Fryar notes, GLY 110

(Jay Ghosh (TA) lectured while Fryar was at Association of Engineering Geologists' meeting in Colorado.)

Starting Ch. 6—Soils and weathering

1. (Diagram of pedosphere relative to other Earth spheres [Merritts et al., 1998])

2. What is soil? Depends on whom you ask: engineer, geologist, soil scientist, or farmer

Our definition: an internally organized, natural body of weathered minerals and organic matter (humus) capable of supporting life

Soil forms by weathering of regolith—fragmented rock

3. (Diagram of showing composition of a typical soil by volume [Fig. 6.1])

4. Soil-forming factors—

Climate—amount of water, temperature, wind, etc.

Organic activity—examples: plants, burrowing animals (such as earthworms)

Relief—how steep are slopes? (Soils are more likely to accumulate in flat areas)

Parent material—what type of rock or sediments are soils developed on?

Time—soils tend to form slowly

5. Weathering—

Physical disintegration and chemical decomposition of Earth materials at or near land surface

Physical weathering dominates in dry areas, chemical weathering in humid areas

Compare with erosion: removal and transport of weathered and unweathered material by wind, water, or ice (glaciers)

Construction analogy: weathering is like blasting, erosion is like a dump truck that hauls material away

6. (Diagram showing how surface area increases as grain-size decreases during physical weathering [Merritts et al., 1998])

7. Types of physical weathering—

Frost wedging (freeze-thaw damage)—what does this do to pavement?


Root wedging (showed Fig. 6.2)

8. Exfoliation as a special case of physical weathering—rocks exposed by erosion expand and crack
(showed photo from Merritts et al. [1998])

9. (Diagram of chemical weathering [Merritts et al., 1998])

10. More about chemical weathering—

General form of weathering reaction:
primary (rock-forming) minerals + acids + oxygen --> sediments + dissolved ions

One example of solution weathering—carbonation:
common in Kentucky and other places with limestone; forms sinkholes and caves
CaCO3 (calcite—makes up limestone) + H2CO3 (carbonic acid—like club soda) --> Ca2+ + 2HCO3-

11. (Satellite photo of sinkhole lakes in central Florida [Merritts et al., 1998])

12. (Photo from inside of Owl Cave, KY, looking out)

13. Other types of weathering reactions—

Oxidation: transfer of electrons (example—iron rusting)
Fe-mineral + O2 --> Fe oxide or hydroxide, such as hematite (Fe2O3)—makes soils red

Hydrolysis: H2CO3 + silicate minerals --> clay minerals (like kaolinite [potter’s clay] or vermiculite)
(clay minerals consist of microscopic layers; clays tend to hold water well and crack when dry)

14. (Electron microscope image of kaolinite—crystal looks like book [Merritts et al., 1998])

15. (Diagram of soil horizons [Fig. 6.8])

16. (Diagram of how soil profiles can vary with climate [Merritts et al., 1998])

17. (Photo of David Butler, former geology grad student, sampling soil on the Ohio River flood plain)

18. (Photo of Ohio River flood-plain soil from different depths)

19. (Photo of residual soil on limestone bedrock, Mammoth Cave N.P.)

20. (Photo of transported soil with caliche, High Plains, Texas)