1 / 48

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

xarles
Download Presentation

Chapter 5 Weathering and Soil

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related