SOIL FORMATION - Soil Ecology - page 155

the parent materials has been considered separately as pedochemical weathering because
of the considerable influence of the biomass in the production of, inter alia, complexing
agents, substances that attack clay minerals or form organo-mineral complexes.
However, biological weathering influences may extend well into the underlying regolith
(Graham et al., 1994), although increasingly attenuated with depth.
While physical weathering is often considered separately from chemical weathering, in
reality they operate together, often in a synergistic way (see, for example, Ugolini, 1986b).
Weathering involves the simultaneous activities of a range of processes including
physical fragmentation, inorganic chemical processes (hydrolysis, oxidation, hydration
and dissolution) and biologically-mediated processes ( e.g., acidolysis and acido-
complexolysis). The weathering processes predominating at a site are determined by
climatic, biological and lithological factors and the degree of evolution of the soil
(Jenny, 1980).
In all weathering systems, water plays dominant physical and chemical roles. In the
solid phase it is a major agent of landscape sculpting and transport while as a liquid,
it is also an important agent for the diffusion and transport of materials. It is a potent
medium of physical disruption through volume change, both as a consequence of phase
change and through involvement in hydration and related reactions. Chemically, it is
an effective solvent, a component of many reactions and of neoformed products, and
an important buffering agent (Chesworth, 1992). Little chemical weathering occurs in very
dry and frozen environments.

3.1.1 PHYSICAL FRAGMENTATION

Physical fragmentation of the parent rock is linked to the development of stresses inside
the rock or individual minerals. These initially lead to the development of micro-fissures,
then to systems of fissures and extensive fracturing (Robert and Tessier, 1992) thereby
opening the materials to chemical weathering processes. Fragmentation may result
from a range of mechanisms although the relative contributions of chemical and
physical processes are still a matter of some debate (Ugolini, 1986b).
Water plays a major role in physical weathering both directly through abrasion and
in transporting the products of breakdown. Past glaciation, in particular, has been broadly
effective in modifying landscapes and producing till which is a common parent material
in some parts of the world. In such cold, arid environments as Antarctica, the efficiency
of fluid water-based physical weathering is substantially limited by the scarcity of water
and the brief period during the year when it is available for transport and other processes
(Campbell and Claridge, 1987).
On freezing, water increases in volume by approximately 9 % and when it penetrates
rocks and subsequently freezes, sufficient pressure may be exerted to fracture rocks along
lines of weakness. The effects of freezing and thawing differ between rock types but are
most effective where this boundary is crossed frequently and in rocks that contain
considerable water. In certain rocks, such effects result in the production of considerable
silt but also clay-sized materials (Ugolini, 1986b). Legros (1992) reported that the freeze:
thaw boundary is crossed ca. 165 times per year at an elevation of 2500 m in the French
Alps and leads to the fragmentation of exposed rocks and scree formation.