As you look up the mountain to the east notice the change
in the color of the rocks from the channel floor up the
mountain. Down in the channel, the rocks are generally light
colored. Up on the mountain most rocks are shiny and dark,
almost black, to reddish brown. Half way up the channel you
can see rocks that grade from light to dark coloration. The
dark color on the rocks is a surface coloration called Desert
Varnish.
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 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).
Logging requirements:
Send me a note with :
- The text "GCZ5XJ Desert Varnish - Anza Borrego SP" on the first
line
- The number of people in your group.
- Send me a note explaining the difference you see in the color
of the rock surfaces in the channel and up on the slopes.
- Relate it to the desert varnish.
- Include in the e-mail the number of the nature trail stop at
the coordinates or post a picture to show your were at the
site.
The following sources were used to generate this
cache:
- 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