Field data

Field researchers have long realized that ice masses are characterized by a high degree of spatial variability in their physical com position. Bulk ice properties other than temperature that are known to vary include ice crystallography (size, orientation and shape), and the concentrations of included gas bubbles, debris, water and dissolved impurities. Each of these properties exerts some control over ice flow rate, resulting in large-scale spatial variations in enhancements in flow rate over that of isotropic ice as described by Glen's flow law (Equation 3). Hereafter, such variations in flow rate will be generalized as ice 'softness'. Although quantifying this effect still largely eludes glaciologists, important progress has been made in certain fields. This situation reflects an improving, but still incomplete, understanding of both (i) the exact influence of covarying suites of physical properties on ice softness, and (ii) the precise distribution of those ice properties within real ice masses. Ice cores recovered from large polythermal ice masses have shed some light on this matter, indicating the presence of a basal ice layer, often of Pleistocene age, that is typically between three (Dahl-Jensen, 1985) and 100 (Shoji & Langway, 1984) times softer than the overlying Holocene ice. Analysis of these ice cores indicates that this enhancement probably results from a suite of physical properties, although a preferred ice crystal orientation is central to most explanations (Thorsteinsson et al., 1999). It is also probable that the liquid water content of ice (principally held within the intercrystalline vein system) also plays an important role in this context. Laboratory studies by Duval (1977) indicate that a 1% increase in water content increases a sample's strain rate by ca. 400%.

In contrast to the Earth's larger ice masses, very little is currently known about the internal physical structure of temperate valley glaciers, where ice flow is sufficiently rapid that no ice of Pleistocene age survives, and from where few ice cores have been retrieved. However, recent ice-core evidence suggests that the physical structure of temperate valley glaciers also varies systematically. Analysis of the crystallography, ionic composition, included debris and included gas of a series of ice cores from Tsanfleuron Glacier, Switzerland, led Hubbard et al. (2000) and Tison & Hubbard (2000) to argue for the presence of three distinctive zones at this temperate valley glacier. The Basal Zone has a thickness of ca. 1.5 m above the ice-bed interface, the Lower Zone extends for ca. 15 m above this, and the Upper Zone extends from the top of the Lower Zone to the glacier surface (Fig. 67.1).

Finally, it is worth noting that all glaciers are characterized by some degree of discrete brittle failure. Crevasses are common and indicative of complex processes, for example, at the glacier bed, within an ice fall or at a calving ice margin, and represent a boundary across which stresses cannot be transferred. They become a significant stumbling block to the application of continuum mechanics on which Glen's law is based, although void theory is one means by which the effect of crevassing can be incorporated realistically into models of ice motion. Still, very little is known about the integrity of ice at depth. For example, although crevasse depth is generally restricted to some tens of metres, hydrofracturing may extend the failure deeper, which will be critical in controlling iceberg calving as an outlet glacier approaches flotation and basal traction reduces towards zero. Further, healed crevasses can represent planes of weakness for subsequent exploitation. Indeed, numerous recent field studies have focused on the role of thrusting along slip planes within

Figure 67.1 Long section of Tsanfleuron Glacier, Switzerland, illustrating its constituent ice zones (UZ = Upper Zone; LZ = Lower Zone; BZ = Basal Zone) as reconstructed from core characteristics. (After Hubbard et al. (2003) with the permission of the International Glaciological Society.)

Distance along profile (m)

Figure 67.1 Long section of Tsanfleuron Glacier, Switzerland, illustrating its constituent ice zones (UZ = Upper Zone; LZ = Lower Zone; BZ = Basal Zone) as reconstructed from core characteristics. (After Hubbard et al. (2003) with the permission of the International Glaciological Society.)

glacier ice, particularly at polythermal glaciers (e.g. Hambrey et al., 1997, 1999).

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