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– 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 sulfatesulfide oxide mineral) ions ionsmineral)

• 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