Sources Of Glacial Meltwater

Glacial meltwater is derived from the melting of ice in one of three positions: supraglacial, which means on the ice surface, subglacial, at the bed, and englacial, which means within the glacier (Figure 4.1). Melting occurs whenever there is sufficient heat to turn the ice back into water, and this heat can be supplied by: (i) solar radiation; (ii) friction generated by ice flow; and (iii) heat derived from the Earth's crust beneath the glacier (geothermal heat). Melting on the ice surface is the most important source of glacial meltwater on many glaciers. On temperate glaciers it is often measured in metres per year, whereas englacial and subglacial melting may contribute only millimetres per year. Ice-surface melting is of course highly

Figure 4.1 Examples of glacial meltwater drainage. (A) Supraglacial stream on the surface of Austre Br0ggerbreen in Svalbard. (B) Moulin on the surface of Austre Br0ggerbreen in Svalbard. (C) Englacial tunnel or conduit melting out of stagnant ice in front of Fox Glacier, New Zealand. Note the rounded and subrounded material melting out of the conduit. (D) Subglacial tunnel and meltwater emerging from the snout of Fox Glacier, New Zealand. [Photographs: N. F. Glasser]

Figure 4.1 Examples of glacial meltwater drainage. (A) Supraglacial stream on the surface of Austre Br0ggerbreen in Svalbard. (B) Moulin on the surface of Austre Br0ggerbreen in Svalbard. (C) Englacial tunnel or conduit melting out of stagnant ice in front of Fox Glacier, New Zealand. Note the rounded and subrounded material melting out of the conduit. (D) Subglacial tunnel and meltwater emerging from the snout of Fox Glacier, New Zealand. [Photographs: N. F. Glasser]

seasonal because surface melting by solar radiation generally requires positive air temperatures (Figure 4.2). The volume of water within the meltwater system also depends on the amount contributed by rainfall, snowmelt on the glacier surface and from valley-side streams, all of which can add significant quantities of externally derived water.

Figure 4.2 Sources of meltwater and principal transfer routes in a typical temperate alpine glacier. In the accumulation zone, water percolates down through the snow and firn to form a perched water layer on top of the nearly impermeable ice and then flows from the from perched water layer into crevasses. In the ablation zone, once all the seasonal snow has melted, water flows directly across the glacier surface into crevasses and moulins. [Modified from: Fountain and Walder (1998) Reviews of Geophysics, 36, figure 1, p. 300]

Figure 4.2 Sources of meltwater and principal transfer routes in a typical temperate alpine glacier. In the accumulation zone, water percolates down through the snow and firn to form a perched water layer on top of the nearly impermeable ice and then flows from the from perched water layer into crevasses. In the ablation zone, once all the seasonal snow has melted, water flows directly across the glacier surface into crevasses and moulins. [Modified from: Fountain and Walder (1998) Reviews of Geophysics, 36, figure 1, p. 300]

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