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High precision, multi-year survey data from the Galena Creek rock glacier, Wyoming

Sarah K. Konrad and Neil F. Humphrey (U. Wyoming), Noel Potter, Jr. and Don J. Hartman (Dickinson College), W. Tad Pfeffer and Daniel P. Costello (U. Colorado), Eric J. Steig (U. Pennsylvania), and Douglas H. Clark (W. Washington U.),

 

Abstract

In 1997, we placed more than 100 small bolts into surface boulders of the Galena Creek rock glacier (GCRG), a valley-type rock glacier in the Absaroka Mountains, Wyoming. We surveyed the initial bolt locations, and resurveyed in 1998 and 1999. By using high-precision survey techniques we were able to establish a surface velocity field accurate to 0.02 m/yr. Measured velocities range from 0 to 0.95 m/yr. Ablation rates of the ice underlying the surface debris can be inferred from the vertical surface velocity and local surface slope, and range from 0 to 0.20 m/yr. We directly measured depth of the surface debris at the bolt locations, and observed a negative correlation between local debris thickness and inferred ablation rate. Unlike a typical ice glacier, ablation rates decrease downglacier due to an increase in surface debris. Flow models based on fundamental glaciologic principles provide good approximations of the observed GCRG velocities and ablation rates, and support a glacigenic origin of the rock glacier.

 

Location Map

 

Surface Velocities and Ablation Rates

 

1997-1998

1998-1999

1997-1999

 

 

Radiocarbon Ages

 

Steady State Flow Model and Age Predictions

 

Input Parameters

* ablation zone length = 1250 m

* accumulation zone length = 200 m

* width = 300 m

* slope = 15°

* ice density = 900 kg/m^3

* A = 3.0 * 10 -24 1/(s*Pa)

* n = 3

* ablation = e-2*(debris thickness)

* debris thickness:

Debris thickness is determined by allowing a rock glacier with a constant debris supply to flow until it approaches steady state (i.e.,balancing debris cover with ablation). The initial debris cover is 1.3 m. The resulting debris cover pattern also smooths the rectangular borders of the rock glacier.

 

Constructing the Model

We chose to create a steady state model of rock glacier flow because there are far toomany unknowns at our current level of knowledge to sufficiently control a transient model. Although we are certain the rock glacier is not in steady state, we believe that steady state modeling can help to constrain the rock glacier "unknowns." In order to simplify the complex geometryof the Galena Creek rock glacier, we createa rectangular glacier of constant slope with dimensions chosen to approximate GCRG. We specify a parabolic cross section, the dimensions of which are determined by balancing the flux in and out of a given cross section. The flux in turn is determined by the input parameters.

 

The resulting glacier has a maximum depth of ~50 m and a maximum surface velocity of ~2 m/yr.

 

Surface Flow Field and Isochrons

 

Comparisons:

Velocity: The model predicts velocities about twice that currently observed.

Depth: The model predicts depths quite similar to depths calculated by simply using the local ice slope and velocity with standard flow parameters.

Age: Although the ages predicted by the model encompass the age range of the organic samples, the model suggests that the rock glacier ice is relatively young along its centerline. The 1680 yr radiocarbon date is from near the center flow line of GCRG. One simple way to increase the predicted ages (and at the same time reduce velocities) would be to modify the shape of the model glacier, making it more flat-bottomed.

Steady State: Although GCRG is likely not in steady state, such modeling provides valuable insight into rock glacier behavior.

 

References

 

Ackert, R. P., Jr., 1998, A rock glacier/debris-covered glacier system at Galena Creek, Absaroka Mountains, Wyoming: Geografiska Annaler, v. 80A, p. 267-276.

Clark, D. H., Steig, E. J., Potter, N. Jr., and Gillespie, A. R., 1998, Genetic variability of rock glaciers: Geografiska Annaler, v. 80A, p. 175-182.

Clark, D., Steig, E., Potter, N., Fitzpatrick, J., Updike, A., and Clark, G., 1996, Old ice in rock glaciers may provide long-term climate records: EOS (Transactions, American Geophysical Union), v. 77, p. 217-222.

Fitzpatrick, J., Steig, E. J., Clark, D. H., Potter, N. Jr., Updike, A., Clark, G. M., and Jacobsen, S. (1995). Rock glaciers as archives of old glacier ice: evidence from an ice core at Galena Creek, Wyoming. EOS, Transactions, 76, p. F2141.

Gillespie, A. R., Clark, D. H., Steig, E. J., and Potter, N., Jr., 1997, Chapman conference delves into the significance of rock glaciers: EOS (Transactions, American Geophysical Union), v. 78, p. 208-209.

Konrad, S. K., Humphrey, N. F., Steig, E. J., Clark, D. H., Potter, N., Pfeffer, W. T., and Costello, D., 1998, Flow rates, age constraints, and accumulation zone observations from the Galena Creek rock glacier, Wyoming: Abstracts, American Geophysical Union Annual Fall Meeting.

Potter, N. (1972). Ice-cored rock glacier, Galena Creek, Northern Absaroka Mountains, Wyoming. Geological Society of America Bulletin 83, 3025-3058.

Potter, N. Jr., Steig, E. J., Clark, D. H., Speece, M. J., Clark, G. M., Updike, A. B., Galena Creek Rock Glacier revisited--new observations on an old controversy. Geografiska Annaler 80A: 251-266 (1998).

Steig EJ, Clark DH, Potter N Jr. & Gillespie AR. The geomorphic and climatic significance of rock glaciers. Geografiska Annaler 80A: 173-174 (1998).

Steig EJ, Fitzpatrick JJ, Potter N Jr., Clark DH. The geochemical record in rock glaciers. Geografiska Annaler 80A: 277-286 (1998).

(New!)

Konrad, S. K., Humphrey, N. F., Steig, E. J., Clark, D. H., Potter, N. Jr., and Pfeffer, W. T., in press, Rock glacier dynamics and paleoclimatic implications. (Expected in the December, 1999 issue) Geology Paper