Rock varnish

Article By:

Liu, Tanzhuo Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.757535

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

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