Rock Varnish

Rocks that are exposed to harsh conditions in desert and arid landscapes often develop a hard, sometimes reddish-brown but usually dark brown to almost black color coating. This outer layer substance which forms over time and darkens the surface of rocks is called 'desert varnish' or 'rock varnish.' Rock varnish can also form on the surface of desert soil if it remains undisturbed for thousands of years. However, rock varnish coatings are not unique to arid climates. The phenomenon is more widespread and also occurs at high-altitudes and in tropical environments. The pictures below show the appearance of rock varnish.

What Is It, Really?

The orange, gray, brown, or black veneer coatings have physiochemical and biological origins. Rock varnish is made up of a coating of clay particles (approximately 70%) combined with mineral deposits of iron and manganese oxides - - all probably wind-blown material that settled on the surface of rock. Other minerals mixed into varnish composition include hydroxides plus silica and calcium carbonate. These ingredients are cemented to the rock surface by living bacteria. The bacteria reside within and beneath the microscopic layers of varnish, and are usually absent from the exposed surfaces. Exactly how rock varnish is formed is not completely known, but one theory is that varnish formation is a means by which these microbes protect themselves in their exposed, extreme environments. Manganese oxides in rock varnish block the transmission of ultraviolet radiation. Could the rock-dwelling bacteria be simply be creating their own effective sunscreen?

Rock varnish forms very slowly. The varnish becomes thicker and darker as it ages. Some varnished rock surfaces that have been untouched for tens of thousands of years. Many older deposits become almost black. By closely examining and measuring the varnish coat, geologists are able to measure how long the rock's surface has been undisturbed.

Ancient peoples used hard stone tools to chip designs through the dark patina of rock varnish to expose the lighter core (Left photo: 'Moab Man' petroglyph, Utah). Varnished rock surfaces became a dark-colored canvas for their artistic work. Many of these petroglyphs, created by chipping away the surface varnish, can still be seen in the American Southwest as well as many other areas.

Over time, rock varnish accumulates over petroglyphs too. The ancient art can become as dark as the rock, sometimes making it very hard to see.

More Information

If you want to learn more about rock varnish, visit the following websites:

  1. Desert Varnish
  2. Desert Varnish & Lichen Crust by W.P. Armstrong from Desert USA
  3. Rock Varnish from Arizona-Sonora Desert Museum
  4. What is Rock Art?

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Note: All photographs taken with a digital camera in Utah.
Developed by Annette Lamb and Larry Johnson, 04/02.