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Chapter 5 Weathering and Soil

GEOL 101 Introductory Geology. Chapter 5 Weathering and Soil. Earth’s external processes. Weathering – the physical breakdown (disintegration) and chemical alteration (decomposition) or rock at or near Earth’s surface

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Chapter 5 Weathering and Soil

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  1. GEOL 101 Introductory Geology Chapter 5Weathering and Soil

  2. 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. Weathering • Types of weathering • Mechanical weathering • Chemical weathering • Other function together • Weathering Processes • Dissolution • Oxidation • Hydrolysis

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

  5. Mechanical Weathering Increased surface area

  6. Types ofWeathering • 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. Frost Wedging

  8. Unloading - Exfoliation

  9. Exfoliation

  10. Thermal Expansion

  11. Biological

  12. 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. 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. Simple Dissolution Halite (salt) dissolving in water

  15. Dissolution by Acidic Water • Marble and limestone (calcium carbonate) decompose due to acidic water Calcium Carbonate + Acidic Water CaCO3 + 2[H+(H2O)] Ca2+ + CO2 + 3(H2O) Soluble Calcium + Carbon Dioxide + Water • Acid rain caused by air pollution

  16. Iron (Fe) minerals rust when exposed to water and oxygen • Oxidation: loss of electron from element • Iron oxidation produces • Hematite (Fe2O3): reddish brown rust • Limonite [FeO(OH)]: yellowish rust Oxidation Iron + Oxygen 4Fe3+ + 3O22-2Fe2O3 Hematite (iron oxide)

  17. Hydrolysis • Silicate minerals decomposed by water due to hydrolysis • Produces clay, soluble salt, silica Potassium Feldspar + Carbonic Acid + Water 2KAlSi3O8 + 2(H+ + HCO3-) + H2O Al2Si2O5(OH)4 + 2K+ + 2HCO3 -) + 4SiO2 Kaolinite + Potassium + Bicarbonate + Silica (clay) (salt) in solution

  18. Products Weathering

  19. 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. 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. Chemical WeatheringRate variation by rock type Granite Marble

  22. Weatheringrates • 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. Rock Characteristics Bowen’s Reaction Series

  24. Weathering Processes • 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. 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. Differential Weathering

  27. Controlled by jointing patterns Differential weathering

  28. Joint-controlled weathering in igneous rocks

  29. 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. Typical soil components

  31. Soil • Soil Formation • Soil Profile • Soil Types • Soil Erosion

  32. 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. 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. 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. Soil Formation

  36. Soil development variations due to topography

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

  38. Soil Profile • 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

  39. Idealized soil profile O A E B C Parent

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

  41. Soil profile showing Solum O A E B

  42. Soil Development

  43. 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. 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. Soil Types Pedalfer Pedocol Laterite Arid Tropical Humid

  46. 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. 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. 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

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