Lecture14 weathering
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Lecture14 weathering

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  • Preferential weathering along joints can create very important influences on landforms. Arch at left is NOT the product of eolian abrasion, as many people think (wind-blown sand seldom rises more than 2-3 m above the ground surface; transport at greater heights is primarily silt and clay, which isn’t very abrasive), but rather is solutional weathering along joints. In upper right, massive joints help keep upper layer relatively resistant to weathering and erosion, allowing it to serve as a caprock for weaker underlying siltstone/shale units, and limiting rate of cliff retreat
  • Examples of spheroidal weathering, which is typically best expressed in granite, sandstone, or basalt (all of which tend to have orthogonal joints). Drawing is from Ritter textbook
  • Examples of orthogonal joints, which are very common in granites, although obviously these are not evenly spaced or truly orthogonal. Can emphasize that differences in joint density at scales of tens to thousands of meters in granite terrains creates hills/tors/high points and depressions/valleys (more densely spaced joints = greater surface area/volume ratio = faster weathering). Photo scale at upper left. Wyoming photos are at Veda Vu
  • These are all photos of the combined effects of different physical weathering processes (thermal expansion, unloading, hydration) that are expressed as joints parallel to the ground surface. This is a typical expression of differential pressures because surface expands more rapidly and completely under unloading, hydration, etc than does subsurface. Where relatively closely spaced orthogonal joints are present, this can create 3d checkerboard effect, with corners and edges weathering most rapidly because of greater surface area/volume ratio, leading to spheroidal weathering
  • These are carbonate-cemented sandstones in arid environments, so there’s enough water to dissolve some of the matrix, but not enough for large-scale dissolution (as there likely would be in a wetter climate). Also note preferential dissolution along bedding planes and joints. This type of solutional weathering is known as taphoni. In the Australian example, there is a weathered surface layer in front of the solutional weathering, that has been locally removed (particularly at top of rock face).

Lecture14 weathering Lecture14 weathering Presentation Transcript

  • Announcements Problem Set 1 Answer Key is posted. Check your graded set. If there are any discrepancies talk to me. Exam 1 review session: Monday at 5pm, here in McG-S room 20. Bring questions! Exam 1 itself: Wednesday the 23 rd
  • Weathering
    • Bedrock to regolith to soil
  • Endogenic vs. exogenic Internal vs. external systems plate tectonics folding faulting volcanism weathering water erosion wind erosion Building up Breaking down tug of war glacial erosion
  • Denudation
    • = wearing away of landforms
    • Involves
    • Weathering
    • Mass wasting
    • Erosion
    Agents: moving water, air, waves, ice
    • Weathering – breakdown of rock materials
    • Erosion – transport of broken-down materials
    Processes that weather/alter/destroy bedrock (P w ) versus Processes that expose bedrock (P e )
  • Yosemite, California P w < P e
  • P w > P e West Virginia
  • Champaign, France
  • Moab, Utah
  • Wind River Range, Wyoming
  • Illinois
  • Amazon Basin, Brazil
  • Svalbard Islands, Norway
  • Canadian Rockies
  • Weathering
    • Disintegration and decomposition of rock near Earth’s surface
    • Regolith = rock fragments made by weathering
    • Bedrock = parent rock producing regolith
    bedrock regolith soil
  • 2 Types of Weathering
    • Chemical alteration
    • yes no
    • Physical Weathering
      • (disintegration)
    • Chemical Weathering
      • (decomposition)
     
  • What controls weathering?
    • Bedrock
      • hardness,
      • jointing , solubility, etc.
    • Water availability
      • - precipitation, water table, & movement
    • Climate
      • - precipitation, temperature, freeze-thaw
    • Vegetation
    • Aspect (Orientation)
      • - exposure to sun & weather
    Resisting Force Driving Forces
  • Joint-controlled weathering Joints in sandstone, Canyonlands NP, Utah Sandstone, Escalante basin, Utah Sandstone, Rainbow Arch, northern Arizona
  • Sandstone, central Australia Granite, central Australia Granite, Missouri
  • Granite, Vedauvoo, Wyoming Granite, Sinai peninsula, Egypt Joint-controlled weathering
  • Aspect (Orientation)
  • Aspect (Orientation) Biological differences w/ slope aspect N S “ South Facing” “ North Facing”
    • depends on climate
    Physical or chemical? precipitation temperature physical chemical This combination of temp and precip doesn’t occur anywhere both, but at very low rates
  • Physical or chemical? precipitation temperature physical chemical This combination of temp and precip doesn’t occur anywhere both, but at very low rates
    • depends on climate
    • Physical = cooler, drier Chemical = warmer, wetter
  • Physical Weathering: Water and Ice Mechanism for fracturing rocks?
  • Physical weathering processes
    • Frost action
    • repeated freeze-thaw breaks rocks
  • Frost action
  • Freeze/Thaw Adequate Water
    • Salt crystal growth
      • salt crystal growth
      • caused by evaporation of H 2 O in dry climates
    Physical weathering processes
    • Unloading (e.g., granites)
      • as surface rock erodes, pressure on buried rock decreases
      • sheets form & slide off = exfoliation
    Physical weathering processes
  • Unloading Granite, Yosemite National Park, California Sandstone, Arches National Park, Utah Granite, Poudre drainage, Colorado
  • Thermal Expansion Granite block
  • Wedging by Vegetation
  • Chemical weathering
    • = involves reactions between air, water, & minerals
  • Chemical weathering processes
    • Hydrolysis
      • minerals + H 2 O
      • silicate minerals
    • 2. Oxidation
      • metals + O
      • e.g., rusting
  • Chemical weathering processes
    • Acid action
      • minerals dissolve into solution
      • often involves C
      • e.g., marble tombstones, limestone caves
  • Solution Sandstone, Zion Canyon National Park, Utah Sandstone, Uluru, central Australia
  • Chemical Weathering
    • Geochemical weathering: Natural, inorganic processes, that break down bedrock into regolith
    • Pedochemical weathering: Natural organic and inorganic processes, that lead to soil formation