Geology Reference
In-Depth Information
Figure 6.14. Genesis and growth of hydrostatic (closed) system pingos. (A) A large lake is under-
lain by unfrozen saturated sand. (B) Rapid lake drainage causes permafrost aggradation, pore-water
expulsion, and development of hydrostatic pressure beneath a residual pond where permafrost is
thin. (C) A growing pingo is underlain by a sub-pingo water lens whose downward freezing results
in intrusive ice. (D) The total volume of ice required to grow a pingo is equal to the pingo volume
above that of the bottom of the residual pond in which growth commenced. (E) Pingo collapse from
partial thaw of pingo ice beneath the central pond that is surrounded by a pingo rampart. From
Mackay (1998). Reproduced by permission of Les Presses de L'Universite de Montreal.
bulganniakh growth in Siberia are lacking, similar growth patterns are observed. For
example, some hydraulic (open) system bulganniakhs have developed in recently-drained
thermokarst (alas) depressions within the last 50 years. Eye-witness accounts indicate
early growth rates of approximately 0.5-2.0 m/year (Soloviev, 1973a).
As pingos become older, their growth rates decrease. Some of the largest pingos, such
as Ibyuk Pingo, are over 1000 years old and growing at a rate of only 2-3 cm/year (Mackay,
1986b). As a rough estimate, Mackay suggests that, for the Mackenzie Delta region, pos-
sibly 15 pingos may commence growth in a century. Probably, only approximately 50 are
actively growing today.
Hydrostatic (closed) system pingos often exhibit pulsating patterns of heave or growth
(Mackay, 1977a). This is caused by the build-up of water lenses that develop under pres-
sure beneath the growing pingo (Mackay, 1978b). These pressures are released when water
escapes to the surface, usually towards the periphery of the pingo where overburden
strength (i.e. permafrost thickness) is least. This may result in seasonal frost mounds
forming on the fl anks of growing pingos (Mackay, 1979a, pp. 18-24). If penetrated by
drilling, these water lenses can cause temporary artesian fl ow and geysers several meters
high.
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