what are pingos?

Pingos are a characteristic geomorphologic feature of certain high latitude and also some high altitude regions. They take the form of dome-shaped mounds, well often quite large hills, that emerge from areas characterised by having permanently frozen ground within what are otherwise flat landscapes. I have for example seen examples of pingos on the Seward Peninsular in Alaska, not too far from the Arctic Circle but at sea level, and on the Qinghai Plateau in SW China, relatively close to the equator but at around 5000m elevation. Although not always the case they have a tendency to be circular in plan with base diameters that depend upon their height but can be up to 300m across. They often form in areas of recent exposure to the effects of permafrost surface penetration, such as the beds of lakes or the alluvial delta of rivers, which have for various reasons been drained of the overlying water that previously insulated the bed layers from the penetration of permafrost experienced by surrounding terrain. The upward and outward growth of the pingo is gradual occurring over periods measured in 100s to 1000s of years with annual growth spurts generally taking place at certain restricted periods of the year. Annual upward growth spurts depend upon the age of the pingo with maximum rates seemingly occurring over the first few decades of growth. They are generally underlain by layered ice, whose annual increase in layer thickness appears to relate to the annual upward growth of the pingo – much like tree rings reflect the annual growth of trees.

Pingos have a fairly clear life cycle. Gradually the rates of annual upward growth slow down. As the pingo matures, fractures appear over the crown of the dome and the altered drainage characteristics lead to a gradual erosion of the upper areas of permafrost, especially near the crown of the pingo dome. Eventually, the erosion of the permafrost becomes so severe that it might expose the ice core. This together with climatic warming eventually leads to a thawing of the permafrost and a collapse of the ice core leaving a relic pingo taking the form of a central depression sometimes surrounded by a doughnut shaped ring. These relic pingos are important indicators of climate change and provide evidence of the latitudes to which frozen ground (permafrost) extended during one or other of the past glacial periods. For example, relic pingos have been observed in areas such as East Anglia and Wales within the British Isles and New Jersey in the USA.