This photo, of the Ohio

Oxidation chemical weathering

(some material on this page borrowed from USGS)

Chemical Weathering

The two main types of weathering are mechanical (physical) weathering and chemical weathering. This page describes several types of chemical weathering. Also, this section explains the strange but common weathering process called "grusification" and the resultant weathering form called (you guessed it.) GRUS! (some pronounce this word "gruss" as in "Russ", while others pronounce it "grew-ss" as in "truce"). You will see a LOT of grusification when you venture to the Landslide Module!

Rainwater is always slightly acidic (and in some places, very acidic!). Carbon dioxide dissolves in water and becomes carbonic acid, which is the stuff that makes sodas acidic (the concentration of carbon dioxide is much lower in regular water, which is why it doesn't feel or taste acidic). This acid reacts chemically with minerals and either dissolves them or turns them into other minerals.

Here are three types of chemical weathering which I want you to know about:

  • Dissolution/Leaching
    Some rocks dissolve completely when exposed to rainwater; two important ones are rock salt and limestone. When these rocks dissolve, the materials which make them up become ions in solution in the water, and are carried away with it. We will see examples of limestone that has been dissolved (dissolution) in this Module when we stop at Wapatki Ruins. There are Karst features (sink holes, caves, underground drainage systems) in this area.
  • Oxidation/Rusting
    In general, oxidation is when an atom or ion loses one or more electrons. A prime example is when iron rusts - rusting is when iron ions change from one form to another, and they lose one electron along the way. Iron-bearing minerals also ``rust'' as the iron contained in their structures changes form as above. In weathering, when oxygen combines with another substances, oxidation has occurred. Sometimes a change in the color of a rock is an indication that oxidation has occurred. One common example of oxidation rust.


These rocks are rusting... (oxidizing)

  • Hydration
    Some minerals react with water and acid to take up hydrogen and ``kick out'' other cations; this process is called hydration. Feldspars tend to hydrate and change to clay.
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