Geoscience Reference
In-Depth Information
In equatorial Africa, the soil profiles are nearly at equilibrium. Long
periods of time will be necessary before their morphology changes.
On the contrary, in sub-Saharan Africa and in Amazonia, soil profiles
have been subjected to stronger erosion. Their morphology can change
visibly in 50,000 or 100,000 years. Naturally, human activity can set off
still more severe erosion, but that is another question.
The measurement of age by means of radioisotopes further refines the
results obtained from weathering balances. Some lateritic profiles of
Amazonia have an age of 65-70 million years (end Cretaceous). The
laterites of Burkina Faso are almost 50 million years old. They date to
the beginning of the Tertiary, more precisely to the Eocene (Henocque
et al
. 1998). Their dismantlement might have started 400,000 years ago
(Chabaux
et al.
2003).
Radioactive measurements
The palaeomagnetic method also has confirmed the above results. The
method consists of measuring the directions of magnetic orientation of
oxides in the horizons that have not been disturbed and comparing these
directions with the reference time-scale based on geomagnetic polarity.
The rate of descent of the weathering front is estimated to be 11.3
±
0.5
m/My in French Guiana (Theveniaut and Freyssinet 1999).
Palaeomagnetic method
We are certain that: pedogenesis in tropical soils has lasted several tens
of millions of years and involved weathering of rocks over thicknesses
of the order of a hundred metres. The rate of descent of the saprolite
can be estimated to be 10-20 m per million years. “Soil collapse', that is
reduction of thickness of sediments is almost three-fold this in the total
profile, but is as high as nine times in the surface layers (Boulangé
et
al
. 1993). At the surface, the lowering of relief is linked to establishment
of the weathering profile and is of the order of 70 m in Amazonia
(Dubroeucq
et al.
1991).
Summary
5.4 ECOLOGY AND CLASSIFICATION
5.4.1 General Ecology of Ferralsols
Role of climate
Schematically, Africa comprises the following natural environments
going south from the centre of the continent (Leprun 1977; Bertrand
1998):
