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are dilated (incorporation of carbon from the air, hydrogen from water,
reduction in density by living organisms…). But under the influence
of weathering, depletion continues inexorably. S
oil collapse
, mentioned
above, soon intervenes in the B horizons as well, although they are the
seats of different kinds of accumulations (Fig. 3.5).
3.2
MECHANISMS OF EVOLUTION OF SOILS
There are different ways of categorizing the mechanisms operating in
soils. Simonson (1959) distinguished four principal ones:
transformations,
removals
,
translocations
and
additions
. We shall follow that author,
but introduce details in the concept of transformation. Minerals are
transformed through
fragmentation
,
hydration
(e.g. hydration of micas),
hydrolysis
(hydrolysis of feldspars),
oxidation
(modification of the forms
of iron—Chap. 12) and simple
solubilization
(e.g. carbonates—Chap. 7).
3.2.1 Transformation by Fragmentation
We should remember that by multiplying the specific surface area of
the rock exposed to biological and atmospheric agents, fragmentation
accelerates chemical weathering. It is however useful to fight against
two received ideas.
Does fragmentation related to negative temperatures always involve
water as the agent of bursting? No, for we should also calculate the
expansion and the difference in temperature that appears in a boulder
between its surface and the interior, when it freezes in the night and is
warmed in the morning sun. This was shown by work conducted at the
CNRS in Caen (Lautridou 1978). Theoretically, the strain caused is not
enough to burst the rock. In reality, and also in laboratory experiments,
calcareous materials do crack! Actually, the effect of fatigue is created
if the alternations of temperature occur thousands of times. But as
comminution proceeds, it happens that the debris size becomes too small
to create a large difference in temperature between the exterior and the
interior of the fragments because of thermal conductivity. Consequently
the debris end up being protected by their small size. The limiting size
depends on the nature of the rock and also on the characteristics of
the climate. This explains why in a given scree, we observe debris of
relatively uniform dimensions (cf. § 2.4.2).
Does physical weathering always precede chemical weathering?
No, because the two phenomena are linked. For example, before the
weathering of granite, the micas at the periphery are dissolved, which
causes disintegration of the rock (Robert 1970). In this case, disintegration
is mostly of chemical origin!
