Pingo at Tuktoyaktuk.

Pingos are conical, ice-cored hills that grow to 50 m in height and more than 300 m in diameter. Generally pingos occur in basins such as those formerly occupied by small, rapidly drained lakes or former fluvial channels. Depending upon topographic irregularities of the lake bed or channel, more than one pingo may grow in a single basin.

Most pingos in the area occur along the Beaufort coastlands that are underlain by Pleistocene sands and silts east of the modern Mackenzie Delta and comprise the greatest occurrence of pingos in the world. These pingos grow in clusters on Richards Island, along the Beaufort Sea coast to Cape Dalhousie, and on all sides of Eskimo Lakes.

Pingo with exposed ice core.

Professor J.R. Mackay estimated the occurrence of approximately 1400 pingos in this eastern and southern portions of the Beaufort coastlands. Offshore, more than 200 undersea pingos have been mapped by the Canadian Hydrographic Service.

The origin of pingos in the Beaufort coastlands is an intrapermafrost phenomenon that is intimately associated with old lakes and channel beds. Lakes may drain because of shoaling as a consequence of river-bank erosion and the breaching of nearby lake borders, and the subsequent infilling with sediments. In the lakes along the coast, rapid drainage takes place when coastal retreat impinges upon, and breaches the lake borders near the sea coast. In any case, indigenous permafrost aggrades toward the centre of the lake bed and expels pore water ahead of it.

Two mature pingos with ruptured summits on outer Mackenzie Delta. Split Hill Pingo (top) and Ibyuk Pingo (bottom).

At this stage, the basal perimeter of the pingo is frozen and established. The change in volume of the freezing pore water initiates doming of the lake bed and, with continued expulsion and freezing of this pore water, ice begins to grow peripherally within the dome. Because the pore water is under increasing hydrostatic pressure, it begins to rise vertically as it follows the path of least resistance. Upon freezing, an ice core is formed at, or near the centre of the dome. Ice accretion within the dome is upwards as well as sideways, thus enhancing the conical aspect of the growing pingo and establishing its mature form. In this final stage of growth, the overburden on the summit tends to rupture because of the increasing accretion and expansion of ice within the dome. This process forms tensional cracks in the overburden and, eventually, a breached summit.

Life expectancy of pingos along the sea coast may be reduced by processes exclusive of breaching and accompanying decay due to thermal factors. With further coastal retreat or rising sea level pingos are eroded. This destruction is accelerated by continued thawing as the ice is exposed to solar heat. Additionally, the warmer water (0-9°C) of Mackenzie River flows past these exposed cores at sea level, and decay by thermal action is enhanced. All pingos in these coastal waters disappear, and pingo growth is absent.

Compiled by: B.R. Pelletier
Geological Survey of Canada