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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October 9th 2007
Need to address both the source of regolith and its transport.
Both are climate-dependent.
Q = -k dz/dx
But climate and all
physics hides in k
Transforms the hydrologic behavior of the landscape
Richards and Kump
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.
e.g. 10Be, 26Al
with half-lives of
order 1 Myr
Bedrock surface using 10Be
Bottom line: they are VERY slow rates…
Water in the landscape
Courtesy David Furbish
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
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.
The pre-land plant world would have operated
In a very different way. Ditto Mars.
Osborne Mountain, Wind River range
Scale for w = 5 microns/yr!!
rates are 5-10 microns/yr
Or 5-10 m/Ma
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
Note timescale for achieving steady state is several Ma, so must average over glacial-interglacial cycles… (gulp)
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