Page 28 - Inuvik
P. 28

unfrozen ground
residual pond
newly frozen ground (permafrost)
permafrost unfrozen ground
                1. There is a layer of unfrozen ground beneath most arctic lakes because they are too deep to freeze to the bottom in winter and the year-round presence of water thaws the surrounding permafrost.
pingo permafrost
ice unfrozen ground with water under pressure
3. The freezing front advances inward, placing the encapsulated “lens”of water under pressure. The thin layer of permafrost above the lens is pushed upward, and the pingo begins to grow.
2. When a lake drains, a shallow residual pond is often left
behind. The former lake bed begins to freeze, but the
pond slows the development of permafrost beneath it.
As the lake bed freezes, the water in the ground turns to ice
and expands. The extra water cannot escape, so it is pushed inward toward the centre, ahead of the freezing front (see arrows).
pingo ice permafrost
4. The pingo is fully formed (stops growing) when it is frozen solid–the unfrozen ground becomes permafrost and the pingo has a core of almost pure ice.
                                  Ibyuk, the tallest pingo in Canada, near Tuktoyaktuk
Stephen Woolf/NWT Tourism

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