GLG362/598 Geomorphology

Processes, landscapes, and important distinctions

Questions?
Weekend field trips are set

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

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?

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"

"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, V(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, U(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.

Basics of process geomorphology

1) Assume balance between forms and process (equilibrium and quasi-equilibrium)

2) Balance created and maintained by the interaction between energy states (kinetic and potential); force and resistance.

3) Changes in force-resistance balance may push the landscape and processes too far: thresholds of change exist: fundamental change of process and thus form.

4) Processes are linked: processes are linked with multiple levels of feedback.

5) Geomorphic analysis occurs at different temporal scales.

Classic vs. process geomorphology

Classic geomorphology (qualitative, taxonomic classification of the landscape-?stamp collecting?) versus quantitative, process-based approach.

Difficulty in applying a broad, physics based approach to landscape development. TIME and complexity.

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 landforms
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

Let's consider a grain on the floor of a channel: http://www.public.asu.edu/%7Emschmeec/bedanim.html--Link from Mark Schmeeckle

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.

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 Research, 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;

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.