1. Weathering Processes

2. Objectives Explain major processes of mechanical weathering
Describe reactions involved in chemical weathering and nature of weathering products
Discuss concept of biological weathering
Note environmental controls over weathering processes and how they influence geography of weathering

3. Introduction
Weathering – the impact of the elements of weathering on rocks
First step in landscape evolution
Important in soil formation
Types of weathering
Mechanical
Chemical
Biological

4. Mechanical Weathering
Breakdown of rocks through stress
Frost action – freezing/thawing of water breaks down rock
Salt wedging – growth of salt crystal wedges rocks apart
Exfoliation – sheets or rocks form after overburden pressure removed

5. Chemical Weathering Alteration of minerals Water is essential
Feldspars altered more readily than quartz
Water is essential
Types of chemical weathering
Spheroidal – rounded boulders produced by weathering through fractures

6. Chemical Weathering

7. Chemical Weathering Reactions - Carbonation
1. CO2 + H2O  H2CO3  H+ + HCO3-
carbon water carbonic acid hydrogen ionb icarbonate ion
dioxide (gas)
2. CaCO3 + H + Ca2+ + HCO3-
calcite (a car- hydrogen ion calcium ion bicarbonate ion
bonate, solid )
Reaction important in limestone/dolomite dissolution
Acidity increases in soil due to plants/organisms release of of CO2

8. Chemical Weathering Reactions - Hydrolysis
1. 4H2O  4H OH-
water hydrogen ion hydroxide ion
2. 4H Mg2SiO4  2Mg2+ + H4SiO4
hydrogen olivine magnesium silicic acid
ions (a silicate, solid) ions (dissolved)
Silicates mostly weather through hydrolysis

9. Chemical Weathering Reactions - Oxidation
FeS2 + 2H2O O2  FeO(OH) +4H+ + 2SO42-
pyrite (a water oxygen goethite (an hydrogen sulfate
sulfide oxide mineral) ions ions
mineral)
Iron from iron-bearing minerals reacts with oxygen to form iron oxides
Produces acidity

10. Weathering Products Carbonate minerals produce dissolved ions
Silicates produce solids + dissolved ions
Feldspars generate clay (kaolinite) + Na, Ca, and Si

11. Biological Weathering
Contributes to acidity of soils
Tree roots can break down rocks along fractures
Lichen remove nutrients from rocks speeding up weathering

12. Geography of Weathering
Frost wedging mostly in cold area where water freezes
Wedging by salt crystals occurs in arid areas and along coasts
Mechanical weathering can take place everywhere, including mountains and gently sloping area
Most intense chemical weathering occurs in humid tropical climates
Limestones weather in most climates, more resistant in dry
Granites weather in all climates