Desert varnish is but one of more than a dozen rock coatings (Table 1) that drastically alter the appearance of rock surfaces. The better term is rock varnish because this coating occurs in virtually all environments, including alpine, antarctic, arctic, desert, periglacial, stream, temperate, and tropical settings. This paper-thin accretion is characterized by extremely high concentrations, typically more than 10%, of manganese oxides that give it a characteristic black to dark brown appearance. Clay minerals, however, make up the bulk of rock varnish along with iron oxides.
There are four general explanations for how rock varnish accretes on top of rock surfaces, but all these models reject the old idea that the constituents of varnish derive from the underlying rock. The model that has not yet been falsified is the polygenetic model of rock varnish formation (see first series of photos). This explanation combines bacterial enhancement of manganese and iron with abiotic fixation of the manganese by clay minerals. The process starts with bacteria fixing manganese on cell walls. Wetting events dissolve manganese. The desert dust supplies interstratified clay minerals, and the dissolved manganese reprecipitates as nanometer-sized fragments of manganese oxides. These tiny minerals fit into the weathered edges of clays, tightly cementing clays to the rock surface. The effect is a highly layered texture at the micrometer and nanometer scales imposed both by the clay minerals and the cementing manganese-oxides.
The most exciting development in varnish research in the past several decades is the development of varnish microlaminations (VMLs). This tool is based on 12 years of detailed analyses of more than 10,000 microsedimentary basins by Tanzhuo Liu ( www.vmldating.com). Working in the deserts of Western North America, Liu found that black varnish layers correspond with wet events and developed calibrations for the late Quaternary and a separate calibration for the Holocene. The second photo illustrates just one rock varnish VML sequence from Death Valley, California, formed over the past 20,000 years.
Liu and other researchers in different settings are in the process of developing calibrations for other regions of the world. Although there are other dating methods that have been proposed for varnish, VML is the only method that has gone beyond the experimental stage. Subject to successful blind testing, the great power of this method is that it is now possible to determine the minimum age of exposure of the underlying rock surface. Liu and other researchers have applied VML to dating features such as alluvial fans, meteorite impact craters, faulting events, landslides, stone artifacts, and ancient petroglyphs.