surface. This creates a hard, microscopic layer that slowly thickens over time.
The primary components of desert varnish are clay minerals, which can make up over 70% of its volume, along with iron oxides and manganese oxides. The varnish’s color is largely determined by the proportion of these metal oxides; higher manganese concentrations typically result in blacker varnish, while more iron leads to reddish or orange coatings. This continuous, albeit incredibly slow, accretion process, often at a rate of 1 to 40 micrometers per thousand years, builds the characteristic dark layer.
Significance and Research
Desert varnish serves as a natural archive, providing insights into Earth’s history and biological processes. Its layered structure can act like tree rings, with each layer potentially preserving information about past climate conditions and geological events. This layering forms the basis for “varnish microlamination dating” (VML), a technique used to correlate and date geomorphic features in deserts by analyzing these micro-layers.
Researchers also study desert varnish to understand extremophile microorganisms, which are life forms that thrive in extreme environments. The bacteria and fungi within the varnish represent resilient life that can survive periods of dormancy and harsh desert conditions. Exploring these microbial communities helps understand the adaptability of life and its role in shaping geological features.
Beyond its scientific utility, desert varnish has cultural importance, particularly as a canvas for ancient rock art. Indigenous peoples carved petroglyphs by removing the dark varnish to expose the lighter rock underneath. These artworks, found in places like Petroglyph National Monument, offer historical records of human activity and cultural expression that have been preserved for thousands of years by the durability of the varnish itself.