1. Chapter 5 Weathering and Soil GEOL 101 Introductory Geology

2. GEOL 101 Earth’s external processes Weathering – the physical breakdown (disintegration) and chemical alteration (decomposition) or rock at or near Earth’s surface Mass wasting – the transfer of rock and soil downslope under the influence of gravity (Chapter 9) Erosion – the physical removal of material by mobile agents such as water, wind, ice, or gravity

3. GEOL 101 Weathering Types of weathering Mechanical weathering Chemical weathering Other function together Weathering Processes Dissolution Oxidation Hydrolysis

4. GEOL 101 Weathering Mechanical & chemical processes work together Solid rock Chemical weathering attacks susceptible minerals Rock crumbles by mechanical weathering Chemical/mechanical processes form clay and disperse minerals

5. GEOL 101 Mechanical Weathering Increased surface area

6. GEOL 101 Types of Weathering Mechanical weathering – breaking of rocks into smaller pieces Frost wedging – freezing/thawing of water in fractures, disintegration of rocks Unloading – exfoliation of igneous and metamorphic rocks at the Earth’s surface due to a reduction in confining pressure Thermal expansion – expansion and contraction due to heating and cooling Biological activity – disintegration resulting from plants and animals

7. GEOL 101 Frost Wedging

8. GEOL 101 Unloading - Exfoliation

9. GEOL 101 Exfoliation

10. GEOL 101 Thermal Expansion

11. GEOL 101 Biological

12. GEOL 101 Chemical Weathering Breaks down rock components and internal structures of minerals Most important agent involved in chemical weathering is water (responsible for transport of ions and molecules involved in chemical processes)

13. GEOL 101 Chemical Weathering Processes Dissolution –Aided by small amounts of acid in the water –Soluble ions are retained in the underground water supply Oxidation –Chemical reaction where compound or radical loses electrons –Important in decomposing ferromagnesian minerals Hydrolysis –The reaction of any substance with water –Hydrogen ion attacks and replaces other positive ions

14. GEOL 101 Simple Dissolution Halite (salt) dissolving in water

15. GEOL 101 Dissolution by Acidic Water CaCO 3 + 2[H + (H 2 O)] Marble and limestone (calcium carbonate) decompose due to acidic water Calcium Carbonate + Acidic Water Ca 2+ + CO 2 + 3(H 2 O) Soluble Calcium + Carbon Dioxide + Water Acid rain caused by air pollution

16. GEOL 101 Oxidation 4Fe 3+ + 3O 2 2- 2Fe 2 O 3 Iron (Fe) minerals rust when exposed to water and oxygen Oxidation: loss of electron from element Iron oxidation produces Hematite (Fe 2 O 3 ): reddish brown rust Limonite [FeO(OH)]: yellowish rust Iron + Oxygen Hematite (iron oxide)

17. GEOL 101 Hydrolysis Al 2 Si 2 O 5 (OH) 4 + 2K + + 2HCO 3 - ) + 4SiO 2 Silicate minerals decomposed by water due to hydrolysis Produces clay, soluble salt, silica Potassium Feldspar + Carbonic Acid + Water Kaolinite + Potassium + Bicarbonate + Silica (clay)(salt) 2KAlSi 3 O 8 + 2(H + + HCO 3 - ) + H 2 O in solution

18. GEOL 101 Products Weathering

19. GEOL 101 Chemical Weathering Alternations Decomposition of unstable minerals Generation or retention of materials that are stable Physical changes such as the rounding of corners or edges

20. GEOL 101 Weathering Factors Rates of weathering –Advanced mechanical weathering aids chemical weathering by increasing the surface area Rock characteristics –Rocks w/ calcite (marble and limestone) readily dissolve in weakly acidic solutions –Silicate minerals weather in the same order as their order of crystallization Climate –Temperature and moisture, most crucial factors –Chemical weathering is most effective in areas of warm, moist climates

21. GEOL 101 Chemical Weathering Rate variation by rock type Granite Marble

22. GEOL 101 Weathering rates organic activity - organic acids raises wtr rates by orders of magnitude climate - temp and amount of water water flow through rock bedrock composition topography: groundwater flow time - to equilibrium?

23. GEOL 101 Rock Characteristics Bowen’s Reaction Series

24. GEOL 101 Weathering Processes backwards Bowen’s Reaction Series Reaction rates and weathering susceptibilities approximate a “backwards Bowen’s Reaction Series” first minerals (highest P/T) to crystallize are least stable (first to weather) last minerals (lowest P/T) to crystallize are most stable (last to weather) Residual material is altered to a stable composition and physical form

25. GEOL 101 Differential Weathering Masses of rock do not weather uniformly due to regional and local factors Results in many unusual and spectacular rock formations and landforms

26. GEOL 101 Differential Weathering

27. GEOL 101 Differential weathering Controlled by jointing patterns

28. GEOL 101 Joint-controlled weathering in igneous rocks

29. GEOL 101 Soil Soil is a combination of mineral and organic mater, water, and air Results from weathering Regolith: rock and mineral fragments produced by weathering (weathered debris) Soil supports the growth of plants Good soil: mix of decomposed rock and humus (decay plant and animal)

30. GEOL 101 Typical soil components

31. GEOL 101 Soil Soil Formation Soil Profile Soil Types Soil Erosion

32. GEOL 101 Soil Formation Parent material –Residual soil – parent material is the underlying bedrock –Transported soil – forms in place on parent material that has been carried from elsewhere and deposited Time –Important in all geologic processes –Amount of time for soil formation varies for different soils depending on geologic and climatic conditions

33. GEOL 101 Soil Formation Parent material –Residual soil: parent material is the underlying bedrock –Transported soil: forms in place on parent material that has been carried from elsewhere and deposited Time –Important in all geologic processes –time for soil formation varies depending on geologic and climatic conditions

34. GEOL 101 Soil Formation Climate –Most influential control of soil formation –Key factors: temperature and precipitation Plants and animals –Organisms influence the soil’s physical and chemical properties –Also furnish organic matter to the soil Slope –Steep slopes often have poorly developed soils –Optimum terrain is flat-to-undulating upland surface

35. GEOL 101 Soil Formation

36. GEOL 101 Soil development variations due to topography

37. GEOL 101 Soil Profile Soil forming processes operate from the surface downward Vertical differences are called horizons – zones or layers of soil

38. GEOL 101 O horizon – organic matter A horizon – organic and mineral matter –High biological activity –O and A horizons together make up the topsoil E horizon – little organic matter –Zone of eluviation and leaching –The O, A, E, and B horizons together are called the solum (“true soil”) B horizon – zone of accumulation C horizon – partially altered parent material Soil Profile

39. GEOL 101 Idealized soil profile A E B C Parent O

40. GEOL 101 Soil Profile showing horizons A E B C Parent O

41. GEOL 101 Soil profile showing Solum A E B O

42. GEOL 101 Soil Development

43. GEOL 101 Soil Types Characteristics of each soil type primarily depend on prevailing climatic conditions Climate conditions relate to vegetation Three generic soil types Pedalfer Pedocal Laterite

44. GEOL 101 Soil Types Pedalfer –Humid (>63 cm rainfall), temperate –Forest vegetation –Fe oxides and Al-rich clays in the B horizon Pedocal –Arid (<63 cm rainfall), temperate –Dry grasslands and brush vegetation –High accumulations of calcium carbonate Laterite –Tropical climates, hot and wet –Lush grasslands and trees vegetation –Intense chemical weathering

45. GEOL 101 Soil Types PedalferPedocolLaterite Humid AridTropical

46. GEOL 101 Soil Erosion Constant recycling of Earth materials, part of rock cycle Water and wind are powerful erosion forces that move soil components Raindrop like tiny bomb to soil particles sheet erosion: thin sheets of water rills: tiny channels gullies: deeper cuts Sediment: soil that reaches a stream

47. GEOL 101 Soil Erosion Natural rates of soil erosion depend on –Soil characteristics –Climate –Slope –Type of vegetation In many regions, rate of soil erosion is significantly > rate of soil formation Sedimentation and chemical pollution –Related to excessive soil erosion –Occasionally soil particles are contaminated with pesticides, industrial pollutants etc.

48. GEOL 101 Soil Erosion Good soil development critical for agriculture and forestry Agricultural soil conservation measures Planting trees as windbreaks Plowing hill contours and terracing Crop rotation Forestry soil conservation measures Eliminate clear cutting (selective harvest) Careful design of logging roads Harvest away from drainage area