Melting of ice wedges often produces high-centred polygons (Fig. 13.1) or, where thermokarst activity is pronounced, thermokarst mounds (French 1975; Lawson 1986). Mounds 3-15 m in diameter and 0.3-2.5 m high started to form — mainly by thermokarst subsidence — within 2-3 years of vegetation clearance in cultivated fields near Fairbanks as surface water ponded in small disconnected depressions, accelerating thaw of the underlying ice wedges (Pewe 1954, 1982). On eastern Banks Island, Canada, thermal erosion of ice wedges by streams has produced conical thermokarst mounds that may exceed 8 m in height and 2-3 m in summit diameter (French 1974).
Thermal erosion of ice wedges beneath hillslopes often forms gullies and tunnels. In the Tuktoyaktuk Peninsula area, Canada, gullies are initiated by (1) collapse of tunnels formed by water flowing through interconnected ice-wedge cracks during the snowmelt period, (2) surface flow through ice-wedge troughs, (3) overtopping of snow dams followed by rapid erosion at lake outlets, and (4) diversion of lake outlets through ice-wedge systems (Mackay 1974, 1988). The gullies can develop rapidly to depths of several metres when lakes drain catastrophically. In central Yakutia, Siberia, sinkholes and underlying tunnels form where thermokarst mounds collapse into adjacent trenches and disintegrate through thermal erosion (Czudek and Demek 1970).
Thermokarst subsidence can also create or accentuate low-centred polygons. Water seeping into and moistening dry, unfrozen peat in polygon centres increases the thermal conductivity of the peat and thus the depth of thaw. Where the upper layer of permafrost is ice-rich, the resulting thermokarst subsidence may lead to ponding of surface water in the centres of low-centred polygons (Dredge and Nixon 1979). Where the soil is exceptionally ice-rich, thermokarst subsidence beneath the centres and troughs of low-centred polygons can transform them into walled or fortress polygons following a rapid lowering of the water table in ice-wedge troughs (Mackay 2000).
Thermokarst subsidence and thermal erosion sometimes coexist in gently sloping depressions, forming beaded streams: a series of pools linked by short, narrow channels (Higgins et al. 1990). The pools are 0.5-3 m deep, <30 m in diameter and form by melting of ground ice, usually at ice-wedge intersections. The channels tend to form by thaw of individual ice wedges, and therefore have short, straight sections, often with abrupt changes in direction at ice-wedge intersections.
Ice-wedge melting produces voids that often fill with sediment. The process of infilling (ice-wedge casting) and the resulting structures (ice-wedge pseudmorphs, involutions and tunnel fills) are strongly influenced by thaw-consolidation processes (Harris et al. 2005; Murton 2006).
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