1. What are the most important forms of mechanical weathering?  How does each process result in breakdown of rock?

2. What processes besides chemical weathering produce soil profiles?

3. Describe each of the following weathering processes using an example with balanced equations:

hydrolysis of K-feldspar

oxidation of pyrite

dissolution of halite

dissolution of calcite

hydration of anhydrite

4. What are the defining characteristics of each of the following soil horizons?

O, A, B, C, K, E

5. How does soil pH vary with depth?

What controls pH of soils?

6. What are the chemical and structural characteristics of the following clay minerals?

kaolinite, illite, smectite, vermiculite, chlorite, gibbsite

7. What is the relationship between climate, weathering rate and occurrence of different types of clay minerals?

8. What controls the CEC of clay minerals? How are CEC and climate related?

9. Describe the soil profile of a typical spodosol. How are aluminum and iron transported in spodosols?  Describe the relationship between mobility of Fe and Al species and soil pH.

10. How can the isotope characteristics of soil carbonate in ancient soils be used to assess climate and climate change?

Name:____________________

Please answer 5 of 7 questions. Note that questions contain multiple parts. Each question is worth 20 points.

1. In a study of igneous rock weathering in a humid rain forest setting, the weathered residual soil forming on a basalt parent rock was found to be rich in ferric hydroxide (goethite) and aluminum hydroxide (gibbsite), but was poor in silica. Chemical analysis of the weathered residual soil on a granite showed that iron and aluminum hydroxides were abundant, but that SiO₂ was also common in the residual soil.

a. What factors account for this difference?

b. What residual soil-forming minerals would you expect to find in the humid rain forest setting if the parent rock was a pure CaCO₃ limestone? Why?

2. The chemical weathering of silicate minerals consumes acidity in the soil environment.

a. What are the sources of natural acids that promote silicate mineral weathering?

b. Why are the areas in the Adirondacks that are the most prone to watershed acidification underlain by thin soils developed on granitic bedrock?

3. a. What soil environmental factors control whether iron will exist in ferrous or ferric form?

b. Why do we sometimes find that the E-horizon of spodosols contains very little iron oxide or hydroxide, whereas the B-horizon is often strongly enriched in ferric oxides and hydroxides?

4. Farmer Selleck, who lives in the Piedmont region of Georgia (elevation 600’, climate warm and wet) is interested in developing a new field suitable for growing corn, which has a high potassium demand. The upper terrace level on the property is presently a pine forest developed on deeply weathered granite while the lower terrace is hardwood forest developed on moderately weathered alluvial sediment. XRD patterns of clay mineral separates from the upper terrace soil reveal only 7 A reflections. The lower terrace soil clay separates produce 7, 10 and 16 A reflections.

a. Which of the terraces would likely be better for development of a new cornfield? Why?

b. Explain how factors such as climate, forest type and parent material) might account for the differences in the clay mineralogy of the two sites.

5. The graph below illustrates the relationship between the depth to the CaCO₃ – bearing horizon and the carbon isotope composition of the CaCO₃. The samples were taken from well-developed soil profiles in a east-west transect from central Ohio to eastern Colorado, approximately parallel to Interstate 70.

Denver – Lima – Burlington –Topeka-Kansas City -- St. Louis -- Vandalia --- Columbus

5 (cont’)

a. How can you explain the systematic east-to-west variation in depth to the carbonate-cemented horizon?

b. How can you explain the relationship between depth and carbon isotope values of the calcite? Note that the carbon isotope values form two discrete populations.

6. In general, chemically weathered igneous rocks are richer in iron and aluminum and poorer in sodium, potassium and calcium compared to unweathered rock.

a. What compounds tend to contain aluminum and iron in weathered materials?

b. What is the fate of sodium, potassium and calcium?

7. Soils in arid and semi-arid regions often contain a near-surface accumulation of minerals that are usually considered soluble in more humid climate settings.

a. Discuss the factors that control the accumulation of soluble salts (e.g. calcite, gypsum, halite) in soils.

b. Why is the accumulation of salts often related to irrigation of soil for agricultural purposes?