A pingo, also called a hydrolaccolith, is a mound of earth with a core of ice found in the Arctic and subarctic regions, that can reach up to 70 meters in height and up to 600 meters in diameter. Pingos are formed as a result of what is called a “closed” system of unfrozen soil developing within an area of permanently frozen ground. Pingos usually grow a few centimeters per year, and the largest take decades or even centuries to form. Pingos eventually break down and collapse. Evidence of collapsed pingos in an area suggests that there was once permafrost.
Tuktoyaktuk in the Mackenzie Delta of the Northwest Territories in Canada has one of the highest concentrations of pingos, with some 1,350 examples. Pingo National Landmark protects eight of these features. Two of the most famous pingos are Ibyuk and Split. Ibyuk is about 50 meters high and is the tallest pingo in Canada and the second tallest in the world. It is the world’s largest growing pingo, and continues to grow at a rate of about 2 centimeters per year. Ibyuk is estimated to be more than 1,000 years old. Other pingos in the landmark range in height from 5 to 36 meters and represent various stages of pingo development.
The presence of numerous lakes in the Tuktoyaktuk Peninsula, most of which are too deep to freeze solid in winter, is key to the formation of the region's famous pingos. In this area, lakes over two meters deep do not normally freeze solid in winter. The year-round presence of water at the lake bottom protects the sandy lakebed from frigid winter temperatures. As a result, the permafrost beneath the lake thaws, creating a zone of unfrozen ground sandwiched between the lake and the deeper underlying permafrost.
A pingo forms when one of these lakes drains, or partially drains, and the water's warming effect is removed. Exposed to frigid winter temperatures, the waterlogged sand of the former lakebed freezes and its water expands. The increased pressure pushes the surface upwards, supported on a lens of water. A small, cone-shaped hill grows, and after a few years may be recognized as a pingo.
As the lakebed continues to freeze, the hill grows slowly in height, pushed upward by the pressurized water beneath. The pingo continues to grow while there is still unfrozen ground in the lakebed. Throughout its growth, the pingo is supported by the pressure in the water lens. In a process that may take hundreds of years, the lens of water gradually freezes solid and the pingo stops growing.
A pingo will collapse if the ice core melts. This might happen when cracks form on the sides of the pingo that may reach down as far as the ice core, exposing the ice to warm air and sunlight. When the ice core has completely melted, all that remains is a doughnut-shaped ring of raised tundra enclosing a small round lake. In warmer regions, such as Ireland and the Netherlands, remnants of collapsed pingos have helped scientists determine that the climate was once cold enough in these areas to support a permafrost environment.
Aside from Canada, pingos occur in Greenland, Siberia, and the Norwegian island of Spitsbergen. Recent estimates indicate that more than 11,000 pingos exist on Earth, with more than 6,000 alone in northern Asia.
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