Geoscience Reference
In-Depth Information
drainage waters). It thus represents the first stage of concentration of a
residue. It now constitutes the C horizon of the soil that is going to be
formed. The evolution may be depicted at this level by:
R Æ C
Then the transformation of the arènes to soil takes place. It very
often begins with a sort of decomposition of the material leading to the
formation of clay. At the same time, there are losses through drainage
and, thereby, a new concentration process. This can be written as:
C Æ B of weathering
But at the earth's surface, even the clay minerals are unstable and
can be degraded and might disappear. We shall see examples of this.
Only some minerals, still more resistant than these clay minerals, will
survive, for example quartz in the form of sand devoid of any other
substance. This corresponds to
B Æ E
Such transformations of horizons to others seem to be general. It well
explains the evolution of most soils. The sequence is depicted below
(Fig. 3.3).
Redistributions within the profile
R
C
B
E
A
Weathering
Fig. 3.3
General scheme of evolution of a chain of horizons by successive concentration
of residues.
In the figure, the solid arrows represent the transformations and
concentrations
in situ
. The decreasing size of the cubes signifies the
progressive losses of substances. Actually, the A horizon is the small
final residue of the entire evolution. Let us take an example. Consider
a cube of granite that is going to undergo each of the stages we have
described for the equatorial climate. We will end up getting a bauxite.
This is one of the strongest transformations known (Chap. 5). The loss of
material will be between 70 and 90 per cent by weight and the reduction
in volume could be as large as 90 per cent (Soler and Lasaga 2000).
The reverse arrow, dotted, denotes the redistributions from one
horizon to the other under the influence of gravity. We will see that
they are limited.
