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Gradual, Incremental Hillslope Transport: Creep

Gradual, Incremental Hillslope Transport: Creep. Not Creep. Schedule. Conversion of Rock to Regolith on Hillslopes Soil Transport by Creep on Hillslopes. Two Fundamental Distinctions Between Hillslopes. Transport Limited Hillslopes Weathering Limited Hillslopes

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Gradual, Incremental Hillslope Transport: Creep

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  1. Gradual, Incremental Hillslope Transport: Creep

  2. Not Creep

  3. Schedule • Conversion of Rock to Regolith on Hillslopes • Soil Transport by Creep on Hillslopes

  4. Two Fundamental Distinctions Between Hillslopes Transport Limited HillslopesWeathering Limited Hillslopes Strength Limited Hillslopes capacity >> supply bare, rocky landscape supply >> capacity soil mantled landscape (premise from Carlson and Krikby, 1972)

  5. Creep • Classic textbook images

  6. Hillslope erosion: Diffusive Processes Gabilon Mesa, California With and without vegetation, Images from Taylor Perron, MIT

  7. Soil Production • From bedrock to regolith • As uplifted to surface: • Mechanical alteration • Chemical alteration • Regolith: • Mobile regolith • Detached, motion • Vertical and lateral movement • Saprolite • Intact rock structure • Can be dug into • Weathered rock • Not mobilized • Weakly fractured/weathered “Reactive Zone” (Anderson and Anderson, 2010)

  8. Strength and Mobility of Material • From bedrock to regolith • Strain measurement shows the expansion of soil • 0 = no change • 3 = 300% vol. increase • Fracture Production • Deepest are steep (tectonic) • Unweathered • Near-surface are horizontal • Increasingly oxidized • Highly weathered • Exfoliation

  9. Contrasting regolith profiles • Fractured but unweathered rock • Thin saprolite • Thin colluvial soil • Colorado Granodiorite • High Slope • Highly weathered rock • 2m mobile regolith • 5m saprolite • S. India Gniess. • Low Slope

  10. Breaking rock: Fracture formation • Thermal Stress: expansion/contraction • Strains are small (typically <0.1%) • But non uniform stress does most the work: • Thermal expansion coefficient different for each mineral • Non-uniform heating • Produces: • 1. granular disintegration, • 2. spallation (McFadden et al., 2005)

  11. Exfoliation • As rocks approach the surface, constraining stresses are no longer isotropic and stressed rocks delaminate parallel to surface. NOT GLACIAL. Little Shut-eye Pass, Sierra Nevada, California, J. David Rogers

  12. Frost Cracking • Frozen water expands 11% …but not important! • Why? Not a sealed, closed system. • Water migrates toward the freezing front and freezes in existing cracks. • Thin film, unfrozen transport toward crack tips • Freezing opens tip, generating tension in fluid (Walder and Hallet, 1985)

  13. Frost Cracking • Experiment to test this:

  14. What are the soil temperature conditions through the year? (Anderson et al., 2012)

  15. Frost Cracking Depth dependent, not maximum at surface (Anderson, 1998)

  16. Model Comparison: Two mean temperatures (Anderson et al., 2012)

  17. Model Comparison: Two mean temperatures (Anderson et al., 2012)

  18. Frost Cracking Hales and Roering, 2005

  19. Mineral Stress (Expansion and Contraction) • Salt and Expansive Clays Sandstone vs. Mudstone Slaking, New Zealand Salt pits

  20. Seepage and Salt Weathering carves canyon alcoves (Lamb et al., 2006)

  21. Biotic processes • Roots: crack expansion, transmission of wind stress Danjon et al., 2008

  22. Chemical Weathering • (not covered) • Humic acid production by decomposition of organic matter • Mineral conversion (feldspar to clay)

  23. Chemical Weathering (Mudd et al., 2013)

  24. Chemical Weathering (Mudd et al., 2013)

  25. Chemical Weathering (Mudd et al., 2013)

  26. Chemical Weathering (Mudd et al., 2013)

  27. Chemical Weathering (Mudd et al., 2013)

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