Geology Reference
In-Depth Information
( ) Chemical
a
( ) Chemical and mechanical
combined
c
200
Strong
200
150
Strong
chemical
weathering
Moderate
100
150
50
Weak
Moderate
chemical
weathering
with
frost action
0
Moderate
chemical
weathering
-20 -10
0
10
20
30
100
( ) Mechanical
b
200
150
Weak
50
100
Very slight
weathering
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
regolith, leaving a surplus of silica. Under these
conditions, the processes of monosiallitization or
kaolinization form 1 : 1 clays, such as kaolinite and
goethite.
aluminium compounds, alkaline earths, and alkaline
cations being flushed out in preference to silica.
Weathering patterns
3
Intense leaching
leaves very few bases unflushed
from the regolith, and hydrolysis is total, whereas it is
only partial in bisiallitization and monosiallitization.
Under these conditions, the process of
allitization
(also termed soluviation, ferrallitization, laterization,
and latosolization) produces aluminium hydroxides
such as gibbsite.
Given that the neoformation of clay minerals is strongly
influenced by the leaching regime of the regolith, it
is not surprising that different climatic zones nurture
distinct types of weathering and weathering crust. Sev-
eral researchers have attempted to identify
zonal pat-
terns in weathering
(e.g. Chernyakhovsky
et al
. 1976;
Duchaufour 1982). One scheme, which extends Georges
Pedro's work, recognizes six weathering zones (Figure 6.4)
(Thomas 1994):
Soil water charged with organic acids complicates the
association of clay minerals with leaching regimes.
Organic-acid-rich waters lead to cheluviation, a process
associated with
podzolization
in soils, which leads to
1
The
allitization zone
coincides with the intense
leaching
regimes
of
the
humid
tropics
and
is
