WP7_Weathering by hedongchenchen

VIEWS: 3 PAGES: 31

									       Chapter 7:
WEATHERING AND EROSION
                    Objectives
• Describe the two major kinds of rock weathering.
• Identify three end products of weathering.
• Explain the difference between weathering, erosion,
  and mass wasting.
• Describe how ice, water, and air transport regolith
  across Earth’s surface.
• Define and give examples of mass wasting by slope
  failure and/or sediment flow.
Weathering-The First Step in the Rock
              Cycle
 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
   – Regolith
      • A loose layer of fragments that
        covers much of Earth’s surface
   – Soil
      • The uppermost layer of regolith,   The rock in the photo has weathered in
        which can support rooted plants    place with little erosion, forming soil
             Weathering: Two Types

• Mechanical engineering
   • The breakdown of rock into solid
     fragments by physical processes
   • Chemical composition of rock
     NOT altered
• Chemical weathering
   • The decomposition of rocks and
     minerals by chemical and
     biochemical reactions
• Joints
   – A fracture of rock , along
     which no appreciable
     movement has occurred
   – Sheet jointing or exfoliation
   – Frost wedging
• Abrasion
   – The gradual wearing down of
     bedrock by the constant
     battering of loose particles
     transported by wind, water or   The jointing in these rocks has
     ice                             exposed new surface area which has
                                     broken and smoothed due to wind,
                                     water and ice.
 Weathering
exfoliation and frost wedging
Frost wedging and biomechanical weathering
              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      The marble grave marker has been attacked by
    acidity of rainwater acidic rain because of the calcite composition. The
                              grave marker on the right, while old, has not been
                              dissolved because of its granite composition
        Chemical weathering:
ion exchange and the chemical breakdown of feldspar
        Factors affecting weathering
• Tectonic setting
   – Young, rising mountains
     weather relatively
     rapidly
   – Mechanical weathering
     most common
     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
      Factors affecting weathering
• Rock structure
   – Distribution of
     joints influence rate
     of weathering
      • Relatively close
        joints weather
        faster
     Factors affecting weathering
• Topography
  – Weathering
    occurs faster on
    steeper slopes
     • Rockslides
     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
      Factors affecting weathering
• Biologic activity
   – Presence of
     bacteria can
     increase
     breakdown of rock
      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
Factors affecting weathering:
     color dots on map match colors on chart
          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
             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
       Erosion and Mass Wasting

Erosion is the removal of weathering products from the
  source and most often occurs by water
• Erosion
   – The wearing away of bedrock and transport of loosened
     particles by a fluid, such as water
   – Example: Sediment moved along the bottom of a stream
        Erosion and Mass Wasting
• Erosion by wind
   Particles of sand are transported close to the surface.
     finer particles of silt and clay can be transported great
     distances
• Erosion by ice
   – Glacier
      • A semi-permanent or perenially frozen body of ice, consisting of
        recrystallized snow, that moves under the pull of gravity




                                                 Wind-blown fine sediments
                                                 such as this dust cloud can
                                                 Be transported across oceans
Erosion and Mass Wasting




  Left: deposits of unsorted glacial till from glacier
  Right: rock polished and striated by glacier
                Erosion by ice:
     glacier removes, breaks and transports rock pieces
glaciers scour valleys and deposit piles of debris as moraines
         Erosion and Mass Wasting
• Mass wasting
   – The downslope movement of regolith and/or bedrock masses due to the
     pull of gravity
• Slope failure
   – Falling, slumping or sliding of relatively coherent masses of rock
                           Erosion and Mass Wasting:
Rock slide, rock fall, and slumping result in downhill transport of broken rock
        Erosion and Mass Wasting
• Flow: If water or air combines with the downward
  movement, the regolith can “flow” downhill
• Creep
   – The imperceptibly slow downslope flow of regolith
       • Unstable slopes move very slowly over long periods of time
Erosion and Mass Wasting
       Why do major landslides occur near plate
                    boundaries?
• Tectonics and mass
  wasting
  – World’s major historic
    landslides clustered
    near converging
    lithospheric plates
     • High mountains
       undergo rapid
       weathering
     • Earthquakes near plate
       boundaries can trigger
       landslides               This massive slide was triggered by
                                A magnitude 9 earthquake in Alaska
                                near a subduction zone.
              Critical Thinking
• On Earth, clay minerals are the most common
  products of weathering. Samples from the
  Moon do not contain any clay minerals. Why?
• Why are some granite bodies extensively
  jointed, while others are essentially joint free?
              Critical Thinking
• On Earth, clay minerals are the most common
  products of weathering. Samples from the
  Moon do not contain any clay minerals. Why?
• Why are some granite bodies extensively
  jointed, while others are essentially joint free?

								
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