Applied Geomorphology Lecture 5 Weathering Mass Wasting Processes

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

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 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.