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Soils and Geomorphology Bob Anderson October 9th 2007 Hillslopes Convex hilltops G. K. Gilbert’s view of a convex hilltop (1909) Need to address both the source of regolith and its transport. Both are climate-dependent. Regolith balance climate Q = -k dz/dx But climate and all

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soils and geomorphology

Soils and Geomorphology

Bob Anderson

October 9th 2007


G. K. Gilbert’s view of a convex hilltop (1909)

Need to address both the source of regolith and its transport.

Both are climate-dependent.


Regolith balance


Q = -k dz/dx

But climate and all

other interesting

physics hides in k


Occurs throughout the critical zone

Transforms the hydrologic behavior of the landscape

frost cracking
Frost cracking
  • Time spent within the “frost-cracking” window ~ -3 - -8°C
  • Poses a very specific question of temperatures in the subsurface
k a exp e a rt

k = A exp( -Ea / RT)

Arrhenius equation


How do we measure “regolith production” or lowering of the regolith-bedrock interface?

Basin-wide averages from sediment and solute output

But this requires assumptions about steady state…

At a point:

You wait a really really really long time (>>PhD timescale)


You use a long term integrating tool, and measure the concentration of cosmogenic radionuclides.


Cosmogenic radionuclides

e.g. 10Be, 26Al

with half-lives of

order 1 Myr


Dating a Baffin fjord

Bedrock surface using 10Be


Bedrock lowering rates based upon 10Be concentrations

Bottom line: they are VERY slow rates…


But what sets these rates is still up for debate… the connections to climate and tectonic settings are still fuzzy, entangled


Transport of regolith, Q

One example:


Rain bombs!

Courtesy David Furbish


Another example: Frost creep due to repeated freeze-thaw cycling

Single frost event:

• Displacement ~ slope

• Discharge ~ square

of frost depth

Simulation of frost creep

Green = maximum heave; red = post-thaw

Multiple frost events:

• Concave up profile


RSA 2002


But reality is MUCH more complicated and interesting

I = f(S), the saturation state of the soil

So we must allow S to evolve

dS/dt = f(S,P,T) -- i.e. climate again

The California case:

Early storms yield <10% runoff

Late storms yield > 60% runoff

So we need to know the sequence of rain input: the rainfall intensity, the duration of the storm, the interval between storms, and the number of storms per year.


Vegetable matter Vegetation matters.



Infiltration capacity

Root strength…

The pre-land plant world would have operated

In a very different way. Ditto Mars.


High summit surfaces of the Laramide province

Osborne Mountain, Wind River range


Sampling tors for cosmogenic radionuclides

Scale for w = 5 microns/yr!!


High surfaces

Model rules

Cosmogenic radionuclide


Surface lowering

rates are 5-10 microns/yr

Or 5-10 m/Ma


Late Cenozoic features:

Ornamentation of the crests

differential lowering of high surfaces vs glacial canyons

Ornamentation of the front

transient incision of the fluvial system






Front Range high surface


A few landscapes behave themselves…

Note timescale for achieving steady state is several Ma, so must average over glacial-interglacial cycles… (gulp)


Residence time of regolith (or soil) on a landscape:

Estimated by T=h/w. In the case of the high surfaces,

h = 1m, w = 5m/Ma

T=1/5 Ma or 200ka



• The majority of any landscape is hillslopes

• Most of them are cloaked with soils

• The evolution of soil thickness is modulated by both production rate of regolith and its transport

• We can measure soil production using cosmogenic radionuclides

• In high alpine settings transport is dominated by periglacial processes

• The high surfaces of the Rockies are likely steady state surfaces, and residence time is long relative to changes in climate