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of alkaline-earth cations and strong biological activity
slow down weathering, while promoting the neoforma-
tion or the conservation of clays that are richer in silica. In
any climate, clay neoformation is more marked in basic
volcanic rocks than in acid crystalline rocks.
Tropical
red earth
Kaolinite and
iron oxides
Losses of silica
and cations
Black cracking
clay
Topography and drainage
Gains of silica
and cations
The effects of local factors mean that a wider range of
clay minerals occur in some climatic zones than would
be the case if the climate were the sole determinant of clay
formation. Take the case of tropical climates. Soils within
small areas of this climatic zone may contain a range of
clay minerals where two distinct leaching regimes sit side
by side. On sites where high rainfall and good drainage
promote fast flushing, both cations and silica are removed
and gibbsite forms. On sites where there is less rapid
flushing, but still enough to remove all cations and a little
silica, then kaolinite forms. For instance, the type of clay
formed in soils developed in basalts of Hawaii depends
upon mean annual rainfall, with smectite, kaolinite, and
bauxite forming a sequence along the gradient of low to
high rainfall. The same is true of clays formed on igneous
rocks in California, where the peak contents of different
clay minerals occur in the following order along a mois-
ture gradient: smectite, illite (only on acid igneous rocks),
kaolinite and halloysite, vermiculite, and gibbsite (Singer
1980). Similarly, in soils on islands of Indonesia, the clay
mineral formed depends on the degree of drainage: where
drainage is good, kaolinite forms; where it is poor, smec-
tite forms (Mohr and van Baren 1954; cf. Figure 6.5).
This last example serves to show the role played by land-
scape position, acting through its influence on drainage,
on clay mineral formation. Comparable effects of topog-
raphy on clay formation in oxisols have been found in
soils formed on basalt on the central plateau of Brazil
(Curi and Franzmeier 1984).
Smectite
Figure 6.5 Clay types in a typical tropical toposequence.
Source: Adapted from Ollier and Pain (1986, 141)
of tropical climates. More generally, the extent of chem-
ical weathering is correlated with the age of continental
surfaces (Kronberg and Nesbitt 1981). In regions where
chemical weathering has acted without interruption,
even if at a variable rate, since the start of the Cenozoic
era, advanced and extreme weathering products are com-
monly found. In some regions, glaciation, volcanism, and
alluviation have reset the chemical weathering 'clock' by
creating fresh rock debris. Soils less than 3 million years
old, which display signs of incipient and intermediate
weathering, are common in these areas. In view of these
complicating factors, and the changes of climate that have
occurred even during the Holocene epoch, claims that
weathering crusts of recent origin (recent in the sense
that they are still forming and have been subject to cli-
matic conditions similar to present climatic conditions
during their formation) are related to climate must be
looked at guardedly.
WEATHERING AND HUMANS
Age
Limestone weathers faster in urban environments than
in surrounding rural areas. Archibald Geikie established
this fact in his study of the weathering of gravestones in
Edinburgh and its environs. Recent studies of weather-
ing rates on marble gravestones in and around Durham,
England, give rates of 2 microns per year in a rural site and
Time is a further factor that obscures the direct climatic
impact on weathering. Ferrallitization, for example,
results from prolonged leaching. Its association with the
tropics is partly attributable to the antiquity of many
tropical landscapes rather than to the unique properties
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