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

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.

Regolith Discharge on Hilltops and Hillslopes

r

b

Parabolic Hilltop

A Diffusive (or Diffusional?) Landscape

Terrace edges become more diffuse with age

/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:

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 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).

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 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 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.

Summary - Diffusive Hillslope Processes