1. Applied Geomorphology
Lecture 5: Weathering & Mass Wasting Processes
GY301: Weathering and Mass wasting Processes.

2. Weathering Chemical Weathering Physical 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
Ice wedging
Thermal stress
Spheroidal weathering
Biologic activity
Soil creep
Solifluction
Exfoliation
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
Ice wedging
Thermal stress
Spheroidal weathering
Biologic activity
Soil creep
Solifluction
Exfoliation

3. Chemical Stability of Rock Forming Minerals
Silicates correlate with Bowen’s Reaction Series
Silicate Stability and Bowen’s Reaction Series
Silicates correlate with Bowen’s Reaction Series
High T minerals (Olivine, Ca Plagioclase) chemically weather rapidly
Low T minerals (Qt, K-Feld) are chemically stable.

4. Weathering examples Fe-oxide formation from oxidation
Fe oxide (hematite) is a common oxidation product.
In Archean time lower oxygen levels produced magnetite; Banded Iron formations (BIF’s).

5. Weathering Examples cont.
Joint patterns allowing physical and chemical weathering
Weathering Examples: Spheroidal Weathering.
Joint fractures typically form orthogonal patterns.
Chemical and physical weathering is concentrated at corners of cubic rock masses leading to spherical weathering products.

6. Weathering Examples cont.
Biologic activity
Weathering Examples: Biological Activity.
Root growth can exert tensile stress equivalent to ice wedging.
Chemically weathered organic humus produces acidic solutions that attack bedrock.

7. Weathering Examples cont.
Frost wedging
Weathering Examples: Frost Wedging.
Water expands with tremendous force when it expands.
Weak tensile strength of rocks are easily exceeded by frost edging.
Frost edging is most effective in temperate humid climates where T cycles above and below the freezing point.
Products of frost wedging are angular.

8. Weathering Examples cont.
Exfoliation dome formation (Stone Mt. GA)
Weathering Examples: Exfoliation.
Exfoliation forms joint fractures parallel to ground surface as a granite pluton is uplifted and eroded.
Slabs of granite will “peel” of much like layers on an onion.
The reason exfoliation forms well in granite is its homogenous mechanical behavior.

9. Weathering Factors Climate Bedrock type (mineralogy)
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
Weathering Factors.
Climate.
1. Rainfall
2. Average temperature
3. In some climates chemical weathering dominates, in others physical weathering dominates
Bedrock type (mineralogy).
1. Bowen’s Reaction series
Topography (Soil formation).
1. Steep: little or no soil
2. Flat: abundant soil
Duration of weathering process.

10. 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
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.

11. Soil Profile Schematic
A, B and C horizons
Porosity: % void space in material
Permeability: ability to transmit a fluid
Soil Profile Model
A, B and C horizons
Porosity: % void space in material
Permeability: ability to transmit a fluid

12. 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.
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.

13. 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
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

14. Mass Wasting Mass Wasting Classification Schematic
Based on 1. Type of Material, and 2) Velocity of motion.

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

16. Gross Ventre Topographic Map
Note detachment zone scars.
Note the damming of the river.
Note the scale of the movement – approximately one mile.

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

18. Slump Example
Slumps commonly affect man-made structures and are often triggered by poor construction practices
Slump Example
Slumps commonly affect man-made structures and are often triggered by poor construction practices.
Removal of protective vegetation is often a contributing factor.
Slumps are often triggered by seismic events or heavy rainfall.

19. Mudflow Examples Lahar: driven by volcanic eruptions
(Photo courtesy of USGS)
Mudflow Examples
Mud is water-saturated soil and bedrock.
Volcanically triggered mudflows are termed “Lahars”.
Mud is much more dense that water so only a few inches can move a house off of its foundation.

20. Soil Creep
Slow movement of soil downslope due to frost heave or cyclic wetting/drying.
Soil Creep
Common in temperate climates; Needs a freeze-thaw cycle to drive the process.
Moves more material than any other mass wasting process.
Rate is usually several centimeters per year.

21. Solifluction Slow earthflow movement downslope in permafrost regions
In permafrost regions the upper soil profile cannot drain water.
Saturated soil profile will move via earthflow in a downslope direction.
Typically the turf traps the earthflow forming noticeable “lobes” on downslopes.

22. Debris Flow
Moderately fast movement of unconsolidated material downslope
Debris Flows
Moderately fast movement of unconsolidated material downslope
Follows existing drainage channels.

23. Rock Fall Free-fall of rock material at high velocity Rock Fall
Boulders are loosened by ice edging.

24. Mass Wasting “Triggers”
Seismic events/Construction Shocks
Volcanic eruptions (Lahars)
Undercutting/Slope modification
Rainfall
Rapid deposition (submarine turbidity flows)
Mass Wasting “Triggers”
Seismic events/Construction Shocks
Volcanic eruptions (Lahars)
Undercutting/Slope modification
Rainfall
Rapid deposition (submarine turbidity flows)

25. Exam Review
Know chemical weathering reactions and example reaction equations.
Know the types of mechanical (physical) weathering.
Know weathering control factors.
Know the relationship between Bowen’s reaction series and the susceptibility to weathering.
Be familiar with the circumstances that lead to lahar, solifluction, soil creep, rock slides, and slumps.
Be familiar with soil horizons and soil types.
Exam Review
Know chemical weathering reactions and example reaction equations.
Know the types of mechanical (physical) weathering.
Know weathering control factors.
Know the relationship between Bowen’s reaction series and the susceptibility to weathering.
Be familiar with the circumstances that lead to lahar, solifluction, soil creep, rock slides, and slumps.
Be familiar with soil horizons and soil types.