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￿ concentration of a weathering residue (of decarbonatation in this
case) that becomes the soil,
￿ correlative descent of the interface between the unaltered rock
and the weathered rock ( weathering front ) and also descent of the
level of the soil surface.
Concentration of a
decarbonated residue from
the loess
1
Progressive
descent of
the weathering
front of the
unaltered rock
(1, 2, 3 and 4)
and differentiation of
soils in the residue
Cambisol
A
Luvisol
Planosol
A
E
C
Bt
2
3
4
R
Bg
R
R
Evolution over time
Fig. 3.2
Evolution over time of a loess and of the corresponding soil.
These ideas are commonly used by geologists, geomorphologists and
specialists in weathering (Tardy 1969) and also by some pedologists
working in the intertropical zone (Lucas 1989; Boulet et al. 1993). Also,
it is universally accepted that several weathering fronts are liable to
be successively involved in the same material that is being weathered
(regolith). For example, a decarbonatation front, progressing downward,
is followed by a front of kaolinitization of the residual minerals. On the
contrary, these ideas have scarcely entered topics on pedology. However,
we can consider that the weathering fronts, in our field, are nothing but
the boundaries of the horizons of the profiles. This is more evident in the
intertropical environment because the regoliths are very thick and old,
but it is probably valid, at least partly, for the horizons of all soils.
We shall now explain the concepts clearly.
Consider a rock, for example granite. This material, weathered at
the geological timescale, gives an arène by separation and rather
pronounced weathering of the crystals it contains. The arène has already
lost substances (dissolution of certain constituents and export in the
Concentration phenomena
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