What is desert varnish
Desert varnish is a dark coating on rocks found in arid regions. The coating
is composed dominantly of fine-grained clay minerals. Within the clays are
black manganese oxide and red iron oxide. A more general term is rock varnish
which applies to dark coatings on rocks in general.
Varnish can be a prominent feature in many landscapes. It often coats
canyon walls, particularly in the areas where water flows down the the sides
of canyons such as this canyon wall near the Whitehouse at Canyon De Chelly National Park, Arizona, USA, coated with varnish.
Desert varnish plays an important role in archeology. Many petroglyphs are created
by chipping through a dark coat of desert varnish to expose a lighter colored
underlying rock. This petroglyph is in Indian Petroglyph Petroglyph National Monument, New Mexico, USA.
Desert varnish is commonly seen coating rocks in deserts. On the east
side of Death Valley, you can observe canyons with rock slides of different
ages distinguished by the degree of varnish development. The older slides
have a more mature coating of varnish.
Well-developed coatings can form in the splash zone of rivers in arid
regions. The example of a manganese oxide coating in the splash zone of the Rio Grande
River, New Mexico, USA, contains clays and manganese oxide similar to that
found in desert varnish.
Images of desert varnish
This is a side view of a 5.1 cm wide piece of rhyolite coated with desert varnish from
Stoddard Wells, in the Mojave Desert, Southern California. The portion of
the rock that sits above the level of the desert pavement is partially coated
by a black layer a few 10's of micrometers thick of desert varnish. At the
contact with the desert surface, a shiny, dark ground-line band develops containing
a thick layer of varnish which essentially completely coats the region of
the rock in contact with the top layer of the soil. Below the ground line
band in the region where the rock is buried below the surface, an orange
coating develops. The interior of the rock is completely different from the varnish coating.
A footnote: Much of the desert varnish at the Stoddard Wells locality
has now been destroyed by off-road vehicle activity
This is a bottom view
of the rhyolite from Stoddard Wells. Here you can see how the entire underside
of the rock is coated with the red coating
The composition of desert varnish
Desert varnish is dominantly clay. The clay minerals represent the clays
found locally in the region where the varnish develops. In the clay layer,
black manganese oxide (the mineral birnesite) and red iron oxide (the mineral
hematite) add color.
A cross section through
desert varnish shows the contact on the lower right with the colorless quartz
and feldspar grains and black iron oxides of the rock with the deep red varnish
layer. Layering in the clays can be seen to follow the contour of the underlying
The clays appear to be derived from airborne dust. Varnish has a sharp
contact with the underlying rock. In this SEM image layers of clay are seen lying on the
underlying rock. The sample was prepared by fracturing a thin slab of a varnish-coated
rock. The exposed surface filling most of the frame is a fracture through
the varnish layer. A portion of the underlying rock surface can be seen in
the lower left portion of the image. Note how the varnish smoothly goes
around the rounded mineral grain in the bottom center-right side of the image.
Things which look like varnish but which are not
Counterfeit manganese deposits can take various forms. This is an algal stream deposit
from Rock Creek, California. It looks like a coating of manganese oxide, but
contains no manganese at all.
This is a fungal dendritic growth
from sandstone from Meteor Crater, Arizona which also looks like a manganese
deposit to the casual observer.
Material on related forms of manganese oxide in nature can be found in
the manganese dendrite
- Dorn RI, Bamforth DB, Cahill TA, Dohrenwend JC, Turrin BD, Donahue
DJ, Jull AJT, Long A, Macko ME, et al. (1986) Cation-ratio and accelerator
radiocarbon dating of rock varnish on Mojave artifacts and landforms. Science
- Dorn RI, Oberlander TM (1982) Rock varnish. Progress in Physical
- Dorn RI, Oberlander TM (1981) Microbial origin of desert varnish.
- Garvie LAJ, Burt DM, Buseck PR (2008) Nanometer-scale complexity,
growth, and diagenesis in desert varnish. Geology 36(3), 215-218.
- Liu T, Broecker W (2000) How fast does rock varnish grow: Geology.28:183-186.
RS, Lynne BY, Sephton MA, Kolb VM, Perry CC, Staley JT (2006) Baking
black opal in the desert sun: the importance of silica in desert
varnish.Geology 34: 537-540.
- Potter RM, Rossman GR (1977) Desert varnish: the importance of clay
minerals. Science 196:1446-1448.
N. Lee C (2004) Trace element evidence for the origin of desert
varnish by direct aqueous atmospheric deposition. Earth and Planetary
Picture credits to R.M. Potter.
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last updated 22-Nov-2012