NAVIGATION:  BACK TO MODULE TWO INTRODUCTION

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.

Water has dissolved the limestone that once occupied this void.

• Carbonation
  When carbon dioxide in the air dissolves in rain, a weak carbonic acid is formed. This weak acid, while harmless to plants and animals, is able to dissolve some kinds of rocks, like feldspar and limestone, in a process called carbonation.

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Grus and grusification & spheroidal weathering

If you have ever walked the trails of South Mountain (the mountain just south of Phoenix with all the radio towers on it), you have probably seen (or slipped on) grus.  Grus is a loose (unconsolidated) collection of weathered "grains".  It is granular disintegration of rock...  It looks like little angular ball bearings, or very sharp little grains of granite.

The image here shows a retaining wall (in California) designed to hold back the grus...  it is so loose that it tends to drift onto the road!  Photo source is here.  

How does grus form?  I thought you'd never ask!  Basically, it is the granular disintegration (or grusification) of granite due to the hydration of feldspar and biotite minerals which break down into clays.  The swelling of these clay minerals as they abosorb water further weathers the granite and biotite minerals in particular also undergo oxidiation, leading to the reddish color of grus deposits.  Because hydration is the main chemical process breaking granite down, it follows that water plays a big role.  It turns out that granite weathers best in the subsurface where soil moisture is in constant contact with bedrock granite.  This why you often find in desert regions piles of rounded boulders sitting in random piles on the landscape.  Below surface, the granites joints and edges (more surface area) are chemically attacked and the result following exhumation is a pile of random boulders.  This is known as spheroidal weathering and we will see a lot of these on the next module.  In wetter regions you develop really weathered granite outcrops that border on soils and known as sapprolites.

What happens when granite is weathered? (A nice summation from here)

• First, unweathered granite contains these minerals:
  • Na Plagioclase feldspar
  • K feldspar
  • Quartz
  • Lesser amounts of biotite, amphibole, or muscovite
• The feldspars will undergo hydrolysis to form kaolinite (clay) and Na and K ions
• The Na and K ions will be removed through leaching
• The biotite and/or amphibole will undergo hydrolysis to form clay, and oxidation to form iron oxides.
• The quartz (and muscovite, if present) will remain as residual minerals because they are very resistant to weathering.
• Weathered rock is called saprolite.
• Weathered rock fragments are one of the constituents of soil. (See section on SOIL)
• What happens after this
• Quartz grains may be eroded, becoming sediment. The quartz in granite is sand- sized; it becomes quartz sand. The quartz sand will ultimately be transported to the sea (bed load), where it accumulates to form beaches.
• Clays will ultimately be eroded and washed out to sea. Clay is fine-grained and remains suspended in the water column (suspended load); it may be deposited in quiet water.
• Dissolved ions will be transported by rivers to the sea (dissolved load), and will become part of the salts in the sea.

Here's a shot of spheroidal weathered granite producing grus.  Image source is here.

Here's a shot of granite exposed at the surface (out at the McDowell Landslide site).  The sunlight reveals a "rough" sharp (ouch!) profile of "grusified" rock.  The little "chunks" of granite are weathering and accumulating in topographic lows.