Geoscience Reference
In-Depth Information
and concentration of salts. For example, the middle of the profile is drier
and the salts tend to concentrate there. In such a system, horizons are
bound to be born. They will be horizontal.
We could also devote ourselves to a comparable exercise with
temperature. In sum, the horizons are born at the boundary between the
materials and atmosphere. Such as oxidation films, which are parallel
to the interface.
3.1.2 Concept of Weathering Fronts
We shall present here the unifying concept that serves as framework
for the entire topic in its description of pedogenesis. At this stage, for
lack of specific demonstration, it resembles a postulate. It is only the
reading of the different chapters in the second part that will convince
the reader of the relevance of the approach through its ability to explain
the differentiation of profiles in nature.
Let us take an example. We start off with a loess comprising 50 per cent
calcium carbonate (CaCO
3
), 30 per cent quartz and 20 per cent mica that
is easily weathered to clay. Calcium carbonate is a rather soluble salt.
With time, in about ten thousand years, it disappears from a thickness
of several metres of the material. All that remains is a decarbonated
clayey sand residue. The loss of matter is thus 50 per cent. Then the
system continues to evolve. The loess continues its decarbonatation
downwards while the residue, which has become soil, starts its internal
differentiation. The clay appears to abandon the upper part to be
concentrated in the middle part and forms a Bt horizon (= textural
B or
argic
= clay-rich). We shall explain later (Chap. 8) how this is
possible. The material is then in the
Luvisol
stage. The clay enrichment
in the B continues. The rainwater penetrates more and more slowly and
circulates laterally over the top of this horizon. The process depletes the
surface A and E horizons in which remains just a sand cleaned of all
other materials. The degraded soil stage is reached, also called
Planosol
with a Bg (= B
with gleyic properties
) as shown in Figure 3.2. The official
terminology of the
World Reference Base
is shown here in italics. We shall
re-examine it in detail in Chapter 4.
In addition to the specific example, the diagram shows that soil
formation involves the following phenomena:
Highlighting the phenomena
weathering of a considerable thickness of the rock and loss of
substances,
