Surface 8 and 9: Weathering products and associated landforms

Weathering (define + importance)

• the breakdown of rocks into smaller particles through physical, chemical, or biological processes

• it initiates the formation of landforms and influences soil development

2 main types of weathering

• physical weathering → breakdown of rocks without chemical change

• chemical weathering → breakdown of rocks through chemical reactions

Types of physical weathering

• Frost Action

• Unloading

• Heating & Cooling

• Salt-crystal growth

• Exfoliation

• Wetting & Drying

Fatigue

• repeated generation of stress

• causes rocks to fracture at a lower stress level than non-fatigued specimen

Frost Wedging

water occupying pores expands while freezing → expansion = buildup in stress → physical disintegration

• significant in cold climates

• contributes to features e.g. talus slopes

Unloading

erosion removes surface material → easing confining pressure on underlying rock → decreased pressure → mineral grains move further apart → creating voids → rock expands/dilates → produces cracks that run parallel to the surface → encourages rock falls & mass movements

Heating and Cooling

rocks have low thermal conductivities

• heating → exterior heats up much faster than interior → outside expands more than the insides → disintegration & formation of rock flakes, shells, huge sheets

• more prominent for darker crystals

Salt-crystal growth

crystals grow in saline solutions on evaporation → salt crystalizes on the interstices of rocks → produces stress to widen them → granular disintegration

• in coastal and arid regions

• when heated or saturated, salt crystals expand and exert pressure against confining pore walls → producing thermal/hydration stress → salt weathering

Exfoliation

• the process where outer layers of rock peel away due to pressure release or temperature changes

• commonly observed in granite landscapes

Wetting & Drying

• some clay minerals (smectite) swell upon wetting and shrink when they dry out

materials containing such minerals expand upon wetting → inducing microcrack formation, widening of cracks, or the physical disintegration of the rock mass

when drying, absorbed water evaporates → shrinkage cracks form

Types of chemical weathering

• Solution/Dissolution

• Hydration

• Oxidation

• Reduction

• Carbonation

• Hydrolysis

Solution/Dissolution

• the dissociation of molecules into their anions and cations and each ion becomes surrounded by water

• easily reversible → when saturated, some of the dissolved material precipitates

Hydration

• when minerals absorb water molecules without changing the chemical composition of the original material

• assists other weathering processes by placing water deep inside the crystal structures

• brownish/yellowish soils

Oxidation & Reduction

• oxidation → when an atom/ion loses electrons (O₂ combining w/ a substance)

• reduction → if a soil becomes saturated with stagnant water, it becomes oxygen deficient with the aid of anaerobic bacteria → gleying (greyish soils)

Carbonation

• the formation of carbonates

• dominates the weathering of rocks with mainly calcite or calcium carbonate

Hydrolysis

• a chemical reaction where water reacts with minerals in rocks, leading to their breakdown

• e.g. feldspar in granite can bread down into clay minerals through hydrolysis