Rock glacier flow

This article was written by Camryn Kluetmeier.

Rock glaciers are part of a continuum, with clean-ice glaciers at one end, debris-covered glaciers in the middle, and rock glaciers at the far end. Like glaciers, rock glaciers flow downslope, but the speed at which they flow is variable on a timescale of days, seasons and decades. Some rock glaciers are speeding up, while others are slowing down in response to climate change. This can affect how stable they are, and can affect their potential as a water resource or increase their hazard potential.

How fast do rock glaciers flow?

Like ice glaciers, rock glaciers move downslope under the influence of gravity, but there are important differences from ice glaciers in how rock glaciers deform and what factors control their deformation.

Rock glaciers flow much slower than ice glaciers, with speeds reaching a maximum of around several meters per year [1, 2]. In contrast, some glaciers can move several meters a day. Slower rock glaciers may only move several millimeters per year [3].

A rock glacier at te base of a cirque in the Uinta Mountains, Utah, USA. Credit: J. Monroe.

Active and inactive rock glaciers

Unlike ice glaciers, rock glaciers can persist on the landscape as a pile of rocks long after all the internal ice that causes movement downslope has melted away. We therefore divide rock glaciers into active and inactive forms [4].

Active rock glaciers contain enough internal ice to move downslope and often have surface features that indicate motion. A series of ridges and furrows is a common feature that makes active rock glaciers look like a lava flow or a heaping ice cream serving.

Rock Glacier example
The debris-rich lobate structure in the foreground is a typical active rock glacier. The snow and ice beneath the debris indicates the glacier potentially has some ice content, and therefore flows due to internal deformation of the ice. Notice also the steep-sided talus slopes which feed the rock glacier. Image credit: “Rock Glacier” by GlacierNPS is marked with Public Domain Mark 1.0

Very steep frontal and lateral slopes are also a sign of an active rock glacier because internal ice is acting like glue, holding rocks together past the angle where they would normally tumble downslope (like in the above photograph).

Inactive or relict rock glaciers no longer move downslope and have very little or no internal ice. These features often have partial vegetation cover and subdued surface topography when compared with active rock glaciers.

An example of (a) an active rock glacier and (b) an inactive rock glacier in the La Sal Mountains, Utah, USA from Google Earth Imagery. Note the ridges and furrows across the surface of the active rock glacier and trees growing on the inactive one.

How do rock glaciers move?          

Rock glacier movement is primarily driven by a shearing force near the base of the feature [5,6].  An example of a shearing force is pushing a deck of cards one direction at the top and the opposite direction at the bottom, causing the cards to stretch out.

This has been studied by drilling boreholes into rock glaciers and observing horizons that move much faster than the surrounding ice-rock mixture [6].

In addition to a primary basal shear horizon, some rock glaciers have a secondary shear horizon at the base of the active layer. The active layer is the top layer of a rock glacier which freezes and thaws seasonally.

Rock glaciers also experience motion due to plastic deformation of internal ice, just like ice glaciers.

A schematic diagram showing the internal structure and deformation of a rock glacier. Created by C. Kluetmeier based on Brencher et al. (2021) and Kenner et al. (2017) [7,8].  

Speeding up and slowing down

How fast a rock glacier moves varies across many timescales. On an hourly to daily timescale, rock glacier speed up has been linked to heavy precipitation and spring snowmelt events [8].

Seasonally, rock glacier velocities are typically marked by a strong acceleration in late spring to summer followed by a deacceleration in the late autumn. This seasonal cycle is water-controlled, with more liquid water reaching the basal shear horizon in the spring and summer [3,9].

Within the past several decades, some rock glaciers in the European Alps have destabilized, speeding up by several orders of magnitude and then ceasing to move at all [10]. This is thought to be caused by warming permafrost and air temperatures due to climate change.

Summary

Rock glacier motion is highly variable and there is still a lot left to discover about how rock glaciers move and the mechanisms that govern them. It’s important to understand rock glacier activity to gain insights into how they may respond to climate change and assess their potential as a hazard and water resource.

About the Author

Camryn Kluetmeier earned a B.A. in environmental geology from Middlebury College in 2022, completing a thesis on inventorying active rock glaciers in Utah, USA using satellite remote sensing tools. She has delved into the world of glaciology as a student on the Juneau Icefield Research Program (2022) and a Summer Student Fellow at Woods Hole Oceanographic Institution (2021).

Currently, Camryn is working as a research specialist on the calibration and validation of the NASA Surface Water and Ocean Topography satellite based at the University of North Carolina at Chapel Hill (2022-2024). She plans to eventually pursue a Ph.D. linking glacial change to shifting water resources with climate change. You can follow her on twitter at @ckluetmeier.

Further reading

References


1. Wahrhaftig, C. and Cox, A.: Rock glaciers in the Alaska Range, GSA Bulletin, 70, 383–436, https://doi.org/10.1130/0016-7606(1959)70[383:RGITAR]2.0.CO;2,1959.

2. Bertone, A., Barboux, C., Bodin, X., Bolch, T., Brardinoni, F., Caduff, R., Christiansen, H. H., Darrow, M. M., Delaloye, R., Etzelmüller, B., Humlum, O., Lambiel, C., Lilleøren, K. S., Mair, V., Pellegrinon, G., Rouyet, L., Ruiz, L., and Strozzi, T.: Incorporating InSAR
kinematics into rock glacier inventories: insights from 11 regions worldwide
, The Cryosphere, 16, 2769–2792, https://doi.org/10.5194/tc-16-2769-2022, 2022.

3. Delaloye, R., Lambiel, C., and Gärtner-Roer, I.: Overview of rock glacier kinematics research in the Swiss Alps, Geogr. Helv., 65, 135–145, https://doi.org/10.5194\gh-65-135-2010, 2010.
4. RGIK: Towards standard guidelines for inventorying rock glaciers: baseline concepts (version 4.2), 13 pp, 2021.


5. Cicoira, A., Beutel, J., Faillettaz, J., and Vieli, A.: Water controls the seasonal rhythm of rock glacier flow, Earth and Planetary Science Letters, 528, 115844, https://doi.org/10.1016/j.epsl.2019.115844, 2019.
6. Krainer, K., Bressan, D., Dietre, B., Haas, J. N., Hajdas, I., Lang, K., Mair, V., Nickus, U., Reidl, D., Thies, H., and Tonidandel, D.: A 10,300-year-old permafrost core from the active rock glacier Lazaun, southern Ötztal Alps (South Tyrol, northern Italy), Quaternary Research, 83, 324–335, https://doi.org/10.1016/j.yqres.2014.12.005, 2015.
7. Brencher, G., Handwerger, A. L., and Munroe, J. S.: InSAR-based characterization of rock glacier movement in the Uinta Mountains, Utah, USA, The Cryosphere, 15, 4823–4844, https://doi.org/10.5194/tc-15-4823-2021, 2021.
8. Kenner, R., Phillips, M., Beutel, J., Hiller, M., Limpach, P., Pointner, E., and Volken, M.: Factors Controlling Velocity Variations at Short-Term, Seasonal and Multiyear Time Scales, Ritigraben Rock Glacier, Western Swiss Alps, Permafrost and Periglacial Processes, 28, 675–684, https://doi.org/10.1002/ppp.1953, 2017.
9. Liu, L., Millar, C. I., Westfall, R. D., and Zebker, H. A.: Surface motion of active rock glaciers in the Sierra Nevada, California, USA: inventory and a case study using InSAR, The Cryosphere, 7, 1109–1119, https://doi.org/10.5194/tc-7-1109-2013, 2013.
10. Marcer, M., Cicoira, A., Cusicanqui, D., Bodin, X., Echelard, T., Obregon, R., and Schoeneich, P.: Rock glaciers throughout the French Alps accelerated and destabilised since 1990 as air temperatures increased, Communications Earth & Environment, 2, 1–11, https://doi.org/10.1038/s43247-021-00150-6,
2021.

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