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Weathering Processes.

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Presentation on theme: "Weathering Processes."— Presentation transcript:

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

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