CHAPTER 5 WEATHERING AND SOIL

1. CHAPTER 5 WEATHERING AND SOIL

2. Weathering and Soil • Weathering - the disintegration and decomposition of rock at or near the earth's surface. • Mass wasting - the transfer of rock material downslope under the influence of gravity. • Erosion - the incorporation and transportation of material by mobile agents such as water, wind, or ice. • Weathering occurs when rock is mechanicallyfragmented or chemicallyaltered at or near the earth's surface.

3. There are Two Types of Weathering • Mechanical or Physical Weathering • Chemical Weathering

4. Mechanical weathering • Rock broken into smaller pieces - aids further breakdown by chemical weathering • Frost wedging • Unloading • Thermal expansion • Biological activity

5. Mechanical Weathering increases the available area for Chemical Weathering

6. Mechanical Weathering increases the available area for Chemical Weathering

7. Mechanical weathering • Rock broken into smaller pieces - aids further breakdown by chemical weathering . • Frost wedging • Alternate freezing and thawing. • Water expands about 9% when it freezes. • Most pronounced in mountainous regions in middle latitudes. • Debris collects in talus slopes

8. Physical Weathering

9. Frost Wedging and Talus Slope

10. Mechanical weathering Unloading • Rock bodies exposed by uplift and erosion expand. • Concentric slabs of rock break loose as unloading occurs • Sheeting or exfoliation - fracturing parallel to topography. • Exfoliation domes such as Half Dome in Yosemite • Joints - vertical fractures formed by expansion, contraction during cooling or tectonic forces. May be several generations

11. Exfoliation

12. Joints

13. Mechanical weathering • Thermal expansion • Results from daily cycle of temperature change. • Most prominent in hot deserts. • Biological activity • Plants, burrowing animals, etc. • Roots of plants wedge rocks apart

14. Root wedging

15. Chemical weathering • Rock chemically decomposes into materials that are stable in surface environments. • The work of naturally occurring acids • Dissolution • Oxidation • Hydrolysis silicate + carbonic acid --> clay + solution

16. Dissolution of Halite

17. Chemical weathering • Dissolution • Mineral attacked by polar molecules or weak acids. • Ions go into solution. • Examples are halite and limestone (calcite).

18. Chemical weathering Oxidation • Minerals combine with oxygen to form oxides. • Water speeds the oxidation reactions. • Examples are the formation of iron oxides (e.g., hematite) and hydroxides (e.g., limonite) from the iron in iron-rich rocks such as olivine, pyroxene, and hornblende. Often seen as a staining or metallic sheen on the surfaces of rocks with these minerals.

19. Oxidation Iron reacts with oxygen to form iron oxide as seen on these rusted barrels.

20. Chemical weathering Hydrolysis • Mineral reacts with water or stronger acids such as carbonic acid. • Most silicates chemically weather by the process of hydrolysis. • Clays (e.g., kaolinite) are the primary products of the reaction. • Remaining ions go into solution. • Ions and silica in solution are carried away by ground water to eventually precipitate in pore spaces or be carried to the ocean. • Quartz is very resistant to chemical weathering.

21. Chemical weathering Example: 2KAlSi3O8 + 2(H+ + HCO3-) + H2O --> orthoclase + carbonic acid + water --> Al2Si2O5(OH)4 + 2K+ + 2HCO3- + 4SiO2 kaolinite + potassium ion + bicarbonate ion + silica silicate + carbonic acid --> clay + solution

22. Chemical weathering Spheroidal weathering • Jointing + chemical weathering • Rounding of corners (maximum exposure)

23. Spheroidal Weathering

24. Factors influencing rates of weathering Rock characteristics • Mineral composition, solubility • Fractures, bedding planes, voids • Rates of chemical weathering of minerals from least resistant to most resistant follow Bowen's reaction series i.e., those which form at high temperature decompose fastest:

25. Mineral Stabilities(Look familiar?)

27. Factors influencing rates of weathering • Climate • Warm temperatures and abundant moisture promote chemical weathering. • Climates with freeze and thaw promote mechanical weathering. • Topography

28. An examination of headstones reveals the rate of chemical weathering on diverse rock types. The granite headstone (left) was erected four years before the marble headstone (right). The inscription date of 1872 on the marble monument is nearly illegible.

29. End Chapter 5