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By Wayne P.
This article was originally published in The Sand Paper, the membership
newsletter of the
Anza-Borrego Desert Natural History Association
||One of the most remarkable
biogeochemical phenomena in arid desert regions of the world is called desert
varnish. As you descend the steep curves along Montezuma Grade into Borrego
Springs or walk up Borrego Palm Canyon, you are immediately surrounded by
enormous reddish boulders coated with desert varnish. Although it may be only
a hundredth of a millimeter in thickness, desert varnish often colors entire
desert mountain ranges black or reddish brown. Desert varnish is a thin
coating (patina) of manganese, iron and clays on the surface of sun-baked
boulders. Its origin has intrigued naturalists since the time of Charles
According to the classic
paper by Ronald I. Dorn and Theodore M. Oberlander (Science Volume 213, 1981),
desert varnish is formed by colonies of microscopic bacteria living on the
rock surface for thousands of years. The bacteria absorb trace amounts of
manganese and iron from the atmosphere and precipitate it as a black layer of
manganese oxide or reddish iron oxide on the rock surfaces. This thin layer
also includes cemented clay particles that help to shield the bacteria against
desiccation, extreme heat and intense solar radiation.
A number of hypotheses have
been proposed to explain the origin of desert varnish. Although some crustose
rock lichens form thin surface layers on rocks and boulders, they generally
cannot survive the arid, sun-baked conditions favorable to desert varnish.
Several research papers about the microbial origin of desert varnish have
appeared during the past 25 years, including R.S. Perry of the Department of
Earth and Space Sciences, University of Washington who received a Ph.D. on the
subject in 2004. The microbial origin of desert varnish is difficult to
reproduce in the laboratory, particularly the length of time required for a
varnish coating to develop. In addition, it is difficult to reproduce the
remarkable hardness of desert varnish, which is almost as hard as quartz
(nearly 7 on the Mohs scale of mineral hardness). Dr. Perry has authored
several scholarly articles on desert varnish, including his 2003 paper
entitled “Biological and Organic Constituents of Desert Varnish: Review and
New Hypotheses,” which is available on-line. I have summarized some of the
major points of his 2003 article as follows:
1. Desert varnish
is a hard coating (patina) on the rock surface. Contrary to some references,
it is not a weathering product of the rock.
2. The source of
the coating minerals, mostly iron and manganese, is derived from deposits of
dust from the atmosphere. Dust lands on the rock surfaces and the mineral
component becomes concentrated and “glued together” into a hard, microscopic
3. Varnish is
associated with colonies of bacteria and microcolonial fungi. DNA and amino
acids are also associated with some varnish coatings.
4. Varnish bacteria
are involved in the formation of reddish iron oxides and black manganese
oxides through metabolic oxidation processes.
5. Silicic acid may
serve as a cementing agent in some desert varnishes.
Several genera of bacteria
are known to produce desert varnish, including Metallogenium and Pedomicrobium.
Because these bacteria live in extreme environments, they are sometimes placed
in a category called extremophiles. They consist of minute spherical,
rod-shaped or pear-shaped cells only 0.4 to 2 micrometers long, with peculiar
cellular extensions. In fact, the individual cells are smaller than human red
blood cells, which are about 7.5 micrometers in diameter. Because of the
radiating filaments from individual cells and colonies, they are called
All living systems require
the vital energy molecule ATP (adenosine triphosphate) in order to function.
In our cells, ATP is constantly produced within minute bodies called
mitochondria. As electrons flow along the membranes of our mitochondria,
molecules of ATP are generated. The electrons come from the breakdown
(oxidation) of glucose from our diet.
Although varnish bacteria
do not have mitochondria, they do have a similar inner membrane structure
through which electrons flow to generate ATP. In some chemosynthetic bacteria,
the electrons come from the oxidation of elements in their environment. This
is presumably the adaptive advantage for varnish bacteria as they produce
microscopic layers of black manganese oxide and red iron oxide on desert
Varnish bacteria thrive on
smooth rock surfaces in arid climates. According to Ronald Dorn, perhaps
10,000 years are required for a complete varnish coating to form in the
deserts of the southwestern United States. In fact, dating of varnished
surfaces is of enormous importance to the study of desert landforms and to the
study of early humans in America, since many artifacts lying on the ground
become coated with desert varnish. Boulders of the Anza-Borrego Desert region
are covered with a reddish-brown iron oxide, while boulders in parts of the
Alabama Hills near Lone Pine in the Owens Valley are blackened by a manganese
If you scratch through the
varnish layer, the lighter-colored granitic rock is exposed. I have seen one
rock along the trail to Travertine Palms Wash dated at 1945. For thousands of
years Native Americans have used desert varnish for their rock carvings
(called petroglyphs). Throughout the Alabama Hills are acres of elaborate
petroglyphs carved into black desert varnish and Bishop tuff, including
spirals, circles, wavy lines, footprints, men, deer and desert bighorn sheep.
It is fascinating to speculate on the origin and meaning of all these
Native Americans have used varnish-covered rocks as a
canvas for their petroglyphs.
Desert varnish is
widespread on Earth, and its existence on Mars has been proposed based on data
from various space missions to the red planet. If it is present on Mars, is
the process of formation similar to that on Earth? Are extremophile microbes
responsible for the clay and oxide rich coatings? Current research on Earth
indicates that the varnish process may involve microbial action plus inorganic
processes in which the minerals and clays are cemented together and literally
baked onto the rock surfaces. So the next time you walk along one of the many
beautiful canyons in Anza-Borrego Desert State Park, stop and contemplate
about the origin and magnitude of the magnificent varnish-coated boulders
1. Dorn, R.I. and T.M.
Oberlander. 1981. “Microbial Origin of Desert Varnish.” Science 213:1245-1247.
2. Dorn, R.I. and T.M.
Oberlander. 1982. “Rock Varnish.” Progress in Physical Geography 6: 317-367.
3. Perry, R.S. and V.M.
Kolb. 2003. “Biological and Organic Constituents of Desert Varnish: Review and
New Hypotheses.” Available on-line at: http://www.psi.edu/~rperry/perry/SPIE_DV.pdf.
Wayne P. Armstrong is a retired Biology Professor at Palomar College.
He teaches courses in Plant Identification and on-line courses in general
biology and botany. He has a Bachelor’s Degree in Botany and a Master’s
Degree in Biology from California State University Los Angeles plus numerous
additional graduate courses in biology, oceanography and ecology from
Colorado State University, Oregon State University, SDSU and UCSD.
© Anza-Borrego Desert
Natural History Association (ABDNHA), The Sand Paper, Summer 2008.