Hillslopes (Lunar) Apollo 17 Dec. 1972, View of lunarscape at Station 4 with Astronaut Schmitt working at LRV 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. Convex Hilltops Big Sur Convex Hilltops near Freemont, CA
Soil-Mantled Hilltops Gilberts mature hillslope = Our steady-state ~not changing in form through time GKGs reasoning: If regolith (soil) thickness and
form arent 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 theyre parabolic.
Regolith Discharge on Hilltops and Hillslopes b r
Parabolic Hilltop A Diffusive (or Diffusional?) Landscape
Terrace edges become more diffuse with age Show animations of hillslope diffusion: /Users/pna/Work/AnimationLibrary/HillDiff_Up.mov
/Users/pna/Work/AnimationLibrary/HillDiff_Down.mov /Users/pna/Work/AnimationLibrary/HillDiff_UpDown.mov Specific Hillslope Processes Examine processes operating on hillslopes:
1.Look first at the physics and geomorphic results of individual events (e.g. what happens when a raindrop hits a pile of loose sediment) 2.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 drops assault) from 2. Stochastic events, for which we dont know the spatial
distribution or temporal order of events (the distribution of rain drops and the timing of storms). Ejection trajectories of
grains blasted into the air by rainsplash The Raindrop
CTSV Raindrop Size and Effects
Rain Splash Experiments Measuring Net Downslope Rainsplash
Transport Creep General term for slow, downslope
mass movement of material in response to gravity on hillslopes
Creep 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. Frost Heave Results from Displacement of Segmented Dowels in Young Pits
Frost heave displacement 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 freezethaw 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 transport Deterministic component heave profile
associated with a single event. Stochastic component magnitude and timing of freeze-thaw events, which depends on the weather.
Freeze-Thaw Exhumation Gelifluction Measurements
Burrowing Tree Throw
Physical Experiments Hillslope Transport by a Loudspeaker Other Diffusive Hillslope Processes 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.
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