Geoscience Reference
In-Depth Information
De-andosolization corresponds to the partial mineralization of organic
matter. According to Sedov
et al
. (2003) clay appears through relative
concentration (2/1 clay mineral) or through precipitation (halloysite or
kaolinite). The structure becomes coarse polyhedral. Iron is released
giving brown colour and sometimes oxidation-reduction mottles
(redoximorphic features). In some cases the clay can migrate as indicated
by localized coatings visible under the microscope. The soils progress
to Cambisols and sometimes to Luvisols.
When the Andosol is thick, the deeper horizons are largely removed
from the influence of organic matter and drainage is less. Amorphous
materials provided by weathering are often very visible there. These are
yellow isotropic substances coating the coarse elements and filling up
the voids. They are easily transformed to halloysite (Parfitt and Wilson
1985; Quantin 1992; Dubroeucq
et al
. 1998).
De-andosolization
10.3.2 Descent of Weathering Fronts
Except where perturbation brings up to the surface fresh pyroclastic
materials, the surface horizons of Andosols are more weathered than
the B horizons as various indices show (Ndayiragije and Delvaux 2003;
van Ranst
et al
. 2008). This conforms to the theory we presented in
Chapter 3.
On the island of Madeira, Weijden and Pacheco (2003) calculated
an average
annual chemical denudation rate
of 37 ± 12 g m
-2
y
-1
based on
the composition of spring waters. All said, this represents a penetration
of 12 m per million years, a figure comparable to that of soils already
discussed (tropical, red, calcareous soils…). But Andosols are young
(a few thousands of years) so that the descent of the weathering fronts
mostly goes unnoticed in their case.
10.3.3
Andosols In Space
At the scale of climatosequences in volcanic soils, Andosols are well
represented in high-altitude areas (cold, humid climate favourable for
accumulation of carbon compounds). On the contrary, at the footslopes
andosolization does not take place and weathering leads directly to
clayey soils, for example, Ferralsols, Vertisols, Cambisols, etc. This is seen
on the Equator in Nicaragua, Mount Cameroon, etc. (Lamouroux 1972;
Role of climate
