Geology Reference
In-Depth Information
Figure 6.13.
Collapsed hydrostatic (closed) system pingo, Sachs River lowlands, Banks Island,
Canada.
6.5.2. Hydraulic (Open) System Pingos
Hydraulic (open) system pingos are hydrological phenomena that develop at sites where
intrapermafrost or subpermafrost groundwater, under artesian pressure, reaches the
surface (Müller, 1959). In general, they occur as isolated features, or as small groups
within the same locality. A few attain considerable dimensions (
20-50 m in height) (see
Figure 6.12). In nearly all instances, hydraulic (open) system pingos form in areas with
topographic relief, such as lower hillslopes, alluvial fans, or valley bottoms. The majority
is clearly associated with groundwater seepage. Typically, springs and icings form on their
fl anks in winter (see Figure 6.12). In East Greenland, groundwater fl ow and discharge
characteristics suggest that structural weaknesses control the taliks that are associated
with the intra-permafrost hydrology (Worsley and Gurney, 1996). Likewise, on Svalbard,
many of the larger pingos are related to geological faults or are fed by high artesian pres-
sures that are constantly being recharged by the melt of ice from the temperate basal zones
of glaciers (Liestol, 1976; Yoshikawa, 1993; Yoshikawa and Harada, 1995). In Tibet, the
large pingo at the head of the Kunlun Shan Pass is thought to have formed by groundwater
rising to the surface along a fault (Wang and French, 1995c).
Despite numerous descriptive studies, there are no known surveyed growth data for
hydraulic (open) system pingos. It is sometimes assumed they grow solely by ice injection.
However, the role of artesian pressure is not to force overlying sediment upwards but
rather to ensure a steady and slow movement of groundwater towards the surface. Pingo
growth solely from injection represents an unstable condition that requires an unlikely
long-term balance between three independent variables: (i) water pressure, which is deter-
mined by conditions external to the pingo, (ii) overburden strength, which varies with time
of year, and (iii) the rate of freezing, which depends upon temperature (Mackay, 1973a,
p. 1000). Since all three may change independent of the others, this balance will rarely
be maintained for the total growth period of a pingo. The implication is that hydraulic
(open) system pingos probably require a certain amount of ice segregation in addition to
ice formed by intrusion of free water. In Yakutia, for example, where both “fl at” bulgan-
niakhs (dome-like elevations 2-5 m high) and upstanding large bulganniakhs (hills 10-
50 m high) exist in close juxtaposition, it is thought that the ice core is of several origins,
produced both by injection from groundwater under pressure from below and by segrega-
tion (Soloviev, 1973b, p. 148-151). The fl at bulganniakhs are primarily of a segregated
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