Page 177 - Fundamentals of Geomorphology

160 PROCESS AND FORM

( ) Chemical ( ) Chemical and mechanical
a
c
200
combined
Strong
200
150
Strong
chemical
Moderate
100 weathering
(cm) 50 Weak 150
precipitation 0 –20 –10 0 10 20 30 weathering Moderate
Moderate
chemical
chemical
with
annual 200 ( ) Mechanical 100 mechanical frost action weathering
b
Mean 150 weathering mechanical
Weak 50 Strong weathering
100 Moderate
Strong Moderate Slight mechanical weathering
weathering
Very slight
50 Absent or
insignificant
0 0
–20 –10 0 10 20 30 –20 –10 0 10 20 30
Mean annual temperature (°C)

Figure 6.3 Louis Peltier’s scheme relating chemical and mechanical weathering rates to temperature and rainfall.
Source: Adapted from Peltier (1950)

2 Moderate leaching tends to flush cations from the aluminium compounds, alkaline earths, and alkaline
regolith, leaving a surplus of silica. Under these cations being flushed out in preference to silica.
conditions, the processes of monosiallitization or
kaolinization form 1 : 1 clays, such as kaolinite and Weathering patterns
goethite.
3 Intense leaching leaves very few bases unflushed Given that the neoformation of clay minerals is strongly
from the regolith, and hydrolysis is total, whereas it is influenced by the leaching regime of the regolith, it
only partial in bisiallitization and monosiallitization. is not surprising that different climatic zones nurture
Under these conditions, the process of allitization distinct types of weathering and weathering crust. Sev-
(also termed soluviation, ferrallitization, laterization, eral researchers have attempted to identify zonal pat-
and latosolization) produces aluminium hydroxides terns in weathering (e.g. Chernyakhovsky et al. 1976;
such as gibbsite. Duchaufour 1982). One scheme, which extends Georges
Pedro’swork,recognizessixweatheringzones(Figure6.4)
Soil water charged with organic acids complicates the (Thomas 1994):
association of clay minerals with leaching regimes.
Organic-acid-rich waters lead to cheluviation, a process 1 The allitization zone coincides with the intense
associated with podzolization in soils, which leads to leaching regimes of the humid tropics and is