Cryospheric systems - Fundamentals of the Physical Environment - page 381

The main stages of closed-system pingo development
are shown in Figure 15.21 . Fig. 15.21a shows the initiation
of a pingo as permafrost develops in the sediments of a
drained lake. Fig. 15.21b s hows the pingo with the freezing
plane at the bottom of the ice core. The ice core is being
nourished by pore water expelled from the freezing
sediments. In Fig. 15.21c the ice core no longer receives
expelled water, but the pingo can continue to grow from
ice segregation and the freezing of pore water in a confined
system. The growth of the pingo is rapid initially, but slows
down as the freezing plane penetrates deeper into the
ground. Mackay (1979) measured growth rates of 16-20
cm per annum for pingos developing on a lake bed
drained twenty-two years previously, while for more
mature pingos the growth rates were 0.5-8.0 cm per
annum. Rates of growth can be an index of age; some
pingos in the Mackenzie Delta are as old as 1,100 years,
but most are in the range 27-310 years. The famous Ibyuk
pingo near the village of Tuktoyaktuk is one of the largest,
at 49 m in height, and is estimated to be about 1000
years old and currently growing at 2.8 cm per annum
( Plate 15.23 ).

PERMAFROST AND THE
CONSEQUENCES OF CLIMATE
CHANGE

Large areas of ground ice in polar regions exist at
temperatures within 1-3 C of melting. Even a small
amount of global warming in the second half of the
twenty-first century will have large effects on terrain
conditions, which will probably be compounded by
increases in precipitation also. Although precise effects are
difficult to predict, most discontinuous permafrost would
become unstable, and eventually disappear. In the
continuous zone, ground temperatures would rise, the
active layer would deepen and shallow ground ice would
melt. Thawing leads to subsidence hollows which usually
fill with water to give thaw lakes. Landscapes with
depressions resulting from thaw settlement are termed
thermokarst, denoting karst by ice melting in contrast to
karst by limestone solution. Sloping ground would
become very unstable, giving more flows and slides. In
addition the mechanical behaviour and bearing capacity
of melting soils are different from those of frozen soils,

Plate 15.23 The much studied Ibyuk pingo near Tuktoyaktuk, lower Mackenzie valley, Canada. The pingo is fully instrumented
and there is even an ice tunnel. Large dilation cracks radiate from the apex as a result of the growth of the ice core. Smaller pingos
are visible in the distance.
Photo: Ken Atkinson