Description
Desert varnish is a hard, dark-brown or black, dull or lustrous (if wind polished) coating that accumulates on the exposed surfaces of rock fragments and outcrops. Its composition is independent of the composition of the host rock (Potter and Rossman [1]). It is common on gravel plains, especially on alluvial fans with desert pavement. Tone, intensity, and color of the varnish depend mostly on climate variations (e.g., precipitation) and on the relative age of the exposure of the rock surface. Layers of varnish are relatively thin, commonly 0.005 to 0.5 mm thick, but thickness varies from locality to locality as well as over the surface of a single stone. Varnish gives a similar spectral reflectance signature, or appearance, to rocks of different composition and origin. It develops best on resistant, coarse-grained materials whose surfaces do not disintegrate too rapidly, such as granite, sandstone, basalt and other volcanic rocks, and many metamorphic rocks. Limestones that are not silicified generally do not support a varnish because the surfaces dissolve faster than the varnish can develop. Because varnish is thought to form slowly (over decades), its presence indicates stability of the plains surfaces.
Theories of varnish formation and rates of its development are controversial (Whalley [2]). Varnish commonly forms on rocks that contain little or no iron and manganese, indicating that these elements are carried to the rock surface by wind, water, or both. Further development can take place by the biological action of manganese-oxidizing microorganisms. The development rate is, however, relatively slow, i.e., measured in years, decades, and millennia. One item of interest associated with the lore of desert varnish is that British fuel cans left in the eastern Sahara since World War II have been found that have a well developed layer of varnish (not rust!) on their upwind sides (Eastes, et al. [3]). In general, about 70% of a varnish coat consists of clay minerals. The remainder is composed of oxides of iron (Fe) and manganese (Mn). Variations in the ratios of these two oxides, probably due to variations in climate during their accumulation, cause variations in varnish color, ranging from dark bluish-black (Mn rich) to dark brownish-black (Fe rich). Whether or not we can reliably differentiate among varnishes of various composition by field and laboratory spectral measurements in the reflected solar spectrum (0.4-2.5 micra), and subsequently do so via multiband and hyperspectral imagery remains to be determined. Complicating the issue from a remote sensing point of view, is the fact that the overall darkness of a varnished desert pavement is also a function of the spacing of the fragments in the pavement. Another characteristic of rock fragments in desert pavements is that their undersides are reddened ("rubified") by bright red clays and iron hydroxides derived from the soils in which they lie.
The dark tones of desert varnish, which can be seen in association with gravel plains in air photos and some Landsat images, generally indicate a firm, stable, and trafficable surface. Except for areas of steep slopes in swales and incised drainages, or of roughness due to boulders, these surfaces can support vehicular traffic--not because of the varnish, but because of the nature of the desert pavement associated with it. Generally, surfaces stable enough to develop varnish also have developed an accretion mantle of fine sand and silt, which immediately underlies the surface layer of varnished rocks or pebbles (see Summary Sheet for Desert Pavement). Repeated passes or vehicles tracking each other can disrupt the surface and expose the underlying dust, producing fines. In some cases, a single pass of a vehicle can leave a track that persists for many years, because overturned rocks leave a briqht trail among the varnished rocks.
(common names are in bold) Desert lacquer, patina, Wustenlack, vernis desertique, patine du desert, weathering rinds (some)
1. Potter, R.M., and G.R. Rossman. 1979. The manganese and iron-oxide mineralogy of desert varnish. Chemical Geology, v. 25, pp. 79-94.
2. Whalley, W.B. 1983. Desert varnish. In Chemical sediments and geomorphology, edited by A.S. Goudie and K. Pye. San Diego: Academic Press, pp. 197-226.
3. Eastes, J.W., P.P. Hearn, Jr., C.S. Breed, and J.F. McCauley, 1988. Weathering of a metal artifact in a Saharan environment: evidence of a novel form of desert varnish. Applied Spectroscopy, v. 42, no. 5, pp. 827-831.