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Geology 110


  WEATHERING


LEARNING OBJECTIVES


Weathering is the physical and chemical alteration of rocks and minerals at or near the Earth's surface.  Combined with erosion, transport and deposition, weathering is a destructive process that act's to change the Earth's surface.

There are two forms of weathering.

1) Physical or Mechanical Weathering: breakdown of rock material with no change in the chemical or mineralogical makeup of the rock.

2) Chemical Weathering:  a chemical or mineralogical change in rock material.

Usually weathering is a combination of physical and mechanical.


Factors that influence the type of weathering that occurs and the degree of weathering that occurs:

  • Rock Structure: the composition, absence/presence of joints, cracks and faults influence the amount of water that is able to penetrate the rock and how susceptible the rock is to weathering.
  • Climate: temperature and humidity are important.  More water and higher temperatures increase the rate of weathering
  • Topography: topography determines the amount of rock surface exposed to weathering.
  • Vegetation: vegetation acts to both physically and chemically weather rock material.

Mechanical Weathering:

  • Thermal contraction and expansion: heating and cooling of rock material causes the individual mineral crystals to expand and contract.  This expansion/contraction acts to wear the edges of the minerals enhancing weathering.
  • Crystal Growth: this includes ice and salt.  Crystals forming in the cracks and crevices of rocks act to push the cracks and crevices wider enhancing weathering.  Frost wedging is the term used if ice is the crystals ice.  Frost heaving refers to the action of ice on unconsolidated materials.  Ice acts to push up or "heave" the unconsolidated material.
  • Abrasion: wind or water moving rock particles against one another acts to chip and round individual rocks eventually wearing them smaller and more rounded.
  • Exfoliation: also called pressure release.  When rocks that have been formed deep under ground are exposed at the surface they are under much less pressure than when they were buried.  The rock tends to "spring back" due to the release of overburden pressure.  The result is weathering in a sheet or rounded pattern.  Sheets spall off much like the skin of an onion.  Exfoliation domes and spheroidically weathered boulders are rounded.  Sheet joints refer to long flat sheet of rock that split from the larger rock.
  • Biological Activity: tree roots act as wedges to split rock material apart.  The activity of animals and insects can also work rock material loose.

Chemical Weathering: As rocks weather chemical changes take place with in the minerals of the rock.  More stable minerals with lower density are produced.  Bulk density within the rock is reduced.  Chemical weathering involves complex chemical reactions that can be summarized as follows:
 

  • Oxidation: reaction with oxygen (rust).
  • Hydration: absorption of water.
  • Acids and bases: naturally occurring acids and bases react with the minerals to break them down.
  • Solution: dissolution of minerals within the rock.

Examples of common weathering products:

Quartz: quartz is relatively inert and tends to remain in the environment a long time (sand).  Under some conditions silica will dissolve into solution.

Feldspars: weather to form clay minerals.  Potassium, sodium and calcium is released to the environment.

Ferromagnesium minerals: weather to form clay minerals.  Iron released to the environment, or iron minerals such as limonite and hematite can form.

Carbonates: dissolve into solution.


Differential Weathering:  in a landscape there are many types of rocks containing many different minerals all weathering at a different rate.  Often the more resistant rocks are left standing on a landscape as a butte, mesa or pinnacle.
Monument Valley* is an example of differential weathering.
 

*picture is by John Cleteroe