Desert Varnish
The petroglyphs of Newspaper Rock are etched into dark desert
varnish to expose the lighter color of the rock beneath.
Desert Varnish is made up of a thin veneer of manganese, iron
and clay that covers the exposed surfaces of the rocks. According
to the prevailing theory (by Ronald I. Dorn and Theodore M.
Oberlander) proposed in 1981, desert varnish is created by
microscopic bacteria over thousands of years.
The bacteria oxidize the tiny amounts of manganese
and iron in the atmosphere to obtain energy. After the bacteria
oxidize the manganese and iron, it deposits the manganese oxide or
iron oxide on the surface of the rock as the black or reddish
coating you see. Clay particles, probably obtained by wind are also
included in the varnish and likely help protect the bacteria from
drying out and solar radiation. It is thought that it could take as
much as 10,000 years for a mature desert varnish to form on these
desert rocks.
However, the complete mechanisms and processes that create
desert varnishes have not been fully explained. Recent research and
past works continue to propose alternate mechanisms for its
formation and explain all the complexities of its formation. Some
of these other theories include a completely inorganic source,
mineralization of microbe spores after they begin to break down,
photochemical (sun driven chemical reactions), and silicic acid
(fromed from quartz or silica).
A much referenced work on the subject is Dorn, R.I. and T.M.
Oberlander. 1981. "Microbial Origin of Desert Varnish." Science
213: 1245-1247 (but I was unable to locate for this
earthcache).
Cliff Retreat
The rock that makes up the mesa you are standing on is the
Newspaper Rock Sandstone. While this rock appears quite stable, it
is highly fractured. Beneath the sandstone are mudstones that erode
away easily. As the mudstone erodes out from beneath the sandstone,
the fractures in the sandstone begin to widen as the block of
sandstone slowly begins to fall out from the cliff. Eventually, the
sandstone block will fall down the cliff. The trail down to
Newspaper Rock was destroyed in 1984 by such a process.
Logging requirements:
Send me a note with :
- The text "GCZCNM Newspaper Rock Desert Varnish" on the first
line
- The number of people in your group.
- Provide a short explanation as to why the recently exposed
rocks are not as dark as the boulders that have the
petroglyphs
The above information was compiled from the
following sources:
- Bezy, John V. and Arthur S.
Trevena, 2000, Guide to Geologic Features at Petrified Forest
National Park, Arizona Geological Survey Down-to-Earth
10
- Woody, Daniel T., 2006, Revised Stratigraphy of
the Lower Chinle Formation (Upper Triassic) of Petrified Forest
National Park, Arizona, in A Century of Research at Petrifed Forest
National Park, Museum of Northern Arizona Bulletin No. 62, Parker,
W.G., Ash, S.R, and Irmis, R.B., eds., 2006
- Ash, S.R. 1987, Petrified Forest National Park,
Arizona, Geological Society of America Centennial Field Guide
– Rocky Mountain Section, 1987
- Anza Borrego Dessert State Park 2003, Nature
Trail Guide, Palm Canyon Trail
- Wayne P. Armstrong, Desert Varnish & Lichen
Crust Microscopic Organisms Color Rocks & Boulders,
http://waynesword.palomar.edu/pljan98.htm
- Randall S. Perry and Vera M. Kolb, undated
Biological and organic constituents of desert varnish: review and
new hypotheses, pdf from http://www.psi.edu/~rperry/perry/
SPIE_DV.pdf
Placement approved by the
Petrified Forest National Park
