Geology Reference
In-Depth Information
(Folk 1993, 1994; Folk, Noble, Gelato and McClean, 1995) are exceedingly important in the
weathering of all rocks, including granite (Pedersen, 1997).
Erosional offloading is widely cited as another cause of physical breakdown, resulting in sheet
fractures and sheet structure, but the arguments against it are several, and have been outlined in
Chapter 2.
3.3
CHEMICAL ALTERATION
There is some suggestion that chemical reactions take place at grain boundaries in dry conditions,
but infiltration by moisture and gases produces pronounced and widespread alteration by such
processes as oxidation, reduction, carbonation, solution, hydration and hydrolysis. Because of its
molecular structure, water is an ideal solvent. No other liquid can dissolve such a variety and vol-
ume of solutes. It has been claimed that solution is essential to chemical weathering, not only
because of its widespread direct effects, for all minerals are soluble to some extent, but also
because it prepares crystal structures for further reactions. Some workers emphasise solution, oth-
ers consider hydrolysis to be the most effective of these water-related processes, and yet others
favour hydration. Suffice is to say that all play their part and that all may be especially important
in particular circumstances. Hydration implies dissociation of water and the release of hydrogen
ions. Because of their high energy and small ionic radius they are active in substitution and they
readily enter and disrupt crystal lattices. Plant roots assist by concentrating hydrogen ions. On the
other hand, alkaline conditions are prevalent in arid and semi-arid regions like the Australian inte-
rior, and silicate minerals and quartz, which are important constituents of granitic rocks, are, as
Joly (1901) showed a century or so ago, more reactive to alkaline than to neutral solutions.
In general terms, a granitic rock consisting of quartz, a potassium feldspar, some plagioclase and
a mica is first broken down to a granite sand or grus (or fine gravel - see Chapter 5) consisting of
fragments of quartz and feldspar. Some workers differentiate between mechanically disintegrated
grus and chemically altered growan, but there is a gradation between granite sand containing only
slightly altered minerals, and the gritty clay that is the common end-product of the weathering of
granite; it is difficult to distinguish between the two types of weathering product in the field. The
quartz persists for a long time, suffering only slow dissolution, but eventually it is too dissolved
because at some sites quartz fragments present in some horizons are absent from those above.
Also, as is described in Chapter 10, siliceous speleothems are deposited in open sheeting fractures
and other apertures, demonstrating that silica must go into solution before being reprecipitated.
Much, perhaps most, of the silica is derived from the breakdown of silicates such as the feldspars,
but siliceous speleothems are also found in openings in sandstone in many parts of the world and
notably in the Roraima Plateau of Venezuela, showing that quartz goes into solution.
Water reacts with the mica and feldspars to produce clays. The character of the clay varies with
local and regional circumstances (such as whether the system is open or closed, whether the pro-
file is well drained or not) so that if potash resulting from the hydration of orthoclase or microcline
remains in the system, illite is produced, but if it is evacuated kaolinite is formed (McFarlane and
Heydemann, 1984). Most commonly, the weathering of granite through contact with water results
in the formation of kaolinite.
3.4
THE COURSE OF WEATHERING IN GRANITE
Near the land surface in contact with soil, the differential weathering of the various constituents of
granite, and in particular the preferential weathering of feldspar and mica, leaves crystals of
quartz, and phenocrysts of orthoclase and microcline, in microrelief, producing a pitted surface
(
Fig. 3.5).
Such surfaces denote recent exposure, though how recent is not known and it may well
vary from one environment to another. The extent, and particularly the depth, of such recent expo-
sures as indicated by pitting, is in some places surprising. For example, the sidewalls of a valley
