ABSTRACT
Weathering is the chemical and physical degradation of rock, causing it to lose coherence or chemical identity. It is commonly a preliminary to, but by definition does not include, erosion which is the physical removal or displacement of material. Chemical agents of weathering (water, acids, salts, oxygen) do not necessarily act independently of physical agents (thermal changes, including freezing and melting of water, penetration by plant roots). Both may be accompanied by erosion processes, driven by wave action, stream flow, wind, heavy rainfall or gravitational instability of slopes. However, particularly in the case of chemical weathering, the residua may not erode but remain in situ as weathered rock (such as laterite). The vulnerability of a rock to erosion is referred to as its erodibility. This increases with weathering, but it is not a specific rock property as it depends on the erosivity (effectiveness) of eroding influences to which it is subjected and land, slope, vegetation and soil cover. Although these concepts are not reliably quantified, in principle with a suitable choice of units, erosion will occur if the product of the erosivity of an eroding agent and the erodibility of a target rock exceeds a specified value. However, these concepts are not readily applied to large-scale erosion processes, such as landslides, for which a general term mass wasting is used (Section 17.5).
