Page 299 - Fundamentals of Geomorphology

282 PROCESS AND FORM

Water-filled
polygonal crack network

Pond
Ice
wedge

Pond

Thaw zone

Permafrost

Figure 11.3 Ice-wedges, ice-wedge polygons, and raised rims.
Source: After Butzer (1976, 342)

Box 11.1

PINGOS

Pingos are approximately circular to elliptical in plan unfrozen ground (Figure 11.4a). Freezing of the lake
(ColourPlate14,insertedbetweenpages208and209). surface will then cause permafrost to encroach from
They stand 3 to 70 m high and are 30 to 7,500 m the lake margins, so trapping a body of water. When
in diameter. The summit commonly bears dilation the entrapped water freezes, it expands and causes the
cracks, caused by the continuing growth of the ice overlying sediments and vegetation to dome.The same
core. Where these cracks open far enough, they may process would occur when a river is diverted or a
expose the ice core, causing it to thaw. This process lake drained. This mechanism for the origin at cryo-
creates a collapsed pingo, consisting of a nearly cir- static pressure is supported by pingos in the Mackenzie
cular depression with a raised rim. Young pingos may Delta region, NorthWestTerritories, in Arctic Canada,
grow vertically around 20 cm a year, but older pin- where 98 per cent of 1,380 pingos recorded lie in, or
gos grow far less rapidly, taking thousands of years to near to, lake basins. A second plausible mechanism for
evolve. The growth of the ice at the heart of a pingo forcing water upwards arises in open-system pingos
appears to result from pressure exerted by water being (Figure 11.4b). Groundwater flowing under hydro-
forced upwards. Water may be forced upwards in at static pressure may freeze as it forces its way towards
least two ways, depending on the absence (closed- the surface from below a thin permafrost layer. How-
system pingos) or presence (open-system pingos) of ever, unconfined groundwater is unlikely to generate
a continuing source of unfrozen water after the for- enough hydrostatic force to raise a pingo, and the
mation of the initial core. First, in closed-system open-system mechanisms may occur under temporary
pingos, a lake may be in-filled by sediment and veg- closed-system conditions as open taliks are frozen in
etation, so reducing the insulation of the underlying, winter.