Rock-Mass Characteristics - Geotechnical Engineering Investigation - page 467

TABLE 6.11
Climate vs. Rock Decay

Climate Weathering Activity

Cold, dry Strong mechanical Freezing temperatures cause rock breakup

weathering

Cool, wet Moderate chemical Organic material decays to produce active humic

weathering, some acid to react with the parent rock and cause decay

mechanical

Hot, wet Strong chemical High moisture and high temperatures cause rapid

weathering decay of organic material and an abundance of

humic acid to cause rapid rock decay

Continuously wet Some chemical Water movement is downward, causing leaching by

weathering removal of soluble salts and other minerals

Alternating wet and dry Retarded Prevailing water movement may be upward during

the dry period, concentrating and fixing oxides and
hydroxides of iron (laterization), which eventually
results in a barrier preventing downward movement
of water and retarding decay below the laterized
zone (see Section 7.7.2). In predominantly dry
climates evaporites such as caliche form (see
Duricrusts, Section 7.7.2)

Hot, dry Very slight Chemical and mechanical activity is very low

FIGURE 6.68
Diagram of climatic boundaries of regions and intensity of various types of weathering. (After Fookes, P. G.
et al., Quart. Eng. Geol ., 4, 1972.)

Mineral Stability
The relative stability of the common rock-forming minerals is given in Table 6.12. Clay
minerals are usually the end result, unless the parent mineral is stable or soluble.
Quartz is the most stable mineral; minerals with intermediate stability, given in decreas-
ing order of stability, are muscovite, orthoclase feldspar, amphibolite, pyroxene, and pla-
gioclase. Olivine is the least stable, rapidly undergoing decomposition.