Environmental Engineering Reference
In-Depth Information
FIGURE 6.80
Granite boulder 10 m in diameter at base of slope (Rio-Santos Highway, Brazil).
Quartz-Poor Mafic Rocks
In general, the quartz-poor mafic rocks develop a weathering profile, also typical of many
metamorphic rocks, which is characterized by four zones:
1.
An upper zone of residual soils that are predominantly clays with small amounts
of organic matter (equivalent to the A and B horizons of pedological soils, see
Section 7.8.1)
2.
An intermediate zone of residual soil, predominantly clayey, but with decompo-
sition less advanced than in the upper zone
3.
A saprolite zone in which relict rock structure is evident and the materials are
only partially decomposed, which grades to a weathered rock zone
4.
The weathered rock zone where rock has only begun alteration
Clay soils are the product of decomposition. The clay type is related strongly to the rain-
fall and drainage environment (Grim, 1962) as follows:
Low rainfall or poor drainage; montmorillonite forms as magnesium remains.
●
High rainfall and good drainage; kaolinite forms as magnesium is removed.
●
Hot climates, primarily wet but with dry periods; humic acids are lacking, silica
is dissolved and carried away, and iron and aluminum are concentrated near the
surface (laterization).
●
Cold, wet climates; potent humic acids remove aluminum and iron and concen-
trate silica near the surface (silcrete).
●
A decomposition profile in basaltic rocks in a warm, moist climate is given in
Figure
and annual rainfall of about 50 in. (1300 mm), most of which falls between September and
April. These climatic conditions favor the maximum development of the porous clays (see
Section 10.5.2),
and produce some laterites. Laterite development is stronger to the north
of the area where rainfall exceeds 60 in. (1500 mm) annually and temperatures are higher
on an average than 80
o
F (26
o
C)
(Section 7.7.2).
