Geology 111G/Lecture 7                                                                     

 

Weathering

            Mechanical

            Chemical

Spheroidal Weathering

Weathering Products

Soil

 

I.  Weathering:  Breakdown of surface and near surface rock material

A.  Chemical weathering:  breakdown of rock through alteration of mineral components; decomposition.  There is a resulting change in mineral composition.  Water is a key agent in all chemical weathering reactions.

1.  Dissolution: breakdown of mineral into its ionic constituents in solution:       

example: CaSO4x2H20 = Ca2+ + SO42- + 2H2O

 

Example here is for an evaporite mineral, but silicates and others undergo dissolution.

2.  Hydrolysis:  a hydroxyl group or a hydrogen ion added to structure of mineral, replacing other ions to yield a new mineral.  Example is weathering of potassium feldspar to form kaolinite:

2KAlSi3O8 + H2O + 2H+ = 2K+ + Al2Si2O5(OH)4 + 4SiO2

 

3.  Oxidation:  Oxygen is added to structure of mineral, making it another mineral.  Example is magnetite to hematite:

2Fe3O4 + 3H2O = 3Fe2O3 + 2H2O + 2H+

B.  Mechanical (Physical) weathering:  Breakdown of rock by energy of physical processes; no change in mineral composition; disintegration.

1.  Frost wedging:  ice occupies more volume than equivalent quantity of water from which it froze (expansion is approx. 9%). This expansion exerts pressure on walls of fracture and enlarges it.

2.  Exfoliation: large sheets of rock are detached from the larger mass, probably by release of pressure or unloading.

a.  Joint: the resulting fracture, as opposed to a fault.

b.  Examples from Glen Canyon Dam and Yosemite.

3.  Rootlet expansion:  plant roots may enlarge fractures.

C.  Spheroidal weathering:  both chemical weathering and exfoliation tend to produce rounded rocks.

1.  Chemical weathering attacks surfaces and there is more surface area at corners and edges.

D.  Weathering Products: chemical weathering of both feldspars and ferromagnesian silicates yields three important products.

1.  Clays

2.  Soluble salts

3.  Silica in solution or quartz.

E.  Soil: a residue of weathering products that accumulates on a surface exposed to weathering for some time.  Type influenced by time, climate and parent material.