rock coatings

rock coatings Approximately one-sixth of the Earth's land surface consists of ‘bare’ rock surfaces. Yet the true mineral faces of the rock are rarely seen. Instead, a variety of paper-thin accretions coat rocks in all terrestrial environments. Since weathering includes the formation of new compounds that are more in equilibrium with the environment at and near the Earth's surface, rock coatings fall under the interdisciplinary field of weathering. Intellectual curiosity about the physical and chemical characteristics, origin, geography, and utility of these encrustations has spawned over 3000 scientific papers. A wide variety of rock coatings is found on landforms at the Earth's surface.

Rock varnish

Rock varnish is the most studied rock coating. Although it is found in all terrestrial settings, it is most easily recognized on the plentiful bare rocks seen in deserts (hence another common name is ‘desert varnish’). Also, the physical and chemical stability of rock surfaces in deserts allows sufficient time for this slow-growing accretion to form. It is structured much like a brick wall, but its thickness (ranging from less than 5 micrometres (5 μm) to almost 500 μm) is typically less than 30 μm. The clay materials that comprise the bulk of the structure (about 50 to 70 per cent) are cemented to the rock by hydroxides of manganese and iron (15–50 per cent). The enigma of varnish is its great enrichment in manganese, typically over 50 times compared with the adjacent environment (soils, underlying rock, dust).

The debate over the origin of rock varnish revolves around the source of the manganese and the mechanisms of its enrichment. Until the late 1970s and the more common use of scanning electron microscopy, the majority view was that the manganese and the varnish were derived from the underlying rock. Now, investigators see a clear morphological varnish– rock boundary, together with other evidence that shows varnish to be an accretion. Although most investigators now agree on an external origin, there are still two hypotheses on the mechanism of manganese concentration. The abiotic hypothesis holds the small changes in pH can concentrate manganese by geochemical processes. The biotic hypothesis holds that bacteria, and perhaps other micro-organisms, concentrate manganese—an idea supported by culturing experiments and direct observations of bacterial enhancement of manganese.

Silica glaze

The term ‘silica glaze’ covers a broad category of rock coatings that are dominated by amorphous silica with variable amounts of aluminium and iron. They are usually less than 200 μm thick, with a clear white to orange shiny lustre, but they can be darker in appearance. Silica glazes have been noted in warm deserts, in cold deserts like Antarctica, on dry tropical islands, along tropical rivers, in mid-latitude humid temperature settings, and in various archaeological contexts. Silica glazes probably precipitate from soluble Al–Si complexes [Al(OSi(OH)₃)²⁺] that are released from the weathering of phyllosilicate minerals (e.g. micas).

Iron films

Rusty-coloured rocks are readily recognized as ‘iron oxides’ and are often readily dismissed once such a label has been given. Once iron films are examined in detail, they display a wide variety of characteristics, and different types of iron films occur in very different environmental circumstances. For example, subaerial rock surfaces in hyper-arid deserts host iron films characterized by clay minerals cemented with about 10 per cent iron. Subaerial dolerite rock surfaces in the Dry Valleys of Antarctica are rimmed by iron oxyhydroxides that form both an accretion and a weathering rind over a millimetre thick. Rocks in acid streams in Arctic and alpine settings are often impregnated with iron hydroxides that can physically separate pieces of the rock, much like salt or frost weathering.

Other rock coatings

Other rock coatings include biofilms, which are organic coatings, such as lichens, moss, fungi cyanobacteria, and algae. Carbonate skins are composed primarily of carbonate, usually calcium carbonate, but the carbonate is sometimes combined with magnesium. They can occur as tufas, caliche exposed by rock spalling, or even as coatings on urban buildings. Case-hardening agents represent the addition of cementing agents to rock-matrix material; the agent may be manganese, sulphate, carbonate, silica, iron, oxalate, organisms, or anthropogenic. Dust films are a light powder of clay- and silt-sized particles that adhere to rough surfaces and rock fractures. Heavy metal skins form on rocks downstream from mine tailings; they can have high concentrations of copper, lead, zinc, manganese, iron, cadmium, mercury, and other heavy metals. Nitrate crusts of potassium and calcium nitrate are most often found in caves and rock shelters in limestone areas.

Oxalate crusts are typically composed of calcium oxalate and silica with variable concentrations of magnesium, aluminium, potassium, phosphorus, sulphur, barium, and manganese. They are often less than a millimetre thick and can be found forming near or alongside lichens. Phosphate skins are comprised of various phosphate minerals (e.g. iron phosphates or apatite) that are mixed with clays and sometimes manganese. Polish films form where aeolian and glacial abrasion produce micrometre-scale films that reflect the composition of the underlying rock. Sulphate skins are the superposition of sulphates (e.g. barite, gypsum) on rocks; these are distinguished from gypsum crusts in the soils literature.

Applications

Geomorphologists study rock coatings in part because of the possibility of using them as indicators of time or environmental change. Two developments stand out. The first is the radiocarbon dating of organic matter trapped under rock coatings such as silica glaze; the organic compounds may be directly underneath the coating or encapsulated in the weathering rind of the host rock. Radiocarbon dating of organic compounds trapped by rock coatings has been used experimentally in the dating of such features as alluvial fans, colluvium, and landslides, as well as archaeological rock engravings. Another development is the study of microlaminations; various different layers in rock varnish have been used to correlate rock surfaces in Death Valley (California) and Tunisia (North Africa) with dry and wet episodes of climatic change.

Ronald I. Dorn

Bibliography

Dorn, R. I. (1998) Rock coating. Elsevier, Amsterdam.
Konhauser, K. O.,, Fyfe, W. S.,, Schultze-Lam, S.,, Ferris, F. G.,, and and Beveridge, T. J. (1994) Iron phosphate precipitation by epilithic microbial biofilms in Arctic Canada. Canadian Journal of Earth Science, 31, 1320–4.
Liu, T. and and Dorn, R. I. (1996) Understanding spatial variability in environmental changes in drylands with rock varnish microlaminations. Annals of the Association of American Geographers, 86, 187–212.
Robinson, D. A. and and Williams, R. B. G. (1992) Sandstone weathering in the High Atlas, Morocco. Zeitschrift für Geomorphologie, 36, 413–29.