WEATHERING 581 WEATHERING W B Whalley and P A Warke, Queen’s University Belfast, Belfast, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction Weathering can be defined as the irreversible structural and/or mineralogical breakdown of rock through the cumulative effects of chemical, physical, and biological processes operating at or near the Earth’s surface (Table 1) However, this seemingly straightforward definition masks the complexity of rock weathering, in which interactions between the many different components of the weathering system give rise to an element of unpredictability that is characteristic of non-linear systems The weathering behaviour of rock is a response to subaerial (Earth surface) conditions, where temperatures and pressures differ from those under which the minerals were formed Consequently, adjustment to surface environments is manifest through rock breakdown, the rate of which is controlled by many factors: characteristics of the rock itself, the availability of weathering agents such as salt and moisture, biological agents such as lichens, and, especially, the microclimatic environment to which the rock is exposed Without weathering and, in particular, the breakdown of one mineral type into another, there would be no soils of any significance and little scope for widespread development of flora and fauna on land Thus, long-term weathering is of paramount importance to the biosphere and is a crucial element of both long-term and short-term landscape development Nonequilibrium Conditions and the Lithological Cycle – General Significance The lithological cycle provides a useful starting point when considering the role of weathering in landscape development Erosion is generally preceded by a combination of weathering processes, which are usually crucial in the formation of silt, clays, and resultant solutes and in the release of residual components of crustal materials As rock approaches the Earth’s surface, either through tectonic uplift or through erosion of the overburden, associated changes in pressure and/or temperature mean that it is no longer in a state of equilibrium In this context, ‘surface’ should be taken to include the range of locations where the hydrosphere, atmosphere, and biosphere interact with the lithosphere; the maximum extension of these interactions is ca 100 m (e.g where there is tropical deep weathering), although it is normally much less than this Differences in pressure and temperature at or near the Earth’s surface give rise to important (intrinsic) aspects of rock breakdown that can create or reinforce positive-feedback conditions in weathering systems Thermodynamically and chemically different conditions at the Earth’s surface can destabilize minerals, thus increasing their susceptibility to subaerial weathering processes Changing chemical and thermodynamic conditions may be accompanied by volume changes resulting from decreased overburden pressures, which lead to differential stresses that are realized as discontinuities at various scales, from joints and cracks to microcracks Volume changes can also occur when one mineral is altered to another When the rock arrives at the Earth’s surface, the interplay of hydrosphere, atmosphere, and biosphere provides more complexity, although, for the most part, we can reduce this to a small number of extrinsic factors, namely water (in all three phases), temperature (usually between 40 C and ỵ40 C at or near the Earth’s surface), and biotic activity (which depends on water and temperature) The interplay of these factors has had important consequences for long-term global climate change as well as landscape development, as discussed below Joints, Cracks, and Microcracks One major result of the uplift of rocks to the Earth’s surface is that, as pressure decreases, there is a volumetric expansion The most significant way in which this manifests is through the creation of crack systems, from joints to microcracks The intersection of joints, which can be many metres in length and depth, can substantially weaken a mass of rock This gives rise to the concept of ‘rock-mass strength’ (as opposed to the ‘intact strength’ of small blocks) Therefore the ‘strength’ of the rock on a face in a quarry (rock mass) will differ from the crushing strength of the aggregate (intact strength) At a smaller scale, microcracks are plentiful in many rocks; they are usually a few micrometres wide and perhaps a few centimetres long