Sandstone Varnish Factoids

On a recent desert trip an old discussion (between us) arose: How does sandstone varnishing occur? This time when I returned home I looked it up. . . . Most of you may have already known the answer, but it wasn't one that our group understood!
Merry Christmas!
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GEOLOGY — Desert Varnish / Color

COLORED SURFACE COATINGS
Desert varnish is the thin red to black coating found on exposed rock surfaces in arid regions. Varnish is composed of clay minerals, oxides and hydroxides of manganese and/or iron, as well as other particles such as sand grains and trace elements. The distinctive elements are Manganese (Mn) and Iron (Fe).
The color of rock varnish depends on the relative amounts of manganese and iron in it: manganese-rich varnishes are black; manganese-poor, iron-rich varnishes are red to orange; those intermediate in composition are usually a shade of brown. Varnish surfaces tend to be shiny when the varnish is smooth and rich in manganese.
Desert varnish consists of clays and other particles cemented to rock surfaces by manganese emplaced and oxidized by BACTERIA living there. It is produced by the physiological activities of microorganisms which are able to take manganese out of the environment, then oxidize and emplace it onto rock surfaces. These microorganisms live on most rock surfaces and may be able to use both organic and inorganic nutrition sources. These manganese-oxidizing microorganisms thrive in deserts and appear to fill an environmental niche unfit for faster growing organisms which feed only on organic materials.
The sources for desert varnish components come from outside the rock, most likely from atmospheric dust and surface runoff. Streaks of black varnish often occur where water cascades over cliffs. No major varnish characteristics are caused by wind.
Thousands of years are required to form a complete coat of manganese-rich desert varnish so it is rarely found on easily eroded surfaces. A change to more acidic conditions (such as acid rain) can erode rock varnish. In addition, lichens are involved in the chemical erosion of rock varnish.

Cameron,

This is just an interesting sidebar to what you posted about the origins of "desert varnish".

I heard a really interesting discussion on public radio, about a year and a half ago, discussing a new technique for dating petroglyphs. Some retired guy, who was a scientist but not an archeologist, has come up with this very accurate method for determining the age of petroglyphs. His method is based upon very high power microscopic examination of the petroglyphs to determine how much varnish has grown back where the petroglyph has been scratched. Because the varnish is the result of living bacteria, the rate that the bacteria will emplace and oxidize can be very accurately determined. Thus, by measuring the amount of emplacement and oxidation that has taken place over the petroglyph, its age can be determined.

When this guy came up with this idea, most archaeologists thought he was nuts, but testing has proved his method right and it is now considered the definitive way to date petroglyphs. You can check it out further at srs.dl.ac.uk/arch/ssrl/petr…

Thanks for the memory prod.

Thanks for the input Mike. I've heard of patina testing being used in the past to determine petroglyph and pictograph age, but not this "varnish growth". . . fascinating.

But wouldn't the X-rays kill the bacteria that establish the varnish? Or is that what you kinda want to keep the petroglyphs in place?

I don't know tenesmus, I'm guessing that x-rays would kill micro-bacteria. . . but using a high-powered microscope wouldn't mean subjecting the bacteria to x-rays. I'm wondering what kind of microscope could be used on vertical rock, or if they have to remove the sample for testing on a platform or in a lab. Maybe there is somebody out there who can fill us in!

I've been trying to find the exact reference that I heard about on the radio. I haven't found it yet, but here is a link I found that gives a little insight into one of the methods used for dating. adsabs.harvard.edu/abs/2003… This link claims that the testing is not detrimental to the microorganisms that are making the varnish. I'm going to call the local public radio station tomorrow and see if I can track down the actual story I heard.

This is fascinating. The forces that bind sandstone together are tenuous at best. The way the layers are cross-bedded and frozen and then differentially eroded.

Last month I found the coolest frozen wave of sandstone in a dihedral in the swell. One side was serrated and overlapping the other, yet separated from the wall in a curved shape. We climbed the next dihedral over and I can't wait to rap down that side and examine that thing closer. Its pure fascination.