Rock glacier

Rock glacier with multiple flow lobes, Chugach Mountains, Alaska.

Rock glaciers are distinctive geomorphological landforms, consisting either of angular rock debris frozen in interstitial ice, former "true" glaciers overlain by a layer of talus, or something in between. Rock glaciers may extend outward and downslope from talus cones, glaciers or terminal moraines of glaciers.^[1] There are two types of rock glaciers: periglacial glaciers, or talus-derived glaciers, and glacial rock glaciers, such as the Timpanogos Glacier in Utah, which are often found in the former places of glaciers. Possible Martian rock glacier features have been identified by the Mars Orbiter spacecraft.^[2] A rock glacier, especially if its origin is unclear, can be considered as a discrete debris accumulation.

Formation

The two known factors that must be present in order to create rock glaciers are low ice velocity and permafrost. Most glacial rock glaciers are created by the recession of debris covered glaciers. Glacial rock glaciers are often found in cirque basins where rocky debris falls off the steep sides and accumulates on ice glaciers.^[3] As the glaciers shrink, their composition changes as they become increasingly covered with debris. Eventually, the glacial ice is replaced by ice cored rocks.^[4] With the exception of ice-cored rock glaciers, rock glaciers are a periglacial process. This means that they are a nonglacial landform associated with cold climates, particularly with various aspects of frozen ground. Periglacial rock glaciers require permafrost instead of glacial ice in order to form. Instead, they are caused by continuous freezing occurring within a talus lobe.^[5]

Movement

Rock glaciers move downslope by deformation of the ice contained within them, causing their surface to resemble those of glaciers. Some rock glaciers can reach lengths of 3 km and can have terminal embankments of 60 m high. Blocks on the surface can be up to 8 m in diameter. Flow features on the surface of rock glaciers may develop from:

• Deformation of the ice core.
• Movement of the debris cover along the debris-ice interface.
• Deformation from a period of glacial advance.
• Changes in the hydrologic balance.

Their growth and formation is subject to some debate, with three main theories:

• A permafrost origin, which implies that the features are related to permafrost action rather than glacial action;
• A mass wasting or landslide origin which does not require the presence of ice and suggests a sudden catastrophic origin with little subsequent movement.

Rock glaciers may move or creep at a very slow rate in part dependent on the amount of ice present.

According to recent studies, rock glaciers positively influence the streams around them ^[6]

Human use

Rock glaciers in the Chilean Andes help supply the water for much of Chile, including the capital of Santiago. Mining operations in the high mountains have led to the degradation and destruction of more than two rock glaciers. Several copper mines dump their waste rock onto rock glaciers, which results in faster melting and higher velocity movement of these rock glaciers. The dumping of waste rock on the rock glaciers may lead to their destabilization. In 2004, protesting irrigation farmers and environmentalists changed rules so new mining projects can no longer damage or alter rock glaciers in Chile.^[7]

References

1. Fred H. Moffit; Stephen R. Capps (1911). Geology and Mineral Resources of the Nizina District, Alaska, USGS Bulletin 448. U.S. Government Printing Office. p. 54-55.
2. W. Brian Whalley and Fethi Azizi, Rock glaciers and protalus landforms: Analogous forms and ice sources on Earth and Mars, Journal of Geophysical Research: Planets, Volume 108, Issue E4, April 2003 (abstract)
3. Easterbrook, D. J (1999). Surface processes and landforms. Prentice Hall. p. 405.
4. Gürgen, Gürcan, et al. "Debris Covered Glaciers and Rock Glaciers," e-Journal of New World Sciences Academy (NWSA) 5.1 (2010): 32-45.
5. • Dale Ritter, R.Craig Kochel, Jerry. Miller (1995). Process Geomorphology, 3rd Ed. Wm. C Brown Communications, Inc. pp. 383–385.
6. http://www.bioone.org/doi/abs/10.1657/1938-4246-46.3.645?journalCode=aare
7. • Orlove, Ben (2008). Darkening Peaks: Glacier Retreat, Science, and Society. Berkeley: University of California Press. pp. 196–202.

• Douglas Benn & David Evans (1998). Glaciers & Glaciation. London: Arnold. pp. 257–259.
• Hausmann, H.; K. Krainer, E. Bruckl, W. Mostler (2007). "Internal structure and ice content of Reichenkar rock glacier (Stubai Alps, Austria) assessed by geophysical investigations" (PDF). Permafrost and Periglacial Processes 18 (4): 351–367. doi:10.1002/ppp.601.

External links

• Earth Observatory image and explanation of a rock glacier on Sourdough Peak, Alaska
• USGS Glossary of Glacier Terminology
• AGU - Terrestrial Models of Rock Glacier and Protalus Lobe Formation