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In other words, when half-saturation is reached (1- W = 0.5), the
aggressiveness of the soil solution does not exceed 0.5 3 = 0.125, all other
things being equal. Thus it is not surprising that the solutions would
mostly be unsaturated and non-aggressive as well.
Attainment of equilibrium is slow when different phases are involved.
According to Durand (1980b), the equilibration of soil water with the
atmosphere requires 24 hours.
7.3.4 Dissolution Shapes/Forms
Let us now examine the dissolution of CaCO 3 from the scale of the
landscape to that of the electron-microscope.
Water of course dissolves carbonate rocks, but it is not the sole agent
responsible for the phenomena of karstification and the appearance of
caves, despite what is often stated! The cavities seen in limestone terrain
have often originated from geological faults that water exploits, corrodes
and scoops out. This is why most of these caves have, beyond the
chambers and galleries that compose them, an overall linear structure.
On the surface too fractures play a large role in channelling the water
and initiating the dissolution.
In the tropical environment, the solubility of carbonates is such that
karst forms are rare. Where they occur, the forms are pure marvels of
nature: turrets of the Along bay in Vietnam, of the Stone Forest (Shilin)
in China…
Karstification of landscapes
3 from the fine fractions
We know that dissolution is a function of the ratio surface-area/mass.
The largest particles are thus the most resistant (Chap. 3, § 3.4.3).
In the mountains, under forest, in an environment undisturbed by
erosion, the following distribution of carbonates was seen in a profile
of fine earth on the way to acidification (Bottner and Paquet 1972): at
the base of the soil, carbonates are found in all the fractions; in the
middle, they survive only in the sands; at the top they no longer exist.
Most often, only the coarse particles are calcareous. They are rounded,
for example in ancient tills (Fig. 7.9).
Disappearance of CaCO
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