Glacier surging

Abstract

Surge-type glaciers have a surge cycle that involves long periods of quiescence, with flow velocities much lower than the balance velocity, interspersed with short surges, in which 10–1000-fold increase of flow results in average velocities greatly exceeding the balance velocity. It is still elusive what exactly triggers these sudden switches in flow mode, but controls that have been identified and for which field evidence exists are thermal regime, configuration of subglacial hydrology, and bed deformation. Basal shear stress, the amount and distribution of subglacially stored water, and type of subglacial drainage system (within or above a soft deformable bed) appear crucial in the process of surging. Two main surge mechanisms have been verified by observations on surging glaciers in different regions: the Alaskan-type mechanism, a hydrological instability in temperate, soft-bedded glaciers, caused by till weakening and a switch in the subglacial drainage system, and the Svalbard-type mechanism, a thermally induced soft bed instability in polythermal (subpolar) surge-type glaciers. Distinguishing surge-type from normal glaciers by remote sensing can be achieved for glaciers with clear development of the following surface features diagnostic for surging: • A surge advance • Elongated “tear-shaped” moraine loops and/or sheared off tributaries • Trimlines of former ice surface hanging above the present ice surface • A marked surge bulge • A completely crevassed glacier (that was not entirely crevassed before) • A sudden increase in iceberg production • Potholes covering large parts of the glacier Only a small percentage of glaciers worldwide are of surge type, and these tend to occur in geographical clusters. No single factor or combination of glacial or environmental factors has been found conclusive determinants of surging. Instead, controls on surging might be regional and related to specific surge mechanisms. There still remain a number of unresolved questions in the understanding of surge behavior, and many that were posed in Raymond (1987) have not been conclusively answered. Some key questions include: (1) what are the triggers of the flow instabilities leading to surges; (2) which factors control surge initiation, propagation, and termination; (3) what are the critical thresholds between real surges and slow surges, and (4) how are restrictions in outflow related to surging? In the historic line of asking the questions: What are glacier surges? (Meier and Post, 1969), How do glaciers surge? (Raymond, 1987) andHow much do we really know about glacier surging? (Harrison and Post, 2003) perhaps future overview papers could address the questions Why and when do glaciers surge? and Why don’t all glaciers surge?.