Applied Geomorphology Lecture 5 Weathering Mass Wasting Processes
- Slides: 28
Applied Geomorphology Lecture 5: Weathering & Mass Wasting Processes
Weathering • Chemical Weathering – Hydration: chemical reaction that consumes H 2 O [ 2 KAl. Si 3 O 8 + 2 H 2 CO 3 + H 2 O = Al 2 Si 2 O 5(OH)4 + 4 Si. O 2 + 2 K+ +2 HCO 3 - ] – Oxidation: chemical reaction that consumes oxygen [ Fe 2 Si. O 4 + ½O 2 = Fe 2 O 3 + Si. O 2 ] – Dissolution: dissolving of minerals into solution [ Na. Cl + 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 Forming Minerals • Silicates correlate with Bowen’s Reaction Series
Chemical Weathering Examples: Silicate Hydrolysis Stability • Those silicates that crystallize at low temperatures on Bowen’s Reaction Series have the best chemical stability to resist hydrolysis at Earth’s surface temperatures. • Proof of this is the dominance of quartz grains in sediment that has a long transport history in a stream- i. e. beach sand along Gulf Coast.
Chemical Weathering Examples: Oxidation • Fe-oxide formation from oxidation of ferromagnesian silicates.
Physical Weathering Examples: Spheroidal Weathering • Joint patterns allowing for physical and chemical weathering
Physical Weathering Examples: Biological Activity • Biologic activity
Physical Weathering Examples: Ice wedging • Frost/Ice wedging.
Physical Weathering: Thermal Stress • In climates with extreme ranges in temperature (deserts, arctic) the cycle of thermal expansion and contraction will cause fractures to propagate in the rock material. • Thermal stress is the only type of weathering on the Earth’s moon because of the absence of an atmosphere.
Physical Weathering Examples: Exfoliation Domes • Exfoliation dome formation (Stone Mt. GA).
Physical Weathering: Solifluction • Solifluction occurs in permafrost regions where the frozen non-permeable soil profile traps liquid water causing top of soil profile to become oversaturated. • The oversaturated soil will flow as a confined mudflow.
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 Ca. CO 3 in B horizon; All horizons are poorly developed. • Laterites: originate in humid tropical climates. Contain mainly Al 2 O 3. 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” • • • 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.
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