Article from the November/December 2004 issue of
ON THE SURFACE
Pingos
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Two mature pingos with ruptured summits on outer
Mackenzie Delta. Split Hill Pingo (top) and Ibyuk Pingo (bottom). |
When a thermokarst lake drains (see the May column), it may give rise to another splendid arctic landform: a pingo.
Pingo is an Inuit word for the conical hills typical of the Mackenzie Delta in the Northwest Territories of Canada and is now used to describe ice-cored mounds in regions of continuous, discontinuous and altitudinal permafrost. They are true permafrost features as they grow below the active layer where the annual freeze-thaw cycle operates. As well as in Antarctica, Pingos occur in Canada, Alaska, Greenland, Spitsbergen and Siberia, where they are also called bulganniakh. Pingos are usually classified into two main types: hydrostatic (closed system) or hydraulic (open system) pingos, depending on their genesis. The pingos of the Mackenzie Delta are almost all of the hydrostatic variety, and it is their type locality. One of the largest active pingos is the 48 m high Ibyuk Pingo near the village of Tuktoyaktuk. The precursor of hydrostatic pingos are often thermokarst or thaw lakes in areas of continuous permafrost. The deepest of these lakes (those of more than three meters depth) survive during the winter under an insulating cap of ice, and are underlain by a lens of talik, which is ground that remains unfrozen due to the thermal insulation of the lake. During the summer these lakes expand and once they breach the high ground confining them, they are often drained very rapidly, sometimes in no more than 24 hours. Once the lake is drained, a layer of permafrost develops on top of the unfrozen talik, so that it is now closed and surrounded by permafrost on all sides. As the permafrost gradually advances, the remaining water in the talik is pressurized due to freezing and expansion of the newly formed ice. This water is then expelled in the direction of least resistance, and it starts to dome up the relatively thin permafrost layer on top of the talik, where it slowly freezes to form the solid ice core of a pingo. The hydrostatic pressure can be considerable, with boreholes producing fountains of up to three meters in height. When the term ‘closed system’ was first coined, it was assumed that the pingo grew from a single talik beneath it. However, this is not always the case as different taliks may interconnect to form a trough-talik. Therefore the term hydrostatic is more accurate, meaning that the water is derived locally and driven by hydrostatic pressure resulting from the local advancement of permafrost. In contrast, in hydraulic pingo formation at least part of the water is derived from elsewhere, and delivered to the growing pingo by groundwater movement. The type locality for hydraulic pingos is east Greenland, but they also occur in Alaska and Spitsbergen. Whereas hydrostatic pingos are usually solitary features, hydraulic pingos tend to occur in clusters, occupying areas with a suitable supply of groundwater. Hydraulic pingo formation is less well understood and it is not always clear how the groundwater is able to move about in an area of continuous permafrost, or how the required pressure is generated to initiate pingo growth. In mountainous areas such as east Greenland the water may be sourced from permanent snow or small glaciers higher up the mountains, with the difference in altitude generating the required artesian pressure. The existence of taliks beneath these glaciated areas may provide a pathway for the water to move underground without freezing solid. Hydraulic pingos are also associated with fluvial systems, which may also sustain taliks beneath them. When a river channel migrates laterally or is abandoned altogether, the permafrost advances on the sub-channel talik, thereby initiating pingo growth. Faults in the bedrock can also provide pathways along which groundwater moves towards the pingos. For example, the pingos of the Brooks Range of northern Alaska are found above areas with shallow bedrock and their distribution seems to be controlled by local fracture systems. The existence of high geothermal heat flows leading to increased groundwater upwelling along faults and fractures has also been invoked for the predominance of hydraulic pingos in east Greenland. Not all pingos can be understood in terms of hydraulic or hydrostatic genesis. Therefore it is possible that other mechanisms are also able to create these landforms. The morphology of both hydrostatic and hydraulic pingos is quite similar. They are generally circular or oval, but elongate ones exist as well. Average basal diameter is about 200 m, but it can be as large as 600 m. The sides often form an abrupt contact with the surrounding terrain. Typical slope angles are 34° to 38°, but rareley more than 45°, which is the theoretical maximum for unconsolidated, predominantly sandy sediments. The largest difference between the two types lies in the nature of their overburden. The hydrostatic pingos of the Mackenzie Delta are developed in Pleistocene glaciofluvial sands, covered by till, morainal deposits and finally the deposits from the lake from which the pingo grew. Many of the hydraulic pingos have an overburden of fluvial sediments, or even consolidated deposits such as massive sandstone beds. Although pingos are stable permafrost features, they will ultimately decay and collapse. The best age estimate for mature pingos is about 1000 years. Collapse may be initiated by climate amelioration or rupturing of the overburden by the pressurised water lens, but the most common mechanism is rupturing of the thermally protective overburden.As the diameter of a pingo is established early on, subsequent growth results in an increase in height with little accompanying increase in basal diameter. This stretches the overburden until it develops radial cracks and fails around the summit of the pingo. This causes the overburden to slump radially outward, and exposes the ice core which then decays through melting and ablation. The final stage of decay is reached when all the core ice has been lost, leaving a depression surrounded by a rampart of slumped overburden, which is sometimes called an ognip, because of the inversion in profile. The ramparts of Walton Common in west Norfolk are examples of decayed Pleistocene pingos.
Exposed ice core of an eroded pingo.
| Sources: S. D. Gurney: Aspects of the genesis and geomorphology of pingos: perennial permafrost mounds, Progress in Physical Geography 22,3 (1998) pp. 307-324 Tony Waltham: Pingos of Tuk, Geology Today, Vol 19, number 6 pp. 212-215 . |
By Paul De Schutter
If you have any comments or additional information you want to share about the topics covered in this column, you are welcome to send them to paulds@pi.be.
Other on the surface articles by the same columnist.
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Last modified on Tuesday, May 03, 2005 at 21:21