Weathering

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					Weathering of Rocks
      Why we see weathering

1. Most minerals are not stable at the
   Earth’s surface

2. The Earth wants to be flat – lowest
   energy state

  •   Topography provides energy
  Weathering in the Rock Cycle

          Erosion + Deposition
Igneous                          Sedimentary
                 Melting




            Metamorphic
Three Dynamic Processes of Breaking
        and Removing rock

1. Weathering - the disintegration
   and decomposition of rock at or
   near the surface
2. Erosion
3. Mass wasting
     Types of Weathering

Mechanical – physical breakdown of
 rocks.
Chemical – decomposition of
 rocks by chemical reactions.
            Mechanical
            Weathering


Mechanical weathering breaks
 down rocks/minerals

Smaller particles increases the
 amount of exposed surface area

Does not change the chemical
 composition of the minerals\
Mechanical Weathering - making smaller
               pieces
     Mechanical Weathering

1.   Frost Wedging
2.   Salt Wedging
3.   Biological Wedging
4.   Unloading
5.   Thermal Expansion
      Types of Mechanical
          Weathering
Frost wedging – water penetrates into
  cracks, expands when it freezes.


Must have:
  • Adequate moisture
  • Cracks in rocks
  • Freeze/thaw cycles
Frost Wedging
      Types of Mechanical
          Weathering
Salt wedging – growth of minerals in cracks

  • Desert environments
  • Water evaporates, ions in solution
    combine to form minerals
Salt Wedging
      Types of Mechanical
          Weathering
Biological wedging – plant roots penetrate
  into cracks causing cracks to widen.

Must have:
  • Climate hospitable for plants.
  • Adequate moisture and temperature.
Biological
Wedging
            Unloading

Exfoliation Domes
Removal of overlying material allows
 rock to expands

Top layers expand more than deeper
 layers

Causes sheeting or exfoliation
           Unloading

Sheeting          Sheeting
   Thermal Expansion


1. repeated daily heating and cooling of
   rock; ex. Desert environments

2. heat causes expansion; cooling
   causes contraction.
      Chemical Weathering
• Chemical alteration of minerals.

•   Results in new minerals and ions in
    solution.

• Water and acid are essential.
Types of Chemical Weathering
1. Hydrolysis - any reaction in which water
   participates.
  a) Ion exchange – H+ replaces other cations.
  b) Dissolution - mineral completely dissolves,
     leaving only ions in solution.
  c) Oxidation - reaction in which elements gain
     or lose electrons (example: rust).
      Acid Hydrolysis - Dissolution
Carbon Dioxide + Rain
                    H2O + CO2 H2CO3
    Becomes Acid
                    CaCO3 + H2CO3 Ca2+ + 2HCO3-
Dissolves Minerals (i.e.
carbonates such as calcite)


Carries away--- Ions
  Acid Hydrolysis &
 Secondary Minerals
    Carbon Dioxide + Rain

          Becomes Acid


Dissolves Minerals (i.e. silicates)

           Leaves ----Clay
     Carries away--- Ions
                       Silica
   Dissolution

Minerals dissolve in
 water

For example: salts
      Oxidation
1. Iron silicate
   dissolves

2. Iron oxidizes

3. Oxidized iron
   combines with
   water

4. Leaves iron oxide
    Weathering Products

Regolith – a loose layer of broken rock
 and mineral fragments.

Sediments

Dissolved Ions
          Products of Weathering
       Mineral            Residual Products         Material in Solution


       Quartz                 quartz grains                 silica


      Feldspar                clay minerals         silica, K +, Na+, Ca2+

                         clay minerals, limonite,
Amphibole (hornblende)                                silica, Mg2+, Ca2+
                                hematite

       Olivine             limonite, hematite            silica, Mg2+
      Factors influencing
       Weathering Rates
1. Rock structures – chemical/mineral
    composition, physical features

2. Topography

3. Climate

4. Vegetation
5. Time
Climate and Weathering
    Benefits of Weathering

• Creates soil
• Produces clay, sand, and gravel
• Produces minerals
Soils
           Terminology

Regolith – blanket of loose, weathered
 rock debris covering unweathered
 bedrock.

Soil – uppermost part of regolith.
Rock particles, new minerals, organics
  How soil differs from regolith:

1. More chemical weathering in soil.
2. Soil has structure (layers called
  soil horizons).
3. Soil retains nutrients and
  moisture, essential for abundant
  plant growth.
         More Terminology

Soil profile – sequence of soil horizons.


Soil horizon – layer within a soil profile
 that has distinct characteristics.

      Composition, color, texture
Soil Horizons
           A horizon

• Zone of leaching (lots of chemical
  weathering)
• Organic rich, often dark in color
• Decaying organic matter releases
  nutrients
           B horizon

• Zone of accumulation – minerals
  (clay and iron oxide) are washed
  down from above.
• High clay content, reddish color
  from iron.
• Able to retain moisture because of
  clay content
             C horizon

Parent material – grading from
 weathered to unweathered.

Parent material can be:
  1. Bedrock
  2. Stream sediments
  3. Volcanic ash
Controls of Soil Formation

  1. Parent Material
  2. Time
  3. Climate
  4. Plants and Animals
  5. Slope

				
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posted:2/18/2012
language:English
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