Friday Rock Blogging: Desert Varnish

Desert varnish on a cliff face. Also, a tree, with no desert varnish on the tree. Photo courtesy Brent Pearson. If you kick a dark pebble in the middle of the desert, you will sometimes find that it is light underneath. What this means is that you have disturbed a pebble that has been sitting there untouched for thousands of years. During that time, it accumulated a thin coating of windblown gunk - mainly clay dust, and manganese and iron oxides - known as desert varnish.

i-b6f82f1039373942cb450f6e2deaaee0-Valley_of_Fire_petroglyphs.jpg Desert varnish is not difficult to scratch through, and petroglyphic sgraffito is a popular artistic medium for native desert-dwellers and idiot tourists alike.

Desert varnish has a complex internal structure; there are thin sections below the fold.

i-6d06088eb5db6c137549706987cf26cc-desert-varnish-micrograph.jpg These entirely gratuitous micrographs are from a paper in the latest issue of Geology. Three scientists at Arizona State took a careful look at some sections of desert varnish picked up in the Sonoran desert, and find that most of the things that happen to large-scale sediments also happen to desert varnish: chemicals are dissolved at one place and precipitated at another, cracks heal, microbes might eat the delicious manganese.

This all makes it difficult to give a precise age for a varnished surface.

i-26095804307c10df000689015d7d407b-desert-varnish-mnfe.jpg This is a false color image of the distribution of iron and manganese in a desert varnish sample. Red is iron, green is manganese, and darker areas are mostly silicates (clay). The segregation of iron and manganese indicates that something wacky is going on.


Laurence A.J. Garvie, Donald M. Burt, and Peter R. Buseck, 2008, Nanometer-scale complexity, growth, and diagenesis in desert varnish, Geology 36:3 pp.215-218. DOI: 10.1130/G24409A.1.

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The Geo Dept is often brought false meteorites (or "meteor-wrongs") by members of the public, and we nicely examine and/or test their rock or other object, show them why it is not a meteorite, and usually try to figure out what it really is. Something that is commonly mistaken for a meteorite with a fusion crust is a rock with desert varnish. Then one has to explain that sandstones don't come from space.

Desert varnish is one of those things that drives home the concept of deep time to me. "Nanometer-scale complexity" makes me think of that Dr. Suess book where there was a whole world on a pollen speck.

Great post! I grew up in canyonlands, where cliffs coated in desert varnish abound. And yes, lots of petroglyphs, too! Thanks for the brief glimpse of home.

Desert Varnish:

- Organisms found on desert varnish are consuming the varnish, not contributing to it. If they were contributing, they wouldn't be found preferentially in the pits and crevices consuming its minerals.
- On petroglyphs, there's no varnish what-so-ever in the scratches, except for a very few primitive Clovis Petroglyphs where the coating in the scratches is identically the same thickness as on the background rock. This shows that desert varnish is not an ongoing process, but occurred suddenly.
- Desert varnish may be found preferentially (not exclusively) on SW facing desert walls, since the Late Pleistocene comets in North America arrived from the SW.
- Manganese is a large component of comet-crust meteorites like the Tafa primitive achondrites from Niger, including iron (iron sulfides and iron oxides) and organic components from the hard tar on comet crust.
- Mars rocks also show desert varnish where they've likewise been impacted by comets.
- Desert varnish can be found to a small extent in rock layers where the pressure of a local impact has compressed the contamination into the crevices.
- See attached image of a probable Clovis-age skull completely coated in desert-varnish comet spatter.
- Goethite, a component of desert varnish, is sometimes found compressed into nodules, reniform or mushroom shapes by the high pressures generated by comet impacts. When the prestressed goethite is shattered it fractures into shatter cones.
- a major component of desert varnish is clay, because the comet impact has combined terrestrial and comet material.
- Streaks down cliff face were caused by comet material in the water.
- Desert varnish isn't confined to the desert, but is almost ubiquitous in the Americas. Comet spatter, less contaminated by terrestrial components than desert varnish can be black, orange, white and green and frequently appears on sharp-edged boulders fractured from bedrock, particularly on the SW sides of mountains due to the direction and extremely oblique angle of the most recent Late Pleistocene impacts where it can easily be confused with lichen.
- Comet spatter can frequently be found on Clovis spear points.

- Comets were formed beyond the heliosphere, so the Niger primitive achondrites have a large cosmic ray exposure, but no solar exposure.
- Comet impacts create high pressure, low temperature elongated impact basins like a big slush ball hitting a sandbox. The extreme pressure from the impacts compresses sandstone and quartzite into milky quartz which traps the salty fluid inclusions of the comet ice. In large structural basins the milky quartz splashes out only to wash back into the basins after being worn smooth in streams and rivers into milky-quartz pebbles and cobbles that form into conglomerate which is often the basal layer of rock lying unconformally over the basement rock embedded with comet ore.
- Comets have a rocky crust over an icy core with hard tar on the surface. The rocky crust composed of shattered rock crystals on the order of 1 micrometer has random-sized metallic and ice inclusions in a raisin-bread--like composition. Naturally the ice melts on impact leaving voids. Since voids are never found in asteroid meteorites, having been formed far closer to the sun, millions of tons of comet-crust meteorites have been overlooked as iron-furnace slag. Comets were formed in two stages: first the ice core followed by the rocky crust of shattered mafic crystals and iron chunks. Perhaps a binary companion to our sun was itself a close binary that repeatedly collided over millions of years shedding first its hydrogen, then its ice and finally its rock and liquid-iron outer core before the inner cores finally stuck together with their iridium. (Comets have less than 1 ppb iridium.) Originally, our solar companion was closer to our sun, perhaps at the outer edge of the Kuiper Belt, before migrating outward to conserve angular momentum as it spewed jets of ice then rock as the close binary orbs rubbed shoulders in multiple collisions over millions of years. The companion presently is calculated to orbit between 1000 AU at periapsis and 4000 AU at apoapsis with a period of 25,000 years.[1] In this way, it swept out the space between the Kuiper Belt/scattered disc and the Inner Oort Cloud (Hills Cloud) and bombarded the inner solar system with comets over the last 4.5 Ga, resulting in many of the structural basins on earth including Witwatersrand Basin, Cuddapah Basin, Athabasca Basin (and their mineral ores) as well as The Great Unconformity. Comets are likely responsible for initiating the lava flows of the Deccan and Siberian Traps, for dislodging asteroids (such as the Chicxulub impact) and for bringing on most of the major and minor extinctions on the planet.

1. Understanding Precession of the Equinox: Evidence our Sun may be part of a long cycle binary system
Walter Cruttenden and Vince Dayes (2003)

Dave Carlson
Philadelphia, PA
See comet-related images at: