weathering/soils

Weathering and Soils


Contents of Entire Course

 

Physical Weathering
Chemical Weathering
Types of Chemical Weathering Reactions
Weathering of Common Rocks
Weathering Rinds, Exfoliation, and Spheroidal Weathering
Factors that Influence Weathering
Soils
Soil Erosion

 adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane University


Weathering is the process that breaks  rocks down to smaller fragments and alters minerals formed at higher temperature and pressure to those stable under conditions present near the Earth's surface.
 

Geologists recognize two categories of weathering processes

  • Physical Weathering  - disintegration of rocks and minerals by a physical or mechanical process.

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  • Chemical Weathering - chemical alteration or decomposition of rocks and minerals.
  • Although we separate these processes, as we will see, both work together to break down rocks and minerals to smaller fragments or to minerals more stable near the Earth's surface.


    Physical Weathering

    Physical weathering takes place by a variety of processes.  Among them are:

  • Development of Joints - Joints are regularly spaced fractures or cracks  in rocks that show no offset across the fracture (fractures that show an offset are called faults).
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    Chemical Weathering

    Since many rocks and minerals are formed under conditions present deep within the Earth, when they arrive near the surface as a result of uplift and erosion, they encounter conditions very different from those under which they originally formed.  Among the conditions present near the Earth's surface that are different from those deep within the Earth are:
     

  • Lower Temperature (Near the surface T = 0-50oC)

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  • Lower Pressure (Near the surface P = 1 - several hundred atmospheres)

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  • Higher free water (there is lots of liquid water near the surface, compared with deep in the Earth)

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  • Higher free oxygen (although O is the most abundant element in the crust, most of it is tied up bonded into silicate and oxide minerals, at the surface there is much more free oxygen, particularly in the atmosphere).
  • Because of these different conditions, minerals in rocks react with their new environment to produce new minerals that are stable under conditions near the surface. Minerals that are stable under P, T, H2O, and O2 conditions near the surface are, in order of most stable to least stable:
     
  • Iron oxides, Aluminum oxides - such as hematite Fe2O3, and gibbsite Al(OH)3.
  • Quartz*
  • Clay Minerals
  • Muscovite*
  • Alkali Feldspar*
  • Biotite*
  • Amphiboles*
  • Pyroxenes*
  • Ca-rich plagioclase*
  • Olivine*

  • Note the minerals with *.  These are igneous minerals that crystallize from a liquid.  Note the minerals that occur low on this list, are the minerals that crystallize at high temperature from magma.  The higher the temperature of crystallization, the less stable are these minerals at the low temperature found near the Earth's surface (see Bowen's reaction series).

    The main agent responsible for chemical weathering reactions is water and weak acids formed in water

  • An acid is solution that has abundant free H+ ions.

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  • The most common weak acid that occurs in surface waters is carbonic acid.

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    Carbonic acid is produced in rainwater by reaction of the water with carbon dioxide (CO2) gas in the atmosphere.

    carbonicacid.gif


    Types of Chemical Weathering Reactions

      Hydrolysis - H+ or OH- replaces an ion in the mineral. 

    Example:
    hydrolysis.gif
     
    Rock Primary Minerals Residual Minerals* Leached Ions
    Granite Feldspars Clay Minerals Na+, K+
    Micas Clay Minerals K+
    Quartz Quartz  ---
    Fe-Mg Minerals Clay Minerals + Hematite + Goethite Mg+2
    Basalt Feldspars Clay Minerals Na+, Ca+2
    Fe-Mg Minerals Clay Minerals Mg+2
    Magnetite Hematite, Goethite   ---
    Limestone Calcite None Ca+2, CO3-2
     
    *Residual Minerals = Minerals stable at the Earth's surface and left in the rock after weathering.


    Weathering Rinds, Exfoliation, and Spheroidal Weathering

    When rock weathers, it usually does so by working inward from a surface that is exposed to the weathering process.  This may result in:



    Factors that Influence Weathering Soils
    Soils are an important natural resource.  They represent the  interface between the lithosphere and the biosphere - as soils provide nutrients for plants.  Soils consist of weathered rock plus organic material that comes from decaying plants and animals.  The same factors that control weathering control soil formation with the exception, that soils also requires the input of organic material as some form of Carbon.
    When a soil develops on a rock, a soil profile develops as shown below.  These different layers are not the same as beds formed by sedimentation, instead each of the horizons forms and grows in place by weathering and the addition of organic material from decaying plants and plant roots.

  • Caliche - Calcium Carbonate (Calcite) that forms in arid soils in the K-horizon by chemical precipitation of calcite. The Ca and Carbonate ions are dissolved from the upper soil horizons and precipitated at the K-horizon.  In arid climates the amount of water passing through the soil horizons is not enough to completely dissolve this caliche, and as result the thickness of the layer may increase with time.

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  • Laterites - In humid tropical climates intense weathering involving leaching occurs, leaving behind a soil rich in Fe and Al oxides, and giving the soil a deep red color.  This extremely leached soil is called a laterite.

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  • Paleosols - If a soil is buried rapidly, for example by a volcanic eruption, the soil may be preserved in the geologic record as an ancient soil called a paleosol.


  • Soil Erosion

         In most climates it takes between 80 and 400 years to form about one centimeter of topsoil (an organic and nutrient rich soil suitable for agriculture).  Thus soil that is eroded by poor farming practices is essentially lost and cannot be replaced in a reasonable amount of time.  This could become a critical factor in controlling world population.


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