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

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

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.

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).

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.

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.

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.

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.

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.

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.

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

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.

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

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

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

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

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)

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.

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.

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.

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.

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

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

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)

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.