1. Intro to Geomorphology (Geos 450/550) Lecture 8: hillslope processes weathering and sed-transport processes on slopes controls on weathering rates regolith thickness climate rock type/structure slope transport processes

2. Problems with scarp writeups: Survey data that did not show a scarp some can be fixed (inversion of slope profile) “random” profiles cannot be fixed, so use GPS Good data but poor fits need to adjust x 0, b, a, and κT in that order No reporting of results No discussion of diffusion model Confusing scarp age and terrace age

3. Back to the soil production function: Why does it have this shape?

4. Model 1: Solve for the temperature as a function of depth and time given periodic changes in surface temperature.

5. scale factor = 3 m for annual 0.15 m for diurnal

6. Model 2 : Depth distribution of roots

7. Climatic control of weathering rates

11. How does climate and rock type affect P 0 ? Effective energy and mass transfer (EEMT) approach: Is based on a conceptual model of energy input and flow through the soil profile, including thermal (temperature) and material (water and biomass) forms of energy (Rasmussen and Tabor, 2005). Calibration data from Riebe et al. for granite a and b are lithologically-dependent empirical parameters

12. Map of EEMT based on PRISM gridded datasets

13. Relative susceptibility To weathering

14. Structural control of weathering rates

16. Types of Physical Weathering 1.Crack propagation by thermally-induced stresses? 2.Hydrofracturing (crack propagation by freeze/thaw) 3.Salt fracturing (crack propagation by crystallization of salt) 4.Biological wedging (crack propagation by roots)

17. Frost Action: Freezing of water in a Depression in Rock Ice - A 9% Increase In Volume Upon Freezing

18. Climatic control of weathering rates

20. Hillslope transport processes creep: heave+gravity, diffusive, low-slope, high veg continuous creep (solifluction), seasonal drainage mass movements: gravity, weathering-limited bioturbation: movement of animals and growth of plants, coarse-grained raindrop impact: diffusive, fine-grained overland flow: semi-diffusive, low veg rill wash: low veg (badlands) solution: karst piping: the movement of water and sediment in interconnected networks of cracks in the near-surface. fine-grained, low- perm spring sapping: outflow of water at a spring where surface and water table meet. structural control

21. Erosion in fluvial systems: two basic regimes 1.Weathering (detachment) limited low weathering rates relative to high transport rates high erosion rates thin regolith or soil slope landsliding bedrock rivers (downcutting) 2.Transport limited low transport rates relative to high weathering rates low erosion rates thick regolith or soil slope creep alluvial rivers (transporting or depositional)

22. Typical slopes of arid and humid environments Slide courtesy of Mike Poulos

23. Cinder cones provide unusually well-constrained initial condition Slide courtesy Craig Rasmussen

25. South FacingNorth Facing ~70 kyr ~300 kyr ~2,000 kyr ~70 kyr ~300 kyr ~2,000 kyr Slide courtesy Craig Rasmussen

26. Use both topo and soil variations with aspect to discriminate better among competing models In water-limited environments: Bioturbation is 1-2 orders of magnitude more important that freeze-thaw-driven creep. North-facing slopes have greater maximum steepness. Regolith is thicker on north-facing slopes but clay clay accumulation more well developed on south- facing slopes (Rech, 2001; Rasmussen et al.) Modern biomass density is greater on north-facing slopes BUT paleo-biomass density was greater on south-facing slopes (glacial climates are 80% of Quaternary).

27. Blue – North facing Red – South facing Soils more well developed (e.g. clay accum.) on south-facing slopes Slide courtesy Craig Rasmussen

28. McGuire et al., in rev. Biomass greater on north-facing slopes now, but what about the past?

29. McGuire et al., in rev. Paleovegetation a key element of the story – South-facing slopes had greater biomass for 80% of Quaternary

30. Pelletier et al. (2013); McGuire et al. (in rev.) conceptual model For water-limited environments and slopes (>2 km a.s.l. in warmer parts of western U.S., 1-2 km a.s.l. in colder parts): Greater soil water availability on north-facing slopes drives faster regolith production. Greater biomass on south-facing slopes (during glacial climates) increased colluvial transport rates compared with north-facing slopes, leading to lower max gradients on north-facing slopes over time. Greater biomass led to greater dust (clay, quartz) accumulation in soils on south-facing slopes.