Rock Surface and Weathering: Process and Form

Rock Surface and Weathering: Process and Form

Weathering processes involve a range of physical, chemical and biological processes that generally weaken and break down rock materials, although some may, in the short term at least, strengthen rocks through, for example, the development of surface weathering crusts or rinds (Day, 1980; Conca and Rossman, 1982, 1985; Robinson and Williams, 1987). Although it is possible to identify distinct weathering processes, in most natural environments rocks are attacked simultaneously by more than one process often acting synergistically.

Weathering processes are fundamental to geomorphology. Occuring in situ, they release compounds in solution and prepare rock materials for removal and transportation by erosion and are, in themselves, directly responsible for the creation of characteristic landform features and landform evolution. Weathering processes are fundamental also for the creation of soils, determining many of their physical and chemical properties. Because weathering influences soil fertility, slope stability and the durability of engineering structures and building materials, it is a multidisciplinary field of study and there are varied approaches to research and the advancement of knowledge (e.g. Birkeland, 1984; Ollier, 1984; Ollier and Pain, 1996; Bland and Rolls, 1998; Whalley and Turkington, 2001; Lee and Fookes, 2005).

This review focuses on the weathering processes that act on exposed rock surfaces, on the retreat of rock surfaces and on the evolution and development of the resulting surface weathering features. The emphasis is on surface and immediate sub-surface change rather than on the geochemical alteration of minerals at depth as a result of chemical disequilibria between the conditions under which many minerals are formed and those encountered at Earth's surface (see Chapter 16).

Early interest in rock surface weathering focused on the description and documentation of weathering features, and to a lesser extent the products of weathering. This was particularly widespread from the early 1800s when scientists began to explore and map areas of the world very different from their home environments (Turkington and Paradise, 2005). Description of weathering features was accompanied by speculation as to their origin, often through deductive reasoning from field evidence. This remains a significant approach in weathering research, especially for the explanation of large-scale weathering features and landforms (Twidale, 1982; Young and Young, 1992; Robinson and Williams, 1994).

Attempts to understand the origin of features led to experimental simulations of weathering designed to test the efficacy of suggested processes. Salt, often very visible on the surface of disintegrating rocks, was the first, with the simulation of salt weathering attempted in 1828, followed later by tests of insolation and wetting and drying (Turkington and Paradise, 2005). Experimental weathering has become a major research field, greatly aided by continually improving environmental control and monitoring equipment linked to greatly enhanced tools for studying the results. For example, the application of electron microscopy has helped to advance understanding of the operation of biological weathering processes (Moses and Smith, 1993; Viles, 1995; Seaward, 1997; Robinson and Williams, 2000a; Carter and Viles, 2004, 2005). Significant developments in experimental weathering have resulted from studies of the durability of building stones (Schaffer, 1932; Smith et al., 2008) as well as geomorphology. Partly as a result of this, but also because of the distinctive suites of landforms often associated with them, weathering research has tended to focus on a limited range of rocks. Limestones, sandstones, gritstones and granites, have all been studied intensively, shales, schists and some other igneous rocks rather more rarely whilst many other rock types have been largely ignored.

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