Desert varnish is a dark, shiny coating often observed on exposed rock surfaces in arid and semi-arid regions worldwide. This natural phenomenon creates dark layers, ranging from reddish-brown to black, on boulders and canyon walls, sometimes coloring entire desert landscapes. Its precise formation mechanisms have long puzzled scientists, making it a subject of ongoing inquiry.
What Desert Varnish is Made Of
Desert varnish primarily consists of fine-grained clay minerals, along with significant amounts of iron and manganese oxides. These components are not derived from the underlying rock but rather originate from external sources, predominantly airborne dust particles. Aeolian dust settles on rock surfaces, providing material for the varnish. Clay minerals often comprise over 70% of the varnish, with iron and manganese oxides making up the rest.
The Role of Microorganisms
Microorganisms play a role in desert varnish formation, particularly in concentrating manganese and iron. Specific types of bacteria and fungi, notably manganese-oxidizing bacteria, are instrumental in this process. These microbes absorb trace amounts of manganese and iron from the environment, transforming soluble forms into insoluble oxides like black manganese oxide and reddish iron oxide. This biological activity facilitates the deposition and binding of these metals onto the rock surface. These microorganisms are consistently found within varnish layers, suggesting their contribution to its development.
The Formation Process Explained
The formation of desert varnish begins with airborne dust particles, rich in clay and metal oxides, adhering to stable rock surfaces. Intermittent moisture, such as dew or light rain, helps bind these dust particles to the rock. Microorganisms on the rock surface then initiate the oxidation of manganese and iron. These microbial processes concentrate the metals, converting them into insoluble forms that precipitate and accumulate.
These metal oxides, along with additional layers of dust and clay, accrete onto the rock. This layer-by-layer deposition, cemented by the iron and manganese compounds, builds up the dark, shiny coating. The varnish color varies depending on the predominant metal; manganese-rich varnish appears black, and iron-rich varnish appears reddish-brown. This continuous process results in a thin yet durable surface layer distinct from the rock itself.
Environmental Factors and Time
Desert varnish forms under environmental conditions that include intermittent moisture, stable rock surfaces, and low rates of erosion. Moisture, whether from dew or occasional rainfall, is necessary for initial dust adherence and microbial activity. Stable rock surfaces that are not frequently subject to fracturing or heavy abrasion allow varnish layers to accumulate. The process is slow, often taking thousands to hundreds of thousands of years to form a noticeable coating. A typical varnish layer can be less than a micrometer thick per millennium.
Why Scientists Study Desert Varnish
Scientists study desert varnish for several reasons, primarily for its use in dating exposed rock surfaces and artifacts. The slow, cumulative nature of varnish formation allows researchers to estimate the age of rock art, such as petroglyphs, by analyzing the varnish layers that form over them. Additionally, the composition and layering of desert varnish also indicate past environmental and climatic conditions. Variations in the chemical makeup of varnish layers provide insights into historical temperature, humidity, and precipitation patterns, offering valuable data for paleoclimate reconstruction.