Icemarginal Terrestrial Landsystems Active Temperate Glacier Margins

David J.A. Evans

2.1 INTRODUCTION

Temperate glacier margins are mainly wet-based for at least part of the year and are located in terrain that contains discontinuous or no permafrost. Such glaciers are considered as active when they are capable of forward momentum even during overall recession. This is manifest in the small winter readvances that characterize receding outlet glaciers in places like Iceland (e.g. Sharp, 1984; Boulton, 1986; Krüger, 1995). Cold winter conditions result in the penetration of a seasonal cold wave from the atmosphere through the thin ice. This produces a narrow marginal frozen zone thought to be significant in the production of some frontal moraines. In some settings the marginal frozen zone may persist for several years and is therefore technically discontinuous permafrost. The processes and major landform-sediment associations of active temperate glacier margins have been studied in great detail (e.g. Sharp, 1982, 1984; Harris and Bothamley, 1984; Krüger and Thomsen, 1984; Krüger, 1985, 1993, 1994a, 1997; Boulton, 1986; Boulton and Hindmarsh, 1987; Benn, 1995; Evans and Twigg, 2002), thereby informing models of landform production (e.g. Boulton and Eyles, 1979; Gustavson and Boothroyd, 1987; Krüger, 1987; Benn and Evans, 1998).

The impact of any glacier on landform development relies heavily upon its ability to transport debris. Although debris-rich basal ice sequences are typically thin or absent beneath temperate glaciers (Hubbard and Sharp, 1989), concentrations of debris are observed in the basal ice facies of glaciers whose subglacial meltwaters are influenced by supercooling in overdeepenings (e.g. Alley et al., 1998, 1999; Lawson, et al., 1998; Evenson et al., 1999). Any debris-rich ice that does occur in a temperate glacier snout can be transported to englacial and supraglacial positions by compressive flow (Fig. 2.1). In relative terms, however, the volume of material transported through an active temperate lowland glacier by these mechanisms is small and so, consequently, supraglacial landforms and widespread ice stagnation topography are uncommon compared with temperate glaciers in high-relief settings. Exceptions to this rule occur in situations where unusually high concentrations of debris-rich ice are produced (e.g. through freezing of englacial drainage systems and/or supercooling, Spedding and Evans, 2002), or where complex marginal oscillations have led to proglacial thrusting and the re-incorporation

Figure 2.1 Debris entrained in active temperate glacier snouts. A) Debris-rich ice in the snout of Kviarjokull. This originates either by freezing-on, by the process of supercooling over a subglacial overdeepening, or the freezing of sediment-charged englacial drainage networks (Spedding and Evans, 2002) and/or by freezing in fractures produced by pressurized meltwater discharges (e.g. Roberts et al., 2000, 2001; Ensminger et al., 2001); the sediments are then exposed on the glacier surface by compressive flow; (ice axe in foreground for scale). B) Typically sparse debris concentrations in the snout of Flaajokull. This view also shows the large amount of debris being transported as a subglacial deforming layer and used to construct push moraines in the foreground.

Figure 2.1 Debris entrained in active temperate glacier snouts. A) Debris-rich ice in the snout of Kviarjokull. This originates either by freezing-on, by the process of supercooling over a subglacial overdeepening, or the freezing of sediment-charged englacial drainage networks (Spedding and Evans, 2002) and/or by freezing in fractures produced by pressurized meltwater discharges (e.g. Roberts et al., 2000, 2001; Ensminger et al., 2001); the sediments are then exposed on the glacier surface by compressive flow; (ice axe in foreground for scale). B) Typically sparse debris concentrations in the snout of Flaajokull. This view also shows the large amount of debris being transported as a subglacial deforming layer and used to construct push moraines in the foreground.

of stagnating ice in a fashion similar to the surging glacier landsystem (e.g. Kotlujökull, Iceland; Krüger, 1994a). Because later chapters develop the theme of temperate glacier lobes in mountain topography, this chapter concentrates on lowland outlet glaciers and uses mainly Icelandic case studies for illustration.

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