Agriculture Reference
In-Depth Information
Insolation and fire are other temperature-related factors involved in rock fragmentation
through the differential expansion of rock constituents and layers. Disruptive pressures
may also build up through volume increases consequent on the weathering of such
minerals as biotite and plagioclase to clays (Birkeland, 1984). Volume increases in rock
constituents may also result from such salt weathering effects as crystallisation, changes
in the level of hydration and thermal expansion, and through chemical reactions such as
the formation of iron oxides (see, for example, Robert and Delmas, 1984).
Wind erosion is highly effective in desert environments through both particle trans-
port and abrasive detachment, that is a 'sand-blasting' effect. In the northern polar desert
and the cold desert of Antarctica, this is evidenced by the preferential erosion of the sides
of boulders facing the prevailing winds (ventifacts) and the patterns of transport and
sorting of surrounding finer particles (Campbell and Claridge, 1987). It is also evidenced
by the widespread formation in these areas of lagged surfaces (desert pavements) in which
a layer of small hard rocks forms a protective cover over the finer underlying materials.
Measured rates of wind erosion on exposed rocks are of the order of 1 mm per year
(Ugolini, 1986b).
Fungal hyphae are capable of penetrating intact rock surfaces thereby accelerating
micro-division (Robert and Berthelin, 1986). Lichens are common inhabitants of rock
surfaces in many environments and secrete chelating and other agents which affect rock
surfaces. In a study of the lichen:rock surface interface of a volcanic rock, Adamo and
Violante (1991) recorded the extensive physical disintegration of the rock surface with
separation of surface particles and widespread etching of the mineral grains.
They also found a mixture of rock-forming minerals, clay minerals and poorly ordered
alumino-silicates thus demonstrating the close interactions between physical and
chemical weathering. More complex communities of bacteria, algae and fungi exist
in lithic communities (Vincent, 1988) although their roles in rock weathering remain
to be defined.
Roots and burrowing soil animals may penetrate the matrix of the weaker and
chemically-weathered rocks and indurated layers. As an example of the latter situation,
Johnson
et al.
(1987) reported the widespread local disruption and surface deposition
of a dense 0.5m-thick calcrete (Bkm) horizon and the underlying C horizon through
the burrowing activities of wombats (Marsupalia: Vombatidae) in South Australia.
In stronger rocks, roots may penetrate and enlarge cracks and fissures thereby exposing
enlarged areas of reactive mineral surface to the actions of the major chemical weather-
ing agents
and water (Graham
et al.,
1994).
3.1.2
CHEMICAL PROCESSES
Chemical weathering results from the reactions that take place between primary and
secondary minerals and a weathering solution (Figure II.2). The primary minerals are
degraded, secondary minerals including clays and clay-humus complexes are formed,
some of the weathering solution is retained and a leachate is produced. Silica is exported
and new crystalline minerals may be precipitated from materials dissolved in both
the retained solution and the leachate through the process of neoformation (Pédro, 1979):
