Debris Entrainment

The evacuation and entrainment of rock fragments at the glacier bed were briefly outlined in Section 5.2.2 but before we consider low-level debris transport in depth we need to consider the main processes of basal debris entrainment.

Figure 7.5 Supraglacial debris stripes emerging from longitudinal foliation on the surface of Midre Lovenbreen, Svalbard. The glacier has receded in recent years and the debris stripes can be traced onto the glacier forefield. [Photograph: N.F. Glasser]
Figure 7.6 Photograph of supraglacial debris on the surface of a glacier in the Cordillera Blanca, Peru. Note that the debris is coarse, angular and contains little in the way of fine material. A supraglacial stream is beginning to rework the debris. [Photograph: N.F. Glasser]

1. Regelation. This process involves entrainment of debris at the bed by a combination of pressure melting and refreezing (regelation), to create a basal debris layer (Figure 7.7). It occurs under both temperate and polythermal glaciers. The matrix of this material is commonly fine-grained, comprising clay or silt. Clasts up to boulder-size, with subangular and subrounded shapes, faceted surfaces and striations, also occur.

Figure 7.7 The subglacial debris layer of a temperate glacier in Patagonia. Ice flow left to right.

Note how the thickness of the basal debris layer changes as the glacier flows across the bed, particularly where it is compressed against the bedrock obstacle. Person in lower left indicates scale. [Photograph: N.F. Glasser]

Figure 7.7 The subglacial debris layer of a temperate glacier in Patagonia. Ice flow left to right.

Note how the thickness of the basal debris layer changes as the glacier flows across the bed, particularly where it is compressed against the bedrock obstacle. Person in lower left indicates scale. [Photograph: N.F. Glasser]

2. Entrainment associated with ice deformation. Ice deformation can result in reorganisation of the debris that is already incorporated at the ice surface or bed. In polythermal glaciers in Svalbard, for example, three main modes of entrainment have been recorded in addition to regelation: (i) incorporation of angular rockfall material within the stratified sequence of snow, firn and superimposed ice; (ii) incorporation of debris of both supraglacial and basal character within longitudinal foliation; and (iii) thrusting, where debris-rich basal ice (including regelation ice) and subglacial sediments are uplifted into an englacial position, sometimes emerging at the ice surface.

3. Entrainment associated with glaciohydraulic supercooling. In recent years it has been suggested that supercooled meltwater flowing beneath glaciers is capable of entraining large quantities of sediment as it refreezes to the glacier sole on the upslope side of bed overdeepenings (see Section 2.4). As this debris-entrainment process involves the refreezing of supercooled meltwater it is size-selective and favours the accretion of silt-size debris, which is typically present in meltwater. Typical ice facies include debris-rich frazil ice and anchor ice.

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