D: Hey Yaël, have you ever heard of pingos?
Y: Is that a type of dog?
D: No, they are like small mountains that form in the permafrost.
Y: I've never heard of them.
D: They are circular or elliptical formations that grow up to two hundred thirty feet in height and two thousand feet in diameter.
Y: Are they like volcanos?
D: No. They are actually giant lumps of ice that grow under the soil.
Y: The ice grows?
D: Yes. Pingo ice comes from two sources. Hydrostatic or closed-system pingos form in drained lakes or river channels. As permafrost rises toward the unfrozen floor, water is expelled above it. That water turns into an icy core. The shape and size of the pingo is determined by the previous water body, round shaped in lakes and more elongated in rivers.
Y: So, the water freezes between the permafrost and the ground above?
D: Exactly. Other pingos are hydraulic or open-system pingos. They form when groundwater flows in from an outside source. They are usually found at the base of slopes. The groundwater is under pressure and forces the ground up, making an expanding ice core. Those pingos are usually oval or oblong shaped. Sometimes if the water pressure is high enough, it can lift the original pingo up and another sub-pingo can form beneath it.
Y: Do they ever melt.
D: Sure. The ground above the icy core acts as insulation. If it erodes or is removed by human activity, then the ice below will begin to melt and the pingo will collapse forming a crater.