Chapter 5 Summary

The topics covered in this chapter can be summarized as follows:

5.1 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.
5.2 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.
5.3 The Products of Weathering and Erosion The main products of weathering and erosion are grains of quartz (because quartz is resistant to chemical weathering), clay minerals, iron oxide minerals, rock fragments, and a wide range of ions in solution.
5.4 Weathering and the Formation of Soil 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.
5.5 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.
5.6 Weathering and Climate Change The geological carbon cycle plays a critical role in balancing Earth’s climate. Carbon is released to the atmosphere during volcanic eruptions. Carbon is extracted from the atmosphere during weathering of silicate minerals and this is eventually stored in the ocean and in sediments. Atmospheric carbon is also transferred to organic matter and some of that is later stored in soil, permafrost, and rocks. Our use of geologically stored carbon (fossil fuels) upsets this climate balance.

Questions for Review

  1. What has to happen to a body of rock before exfoliation can take place?
  2. The climate of central B.C. 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?
  3. What are the likely products of the hydrolysis of the feldspar albite (NaAlSi3O8)?
  4. Oxidation weathering of the sulphide mineral pyrite (FeS2) can lead to development of acid rock drainage (ARD). What are the environmental implications of ARD?
  5. 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?
  6. What ultimately happens to most of the clay that forms during the hydrolysis of silicate minerals?
  7. Why are the slope and the parent materials important factors in soil formation?
  8. Which soil constituents move downward to produce the B horizon of a soil?
  9. What are the main processes that lead to the erosion of soils in Canada?
  10. Where in Canada would you expect to find a chernozemic soil? What characteristics of this region produce this type of soil?
  11. Where are luvisolic soils found in B.C.?
  12. Why does weathering of silicate minerals, especially feldspar, lead to consumption of atmospheric carbon dioxide? What eventually happens to the carbon that is involved in that process?