Pingos Pop Up in Alaska's North Country
Many people have no doubt puzzled at the word "pingo" after spotting it on a topographic map. After you see one, the name seems to fit.
When viewed from an airplane, pingos look like mosquito bites on flat ground surfaces of the Arctic and subarctic. As Geophysical Institute Senior Engineer Emeritus John Miller explained in this column 20 years ago, pingos are actually earthen mounds with a core of ice that can measure from 10 to 500 feet across. They often support trees or bushes on top.
The word "pingo" is an Eskimo word meaning "small hill." A.E. Porslid, the first western scientist to borrow the word pingo to describe the mounds, found arctic pingos rising on sandy slopes and from old lake basins. The pingos that emerged from sandy slopes were small and often ruptured at the top, Porslid noted in a 1938 report. The novice pingo hunter might not even recognize them.
The larger pingos, the ones that rise from flat ground, are sometimes taller than 200 feet. They form where there was once a lake that has since dried up or drained elsewhere, Porslid wrote. The thawed, saturated soil under the former lake slowly freezes. As the soil freezes, the expanding ice lifts the soil above it until a volcano-like cone appears. Miller compared the process, which takes hundreds of years, to a closed tin can full of water that bulges when the water freezes.
Though the Arctic contains the largest and most conspicuous pingos, Interior Alaska also has its share. Interior pingos range in height from about 10 feet to 100 feet, and in circumference from about 50 to 1,500 feet, according to "Pingos in Central Alaska," a Geological Survey Bulletin by William Holmes, David Hopkins, and Helen Foster.
The Interior's pingos are smaller than the Arctic's because the Interior pingos form differently. Rather than arising from old lake beds, the pingos of the Interior pop up in wet, poorly drained areas underlain by permafrost.
Permafrost can create a pingo when it sometimes acts as a barrier that doesn't allow groundwater to seep through. When a well is dug through large chunks of permafrost, for example, water sometimes rushes up and shoots out the well hole like a fountain. This phenomenon, called an artesian well, happens in low areas as a result of the pressure from groundwater in the surrounding hills. With its impermeable bulk, permafrost can hold water below the level gravity wants to push it. Interior pingos form when cracks in the permafrost allow water through. Water moves away from the relatively warm earth, comes closer to cold air and freezes. This process of freezing and expansion continues until a mound of earth is created-a pingo.
Interior pingos often catch the eye because they attract different species of trees than the surrounding lowlands. Because pingo soil has been thrust upward by the ice below, the soil is often well-drained. Pingos often sport a cover of birch and aspen, while down below the ground wears a boggy coat of willows, alders and black spruce.
