Presto, Chango, Pingo, But on a Very Long Time Scale

Surface blisters or ice mounds 50 to 500 feet across that thrust up from otherwise flat terrain are called pingos. In the Arctic, pingos tend to form on the sites of drained lakes as the thawed silt beneath the former lake gradually refreezes over many hundreds of years. Like a closed tin can full of water that bulges or bursts as the last of the liquid freezes, the earth's surface yields from the freezing expansion of watery-silt below the lake bed.

In the valleys of the interior, a slightly different process is involved and the pingos are smaller. Here pressure from a "head" or reservoir of water underground in the uplands occasionally will uplift the surface soil and vegetative cover, particularly if the soil is silty and not well-drained. Cracks form, water finds its way near enough to the surface to freeze, forming more cracks and still more freezing and expansion until a mount is created 5 to 15 feet above the local elevation. In the process the original soil can become well-drained owing to the pingo elevation and sometimes vigorous stands of poplar, birch or aspen will gradually dominate the pingo in stark contrast to the wetland willows, moss and scrub spruce that otherwise typify the silty valley floor.