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thus desiccated and into the next zone of pore water, beyond initial reach. The process is
repeated to depths at which downward penetration of the cold wave ceases or is matched
by geothermal heat flow (Figure 15.3). Ground contraction on cooling and local
desiccation counter expansion due to frost heave and generate cracks which may become
sites of vertical lenses or ice wedges . Heaving, contraction and seasonal melt drive
permafrost processes, as we shall see below.
Permafrost, or perennially frozen ground, consisting of segregated and interstitial ice
zones and desiccated lenses up to 400 m thick, is found in the Arctic basin. It forms
continuous cover on non-glacial polar land surfaces and cold, arid continental interiors
but thins equatorwards and coastward. Discontinuous or sporadic forms occur as the
extent of talik or unfrozen ground increases and account for 45 per cent of approximately
40 million km 2 of global permafrost ground (Figure 15.4). Seasonal melt during summer
months with temperatures above 0° C develops a saturated, surface active layer 0·1-3·0
m thick. Meltwater drains laterally but is unable to penetrate the frozen substrate and the
layer refreezes in winter. The roles of microclimate, relief, slope, material porosity and
Figure 15.3 Sections through permafrost: (a) north-south
section through principal zones from North West Territories
to northern Alberta, Canada; (b) formation of interstitial and
segregated ice lenses by downward migration of the freezing
plane. Note that migration of water to the freezing plane
leaves a desiccation layer
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