Geology Reference
In-Depth Information
3
Weathering
3.1
DEFINITION AND SIGNIFICANCE
Weathering may be defined as the disintegration or decay of rocks
in situ
and in the range
of ambient temperatures found at and near the Earth's surface (Winkler, 1965). Some weathering
processes are physical or mechanical and result in the break down or fragmentation of the rock.
Others are chemical and involve the alteration of one or more of the constituent minerals. Biota
also make important contributions to both types of weathering, and indeed, several processes, of
various kinds, commonly work together to produce a weathered mantle or regolith, which with the
addition of organic materials becomes a soil. The lower limit of significant or detectable weathering
is called the weathering front.
Weathering is an essential precursor to erosion: without preliminary weathering of the rock,
there would be little erosion. Some weathering processes, however, result not in the weakening of
the rock but rather in its cementation and induration, through the development of concentrations
of minerals called duricrusts of which laterite, ferricrete, bauxite, silcrete, calcrete and gypcrete
are well-known and widely developed examples. None, however, is peculiar to granitic terrains.
On the other hand, many of the minerals forming the duricrusts are allochthonous, being trans-
ported in groundwaters, in rivers, or on the wind, so that, combined with locally derived contribu-
tions, laterite and silcrete, for example, are well represented in granite landscapes. Where the
duricrusted surface is dissected, plateau forms are characteristic (Fig. 1.1h). Where intact, the
duricrust forms a protective carapace, as for example on northern Eyre Peninsula, South Australia,
where the rolling granite plain carries a veneer of calcrete which has not only stabilised the sur-
face but has induced a weak karst.
3.2
PHYSICAL DISINTEGRATION
Earlier investigators set great store by physical processes, and temporal variations in insolation,
involving alternations of heating and cooling (Griggs, 1936), for example, were widely held respon-
sible for granular disintegration and for flaking and spalling (widely referred to as exfoliation) in
granite. The argument is plausible. Granite consists of minerals of different colours with different
coefficients of heating and therefore of expansion. The stresses generated by alternations of heat-
ing and cooling were considered sufficient to cause fragmentation. In addition, rocks are poor con-
ductors of heat, so that rocks exposed to the Sun would be heated and would expand, whereas
deeper sectors would not, and it was considered that as a result the outer skin would separate from
the inner, resulting in flaking and spalling. It has long been realised that granite expands on heat-
ing. Indeed, this knowledge was applied in quarrying in ancient Egypt and in India. Also, the
intense, though ephemeral, heat of forest or bush fires unquestionably causes flaking of exposed
surfaces (
Fig. 3.1)
;
and the heat generated in nuclear explosions has similar effects.
