Geoscience Reference
In-Depth Information
Certain observations confirm these figures. In Spain, rubefaction is
distinct for the Riss-Würm interglacial period or older (Vaudour 1969).
In Australia, the red soils on sandy aeolian deposits seem to be older
than 100,000 years (Mee
et al.
2004). But many other data reveal more
rapid development. In the French Jura, red soils are present on moraines
of Würm age (Bresson 1976; Jouaffre
et al
. 1991). It is the same in the
Rhine valley: in Alsace, the red soils occupy a vast Würmian terrace
situated on the left bank, between Bale and Colmar (Marocke 1963;
Mettauer 1967, 1969). Near Rome, at the foot of the Vico volcano, an
Alisol Gleyic and Rhodic has been formed on a flow slightly less than
100,000 years old (Lulli
et al
. 1988). As pointed out in Chapter 8 (§ 8.4.3),
these inconsistencies are because of the climate (weathering demands a
certain minimum quantity of water) as well as the texture (weathering
starts more slowly if the particles are coarse or very coarse to begin
with).
The question is raised whether rubefaction still happens today in
the Mediterranean environment. Some authors have answered in the
affirmative (Lamouroux 1971). However, in southern France (Languedoc)
and in Spain it is seen that the red coating of the landscape is
undergoing degradation over millions of ha. In these regions, it survives
only on strips corresponding to exceptionally stable high shelves. The
landscape is not red overall, while it would have been so according to
the hypothesis that the rocks will directly yield a coloured soil. The
recent colluvia generated by erosion are brown. All said, red soils are
of course typical, but are rare in the landscape. Even if rubefaction were
current, it certainly is very slow. Anthropic erosion proceeds faster and
appears to erase it completely. Besides, the last interglacial (Riss-Würm)
period could have been slightly warmer than the present and thus had
favoured rubefaction…
Present state of rubefaction
9.2.2
Role of Parent Material and Climate
Red soils are found on parent materials that contain iron and in a
situation where this iron is released (process favoured by a humid
season) but not evacuated and is concentrated in place (favoured by a dry
season). Examination of the solubility curves of iron (Chap. 3, Fig. 3.7)
shows that this precipitation of iron is facilitated by a pH between 6
Generalities
