ABSTRACT
Glacier surging represents a cyclic flow instability that is triggered from within the glacier system rather than by external climate forcing. The active phase of a surge involves the transfer of ice from a reservoir area to the snout of a glacier and can produce ice flow velocities up to one thousand times the flow rate of intervening non-surge phases (Clarke et al., 1984; Raymond, 1987). This may result in the rapid advance of the glacier front and a concomitant thinning of the reservoir area. Between surges, periods of slow flow, or quiescent phases, are characterized by snout stagnation and ice build up in the reservoir area. Although individual surging glaciers display uniform return periods there are large variations between glaciers and regions (Post, 1969; Clarke et al., 1986; Dowdeswell et al., 1991; Hamilton and Dowdeswell, 1996). A climatic linkage to surging was modelled by Budd (1975), who suggested that a continuously fast-flowing glacier is capable of discharging its annual mass balance, whereas a surging glacier has a total mass throughput that is too small to sustain fast flow but too large to be discharged by slow flow alone, thereby initiating a regular surging cycle. As the reservoir zone builds up, a thermal boundary may exist at the down-glacier end, and water storage increases at the bed. Thus, it should be noted that, while there is a climatic linkage, ultimately surging is the result of oscillations in the internal dynamics of the glacier. Specifically, the large changes in glacier velocity are driven by reorganizations in the subglacial drainage system (Clarke et al., 1984; Kamb et al., 1985; Clarke, 1987; Fowler, 1987; Kamb, 1987). In Iceland, surging is likely associated with geothermal activity as this can allow cyclical build up and release of large subglacial meltwater reservoirs (Björnsson, 1975, 1992). However, many of the surging glaciers may be predisposed to this type of behaviour. For example jökulhlaups drain regularly from Grimsvötn lake out beneath Skiedararjökull (intervals of 1–10 years) but these do not induce the glacier to surge, an event which occurs independently (Björnsson, 1998). Regardless of the trigger mechanism, the landform-sediment assemblages produced by surging glaciers appear to be consistent and predictable. Significantly, moraines deposited by surging glacier margins cannot be modelled as the products of steady-state glaciers in equilibrium with climate. Moreover, the identification of former surging within Pleistocene ice sheets remains a problem area that glacial geologists continue to tackle through the identification of diagnostic palaeo-surge signatures in the geomorphological, sedimentological and stratigraphical record.
