Hillslopes (Lunar)

1. Hillslopes (Lunar) Apollo 17 – Dec. 1972, View of lunarscape at Station 4 with Astronaut Schmitt working at LRV

2. Hillslopes (Madagascar) Tsingy de Bemaraha national park in Madagascar has a forest of limestone pillars. The word tsingy means “where one cannot walk barefoot.” Water eroded caves and passages through the land, the roofs of which eventually collapsed and left the pillars standing up to 70 meters tall. The tops of the rocks have a vastly different ecosystem from the valleys, and from the surrounding savannahs. The stone forest is home to thousands of species not seen outside of Madagascar. Image by Stephen Alvarez for National Geographic.

3. Convex Hilltops – Big Sur

4. Convex Hilltops – near Freemont, CA

5. Gilbert’s “mature hillslope” = Our “steady-state” ~not changing in form through time GKG’s reasoning: If regolith (soil) thickness and form aren’t changing w/ time 1. The Quantity of regolith passing each point must increase downslope, because it is continuously being generated by weathering. 2. Uniform regolith thickness implies that the average velocity of downslope soil transport must increase downslope. 3. Since velocity must increase downslope, and gravity drives soil-creep, slope at any point on hillslope is adjusted to provide just the right amount of increased regolith transport required by the mass balance. In fact, slope increases LINEARLY from divide, so these “mature” hillslope forms are not just convex – they’re parabolic. Soil-Mantled Hilltops

6. Regolith Discharge on Hilltops and Hillslopes

8. rb rr

10. Parabolic Hilltop

11. A Diffusive (or Diffusional?) Landscape

12. Terrace edges become more diffuse with age

13. /Users/pna/Work/AnimationLibrary/HillDiff_Up.mov /Users/pna/Work/AnimationLibrary/HillDiff_Down.mov /Users/pna/Work/AnimationLibrary/HillDiff_UpDown.mov Show animations of hillslope diffusion:

14. Examine processes operating on hillslopes: • Look first at the physics and geomorphic results of individual events (e.g. what happens when a raindrop hits a pile of loose sediment) • Then we can look at how individual events, of different magnitudes, stack up in time. • Differentiate: • 1. Deterministic events, those that are predictable once we know the conditions (e.g. the trajectory of an ejected grain from a rain drop’s assault) from • 2. Stochastic events, for which we don’t know the spatial distribution or temporal order of events (the distribution of rain drops and the timing of storms). Specific Hillslope Processes

15. Ejection trajectories of grains blasted into the air by rainsplash

16. The Raindrop

17. CTSV

18. Raindrop Size and Effects

19. Rain Splash Experiments

20. Measuring Net Downslope Rainsplash Transport

21. Creep General term for slow, downslope mass movement of material in response to gravity on hillslopes

22. Creep

23. Solifluction – results from frozen soils attaining excess water during the freezing process by the growth of ice lenses. This aids downslope movement by supersaturating near-surface soil upon thawing.

24. Frost Heave Results from Displacement of Segmented Dowels in Young Pits

25. In regions that freeze and thaw regularly (i.e. periglacial landscapes), material moves seasonally down a hillslope, by flexing upward during freezing and collapsing upon thaw. Expansion is normal to the surface, but collapse is vertical, so the total cycle of motion is a downslope racheting. The total downslope movement is set by the height of the heave and the local slope. To get the total displacement downslope over a period of time, one needs to know the timing and magnitude of freeze-thaw events, and ground water content. The profile appears exponential, because the frequency of shallow freeze events is so much greater than the deep freeze events. It is the product of discharge per event and the probability distribution of thaw depths that must be integrated. Frost heave displacement

26. Deterministic component – heave profile associated with a single event. Stochastic component – magnitude and timing of freeze-thaw events, which depends on the weather. Frost heave transport

27. Freeze-Thaw Exhumation

28. Gelifluction Measurements

29. Burrowing

30. Tree Throw

31. Physical Experiments – Hillslope Transport by a Loudspeaker

32. Result from simple conservation of mass and Determining the right transport rule… Which is: a linear dependence on slope Combine deterministic physical processes with stochastic probability distributions. Other Diffusive Hillslope Processes

33. Summary - Diffusive Hillslope Processes