Lecture 4: Weathering and Mass Wasting

Applied GeomorphologyLecture 5: Weathering & MassWasting Processes

Weathering• Chemical Weathering– Hydration: chemical reaction that consumes H 2 O [ 2KAlSi 3 O 8 +2H 2 CO 3 + H 2 O = Al 2 Si 2 O 5 (OH) 4 + 4SiO 2 + 2K + +2HCO 3- ]– Oxidation: chemical reaction that consumes oxygen [ Fe 2 SiO 4 +½O 2 = Fe 2 O 3 + SiO 2 ]– Dissolution: dissolving of minerals into solution [ NaCl + H 2 O =Na + + OH - + H + + Cl - ]• Physical Weathering– Ice wedging– Thermal stress– Spheroidal weathering– Biologic activity– Soil creep– Solifluction– Exfoliation

Chemical Stability of Rock FormingMinerals• Silicates correlate with Bowen’s ReactionSeries

Weathering examples• Fe-oxide formation from oxidation

Weathering Examples cont.• Joint patterns allowing physical andchemical weathering

Weathering Examples cont.• Biologic activity

Weathering Examples cont.• Frost wedging

Weathering Examples cont.• Exfoliation dome formation (Stone Mt. GA)

Weathering Factors• Climate– Rainfall– Average temperature– In some climates chemical weathering dominates, in othersphysical weathering dominates• Bedrock type (mineralogy)– Bowen’s Reaction series• Topography (Soil formation)– Steep: little or no soil– Flat: abundant soil• Duration of weathering process

Soil ProfileSchematic• A, B and C horizons• Porosity: % voidspace in material• Permeability: abilityto transmit a fluid

Soil Types• Pedalfer: originate in temperate humid climatezones. Well developed A, B and C horizons.• Pedocal: originate in arid and semi-aridtemperate climates. Contain abundant CaCO3 inB horizon; All horizons are poorly developed.• Laterites: originate in humid tropical climates.Contain mainly Al2O3. Horizons are poorlydeveloped.

Mass Wasting• Movement of material down-slope as the resultof gravity• Classified based on 2 parameters:– Type of material• Rock• Debris, earth• Mud– Velocity of motion• Fall, avalanche• Slide• Flow

MassWasting

Rock Slide Example• Gross Ventre (near Jackson Hole,Wyoming)(photograph courtesy of A.G.I)

Gross Ventre Topographic Map

Slump Example• Upper portion of slump is a slide producinga noticeable “scar”• Lower portion is generally an earthflow(photograph courtesy of A.G.I)

• Slumps commonly affectman-made structures andare often triggered bypoor constructionpracticesSlump Example

Mudflow Examples• Lahar: driven by volcanic eruptions(Photo courtesy of USGS)

Soil Creep• Slow movement of soil downslope due to frostheave or cyclic wetting/drying.

Solifluction• Slow earthflow movement downslope in permafrostregions

• Moderately fastmovement ofunconsolidatedmaterial downslopeDebris Flow

• Free-fall of rockmaterial at highvelocityRock Fall

Mass Wasting “Triggers”• Seismic events/Construction Shocks• Volcanic eruptions (Lahars)• Undercutting/Slope modification• Rainfall• Rapid deposition (submarine turbidityflows)