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WEATHERING, EROSION AND MASS WASTING

WEATHERING, EROSION AND MASS WASTING. What is Weathering?. Weathering is the combined actions of all processes that cause rock to disintegrate physically or decompose chemically. Weathering processes include ‘physical’ weathering or ‘chemical’ weathering

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WEATHERING, EROSION AND MASS WASTING

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  1. WEATHERING, EROSION AND MASS WASTING (c) Vicki Drake, 2010

  2. What is Weathering? • Weathering is the combined actions of all processes that cause rock to disintegrate physically or decompose chemically. • Weathering processes include ‘physical’ weathering or ‘chemical’ weathering • Physical Weathering – breaks rocks down into smaller and smaller pieces • Chemical Weathering – completely alters minerals in rocks; creates new minerals (c) Vicki Drake, 2010

  3. Weathering Factors • Factors that affect weathering include: • Mineralogy of parent rock • Mafic minerals least resistant to weathering • Felsic minerals more resistant to weathering • Climate • Humid climates = more chemical weathering • Dry climates = more physical (mechanical) weathering • Time • Longer the exposure time, the greater the weathering opportunity • Number of fissures or openings • More joints, cracks, and openings allow for greater weathering processes to occur • Degree of slope • Steeper slopes encourage greater weathering of exposed rocks (c) Vicki Drake, 2010

  4. .Why and Where Does Weathering Occur? • Weathering occurs at the Earth’s surface as rock materials are exposed to the environment. • Weathering is possible because all rock materials – no matter how ‘solid’ - have openings (pore spaces) that allow air, water and other materials to do their work. (c) Vicki Drake, 2010

  5. PHYSICAL (MECHANICAL) WEATHERING • Physical Weathering does not change the basic mineralogy of rock • rocks are disintegrated into smaller and smaller pieces, ready for transport • Key ingredient in physical weathering: water (c) Vicki Drake, 2010

  6. TYPES OF PHYSICAL WEATHERING • Frost (Ice) Action: the role of water in weathering when it freezes • Frost shattering (ice wedging) • Frost heaving (ice heaving) • Stone polygons/rings in high latitude tundra • Pingos • Salt Crystal Growth • Unloading and Exfoliation • Bioturbation • Vegetation • Animal (c) Vicki Drake, 2010

  7. FROST ACTION • Frost Action is the repeated growth and melting of ice crystals in pore spaces of soil and within rock fractures. (c) Vicki Drake, 2010

  8. FROST SHATTERING/ICE WEDGING • This type of physical weathering occurs at high altitudes where there are definitive cycles of summer and winter. • Granites tend to be the most susceptible to this type of weathering • Most high altitude mountains are granitic (c) Vicki Drake, 2010

  9. FROST SHATTERING/ICE WEDGING • During a thaw cycle (summer, for instance), water from groundwater or precipitation finds its way into the fissures of rocks. • In winter, the water freezes and expands by up to 9% in volume pushing the fissure apart even further. • Over many freeze-thaw cycles, rock will break into smaller pieces. (c) Vicki Drake, 2010

  10. Frost Shattering/Ice Wedging (c) Vicki Drake, 2010

  11. FROST SHATTERING/ICE WEDGING (c) Vicki Drake, 2010

  12. FROST/ICE HEAVING • This type of physical weathering occurs in high latitude regions of the ‘arctic tundra’ • Tundra are the extremely high latitude vast open spaces, covered with low-growing grasses and sparse vegetation. • Some parts of tundra are more ‘bog-like’ • Underlain by permanently frozen soils at depth - permafrost • Only upper layers thaw during brief weeks of ‘summer’. (c) Vicki Drake, 2010

  13. MAP OF TUNDRA LOCATIONS (c) Vicki Drake, 2010

  14. Tundra in summer Tundra in winter (c) Vicki Drake, 2010

  15. PERMAFROST • Permafrost is permanently frozen soil, sediment, or rock. • Permafrost has a number of different layers, of which frozen ground is just one portion • The 'active layer' is ground that is seasonally frozen, typically lying above the perennially frozen permafrost layer. • This is the layer involved with ‘frost heaving’ (c) Vicki Drake, 2010

  16. PERMAFROST The Active layer goes through repeated cycles of freezing and thawing Frost heaving occurs in the ‘active’ layer (c) Vicki Drake, 2010

  17. HOW DOES ICE HEAVING WORK? • Summer thaw of upper ‘active’ layer allows water to migrate down through soil layers under gravity • Water ‘pools’ against the more permanently frozen soil layers • Winter freeze and water expands vertically, lifting up overlying soil layers (c) Vicki Drake, 2010

  18. ICE/FROST HEAVING (c) Vicki Drake, 2010

  19. RESULTS OF ICE HEAVING Roads can be affected by ice heaving by warping the surface As the water freezes, it expands vertically, pushing up the overlying layers (c) Vicki Drake, 2010

  20. FROST HEAVING: PATTERNED GROUND AND STONE RINGS • Under the right conditions over hundreds of years, stone and soil organize themselves into patterns, through cycles of freezing and thawing. • The frost heaving activity found in the tundra can produce small hills with center depressions (up to 18 inches tall and 3-4 feet across) (c) Vicki Drake, 2010

  21. Patterned Ground: Stone Polygons (c) Vicki Drake, 2010

  22. PINGOS • Pingos are ice-cored hills forming in the tundra • In time, the expanding ice forms an isolated mass • its volume increases and it pushes up the overlying tundra • Pingos grow at a rate of approximately one-half inch per year • The tallest pingo in the world (in the western Arctic) is 16 stories (192 feet) high. (c) Vicki Drake, 2010

  23. PINGOS (c) Vicki Drake, 2010

  24. PINGO IN NW ALASKA

  25. Salt Crystal Action • Similar to ice-crystal growth – salt crystals grow instead • Susceptible rocks: • Sandstones in dry and arid regions • Process: • Groundwater moving down under gravity through permeable sandstone, naturally high in salts. • Water hits an impermeable layer (shale, for instance) • Water flows along impermeable shale to an opening • Water exits rock leaving salt crystals behind. • Salt crystals grow at base of cliff, producing niches (caves) • Base of cliff wears away, rest of cliff collapses and process begins again (c) Vicki Drake, 2010

  26. WEATHERED SANDSTONE

  27. Cliff Dwellers (c) Vicki Drake, 2010

  28. UNLOADING AND EXFOLIATION • Large sections of granitic rock, formed at great depth under pressure, brought to surface through plate tectonics. • At the molecular level, the granite expands in lower pressure environment • Develops fractures in form of thick shells that peel away from rock • Forms rounded features: domes, for instance (c) Vicki Drake, 2010

  29. EXFOLIATION (c) Vicki Drake, 2010

  30. OTHER PHYSICAL WEATHERING • Surface Heating and Cooling • Expansion and contraction of rock over time • Fire fracturing • Bioturbation • Plant roots • Animal burrowing (c) Vicki Drake, 2010

  31. BIOTURBATION Fire Roots Burrowing animals (c) Vicki Drake, 2010

  32. CHEMICAL WEATHERING • Chemical weathering alters the minerals of rocks – in some cases, the minerals are dissolved. • Types of Chemical Weathering • Hydrolysis • Oxidation • Dissolution: Carbonic Acid Action (c) Vicki Drake, 2010

  33. Hydrolysis • The addition of water at the molecular level to silicates. • Creates a grain-by-grain breakup of the minerals in Granite into a clay called Kaolinite • Kaolinite used in manufacturing of spark plugs and ceramic casings for lights. (c) Vicki Drake, 2010

  34. Oxidation • The addition of oxygen molecules (a hydroxyl radical) to metallic minerals (such as iron) (think: RUST) • Results in decay of igneous and metamorphic rocks down to 100 meters or more in tropical areas (c) Vicki Drake, 2010

  35. Dissolution: Carbonic Acid Action • The mixing of CO2 and water creates carbonic acid – a weak acid • Carbonic acid attacks limestones and marbles: rocks composed of calcium carbonate (CaCO3) • In regions underlain by limestone, removal of CaCO3 results in development of karst topography (c) Vicki Drake, 2010

  36. Karst Topography • Regions with limestone bedrock being weathered out. • Results in landforms such as caverns, sinkholes, disappearing streams, and low elevation. • Karst is a German name for an unusual and distinct limestone terrain in Slovenia, called Kras. (c) Vicki Drake, 2010

  37. Karst Topography (c) Vicki Drake, 2010

  38. EROSION AND MASS WASTING • Erosion: Movement of weathered rock over long distances by water or wind. • Mass Wasting: Downslope movement of weathered rock over short distances due to gravity (c) Vicki Drake, 2010

  39. MASS WASTING • Main force moving weathered materials down slope is gravity. • Factors that control mass wasting: • Steepness of slope • Water content of materials • Presence (or absence) of native vegetation • Human activities (c) Vicki Drake, 2010

  40. SLOPE STABILITY: STEEPNESS W = Weight of total mass of earth material (at center of mass). D = Vector component of W parallel to potential movement. N = Vector component of W normal to slip plane. (c) Vicki Drake, 2010

  41. SLOPE STABILITY: WATER (c) Vicki Drake, 2010

  42. SLOPE STABILITY: VEGETATION • Native vegetation (such as chaparral) tend to grow on steep slopes. • Root structures act as binders and stabilizers of loose unconsolidated materials. • Removal of native vegetation through fire or clearing reduce stability of weathered materials on a slope. (c) Vicki Drake, 2010

  43. Vegetation’s role in slope stability: (A, B) Roots support and stabilize soils near surface and at depth C) Upslope soils stabilized by stems and roots close to surface

  44. SLOPE STABILITY: HUMAN ACTIVITIES (c) Vicki Drake, 2010

  45. SLOPE STABILITY: MIINING The Frank Slide: rock avalanche and is composed of limestone blocks mainly. At 4:30 am on April 29, 1903, the face of Turtle Mountain, Alberta, Canada, collapsed onto the coal-mining town of Frank, killing at least 70 people. This landslide has a volume of 30 million cubic meters, and an equivalent weight of 90 million tons (c) Vicki Drake, 2010

  46. TYPES OF MASS WASTING • Rock fall • Talus slope development • Angle of Repose • Bedrock failure • Slides • Material remains coherent and moves along defined surface: joint, fracture, bedding planes • Slumps • Downward rotation of rock/regolith along concave-upward curved surface • Flows • Materials flow down slope – mixture of water, rock and other materials (slow to fast movement) • Slurry: Lahar, Mud Flow, Debris Flow, Solifluction • Granular: Debris avalanche, Earth Flow, Soil Creep (c) Vicki Drake, 2010

  47. ROCK FALLS Fastest form of mass wasting Hundreds of tons of rock free-falling to surface (c) Vicki Drake, 2010

  48. Granitic rock failure

  49. TALUS SLOPES • The pile of rocks that accumulates at the base of a cliff, chute, or slope. • Movements occur whenever the talus slope exceeds the critical angle: “angle of repose” • ‘angle of repose’ is the steepest angle unconsolidated material may remain stable • The exact angle at which failure takes place depends upon the materials, rock size, and moisture content • Dry homogenous materials in a pile experience slope failure when the angle of repose (the resting slope angle) exceeds 33–37° (c) Vicki Drake, 2010

  50. TALUS SLOPES Talus (loose, weathered bedrock) falls to base of mountain building up a ramp that is very unstable (c) Vicki Drake, 2010

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