Weathering and Erosion

1. Weathering and Erosion Tim Davis/Photo Researchers

2. Weathering • Changes that occur in sediments and rocks near earths surface • Two main types: chemical and physical • Not the same as erosion:loosen and transport of soil and rock Physical: fragmentation Chemical: minerals are chemically altered or dissolved

3. Physical and chemical weathering go hand in hand

4. 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

5. Factors Controlling Rates of Weathering Chemical Weathering of Silicates • Quartz: very stable • Feldspars: form clay minerals • Mafic minerals: decompose to oxides

6. Grave stones Limestone, 1852 Slate, 1835

7. 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

8. Microscopic view of disintegration of a granite 1. Cracks form along crystal boundaries. Feldspar, biotite and magnetite start to decay, while quartz does not 2. The decay progresses, and as the crack opens, the rock weakens and disintegrates

9. Weathered granite

10. Etched and corroded feldspar in the soil zone Fig. 6.2 Berner & Holden, 1977

11. Importance of fractures

12. The climate system 2. Variability in atmospheric carbon dioxide leads to corresponding variability in rate of weathering 1. When rocks weather the climate and lithosphere interact 2. Variability in atmospheric carbon dioxide leads to corresponding variability in rate of weathering

13. Bicarbonate reactions

14. Chemical weathering of carbonates • Easily soluble in water (especially with some acid present) • Ca and Mg taken into solution

15. Weathering reactions Silicates Weathering of feldspar (or other silicates): K-feldspar + acidic water = K+ + kaolinite (a clay mineral) K-feldspar kaolinite K-feldspar mica Weathering of silicates uses carbonic acid which reduces the carbon dioxide level in the atmosphere

16. Mobility • Mobility is the ability of elements to be transported out of the weathering zone • ion mobility Ca>Na>Mg>K>Si>Fe>Al Al considered immobile (bauxite)

17. Relative stabilities • Important factors • Chemical stability: a measure of its tendency to remain in a given chemical form • Solubility: measure of the amount of a substance that can be dissolved in water • Rate of dissolution: measure of the amount of a mineral that dissolves in a given amount of time.

18. Oxide formation Ferrous iron (II) soluble Ferric iron (III) precipitates

19. Weathering Oxides Provide Color to the Desert Landscape Betty Crowell Fig. 6.9

20. Mechanical Weathering: Making Cracks Exfoliation Cracks- cracks planar to earths surface-cause is not well known Tectonic Forces- forces in rocks cause cracks to grow Frost shattering-growth of ice in small cracks Thermal expansion — differential thermal expansion of minerals creates stress in rocks- fires Organic activity — tree roots to micro-organisms Mineral Growth- growth of salts in cracks

21. Exfoliation Dome in Yosemite Fig. 6.14 Tony Waltham

22. Joint-controlled Weathering Fig. 6.11 Jeff Foott/DRK

23. Salt Weathering • Salt accumulates in rock pores and cracks • generally occurs by evaporation • sources include -weathering products in water -precipitation-especially near coasts -eolian dust

25. Role of Organisms in Weathering Fig. 6.12 Peter Kresam

26. Frost shattering • Thought to be dominant in arctic and alpine environments • occurs by the freezing of water in rock void spaces-cracks

27. Gneiss Boulder Fractured by Frost Action Michael Hambrey

28. Typical soil make-up

29. Soil Profile

30. Nomenclature

31. Laterite

32. Pedalfer

33. Pedocal

34. Climate dependence Wet Laterite Pedalfer Dry Pedocal Cold Hot

35. Soil distribution US

36. Florida State Soil MYAKKA SERIES Florida State Soil A--0 to 6 inches; black (10YR 2/1) crushed, sand; weak fine granular structure; very friable; matted with many fine and medium roots; strongly acid; clear smooth boundary. (3 to 8 inches thick) E--6 to 20 inches; white (10YR 8/2) sand; common fine faint vertical dark grayish brown, dark gray, and gray streaks along root channels; single grained; loose; common fine and medium roots; strongly acid; abrupt wavy boundary. (12 to 25 inches thick) Bh1--20 to 24 inches; black (N 2/0) sand; weak coarse subangular blocky structure; many fine and medium roots; sand grained coated with organic matter except for common fine pockets of uncoated sand grains; very strongly acid; clear wavy boundary. (2 to 13 inches thick) Bh2--24 to 32 inches; dark reddish brown (5YR 2/2) sand; common coarse faint vertical tongues of very dark brown (10YR 2/2) weak coarse subangular blocky structure; many fine and medium roots; sand grains coated with organic matter; very strongly acid; clear smooth boundary. (0 to 23 inches thick) Bh3--32 to 36 inches; dark reddish brown (5YR 2/2) sand; weak fine granular structure; very friable; few fine roots; sand grains coated with organic matter; strongly acid; clear wavy boundary. (0 to 16 inches thick) C/B--36 to 56 inches; dark brown (7.5YR 4/4) sand (C); weak fine granular structure; very friable; few fine roots; common medium distinct dark reddish brown (5YR 2/2) Bh bodies; strongly acid; clear wavy boundary. (0 to 36 inches thick) C--56 to 85 inches; dark grayish brown (10YR 4/2) sand; single grained; loose; few fine roots; strongly acid.