Environmental controls and glacier behaviour an overview

Glacier behaviour, and therefore the response of glaciers to environmental change, is fundamentally determined by an interdependent combination of climatic regime and the interaction of the glacier with its immediate geological and topographic environment. Climate controls glacier mass balance through its impact on rates, distributions and types of precipitation and ablation. This mass balance regime in turn is the fundamental ratecontrolling process for dynamics, because the rate of input and output of mass, sometimes known as the 'activity index' (Meier, 1961), determines the velocity needed to achieve equilibrium. Climatic regime is also the major (but not only: see below) determinant of the thermal regime of the ice mass. Thermal regime has a major impact on ice dynamics through the effect of ice temperature on ice deformation rates, and also because of the importance of the presence of liquid water for basal sliding.

The interaction of a glacier with its immediate geological and topographic environment affects glacier behaviour and response in various ways. In terms of the geological environment, the nature of the glacier substrate has a significant impact on glacier behaviour. The most fundamental parameter is the presence or absence of a layer of unconsolidated sediment at the basal interface (e.g. Boulton, 1986). A layer of basal unconsolidated sediment allows more rapid motion than would otherwise occur, and is thought to be crucial in rapid flow in fast-moving ice streams (e.g. Alley, 1986). More specifically, the lithology of the substrate affects glacier behaviour, and there is a widely known association between the subglacial presence of relatively erodible bedrock, and a propensity for relatively rapid ice flow (e.g. Anandakrishnan etal., 1998; Bell etal., 1998; Jiskoot etal., 2003). The presence of erodible bedrock may provide the source of sediment for a subglacial deforming till layer, thereby allowing enhanced flow rates, as thought to be the case at the Siple Coast ice streams (Bell et al., 1998). On hard rock beds, bedrock type affects bed roughness, which in turn affects sliding rate (Hubbard et al., 2000a), and is also a factor in the subglacial erosion rate (Boulton, 1979).

The thermal regime is also partly determined by geological conditions, and in particular subglacial geothermal heat flux. The variability of geothermal heating can lead to temporally variable effects, such as massive subglacial melting beneath Vatnajokull in Iceland associated with volcanic activity, leading to intermittent jokulhlaups (e.g. Gudmundsson et al., 1997), or spatially variable effects, such as the initiation of rapid flow within the Greenland Ice Sheet over areas of thin crust with high geothermal heat flux rates (Fahnestock etal., 2001).

The topographic environment affects glacier behaviour and response most directly through its effect on glacier surface slope and bed slope (see below). At the mountain-range scale, topography has an indirect effect on behaviour through its role as a determinant of climatic regime of the glacier because of oro-graphic effects on precipitation, and also has an effect through the impact of topographic complexity and glacier shape on behaviour (Jiskoot et al., 2003).

These general comments about environmental controls on glacier behaviour can be brought into focus through the lens of the fundamental relationships governing glacier dynamics, as follows. The fundamental driving stress that determines glacier flow is the vertical shear stress T^, which for a glacier confined by valley walls is given by txy = fpgh sin a

where h is overlying ice thickness, p is ice density, g is acceleration due to gravity, and f is the shape factor, which is a function of the cross-section shape of the valley and broadly its width/ depth ratio. Therefore, the two key glacier variables that determine the magnitude of the driving stress are ice thickness, h, and surface slope, a, as well as the cross-sectional topography of the valley. Ice thickness and surface slope, and their spatial distribution, are determined by interactions between the climate and topography, and the way those interactions determine the distribution and rates of precipitation and melting.

The rate of deformation of ice resulting from the applied shear stress txy is determined by Glen's flow law for ice, as follows e = At n

where e„ is the shear strain rate, n is a constant and A is a flow parameter with a value that is sensitive to a range of charac-

teristics of the ice, which are a function of the environmental interactions outlined above. In particular, A is sensitive to ice temperature, water content and presence of physical and chemical impurities, as well as crystal properties such as size and fabric. The temperature dependence of A is strong, and relatively well understood: it is such that the strain rate at 0°C is about 14 times that at — 10°C (Paterson, 1994, p. 97). The sensitivities of A to the other parameters, especially the presence of impurities, are much less well known, although the characteristics of ice can vary significantly through a glacier. The next section addresses this issue.

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