The topics covered in this chapter can be summarized as follows:
|10.1 Mechanical Weathering
|Rocks weather when they are exposed to surface conditions, which in most case are quite different from those at which they formed. The main processes of mechanical weathering include exfoliation, freeze-thaw, salt crystallization, and the effects of plant growth.
|10.2 Chemical Weathering
|Chemical weathering takes place when minerals within rocks are not stable in their existing environment. Some of the important chemical weathering processes are hydrolysis of silicate minerals to form clay minerals, oxidation of iron in silicate and other minerals to form iron oxide minerals, and dissolution of calcite.
|10.3 Soil Formation
|Soil is a mixture of fine mineral fragments (including quartz and clay minerals), organic matter, and empty spaces that may be partially filled with water. Soil formation is controlled by climate (especially temperature and humidity), the nature of the parent material, the slope (because soil can’t accumulate on steep slopes), and the amount of time available. Typical soils have layers called horizons which form because of differences in the conditions with depth.
|10.4 The Soils of Canada
|Canada has a range of soil types related to our unique conditions. The main types of soil form in forested and grassland regions, but there are extensive wetlands in Canada that produce organic soils, and large areas where soil development is poor because of cold conditions.
|10.5 Clay Minerals
|Clay minerals are sheet silicates, made up of tetrahedral and octahedral layers with differing arrangements. They form from the weathering and hydrothermal alteration of other silicate minerals. Clay minerals are soft and weak, and so a mass of clay is typically malleable and can be formed into useful shapes. Clays are almost always fine grained, and they attract ions in solution, and so are important to agriculture and pollution control. A mass of clay is also relatively impermeable.
Answers for the review questions can be found in Appendix 1.
- What has to happen to a body of rock before exfoliation can take place?
- The climate of central British Columbia is consistently cold in the winter and consistently warm in the summer. At what times of year would you expect frost wedging to be most effective?
- What are the likely products of the hydrolysis of the feldspar albite (NaAlSi3O8)?
- Oxidation weathering of the sulphide mineral pyrite (FeS2) can lead to development of acid rock drainage (ARD). What are some of the environmental implications of ARD?
- Most sand deposits are dominated by quartz, with very little feldspar. Under what weathering and erosion conditions would you expect to find feldspar-rich sand?
- What ultimately happens to most of the clay that forms during the hydrolysis of silicate minerals?
- Why are the slope (relief) and the parent materials important factors in soil formation?
- Which soil constituents move downward to produce the B horizon of a soil?
- What are the main processes that lead to the erosion of soils in Canada?
- Where in Canada would you expect to find a chernozemic soil? What characteristics of this region produce this type of soil?
- What is the main component of an octahedral layer in a sheet silicate? What about a tetrahedral layer?
- How does the presence of clay minerals within a rock contribute to slope failure?
- Why are clay minerals effective at absorbing cations, and why might this be important to pollution control?