Weathering and Erosion

1. Weathering and Erosion

2. Weathering • Changes that occur in sediments and rocks near Earth’s surface • Two main types: chemical and physical Chemical: minerals are chemically altered or dissolved Physical: fragmentation

3. Physical and chemical weathering go hand in hand

4. Factors Controlling Rates of Weathering

5. Chemical Weathering

6. Chemical Weathering • This process occurs because minerals formed deep in the Earth’s interior are not stable under surface conditions. • Stability is generally the reverse of Bowen’s reaction series. • The principle agent of chemical weathering is water. • Quartz: very stable • Feldspars: form clay minerals • Mafic minerals: alter to oxides

7. Controls on the rate of weathering Chemical stability: Measure of a substance’s ability to remain in a given chemical form rather than to react to form a different chemical substance. Two controls on chemical stability: solubility and rate of dissolution. Solubility: Amount of the mineral dissolved in water when the solution is saturated. Higher solubility = lower weathering stability. Rate of dissolution: The amount of mineral that dissolves in an unsaturated solution in a given amount of time.

8. Weathering and Making Coffee Water dissolves some of the solid, leavingbehind an altered material, …. ….and produces a solution containing substances drawn from the original solid Fig. 7.4

9. Etched and corroded feldspar in the soil zone Fig. 6.2 Fig. 7.2

10. Importance of fractures Fig. 7.5

11. Microscopic view of disintegration of granite Cracks form along crystal boundaries. Feldspar, biotite and magnetite start to decay, while quartz does not. The decay progresses, and as the crack opens, the rock weakens and disintegrates Fig. 7.3

12. Weatheredgranite

13. Carbon Dioxide Fig. 7.6

14. Bicarbonate reactions Fig. 7.6

15. Weathering reactions • Atmospheric carbon dioxide dissolves in rainwater: • CO2 (carbon dioxide) + H2O (water) --> H2CO3 (carbonic acid) • Weathering of silicates uses carbonic acid, which reduces the carbon dioxide level in the atmosphere: • 2KAlSi3O8 + 2H2CO3 + H20 --> Al2Si2O5(OH)4 + 4SiO2 + 2K+ + 2HCO3- • feldspar + carbonic acid + water --> kaolinite + silica + potassium ions + bicarbonate ions

16. Chemical weathering of carbonates • Carbonates weather fast, especially in the presence of acid. • Calcite acid + carbonic ions --> calcium + bicarbonate ions Weathered limestone in 2500-year-old Greek ruins in Italy (Fig. 7.7)

17. Formation of iron oxide

18. Weathering oxides provide color to the desert landscape Fig. 7.9

19. Mechanical Weathering

20. Some notes • Mechanical weathering = physical weathering. • Physical weathering is most prominent in arid regions (chemical weathering is minimal).

21. How Rocks Break • Natural zones of weakness • Joints: Formed in massive rocks. Large cracks. Formed below surface. Meters apart. • Natural foliation in rocks (surfaces along which rocks part). • Activity by organisms • Frost wedging:Breaking that results from the expansion of water as it freezes after infiltrating into crack. • Mineral crystallization • Alternating heat and cold cycles(desert temperatures drop 43° to 15° in an hour at twilight) • Exfoliation:Physical process where large flat or curved sheets of rock detach from an outcrop. • Spheroidal weathering:Same as exfoliation, but on a smaller scale. • Other forces (glaciers, waves, etc.).

22. Exfoliation Dome in Yosemite Fig. 7.13

23. Joint-controlled weathering Fig. 7.10

24. Role of Organisms in Weathering Fig. 7.11

25. Gneiss Boulder Fractured by Frost Action Fig. 7.12

26. Spheroidal Weathering Fig. 7.14

27. Soil

28. Soil Terminology • Regolith: A layer of loose, unconsolidated material overlying bedrock (“soil” is top layers). • Humus: Organic matter in soil. • Horizon: Layer of soil.

29. Typical soil make-up

30. Fig. 7.16

31. Climate dependence Wet Laterite Pedalfer Dry Pedocal Cold Hot

32. U.S. Soil Distribution