Geoscience Reference
In-Depth Information
The granulodynamic paths of the type represented are found in
young soils, especially those that contain easily fragmented minerals
such as the micas. It is seen that the fragmentation in the middle of a
particle, repeated two or three times, displaces the bulk of that particle
to the next size class. Thus fragmentation is a very effective factor in
textural change during weathering.
3.4.2 Argilluviation
A rg i l luv iat ion, wh ic h w i l l be st ud ied i n det a i l i n Ch apter 8, i s by def i n it ion
the movement of clay from a horizon that is depleted (horizon of the E
type) to a horizon that is enriched in clay (B horizon). A redistribution
of this sort is presumed only to affect the very fine particles and not
the coarser constituents (sands and silts). Therefore, the sand/silt ratio
is not altered. Consequently, in the textural triangle, the depleted E
horizon, the enriched B horizon and the original C horizon must all
be aligned on the same straight-line segment passing through the clay
corner, except for errors. In fact, this straight-line segment corresponds
to a constant sand/silt ratio (Fig. 3.16).
In nature, profiles are indeed observed having horizons with this
feature, but often with a small difference that will be examined later.
Clay
The same
thing in the
equilateral
textural triangle
Enriched B
C representing
the average
initial state
Depleted E
Establishment of the
fixed sand/silt ratio
Sand
Silt
Fig. 3.16
Theoretical position in the right-angled and equilateral textural triangles of the
horizons depleted or enriched by argilluviation.
To assume that only the fine particles are mobile means that
argilluviation can be modelled by a law of the reciprocal of the cube of
the radius, that is of 1/
r
3
, if
r
is the mean radius of the particle (when
r
increases by a very small amount, translocation is no longer possible).
We will see later why we introduce here this way of looking at the
phenomenon.
