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Weathering and erossion

Weathering and Erosion

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Weathering and erossion

  2. 2. WEATHERING AND EROSION Weathering - processes at or near Earth’s surface that cause rocks and minerals to break down Erosion - process of removing Earth materials from their original sites through weathering and transport
  3. 3. WEATHERING AND EROSION Weathering produces regolith (“rock blanket”) which is composed of small rock and mineral fragments. A loose layer of fragments that covers much of Earth’s surface. When organic matter is mixed into this material it is called soil. The uppermost layer of regolith, which can support rooted plants.
  4. 4.  Joints  A fracture of rock , along which no appreciable movement has occurred  Abrasion  The gradual wearing down of bed rock by the constant battering of loose particles transported by wind, water or ice The jointing in these rocks has exposed new surface area which has broken and smoothed due to wind, water and ice.
  5. 5. WEATHERING-THE FIRST STEP IN THE ROCK CYCLE  How rocks disintegrate  Weathering  The chemical and physical breakdown of rock exposed to air, moisture and living organisms The rock in the photo has weathered in place with little erosion, forming soil
  6. 6. WEATHERING Mechanical Weathering - processes that break a rock or mineral into smaller pieces without altering its composition Chemical Weathering - processes that change the chemical composition of rocks and minerals
  7. 7. PROCESSES AND AGENTS OF MECHANICAL WEATHERING These are actions or things that break down Earth materials  frost wedging  thermal expansion and contraction  mechanical exfoliation  abrasion by wind, water or gravity  plant growth
  8. 8. PROCESSES AND AGENTS OF MECHANICAL WEATHERING Frost Wedging – cracking of rock mass by the expansion of water as it freezes in cracks
  9. 9. FROST WEDGING (IN SOIL) Ice crystals
  10. 10. PROCESSES AND AGENTS OF MECHANICAL WEATHERING Thermal expansion and contraction – repeated heating and cooling of materials cause rigid substances to crack and separate
  11. 11. PROCESSES AND AGENTS OF MECHANICAL WEATHERING Exfoliation – As underlying rock layers are exposed, there is less pressure on them and they expand. This causes the rigid layers to crack and sections to slide off. The expanding layers often form a dome.
  13. 13. PROCESSES AND AGENTS OF MECHANICAL WEATHERING Abrasion – Moving sediments or rock sections can break off pieces from a rock surface they strike. The sediments can be moved by wind or water and the large rock sections by gravity.
  15. 15. WIND AND WATER ABRASION Photo Ref: P211442, "IPR/52-34CW BGS©NERC
  16. 16. PROCESSES AND AGENTS OF MECHANICAL WEATHERING Plant Growth – As plants such as trees send out root systems, the fine roots find their way into cracks in the rocks. As the roots increase in size, they force the rock sections apart, increasing the separation and weathering.
  18. 18. PROCESSES OF CHEMICAL WEATHERING dissolving (dissolution) oxidation hydrolysis
  19. 19. PROCESSES OF CHEMICAL WEATHERING Dissolving (dissolution) Water, often containing acid from dissolved carbon dioxide, will dissolve minerals from a rock body leaving cavities in the rock. These cavities may generate sinkholes or cave features such as stalactites and stalagmites.
  20. 20. CHEMICAL WEATHERING  Dissolution  The separation of materials into ions in a solution by a solvent, such as water or acid  Rainwater acts as weak solution of carbonic acid  Anthropogenic actions influence acidity of rainwater The marble grave marker has been attacked by acidic rain because of the calcite composition. The grave marker on the right, while old, has not been dissolved because of its granite composition
  21. 21. CARBON DIOXIDE  Carbon dioxide dissolves in rain water and produces Carbonic acid. This Carbonic acid easily weathers marble and Limestone.
  22. 22. PROCESSES OF CHEMICAL WEATHERING Oxidation Minerals may combine with oxygen to form new minerals that are not as hard. For example, the iron-containing mineral pyrite forms a rusty- colored mineral called limonite.
  23. 23. PYRITE OXIDATION Pyrite Limonite
  24. 24. OXYGEN Water + Oxygen +Iron = RUST When water and oxygen mix with Iron it creates rust. This is called oxidation.
  25. 25. PROCESSES OF CHEMICAL WEATHERING Hydrolysis Minerals may chemically combine with water to form new minerals. Again these are generally not as hard as the original material.
  26. 26. FELDSPAR HYDROLYSIS Feldspar Kaolinite (clay)
  27. 27. FACTORS IN CHEMICAL WEATHERING Climate – wet and warm maximizes chemical reactions Plants and animals – living organisms secrete substances that react with rock Time – longer contact means greater change Mineral composition – some minerals are more susceptible to change than others
  28. 28. EROSION TRANSPORT AGENTS OR FORCES  Water rain streams and rivers ocean dynamics ice in glaciers  Wind  Gravity
  29. 29. STREAMS Flowing water will lift and carry small sediments such as silt and sand.
  30. 30. STREAM EROSION AND DEPOSITION Where water moves more swiftly there will be more erosion. Where the water slows down, sediments will be deposited.
  31. 31. OCEAN DYNAMICS  Tidal action and waves carry away weathered materials.
  32. 32. GLACIERS Glaciers are large ice fields that slowly flow downhill over time.
  33. 33. GLACIERS Glacial ice drags rocky material that scours the surface it flows over . The glacier deposits debris as it melts.
  34. 34. WIND TRANSPORT OF SEDIMENTS Wind will carry fine, dry sediments over long distances.
  35. 35. WIND TRANSPORT OF DUST Photo shows Sahara Desert sand being transported over the Atlantic Ocean.
  36. 36. TRANSPORT BY GRAVITY When sediments are weathered they may be transported downward by gravity. The general term for this is mass wasting.
  37. 37. TRANSPORT BY GRAVITY When sediments are weathered they may be transported downward by gravity as a slump. Slump
  38. 38. TRANSPORT BY GRAVITY Loose sediments transported by gravity are called scree. Scree field
  39. 39. DEPOSITION FORMATION Transported sediments are deposited in layers and generate strata like those found in the Grand Canyon.
  41. 41. FACTORS AFFECTING WEATHERING  Tectonic setting  Young, rising mountains weather relatively rapidly  Mechanical weathering most common
  42. 42. FACTORS AFFECTING WEATHERING  Rock composition  Minerals weather at different rates  Calcite weathers quickly through dissolution  Quartz is very resistant to chemical and mechanical weathering  Mafic rocks with ferromagnesian minerals weather more easily
  43. 43. FACTORS AFFECTING WEATHERING  Rock structure  Distribution of joints influence rate of weathering  Relatively close joints weather faster
  44. 44. FACTORS AFFECTING WEATHERING  Topography  Weathering occurs faster on steeper slopes  Rockslides
  45. 45. FACTORS AFFECTING WEATHERING  Vegetation  Contribute to mechanical and chemical weathering  Promotes weathering due to increased water retention  Vegetation removal increases soil loss Vegetation can both hold water And increase weathering. If removed Rocks may also be vulnerable to abrasion
  46. 46. FACTORS AFFECTING WEATHERING  Biologic activity  Presence of bacteria can increase breakdown of rock
  47. 47. FACTORS AFFECTING WEATHERING  Climate  Chemical weathering is more prevalent in warm, wet tropical climates  Mechanical weathering less important here  Mechanical weathering is more prevalent in cold, relatively dry regions  Chemical weathering occurs slowly here Note: temperate regions such as at the center of the chart undergo both chemical and mechanical weathering, i.e. New York area
  49. 49. PRODUCTS OF WEATHERING  Clay  Tiny mineral particles of any kind that have physical properties like those of the clay minerals  Clays are hydrous alumino-silicate minerals
  50. 50. PRODUCTS OF WEATHERING  Sand  A sediment made of relatively coarse mineral grains  Soil  Mixture of minerals with different grain sizes, along with some materials of biologic origin  Humus  Partially decayed organic matter in soil
  51. 51. Landslides & Mass Wasting
  52. 52. Earth’s Surface is shaped by external processes…
  53. 53. Earth’s Surface is shaped by external processes… In sculpting the Earth’s surface, the two most important agents of erosion are : 1) Mass wasting 2) Running water
  54. 54. PP.490-491 original artwork by Gary Hincks There are a wide variety of manifestations of the downslope movement of materials by gravity, some faster and some slower. All of these processes have destructive effects…
  55. 55. Mass Wasting: Downslope, mass movement of Earth materials Driven by: The pervasive background force of …GRAVITY… Contributing factors:  Saturation of sediments by water  Over-steepened slopes  Removal of vegetation  Earthquakes Water fills pore spaces between sediment grains, reduces internal resistance, adds weight. Plants add slope stability by protection against erosion. Strong ground vibrations. Slopes become unstable once they reach the angle of repose = The steepest angle a slope can attain without slumping.
  56. 56. Stability against gravity depends on the strength of a material, which can be represented by its angle of repose… In sediments, this angle depends on grain and sorting.
  57. 57. In sediments, the angle of repose depends on grain size and sorting of materials…
  58. 58. Mass Wasting  Types of materials: Types of movement:  Rates of movement: Soil/regolith -or- Rock/bedrock Rock Falls - Free-fall of material Rock/Debris Slides - Coherent slabs slide along fracture surfaces Mudflows - Soil and rock mixes with water and becomes fluidized. Earth or Debris Flows - Materials move as a viscous mass. Fastest - Rock falls & avalanches. Avalanches “float” on entrapped air. Slowest - Creep (cm/year). Talus slopes
  59. 59. FIG. 16.12 W. W. Norton Types of mass wasting processes arrayed by typical velocity of movement….
  60. 60. FIG. 16.12 W. W. Norton Rock Fall/Debris Fall
  61. 61. Rock/Debris FallsMASS WASTING Blocks of bedrock break free, and fall from a steep cliff face. Contributing factors: - Steep slopes. - Rocks loosened along joint fractures… …by expansion of water on freezing, …by thermal expansion/contraction, …by biological activity (e.g. root growth) - Ground shaking during earthquakes.
  62. 62. FIG. 16.14 Stephen Marshak
  63. 63. FIG. 16.15 W. W. Norton
  64. 64. FIG. 16.22 W. W. Norton
  65. 65. FIG. 16.27 HI W. W. Norton Steps to mediate…
  66. 66. W. W. Norton Mediation…
  67. 67. FIG. 16.08 Stephen Marshak
  68. 68. Mediation by terracing
  69. 69. FIG. 16.12 W. W. Norton Avalanches
  70. 70. Peruvian Valley Rock Avalanche, May 1970 Before After
  71. 71. FIG. 16.12 W. W. Norton Rock/debris slides
  72. 72. Rock Slides… Beds dip downslope.
  73. 73. Rock SlideMASS WASTING Blocks of bedrock break free, and slide down slope along a fracture surface. Often occurs where strata are inclined, with slip occurring along bedding planes of weak units, like shales. Other important contributing factors: - Slopes become undercut by stream or wave erosion. - Rain or melting snow seeps into deposits and lubricates a slip surface. Often deadly! If materials are unconsolidated called a “debris slide”.
  74. 74. FIG. 16.18 W. W. Norton
  75. 75. Common triggering mechanism: Saturation (water) of a weak, expansive, clay-rich shale unit.
  76. 76. FIG. 16.20 A W. W. Norton Common triggering mechanism…undercutting of slopes by streams or waves…
  77. 77. Rock slides can develop in any type of rock where there is are preferred planes of weakness dipping downslope… Sedimentary Metamorphic Igneous Jointing may facilitate process.
  78. 78. Mediation…
  79. 79. FIG. 16.12 W. W. Norton Mudflow/Debris Flow
  80. 80. Mudflow / Debris Flow: Common in high rainfall areas where fine materials mobilized by abundant water…
  81. 81. FIG. 16.12 W. W. Norton Slumps
  82. 82. Slumps: Rotational TypeMASS WASTING  Mass of material slides downward along a curved surface (slump surface)  Speed is usually intermediate and material doesn’t travel very far.  Slumping often involves several masses that move separately (along diferent slump planes).  Common in weak, water saturated sediments that are over-steepened.  Common in coastal areas where sea cliffs are constantly removed by wave erosion.
  83. 83. FIG. 16.04 B Morphology of a Rotational Slump
  84. 84. Rotational Slump: Headwall shows evidence of backward rotation.
  85. 85. FIG. 16.27 F W. W. Norton Approaches to mediation…
  86. 86. Earthquake-triggered slumps, Alaska EQ 1964
  87. 87. Earth/Debris Flow
  88. 88. Slumps: Earth/Debris Flow TypeMASS WASTING  Common in high rainfall areas.  Occur on hillsides. Develop in rock units rich in clay/silt.  Slow rate of movement.  Stabilized by “toe” and by “dewatering”. Destructive!
  89. 89. Robert L. Schuster/U.S. Geological Survey
  90. 90. W. W. Norton Stabilization of slumps with plant cover…
  91. 91. FIG. 16.27 D W. W. Norton Stabilization by terracing…
  92. 92. FIG. 16.27 C W. W. Norton Stabilization by lowering water table…
  93. 93. FIG. 16.27 B W. W. Norton Reduce slope angle…
  94. 94. FIG. 16.12 W. W. Norton Creep
  95. 95. Soil/Regolith CreepMASS WASTING  Creep - Slow (cm/year) downhill movement of material. - Driven by alternate expansion/contraction of material during freeze/thaw or cycles of wetting/drying.
  96. 96. Gravitational force acts on rocks/soil to move them downslope…
  97. 97. FIG. 16.02 B W. W. Norton
  98. 98. FIG. 16.02 A W. W. Norton Effect of cycles of freeze-thaw on soil/regolith creep…
  99. 99. Soil/regolith creep…
  100. 100. Soil/Regolith Creep Slow! Assisted by: “Frost heaving” (expansion of ice upon freezing). Soil Regolith
  101. 101. Evidence of soil/regolith creep…
  102. 102. FIG. 16.02 C W. W. Norton Tell-tell signs of soil/regolith creep…
  103. 103. Signs of soil/regolith creep…
  104. 104. FIG. 16.02 D (c) Martin Miller
  105. 105. Signs of soil/regolith creep…
  106. 106. FIG. 16.03 A (c) Martin Miller Solifluction: Soil creep in high latitude, cold climate areas where freeze-thaw is active…
  107. 107. Reducing soil creep…
  108. 108. THE SOIL PROFILE
  109. 109. THE 6 SOIL ROLES A Soil’s role includes:  Serving as a foundation  Emitting and absorbing gases  Providing habitat  Interacting with water  Recycling nutrients  Supporting human settlements
  110. 110. THE 5 FACTORS OF FORMATION Soil is formed by…  Parent Material: the original “Mom & Pop” soil transported from elsewhere, usually by wind or water, at different speeds  Climate: the amount, intensity, timing, and kind of precipitation that breaks down parts of ecosystem (i.e. rocks, trees) into soil  Topography: Slope and Aspect affect the angle of the land and position toward/away from the sun that soil will be exposed to  Biological: Plants, animals, microscopic organisms, and humans interact with soil in different ways  Time: the amount of time it takes for the four factors (above) to interact with each other
  111. 111. WHAT IS A SOIL PROFILE?  A Soil Profile is a vertical cross-section of layers of soil found in a given area. Below are two examples of soil profiles.
  112. 112. WHAT IS A SOIL HORIZON?  Soil horizons are the layers in a soil profile used to classify soil types.  Horizons based on color, texture, roots, structure, rock fragments, and any unique characteristic worth noting.  Master Soil Horizons are depicted by a capital letter in the order (from top down): O, A, B, C, and R
  113. 113. O-HORIZON The “Organic Matter” Horizon  Surface-layer, at depths of 0-2 feet  Dark in color, soft in texture  Humus - rich organic material of plant and animal origin in a stage of decomposition  Leaf litter – leaves, needles, twigs, moss, lichens that are not decomposing  Several O-layers can occur in some soils, consisting only of O- horizons
  114. 114. A-HORIZON “Topsoil” or “Biomantle” Horizon  Topmost layer of mineral soil, at depths of 2-10 feet  Some humus present, darker in color than layers below  Biomantle - most biological productive layer; earthworms, fungi, and bacteria live this layer  Smallest and finest soil particles
  115. 115. B-HORIZON The “Subsoil” Horizon  At depths of 10-30 feet  Rich in clay and minerals like Fe & Al  Some organic material may reach here through leaching  Plant roots can extend into this layer  Red/brown in color due to oxides of Fe & clay
  116. 116. C-HORIZON The “Regolith” Horizon  At depths of 30-48 feet  Made up of large rocks or lumps of partially broken bedrock  Least affected by weathering and have changed the least since their origin  Devoid of organic matter due to it being so far down in the soil profile
  117. 117. R-HORIZON The “Bedrock” Horizon  At depths of 48+ feet  Deepest soil horizon in the soil profile  No rocks or boulders, only a continuous mass of bedrock  Colors are those of the original rock of the area