Applied Geomorphology
Lecture 5: Weathering & Mass
Wasting Processes
Weathering
• Chemical Weathering
– Hydration: chemical reaction that consumes H2O [ 2KAlSi3O8 +
2H2CO3 + H2O = Al2Si2O5(OH)4 + 4SiO2 + 2K+ +2HCO3- ]
– Oxidation: chemical reaction that consumes oxygen [ Fe2SiO4 +
½O2 = Fe2O3 + SiO2 ]
– Dissolution: dissolving of minerals into solution [ NaCl + H2O =
Na+ + OH- + H+ + Cl- ]
• Physical Weathering
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Ice wedging
Thermal stress
Spheroidal weathering
Biologic activity
Soil creep
Solifluction
Exfoliation
Chemical Stability of Rock Forming
Minerals
• Silicates correlate with Bowen’s Reaction
Series
Weathering examples
• Fe-oxide formation from oxidation
Weathering Examples cont.
• Joint patterns allowing physical and
chemical 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 others
physical 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 Profile
• A horizon: >50% organic humus mixed
with sand, silt and clay
• B horizon: sand size particles surrounded
by a matrix of soluble residue and clay
minerals
• C horizon: bedrock is weathered but still
recognized
Soil Profile
Schematic
• A, B and C horizons
• Porosity: % void
space in material
• Permeability: ability
to transmit a fluid
Soil Types
• Pedalfer: originate in temperate humid climate
zones. Well developed A, B and C horizons.
• Pedocal: originate in arid and semi-arid
temperate climates. Contain abundant CaCO3 in
B horizon; All horizons are poorly developed.
• Laterites: originate in humid tropical climates.
Contain mainly Al2O3. Horizons are poorly
developed.
Mass Wasting
• Movement of material down-slope as the result
of gravity
• Classified based on 2 parameters:
– Type of material
• Rock
• Debris, earth
• Mud
– Velocity of motion
• Fall, avalanche
• Slide
• Flow
Mass
Wasting
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 producing
a noticeable “scar”
• Lower portion is generally an earthflow
(photograph courtesy of A.G.I)
Slump Example
• Slumps commonly affect
man-made structures and
are often triggered by
poor construction
practices
Mudflow Examples
• Lahar: driven by volcanic eruptions
(Photo courtesy of USGS)
Soil Creep
• Slow movement of soil downslope due to frost
heave or cyclic wetting/drying.
Solifluction
• Slow earthflow movement downslope in permafrost
regions
Debris Flow
• Moderately fast
movement of
unconsolidated
material downslope
Rock Fall
• Free-fall of rock
material at high
velocity
Mass Wasting “Triggers”
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Seismic events/Construction Shocks
Volcanic eruptions (Lahars)
Undercutting/Slope modification
Rainfall
Rapid deposition (submarine turbidity
flows)