Geoscience Reference
In-Depth Information
observed there often have the shape indicated by 'b' in the diagram.
They exhibit a strike slip of thickness
x
, which can be interpreted as
the thickness of the slab that was dissolved after the glacier retreated,
in the absence of protection on the upper surface. Also, some erratic
boulders abandoned by the retreating glaciers stay in place and protect
surfaces that are gradually highlighted in relief. The height
y
of the
erratic boulders is then interpreted as the thickness of the dissolved slab.
These empirical methods give results in agreement with the calculations
presented above. In Flaine, at about 2300 m,
x
is 12-15 cm, whereas
y
often has much smaller values (a few cm).
The rate of ablation on gypsum is much higher and is about 7
metres for 10,000 years at 2500-m altitude, in accordance with the greater
solubility of this mineral and its tendency to flake off from the surface
(Chardon 1996).
7.3.5 Weathering-Front Model
It is by taking the example of soils on loess that we introduced our
model of weathering fronts.
The surface of the unweathered rock at the bottom of soil profiles on
limestone moves downwards. The data and calculations presented above
identify a penetration of about 15 cm in 10,000 years, which makes 15
m over a million years. In spite of very different conditions, we again
find the order of magnitude determined for tropical soils on crystalline
rocks: of 10-20 m per million years (Chap. 5).
The penetration since the last glaciation is negligible. It is, on the
contrary, much greater when we consider the entire Quaternary and
still more on limestones of the Secondary Epoch brought up through
outcropping following uplift of the mountain ranges. Although
extrapolation to similar periods of time is very dangerous, it is sufficient
to demonstrate the power of weathering. This explains all sorts of
observations: penetration of canyons to several hundreds of metres,
cutting of large caves, appearance of ruiniform reliefs of large size,
formation of thick decarbonatation residues on plateaus…
7.4 ACCUMULATION PHENOMENA
7.4.1 Mechanism of Precipitation
Consider the reaction:
Ca
2
+
+ 2
HC
O
3
-
¤
CaCO
3
[
precipitated
] +
CO
2
+
H
2
O
The reaction proceeds to the right if
