Chapter 8 Summary

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

8.1 Mechanical Weathering

Rocks weather when they are exposed to surface conditions. In most cases, conditions at Earth’s surface are very different from the conditions under which the rocks formed. Mechanical weathering processes include exfoliation, freeze-thaw, salt crystallization, and the wedging effects of plant growth.

8.2 Chemical Weathering

Chemical weathering takes place when minerals within rocks are not chemically stable in their existing environment. Chemical weathering processes include 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.

8.3 Controls on Weathering Processes and Rates

Chemical weathering is faster when temperatures are warmer and moisture is present. Physical weathering is more important in regions with frequent freeze-thaw cycles. Weathering rates can depend on the abundance oxygen and carbon, and will vary with the mineral composition of a rock. Weathering accelerates weathering by exposing more surface area to chemical reactions.

8.4 Weathering and Erosion Produce Sediments

Quartz grains are one of main products of weathering and erosion because quartz is resistant to chemical and mechanical weathering. Clay minerals, iron oxide and iron hydroxide minerals, aluminum hydroxide minerals, and ions in solution are common products of chemical weathering. Particles produced by weathering can be described in terms of their composition, grain size, sorting, rounding, and sphericity.

8.5 Weathering and 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.

8.6 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.

8.7 Weathering and Climate Change

The geological component of the carbon cycle affects Earth’s climate over the long term by changing atmospheric carbon dioxide levels. Carbon is added to the atmosphere during volcanic eruptions. It is extracted from the atmosphere when silicate minerals are weathered, and when it is transformed into organic matter by plants. Organic matter can be stored in soil, permafrost, and rocks. Burning of fossil fuels involves moving carbon from geological reservoirs to the atmosphere on timescales much faster than the geological carbon cycle operates.

Review Questions

1. What must happen to a body of rock before exfoliation can occur?
2. Saskatchewan’s climate is consistently cold in the winter and consistently warm in the summer. What times of year would frost wedging to be an important weathering mechanism?
3. What are the products of the hydrolysis of the feldspar albite (NaAlSi₃O₈)?
4. Oxidation weathering of  pyrite (FeS₂) can lead to acid rock drainage (ARD). What are the environmental impacts of ARD?
5. Imagine that you and a friend encounter an old graveyard on a walk through the forest. You see a granite tombstone with the date 1705 carved into it. The tombstone next to it is too badly weathered to read the date. Your friend looks at the badly weathered stone and declares, “Look how weathered this stone is! It must be from way before 1705.” Do you agree?
6. Many sand deposits are dominated by quartz, with very little feldspar. What weathering and erosion conditions are required to get feldspar-rich sand?
7. What ultimately happens to most of the clay that forms during the hydrolysis of silicate minerals?
8. Why are the slope and the parent materials important factors in soil formation?
9. Which soil constituents move downward to produce the B-horizon of a soil?
10. What are the main processes that lead to the erosion of soils in Canada?
11. Where in Canada would you expect to find a chernozemic soil? What characteristics of this region produce this type of soil?
12. Where are brunisolic soils found in Saskatchewan?
13. 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?