Liu, Tanzhuo Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York.
- Links to Primary Literature
- Additional Readings
A dark to dark-brown coating on rock surfaces exposed to the atmosphere (Fig. 1). Rock varnish is probably the slowest-accumulating sedimentary deposit, growing at only a few micrometers to tens of micrometers per thousand years. Its thickness ranges from less than 5 μm to over 600 μm, and is typically 100 μm or so. Although found in all terrestrial environments, varnish is mostly developed and well preserved in arid to semiarid deserts; thus, another common name is desert varnish. Rock varnish is composed of about 30% manganese and iron oxides, up to 70% clay minerals, and over a dozen trace and rare-earth elements. The building blocks of rock varnish are mostly blown in as airborne dust. Although the mechanism responsible for the formation of rock varnish remains unclear, two hypotheses have been proposed to explain the great enrichment in manganese within varnish (typically over 50 times compared with the adjacent environment such as soils, underlying rock, or dust). The abiotic hypothesis assumes that small changes in pH can concentrate manganese by geochemical processes. The biotic hypothesis posits that bacteria, and perhaps other microorganisms, concentrate manganese; this is supported by culturing experiments and direct observations of bacterial enhancement of manganese. See also: Sedimentary rocks
The content above is only an excerpt.
for your institution. Subscribe
To learn more about subscribing to AccessScience, or to request a no-risk trial of this award-winning scientific reference for your institution, fill in your information and a member of our Sales Team will contact you as soon as possible.
to your librarian. Recommend
Let your librarian know about the award-winning gateway to the most trustworthy and accurate scientific information.
AccessScience provides the most accurate and trustworthy scientific information available.
Recognized as an award-winning gateway to scientific knowledge, AccessScience is an amazing online resource that contains high-quality reference material written specifically for students. Contributors include more than 9000 highly qualified scientists and 43 Nobel Prize winners.
MORE THAN 8700 articles covering all major scientific disciplines and encompassing the McGraw-Hill Encyclopedia of Science & Technology and McGraw-Hill Yearbook of Science & Technology
115,000-PLUS definitions from the McGraw-Hill Dictionary of Scientific and Technical Terms
3000 biographies of notable scientific figures
MORE THAN 19,000 downloadable images and animations illustrating key topics
ENGAGING VIDEOS highlighting the life and work of award-winning scientists
SUGGESTIONS FOR FURTHER STUDY and additional readings to guide students to deeper understanding and research
LINKS TO CITABLE LITERATURE help students expand their knowledge using primary sources of information