Calving glaciers

Several mechanisms of glacier calving have been described; however, the fundamental control and relationship with calving rate is poorly known. This makes it difficult to explain the lack of climatic sensitivity of different glaciers, in particular the order-of-magnitude difference in calving rate between tidewater and lake-calving glaciers (Fig. 4.49). The mechanism for 'normal' slab calving includes the development of an overhang before failure occurs as a result of deformation of the glacier close to the ice cliff. Minor normal faults or englacial shear bands have been observed and explained as forward bending of the cliff. Profiles of calving cliffs normally show rotation of the cliff profile about the base.

Kirkbride and Warren (1997) used repeated photographs and field surveys to reveal the mechanism of ice-cliff evolution at Maud Glacier, a temperate glacier calving in a lake in New Zealand. Their study showed that calving is cyclic: (1) waterline melting and collapse of the roof of a sub-horizontal notch at the cliff foot; (2) calving of ice flakes from the cliff face leading to a growing overhang from the waterline upward and cracks opening from the glacier surface; (3) calving of slabs as a result of the developing overhang, returning the cliff to an initial profile; (4) seldom subaqueous calving from the ice foot.

The relationship between climate and calving glaciers is not straightforward, and it is rarely possible to draw reliable conclusions about climate variations from calving glaciers (Warren, 1992). Iceberg calving leads to instability in the glacier, causing the glacier to oscillate asynchronously with climatic variations and with other calving and non-calving glaciers. The rate of calving is primarily controlled by water depth. Calving dynamics are, however, poorly understood. The dynamics seem to be different from temperate to cold/polar glaciers, and between grounded and floating fronts.

The physical processes that control calving rates are complex, poorly understood, and not yet quantified (Bahr, 1995; van der Veen,

1995). There is, however, a strong linear correlation between calving speed and water depth (Fig. 4.49) at grounded, temperate calving fronts. This relationship can be applied in both freshwater and marine environments, but the slope coefficient is some 15 times greater at tidewater termini (Warren et al., 1997). The instability introduced to glaciers by calving introduces heterogeneous glacial responses to climate (e.g. Motyka and Beget,

1996) through the complicated interaction of the calving front with topography and effective water depth (Sturm et al., 1991).

Glacier Uppsala, a freshwater calving glacier in southern Patagonia, has been retreating since 1978 (Naruse et al, 1997). Subsequent to a significant retreat of approximately 700 m in 1994, the recession seems to have ceased in

10 100 1000 Water depth (m)

Figure 4.49 The relationship between water depth and calving rates at the margins of tidewater glaciers (solid dots) and freshwater glacier termini (open circles). (Adapted from Benn and Evans, 1998)

10 100 1000 Water depth (m)

Figure 4.49 The relationship between water depth and calving rates at the margins of tidewater glaciers (solid dots) and freshwater glacier termini (open circles). (Adapted from Benn and Evans, 1998)

1995. A thinning rate of 11m per year was recorded close to the front between 1993 and 1994. They found that temperature alone could not explain the reason for this thinning. It was therefore suggested that calving was the main reason for this extensive thinning at the glacier terminus.

Most Patagonian glaciers are retreating rapidly (Casassa et al, 1997). Glacier O'Higgins, a freshwater calving glacier, has experienced the largest retreat from 1945 to 1986. Climate warming, combined with the detachment of the glacier front from an island, are believed to be the main causes of the retreat (Casassa et al, 1997).

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