The occurrence of topographic forms on Mars similar to terrestrial "rock glaciers" implies that they are suggestive of buried ice and thus indicative of Martian permafrost or glacial conditions [Baker, 2001]. However, due to the different formation mechanisms which are still in dispute a unique definition for this geomorphologic feature is hard to find. Detailed discussions concerning the different terminologies can be found in Martin and Whalley (1987), Whalley and Martin (1992), Hamilton and Whalley (1995) and Whalley and Azizi, (2003). Fig. 1 presents a summary of such different types and terminologies.
Therefore, Whalley and Azizi (2003) propose that the best way of rock glacier definition is a non-generic description which they derived from Potter (1972) and Washburn (1979). Following these classifications a rock glacier is "a tongue-like or lobate body, usually of angular boulders, that resembles a small glacier, generally occurs in high mountainous terrain and usually has ridges, furrows, and sometimes lobes on its surface, and has a steep front at the angle of repose." A rock glacier may be 130 m deep and nearly 1.5 km long [Britannica Concise Encyclopedia (2006); Link] and usually has low flow rates of <1m/year [Martin and Whalley, 1987]. Rock glacier can typically be found above the timberline of mountains, slowly moving down a valley and fed by material fallen from the valley walls, which resembles the terminus material of a glacier [Britannica Concise Encyclopedia (2006); Link]. Fig. 2 presents an example for a terrestrial rock glacier located in Alaska.
The three main models of rock glacier formation involve the permafrost origin, a glacier-derived origin, and a mass-wasting (landslide) origin [Whalley and Martin, 1992; Whalley and Azizi, 2003]. The permafrost model [Wahrhaftig and Cox, 1959; Barsch, 1996; Haeberli, 1985] presupposes that the mean annual air temperature is at most -1.5°C which supports the formation of "congelation" ice from freezing water by ice segregation or water injection under pressure [Whalley and Azizi, 2003]. The prerequisite of the thermal conditions in this model makes rock glaciers to occur "zonal" and thus be indicative for permafrost regions.
The glacial model [Whalley and Martin, 1992] involves the creep of an ice body (<50m), which is preserved thanks to an insulating layer of weathered rock debris. A certain temperature regime is not required because the controls on maintaining this buried ice are related to thickness of debris cover, making rock glaciers to "azonal" features [Whalley and Azizi, 2003]. Following the landslide, or "catastrophic", model [Johnson, 1974, 1984] the presence of ice is not obligatory required. Deduced from the similarity of topographic forms rock glaciers may be derived from rapid landslides or rock avalanches [Whalley and Martin, 1992] and do not flow after emplacement.
Due to the disagreement in rock glacier formation the interpretation of Martian rock glaciers is particularly troublesome. For example the question of nature, volume and significance of the ice bodies following the different hypotheses is unanswered but is very important to understand and classify the Martian forms. Nevertheless the advancement in new orbital data derivation and the possibility to map these features involving digital terrain models and Geographical Information Systems help to identify and discriminate the different features and progresses the understanding of Martian rock glaciers especially concerning their distribution as thermally zoned or age differentiated [Whalley and Azizi, 2003]. Fig. 3 provides an example for a probable Martian rock glacier.
Last update: 07/06/2010 14:37
