Retaining Paterson's (1994) broad definition of ice streams as spatially restricted fast flow surrounded by slower moving ice, then there seem to be two main breeds of ice stream. 'Topographic ice streams' are fixed in position, their location predetermined by bedrock troughs through which they flow. In comparison with others they have higher surface slopes and driving stresses (typically 50—200 kPa), higher basal shear stresses and attain higher velocities. A classic example is Jakobshavns Isbra, in western Greenland, which extends for about 90 km upstream of the grounding line, with a steep surface gradient and high driving stresses (200—300 kPa) producing velocities of 4—7 km a-1 (Echelmeyer et al., 1991). Pine Island and Thwaites glaciers are examples from Antarctica. In contrast, 'pure ice streams' are features with geometry and flow patterns that are not determined by bedrock topography. They are characterized by very low surface gradients and low driving stress with a concave-upwards surface profile (Bentley, 1987). Typically, velocities are of the order of 400 m a-1. In order to flow so fast with such low driving stresses requires efficient lubrication at their base. Their flow geometry is idealized in Fig. 9.3. Ice streams A-E of the Siple Coast of West Antarctica are the most well known and widely studied of pure ice streams. Recent discoveries in the East Antarctic Ice Sheet suggest that there may be a
third breed of ice stream called 'fast flow features', which extend much greater distances into their parent ice sheets, and whose locations seem to be partly controlled by underlying topography (e.g. Bamber et al., 2001).
Although the processes that permit these different categories to flow so fast are poorly known and may differ between different breeds, typical velocities are higher than can be achieved by internal deformation of the ice column alone. Table 9.1 summarizes some characteristics of the more well-known ice streams, in terms of their dimensions and flow velocities.
Given that ice often transports sediment englacially or subglacially and that higher ice fluxes are experienced within ice streams than adjacent to them, then it follows that we should expect higher levels of sediment transport along ice-stream corridors. This may lead to spatially focused sediment delivery at ice-stream termini. Large 'trough mouth fans' or 'till deltas' deposited on submarine continental slopes are examples of these (e.g. Alley et al., 1989; Dowdeswell, et al., 1996; Vorren and Laberg, 1997).
In summary, the basic characteristics of ice streams in the broadest sense are:
• large in dimension (>20 km wide, 150 km long)
• highly convergent onset zones feeding a main channel
• abrupt lateral shear margins
• spatially focused sediment delivery.
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