Editing Desert varnish

{{short description|Orange-to-black rock coating in arid environments}}
[[Image:Desert varnish.jpg|thumb|upright=1.25|Desert varnish on [[Desert pavement|gibber]], Central Australia ]]
'''Desert varnish''' or '''rock varnish''' is an orange-yellow to black coating found on exposed [[Rock (geology)|rock]] surfaces in [[arid]] environments. Desert [[varnish]] is approximately one [[micrometre|micrometer]] thick and exhibits [[nanometer]]-scale layering.<ref>{{cite journal |last1=Krinsley |first1=David |last2=Dorn |first2=Ronald |last3=Tovey |first3=N. K. |title=Nanometer-Scale Layering in Rock Varnish: Implications for Genesis and Paleoenvironmental Interpretation |journal=The Journal of Geology |date=January 1995 |volume=103 |issue=1 |pages=106–113 |doi=10.1086/629726|bibcode=1995JG....103..106K |s2cid=129392419 }}</ref> '''Rock rust''' and '''desert patina''' are other terms which are also used for the condition, but less often.

==Formation==
Desert varnish forms only on physically stable rock surfaces that are no longer subject to frequent [[precipitation (meteorology)|precipitation]], fracturing or [[ventifact|wind abrasion]]. The varnish is primarily composed of particles of [[clay]] along with [[oxidation|oxides]] of [[iron]] and [[manganese]].<ref>{{cite journal |last1=Perry |first1=Randall Stewart |last2=Adams |first2=John B. |title=Desert varnish: evidence for cyclic deposition of manganese |journal=Nature |date=November 1978 |volume=276 |issue=5687 |pages=489–491 |doi=10.1038/276489a0|bibcode=1978Natur.276..489P |s2cid=4318328 }}</ref>  There is also a host of trace elements and almost always some [[organic matter]]. The color of the varnish varies from shades of brown to black.<ref name=Chernicoff>{{cite book |last1=Chernicoff |first1=Stanley |last2=Whitney |first2=Donna |year=2007 |title=Geology: An Introduction to Physical Geology |edition=4th |publisher=Pearson Prentice Hall |page=585 |isbn=9780131751248}}</ref>

It has been suggested that desert varnish should be investigated as a potential candidate for a "[[shadow biosphere]]".<ref>{{cite journal |last1=Cleland |first1=Carol E. |title=Epistemological issues in the study of microbial life: alternative terran biospheres? |journal=Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences |date=December 2007 |volume=38 |issue=4 |pages=847–861 |doi=10.1016/j.shpsc.2007.09.007|pmid=18053938 }}</ref><ref>{{cite news |last=McKie |first=Robie |title=Life on Earth... but not as we know it |url=https://www.theguardian.com/science/2013/apr/14/shadow-biosphere-alien-life-on-earth |work=The Guardian |date=13 April 2013 |access-date=2022-02-25}}</ref> However, a 2008 microscopy study posited that desert varnish has already been reproduced with chemistry not involving life in the lab, and that the main component is actually silica and not clay as previously thought. The study notes that desert varnish is an excellent fossilizer for microbes and indicator of water. Desert varnish appears to have been observed by rovers on Mars, and if examined may contain fossilized life from Mars's wet period.<ref>{{cite journal |last1=Perry |first1=Randall |last2=Sephton |first2=Mark |title=Solving the mystery of desert varnish with microscopy |journal=Infocus |date=6 September 2008 |issue=11 |pages=62–76 |doi=10.22443/rms.inf.1.36 |doi-access=free}}</ref>

==Composition==
[[Image:Desert Varnish on Bishop Tuff-750px.jpg|thumb|left|Desert varnish on [[Bishop Tuff]].]]
Originally scientists thought that the varnish was made from substances drawn out of the rocks it coats.<ref>{{cite journal |last=Blake |first=W. P. |year=1905 |title=Superficial blackening and discoloration of rocks especially in desert regions |url=https://books.google.com/books?id=2lALAAAAYAAJ&dq=%22Superficial+blackening+and+discoloration+of+rocks+especially+in+desert+regions%22&pg=PA371 |journal=Transactions of the American Institute of Mining Engineers |volume=35 |pages=371–375}}</ref>  Microscopic and microchemical observations, however, show that a major part of varnish is clay, which could only arrive by wind.<ref>{{cite journal |last1=Potter |first1=Russell M. |last2=Rossman |first2=George R. |title=Desert Varnish: The Importance of Clay Minerals |journal=Science |date=1977 |volume=196 |issue=4297 |pages=1446–1448 |doi=10.1126/science.196.4297.1446 |jstor=1744384|pmid=17776923 |bibcode=1977Sci...196.1446P |s2cid=31169889 }}</ref>  Clay, then, acts as a [[wikt:substrate|substrate]] to catch additional substances that chemically react together when the rock reaches high temperatures in the [[desert]] sun.  Wetting by [[dew]] is also important in the process.<ref name=Chernicoff/>

An important characteristic of black desert varnish is that it has an unusually high concentration of [[manganese]]. Manganese is relatively rare in the Earth's crust, making up only 0.12% of its weight. In black desert varnish, however, manganese is 50 to 60 times more abundant. One proposal  for a mechanism of desert varnish formation is that it is caused by manganese-oxidizing [[microbe]]s ([[mixotroph]]s) which are common in environments poor in [[Organic matter|organic nutrients]]. A micro-environment [[pH]] above 7.5 is inhospitable for manganese-concentrating microbes. In such conditions, orange varnishes develop, poor in manganese (Mn) but rich in iron (Fe).<ref>{{cite journal |last1=Dorn |first1=R. I. |last2=Oberlander |first2=T. M. |title=Microbial Origin of Desert Varnish |journal=Science |date=11 September 1981 |volume=213 |issue=4513 |pages=1245–1247 |doi=10.1126/science.213.4513.1245|pmid=17744757 |bibcode=1981Sci...213.1245D |s2cid=39581858 }}</ref> An alternative hypothesis for Mn/Fe fluctuation has been proposed that considers Mn-rich and Fe-rich varnishes to be related to humid and [[arid climate]]s, respectively.<ref>{{cite journal |last1=Liu |first1=Tanzhuo |last2=Dorn |first2=Ronald I. |title=Understanding the Spatial Variability of Environmental Change in Drylands with Rock Varnish Microlaminations |journal=Annals of the Association of American Geographers |date=June 1996 |volume=86 |issue=2 |pages=187–212 |doi=10.1111/j.1467-8306.1996.tb01750.x}}</ref>

[[File:Valley of Fire petroglyphs.jpg|thumb|Petroglyphs carved in desert varnish at the [[Valley of Fire]] near [[Las Vegas, Nevada]]. Area shown is about one metre across.]]

Even though it contains high concentrations of iron and manganese, there are no significant modern uses of desert varnish. However, some [[Native Americans in the United States|Native American]] peoples created [[petroglyph]]s by scraping or chipping away the dark varnish to expose the lighter rock beneath.

Desert varnish often obscures the identity of the underlying rock, and different rocks have varying abilities to accept and retain varnish. [[Limestone]]s, for example,  typically do not have varnish because they are too water-soluble and therefore do not provide a stable surface for varnish to form. Shiny, dense and black varnishes form on [[basalt]], fine [[quartzite]]s and [[metamorphic rock|metamorphosed]] [[shale]]s due to these rocks' relatively high resistance to weathering.

Its presence has been cited as a key factor in the preservation of a large number of petroglyphs dating back to the [[Iron Age in the United Arab Emirates|Iron Age]] and earlier in areas such as the [[Wadi Saham]] in the [[United Arab Emirates]].

==See also==

* {{annotated link|Aeolian processes}}
* {{annotated link|Desert pavement}}
* {{annotated link|Manganite}}
* {{annotated link|Shadow biosphere}}

==Gallery==
<gallery>
File:Desert varnish at Capitol Reef National Park.jpg|Desert varnish at [[Capitol Reef National Park]]
File:Petroglyphs at Newspaper Rock, Canyonlands carved into desert varnish.jpg|Petroglyphs at [[Newspaper Rock State Historic Monument]] carved into desert varnish
File:Desert varnish - Valley of Fire State Park.JPG|Desert varnish at [[Valley of Fire State Park]]
</gallery>

==References==

{{reflist}}

==External links==
{{commons category}}

* [http://minerals.caltech.edu/FILES/VARNISH/ Desert Varnish]
* [https://web.archive.org/web/20080513151318/http://alliance.la.asu.edu/dorn/VarnishPages/VarnishPrimerIntro.html Rock Varnish (desert varnish): An Internet Primer for Rock Art Research] by Ronald I. Dorn, Professor of Geography Arizona State University
* [https://web.archive.org/web/20100409105520/http://www.geo.arizona.edu/palynology/geos462/14rockvarnish.html DESERT VARNISH (rock varnish) ]
* [http://www.space.com/scienceastronomy/060630_desert_varnish.html New Way Suggested to Search for Life on Mars] — Space.com
* [http://archives.cnn.com/2001/TECH/space/07/02/mars.rocks/index.html Researcher: Mars rock varnish hints of life July 2, 2001 By Richard Stenger CNN] {{Webarchive|url=https://web.archive.org/web/20060825232537/http://archives.cnn.com/2001/TECH/space/07/02/mars.rocks/index.html |date=August 25, 2006 }}
* [http://www.icamsr.org/docs/spie_4495-13_rock_varnish.pdf Rock Varnish As A Habitat For Extant Life On Mars]

[[Category:Deposition (geology)]]
[[Category:Sedimentology]]
[[Category:Deserts]]
[[Category:Rocks]]
[[Category:Hypothetical life forms]]