Geology Reference
In-Depth Information
nature, composed of icy sediments, while the larger forms possess massive ice cores 5-10 m
thick, formed through the repeated injection of water.
Approximately 500 hydraulic (open) system pingos occur in central Alaska and interior
Yukon Territory (Holmes et al., 1968; Hughes, 1969). They are preferentially located on
lower south- and southeast-facing valley-side slopes. Theoretical calculations by Holmes
et al. (1968) suggest that the average pressure required to overcome the tensile strength
of frozen ground and to subsequently maintain a 30 m high pingo is considerably higher
than most artesian pressure measured in central Alaska. This probably explains why the
majority of open-system pingos in Alaska and Yukon Territory never attain a fully-domed
state but persist largely as doughnut-shaped, semi-circular, or circular ramparts. It
may also explain why many of the hydraulic (open) system pingos on Svalbard and East
Greenland are much larger because relative relief is greater and the hydraulic head,
provided by adjacent sub-glacier melt water, is higher.
6.5.3. Hydrostatic (Closed) System Pingos
Hydrostatic (closed) system pingos result from pore-water expulsion caused by permafrost
aggradation beneath the bottoms of drained lakes that are underlain by saturated sand
(Mackay, 1962, 1985b, 1998). The highest concentration of this type of pingo occurs in
the Tuktoyaktuk Peninsula area of the Pleistocene Mackenzie Delta region of Canada,
but others occur elsewhere in northern Canada (Craig, 1959; French, 1975b; Pissart
and French, 1976; St-Onge and Pissart, 1990; Tarnocai and Netterville, 1976; Zoltai,
1983), northern Alaska (Leffi ngwell, 1919; Walker et al., 1985), and central Siberia
(Soloviev, 1973a).
The high concentration of hydrostatic-system pingos in the Tuktoyaktuk Peninsula area
is the result of several favorable physical conditions. These include: (i) the occurrence of
thick permafrost, (ii) large areas underlain by coarse-grained sediment, and (iii) numerous
thermokarst lakes that drain frequently and easily, either by coastal erosion or by fl uvio-
thermal erosion along ice-wedge polygons (Mackay, 1998, p. 275). Typically, hydrostatic-
system pingos occur within shallow lakes or former lake beds where both upward and
downward permafrost growth occurs in the previously-unfrozen saturated sediment which
comprises the sub-lake talik (Figure 6.14). They usually occur singly and not in groups,
although at least one drained-lake basin is known to contain at least three actively-growing
pingos (Mackay, 1973a, 1979a). On Banks Island, certain hydrostatic system pingos appear
to have formed following the freezing of localized taliks that must have formed beneath
the deeper sections of now-abandoned river channels (Pissart and French, 1976).
Detailed long-term fi eld studies upon the growth of hydrostatic (closed) system pingos
in the Tuktoyaktuk area have been undertaken by J. R. Mackay (1973a, 1979a, 1981c,
1986b, 1988b, 1990b, 1998).
The birth of a small pingo called Porsild Pingo (Mackay, 1988a) is typical of the early
growth cycle. This pingo has grown in a lake which drained catastrophically about 1900.
Birth probably took place between 1920 and 1930 when newly-aggrading permafrost rup-
tured and water was intruded into the unfrozen part of the active layer. A small mound,
approximately 3.7 m high, was photographed by A. E. Porsild in May 1935 and subse-
quently described as part of a paper on “earth mounds” (Porsild, 1938, p. 53). Since then,
Porsild Pingo grew steadily until 1976, at a growth rate approximately linear with height.
After 1976, the growth rate has fallen. A similar pattern of rapid early growth (
1.5 m/yr)
was monitored in a former lake bed that was drained by coastal erosion sometime between
1935 and 1950 (Mackay, 1973a, 1979a, pp. 14-18). Although quantitative observations on
∼
