GLG
362/598 Geomorphology
Processes, landscapes, and important distinctions
Questions?
Saturday field trips
Questions?
Summary
A few definitions
Views of landscapes
So how do you choose a representative piece of the landscape?
Important distinctions
Transport vs. production limited conditions
What drives processes?
What resists them?
Simple concepts of time and equilibrium
A few definitions
Definition of process:
"Action involved when a force induces a change, either chemical or physical,
in the materials or forms at the earth's surface"
or
"Method by which one thing is produced from something else."
-Ritter et al., 1995, p. 3.
Process distinction:
Exogenic:
"Outside," processes working on the surface to bring it to a common level
(gradational, as in aggradation [deposition] or degradation [erosion]).
Weathering, mass wasting, erosion and deposition (running water, wave action/currents/tides,
wind activity, glacial action, groundwater), work of organisms (including
humans). See
lecture 1: the vertical component of geomorphic displacement,
g(x, y, t, H).
Endogenic:
"Inside," processes generally operating within a planetary body acting
to displace material. Tectonism, volcanism, and extraplanetary processes.
See
lecture 1: this is the vertical component of tectonic displacement,
v(x, y, t).
Definition of landscape:
"Distinct association of landforms, as operated on by geological processes
(exo- or endogenic), that can be seen in a single view."
-Glossary of geology
Definition of landform:
"Any physical, recognizable form or feature on the earth's surface, having
a characteristic shape, and produced by natural causes; it includes major
forms such as a plain, plateau, or mountain, and minor forms such as a
hill, valley, slope, esker, or dune. Taken together, the landforms make
up the surface configuration of the earth."
-Glossary of geology
Ideally (IMHO) , the discretization of the landscape into landforms
should be done on the basis of processes operating over the earth's surface.
Views of landscapes
For a given view of a landscape, answer these questions:
What are the landforms that you see?
What are the processes operating within this landscape?
What drives the surficial processes here?
What resists the surficial processes here?
How does energy flow in this landscape?
What can you say about the development of this landscape?
What would be a representative piece of this landscape? How would you
divide it up to get the most important processes and landforms?
So how do you choose a representative piece of the landscape?
What is representative? Depends on the question.
Elements (boxes or landforms/processes?)
Boundaries (spatial and flux)
See the following examples of the key landform elements being hillslopes,
valleys, and channels and the deliniation of drainage basin as representative
piece (Montgomery and Dietrich, 1989, Source areas, Drainage density, and channel initiation, Water Resources Resedarch, v. 25, no. 8, p. 1907-1918 and Montgomery and Dietrich, 1994, A physically based model for the topographic control on shallow landsliding, Water Resources Research, v. 30, no. 4, p. 1153-1171.)
Important distinctions
Transport vs. production limited conditions
Pictures
Dixie Valley transport-limited fault scarp
Babbit Lake (AZ) Production-limited fault scarp
Babbit Lake tranport-limited
Mixed transport- and production-limited fault scarp
Symbolically
The vertical component of geomorphic displacement must be considered in
terms of the material, the soil, that can be eroded. Soil is the material
in which particles are of sufficient size and reduced cohesion for transport
[Rosenbloom and Anderson,
1994]. The soil is the material between
the topographic surface
H(x, t) and the soil/bedrock interface
B(x, t). The elevation of the topography at the end
of a time increment must be greater than or equal to the elevation of the
soil/bedrock interface at the end of the time increment;
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If more soil is available for transport than can be eroded, the situation
is called "transport-limited;" whereas if more could be eroded than is
available, the situation is called "production-limited" [Carson and
Kirkby, 1972]. This material balance is based upon the ideas of Gilbert,
1877, and formulated in the manner of Ahnert, 1970; Anderson
and Humphrey, 1989; and Carson and Kirkby, 1972.
In order to address those possible conditions, we must consider the
volume flow rate per unit width, Q, separately from the transport
capacity of the specified process per unit width, Qt [Anderson
and Humphrey, 1989; Rosenbloom and Anderson, 1994]. In general,
Q < Qt ; Qt depends upon the predefined behavior
of the process transporting material along the profile. If conditions are
completely transport-limited, Q = Qt and the process is transporting
at full capacity. However, by allowing for a production-limited condition,
the process may not carry at full capacity because the amount of erosion
is limited by supply. Thus, Q < Qt.
References for this quote:
Ahnert, F., Brief description of a comprehensive three-dimensional process-response
model of landform development,
Zeitschrift fur Geomorph., Supplementband,
24, 11-22, 1970.
Anderson, R. S. and N. F. Humphrey, Interaction of weathering and transport
processes in the evolution of arid landscapes, in Quantitative dynamic
stratigraphy, edited by T. A. Cross, pp. 349-361, Prentice-Hall, Englewood
Cliffs, New Jersey, 1989.
Carson, M. A. and M. J. Kirkby, Hillslope form and process, 475
p., Cambridge University Press, Cambridge, 1972.
Gilbert, G. K., Report on the geology of the Henry Mountains,
U.S. Geographical and Geological Survey of the Rocky Mountain Region, Washington,
D.C., 1877.
Rosenbloom, N. A. and R. S. Anderson, Hillslope and channel evolution
in the marine terraced landscape, Santa Cruz, California, Journal of
Geophysical Research, Tectonics and Topography Special Volume, 99, 14,013-14,029,
1994.
Transport versus Production-limited simple calculation for a topographic profile. Grey is bedrock, so what is between the solid and dashed lines is transportable.
What drives processes?
Energy is capacity for doing work (kinetic and potential)
1) Gravity
2) Endogenic processes produce relief
3) Climate--Driven by solar energy
What resists them?
Geologic framework: Rocks and structure
Rocks: resistance to weathering (see above; production versus transport
limitation)
Importance of scale: Large scale reflects gross geology of the
area: regional geomorphology
Intermediate scale: Lithologic differences generate differences in
individual alndforms
Small scale: inmohogeneities in a rock unit will generate minor variations
in topography and small features.
Simple concepts of time and equilibrium
Uniformitarianism
Uniformitarianism underpins what we think about the past and the present.
Divided into two components:
-
A substantive uniformitarianism, dismissed as untrue, which postulates
a uniformity of material conditions or rates of processes.
-
A methodological uniformitarianism comprising a set of two assumptions:
(a) that natural laws are constant in space and time, and (b) that no hypothetical
unknown processes be invoked if observed historical results can be explained
by presently observable processes.
-Gould, S. J., 1967, Is uniformitarianism useful? J. Geol. Educ., v. 15,
149-150, as presented by Thorn, C. E.,
Introduction to theoretical geomorphology,
247 p., Unwin & Hyman, Boston, MA, 1988.
Neocatastrophism
"…a modern reintroduction of the notion that rare, large-magnitude events
(even unique events or singularities) appear to have played important roles
in geologic and geomorphic history" (Thorn, 1988) Uniformitarianism
vs. neocatastrophism implies that there is a question of magnitude and
frequency in the significance of geologic or geomorphic events.
Categories of time
Steady time (10-2 - 100 yrs); "Landforms do not change."
Graded time (101 - 104 yrs); "Landforms change,
but offsetting effects maintain an average form"
Cyclic time (105 - 107 yrs); "Landforms progressively
change."
Timescale of interest depends on research question
Time is irreversible and ordered.
GLG
362/598 Geomorphology