rock glaciers

rock glaciers Rock glaciers are alpine features best defined by their morphology and appearance, since there is debate about their origin and significance. They are elongate tongues of rock rubble which flow slowly downhill. The surface velocity is generally less than 1 m year⁻¹, although some with greater velocities (> 5 m year⁻¹) have been described. If no movement can be detected, rock glaciers are generally referred to as ‘relict’; if highly vegetated with subdued features, as ‘fossil’. Flow-related features are commonly seen as ripples on the surface, although it is not known whether these extend to any depth. The length is typically about l km, but many shorter examples exist and some may be up to 3 km long; their width is generally a few hundred metres. The features typically have their heads in corries (cirques) in which there may be a small glacier, but this is not always seen. The elongate forms are best considered as being rock glaciers proper, although the term has also been used for features that are broader than long and which (typically) have their upper sections against cliffs or scree. It has been suggested that the latter features are best termed ‘protalus lobes’ (see Fig. 1).

There are three main theories on the origin of rock glaciers. One suggests that they are glacially derived with a veneer of weathered rock debris over glacier ice. The debris protects the ice from melting and sustains a thin body of ice that would otherwise have melted away. Because this glacier core is thin, it flows only slowly. A second ‘permafrost model’ explains the slow movement as creep of ice dispersed within scree and finds that a glacial derivation is not necessary. It does, however, demand permafrost conditions (usually seen as a mean annual air temperature of less than –1.5 °C) for the formation of the ice and its continued creep. The third model suggests that some rock glaciers (or protalus lobes) are formed by sudden, perhaps catastrophic, failure of scree slopes, but this view is not widely held as a general model. Rock glaciers have been used as indicators of permafrost (or past permafrost if relict features are considered) but this applies only if the permafrost model is valid. Until recently, the origin and flow mechanisms were generally attributed to the permafrost model. Although traces of glacier ice could be seen in some rock glaciers, permafrost conditions were considered to be the only way in which the features could exist. Observations of glacier ice down the length of some rock glaciers have now been established and thus at least some rock glaciers are known to have glacier ice cores. It is possible that modern dating and isotopic techniques will allow ice from such rock glaciers to provide a climatic record. The full implications for recognizing climate change through rock glaciers still require investigation.

W. Brian Whalley

Bibliography

Giardino, J. R., Shroder, J. F., and Vitek, J. D. (eds) (1987) Rock glaciers. Allen and Unwin, London.
Hamilton, S. J. and and Whalley, W. B. (1995,) Rock glacier nomenclature: A reassessment. Geomorphology, 14, 73–80.
Martin, H. E. and and Whalley, W. B. (1987) Rock glaciers: a review, Part 1. Progress in Physical Geography, 11, 260–82.
Whalley, W. B. and and Martin, H. E. (1992) Rock glaciers: II models and mechanisms. Progress in Physical Geography,16, 127–86.