Surging and tidewater glaciers

A surging glacier commonly shows little sign of unusual activity for several years. Then it starts to move rapidly and the glacier surface, especially in the lower part, is transformed from a fairly smooth surface into deep crevasses and ice pinnacles. The frontal part may move several kilometres in a few months or years before it suddenly stops. Ice velocities during most surges are 10-100 times those in normal glaciers. Observed velocities vary from about 100 m per day over short periods, to 200-500 myr-1 for glaciers surging for one to two years. Before a surge, the glacier upstream from the terminus normally consists of stagnant ice covered by debris. During a surge, this part of the glacier becomes dynamically active and thickens. The actively moving front may, however, not proceed beyond the area of the previous stagnant ice mass. In this case it is misleading to speak of a net glacial advance.

Surges seem to start when the glacier reaches a critical surface profile. At the end of a surge, on the other hand, the profile of the glacier surface is much lower than for a steady-state glacier. During a surge, large quantities of ice are moved from an upper to a lower area of the glacier. In the upper part of the glacier, the pre-surge ice surface may be visible along the valley sides as remaining ice with steep ice walls down to the surging ice surface. The high velocities observed in surging glaciers cannot be explained by ice deformation, but by basal sliding and/or bed deformation. Therefore, the basal ice must be at the pressure melting point during the surge. Between the surges, on the other hand, some surging glaciers seem to be frozen to the substratum. The rapid decrease in ice velocity during the later stages of surges have been accompanied by floods in the glacier meltwater streams. Large quantities of silt during these outbursts may indicate that effective subglacial erosion can take place during surges.

Surging glaciers have commonly been related to subpolar or polythermal glaciers of a variety of sizes and types. The majority of the surging glaciers have been described from western North America near the Alaska-Yukon border, to the St. Elias Mountains, the Alaska Range, and to parts of the Wrangell and Chugach Mountains. In Iceland, the main, gently sloping outlet glaciers from Vatnajokull are known to surge. In Asia, surging glaciers have been mapped in the Pamirs, Tien Shan, the Caucasus, Kamchatka, and the Karakoram. Surging glaciers have also been reported from the Chilean Andes, Greenland, Svalbard, and from Arctic Canada (see Paterson, 1994, and references therein). A compilation of data from surging glaciers (Paterson, 1994: 359) suggests that most surges occur at fairly regular intervals. There is no evidence of surging ice sheets, although a surging behaviour may take place in the West Antarctic ice streams.

Recent research aimed at understanding the behaviour of surge-type glaciers has generally taken two different approaches. The first approach has been to carry out extensive process studies at individual glaciers, focusing on the nature of the surge mechanism, surge cycles and basal processes (e.g. Raymond and Harrison, 1988). The second type of approach has been to study several surge-type glaciers in a wider region (e.g. Dowdeswell et al, 1991).

Tidewater glaciers (glaciers standing with their front in a fjord or in the sea) in the fjords of Alaska have retreated significantly during the last two centuries. The most spectacular changes have taken place in the Glacier Bay, one of the glaciers having retreated by reconstruction of ice-surface profiles and calculation of basal shear stress 113

about 100 km since the late eighteenth century. In contrast, some tidewater glaciers have been advancing for the last hundred years. The reported advance rates are, however, significantly less than retreat rates (20-40 myr-1 and 200-1700 myr~\ respectively).

Bakaninbreen began surging in 1985/86, forming a surge front where fast-moving surge ice met non-surging ice (Porter et al., 1997). Up to 1995 the surge front moved 6 km downglacier. Yield strengths calculated for the basal sediments at Bakaninbreen range between 16.6 and 87.5 kPa. The estimates of basal shear stress suggest that sediments upglacier of the surge front will be actively deforming, while only limited deformation will take place downglacier of the surge front.

During the 1985/86 surge of Bakaninbreen, a surge front up to 60 m high was formed (Murray et al., 1997). Shear zones and thrust faults were formed in association with the forward movement of the surge front. A ground-penetrating radar showed several subglacial and englacial debris layers reflecting thrusting by the glacier.

Finsterwalderbreen (35 km2), a polythermal glacier in southern Spitzbergen, last surged around the beginning of the twentieth century (Nuttall et al., 1997). Surface elevations have been measured since 1898, showing thinning and frontal retreat. The accumulation area, however, is gradually building up. At present, velocities increase from about 1 m per year at the front to 13 m per year at the equilibrium line. At the bergschrund, the velocity is about 5 m per year. Radar profiles indicate that the glacier is at the pressure melting point at the base, which is also supported by hydrological studies showing high suspended-sediment loads. The authors suggest that the glacier may be building up towards another surge.

The Columbia Glacier is a large, temperate tidewater glacier that calves into the Columbia Bay in Alaska. Venteris et al. (1997) found regular seasonal cycles in speed and stretching rate. These cycles continued after the glacier retreated off the shoal at the end of the fjord around 1983. They suggested that seasonal change in subglacial water is the main controlling factor of the velocity of the glacier. The change in terminus position (extensive retreat) seems to be linked to thinning caused by longitudinal extension, as proposed by van der Veen (1996).

Five surges of Variegated Glacier in Alaska indicate that they all terminated with their surge fronts in the terminal lobe of the glacier, and that different surges penetrated into the terminal lobe by different amounts (Lawson, 1997). Of five studied surges, the one that occurred in 1905-6 penetrated furthest into the terminal lobe. A surge in 1964-65 affected a greater proportion of the glacier than the 1947-48 and the 1982-83 surges.

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