Weathering Processes

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

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

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

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

Chemical Weathering

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

Chemical Weathering Reactions - Hydrolysis 1. 4H2O  4H+ + 4OH- 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

Chemical Weathering Reactions - Oxidation FeS2 + 2H2O + 4O2  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

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

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

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