Page 179 - Fundamentals of Geomorphology

162 PROCESS AND FORM

of alkaline-earth cations and strong biological activity
Tropical
slow down weathering, while promoting the neoforma- red earth
tion or the conservation of clays that are richer in silica. In
Kaolinite
and
any climate, clay neoformation is more marked in basic Kaolinite and
iron
oxides
iron oxides
volcanic rocks than in acid crystalline rocks.
Losses of silica
Losses of silica
and cations
and cations
Black cracking
Topography and drainage clay
Gains of silica
Gains of silica
cations
and
The effects of local factors mean that a wider range of and cations
clay minerals occur in some climatic zones than would Smectite
Smectite
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 Figure 6.5 Clay types in a typical tropical toposequence.
clay minerals where two distinct leaching regimes sit side Source: Adapted from Ollier and Pain (1986, 141)
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 of tropical climates. More generally, the extent of chem-
flushing, but still enough to remove all cations and a little ical weathering is correlated with the age of continental
silica, then kaolinite forms. For instance, the type of clay surfaces (Kronberg and Nesbitt 1981). In regions where
formed in soils developed in basalts of Hawaii depends chemical weathering has acted without interruption,
upon mean annual rainfall, with smectite, kaolinite, and even if at a variable rate, since the start of the Cenozoic
bauxite forming a sequence along the gradient of low to era, advanced and extreme weathering products are com-
high rainfall.The same is true of clays formed on igneous monlyfound. Insomeregions,glaciation,volcanism,and
rocks in California, where the peak contents of different alluviation have reset the chemical weathering ‘clock’ by
clay minerals occur in the following order along a mois- creating fresh rock debris. Soils less than 3 million years
ture gradient: smectite, illite (only on acid igneous rocks), old, which display signs of incipient and intermediate
kaolinite and halloysite, vermiculite, and gibbsite (Singer weathering, are common in these areas. In view of these
1980). Similarly, in soils on islands of Indonesia, the clay complicatingfactors,andthechangesofclimatethathave
mineral formed depends on the degree of drainage: where occurred even during the Holocene epoch, claims that
drainage is good, kaolinite forms; where it is poor, smec- weathering crusts of recent origin (recent in the sense
tite forms (Mohr and van Baren 1954; cf. Figure 6.5). that they are still forming and have been subject to cli-
This last example serves to show the role played by land- matic conditions similar to present climatic conditions
scape position, acting through its influence on drainage, during their formation) are related to climate must be
on clay mineral formation. Comparable effects of topog- looked at guardedly.
raphy on clay formation in oxisols have been found in
soils formed on basalt on the central plateau of Brazil
(Curi and Franzmeier 1984).
WEATHERING AND HUMANS
Age
Limestone weathers faster in urban environments than
Time is a further factor that obscures the direct climatic in surrounding rural areas. Archibald Geikie established
impact on weathering. Ferrallitization, for example, this fact in his study of the weathering of gravestones in
results from prolonged leaching. Its association with the Edinburgh and its environs. Recent studies of weather-
tropics is partly attributable to the antiquity of many ing rates on marble gravestones in and around Durham,
tropical landscapes rather than to the unique properties England,giveratesof2micronsperyearinaruralsiteand