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Weathering Processes. Objectives Explain major processes of mechanical weathering Describe reactions involved in chemical weathering and nature of weathering.

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Presentation on theme: "Weathering Processes. Objectives Explain major processes of mechanical weathering Describe reactions involved in chemical weathering and nature of weathering."— 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 – 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- carbonwatercarbonic acidhydrogen ionbicarbonate ion dioxide (gas) 2. CaCO3+H+ Ca2++HCO3- calcite (a car-hydrogen ion calcium ionbicarbonate ion bonate, solid ) Reaction important in limestone/dolomite dissolution Acidity increases in soil due to plants/organisms release of of CO 2

8 Chemical Weathering Reactions - Hydrolysis 1. 4H2O 4H+ + 4OH- waterhydrogen ionhydroxide ion 2. 4H+ + Mg2SiO4 2Mg2++H4SiO4 hydrogenolivinemagnesiumsilicic acid ions(a silicate, solid)ions(dissolved) Silicates mostly weather through hydrolysis

9 Chemical Weathering Reactions - Oxidation FeS2+ 2H2O + 4O2 FeO(OH)+4H+ + 2SO42- pyrite (a water oxygen goethite (anhydrogen 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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