Environmental Engineering Reference
In-Depth Information
soluble rocks. Millions of years are required to decompose rocks to depths of 100 to 150 ft.
Erosion and other forms of mass wasting, however, are continually reducing the profile
during formation.
The greatest depths of decomposition occur in tectonically stable areas. In unstable tec-
tonic areas, the weathered zone is thinner because topographic changes increase the ero-
sion activity. Glaciation removes decomposed materials, often leaving a fresh rock surface.
In glaciated areas, the geologic time span for decomposition has been relatively short, and
the depth of weathering is shallow. Examples are the basalt sills and dikes of Triassic age
in the glaciated northeastern United States, which form prominent ridges with very little
soil cover (see Figure 6.19) . To the south, the dikes, which have been exposed for millions
of years longer, have weathered to clayey soils more rapidly than the adjacent crystalline
rocks and form depressions in the general surface as a result of differential erosion.
Although the climate to the south is somewhat warmer, the precipitation is similar to that
of the northeast.
Rock Structure
Differential weathering reflects not only rock type, but also structure. Decomposition pro-
ceeds much more rapidly in strongly foliated or fractured rocks, and along fault zones,
than in sounder masses. The depth of decomposition can be extremely irregular in areas
of variable rock type and structural features as shown in Figure 6.72. Where granite
masses have been intruded into foliated metamorphic rocks, differential weathering pro-
duces resistant domes, termed monadnocks, as illustrated in the topographic map in
Figure 6.73.
Topography and Groundwater Depth
Topography influences the movement of water through materials and the rate of erosion
removing the products of decomposition. On steep slopes rainfall runs off instead of infil-
trating, and oxidation and reduction activity are much less severe than on moderately to
slightly inclined slopes. On flat slopes and in depressions that are almost continuously sat-
urated, oxidation, reduction, and leaching are only feebly active.
Ground surface
Fault
Residual soil
Resistant mass
Residual soil
Massive
Strongly foliated
FIGURE 6.72
Irregular depth of weathering as affected by rock type and structure in the southern Piedmont of the United
States. The rocks are predominantly gneisses and schists into which granite masses have been injected, as well
as basalt dikes. Decomposition often reaches 30m. (From Sowers, G. F., Proceedings of ASCE, J. Soil Mech. Found.
Eng. Div., 80, 1954. With permission.)
 
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