Pingos in the Present Tense

"No, dear," I said to my then-young daughter. "The past tense of pingo is not pingwent." It's the sort of comment likely to be needed in a household with an eager new reader and an array of books on subjects from the trivial to the technical. It led to a conversation about nouns and verbs, tenses and numbers, pingos and volcanoes.

That long-gone conversation came to mind the other day when I read of recent studies that question the customary views of what pingos are and how they grow.

More people have seen pingos than have ever visited the Arctic, for they make photogenic features. A pingo can rise a hundred feet or more above the flatland surrounding it, which makes it useful to the game-seeking Native hunters from whose term for "hill" its name supposedly derives. Though the classic pingo does look a little like a small volcano, a conical protuberance above the surrounding terrain and sometimes with a crater-like hollow on top, it has a completely different cause. Metaphorically speaking, a volcano is the child of fire, but a pingo is the child of ice. Really, a pingo is a frost heave with an attitude.

At least that's what most researchers think. Though pingos come in a few varieties (even fossil ones, found in terrain much colder in the past than it is now), the most conspicuous are the closed-system pingos found in the far north. There, pingos form when lakes die.

In the dynamic Arctic, lake death isn't unusual. Streams wander, removing water sources; marshy vegetation builds from the edges, trapping sediment and eventually closing in. If the dying lake once was deep enough so that it did not freeze all the way to the bottom in winter but now encounters freezing from the sides and bottom as well as the top, the water remaining is in effect boxed into a trap with walls that continue squeezing inward. That generates hydrostatic pressure. Whatever part of the confined water that freezes must expand by nine percent, and that generates yet more pressure. And, since water moves through soil toward the freezing front, still more water will migrate in to add to the ice buildup.

That's a simplified outline of a process with a dramatic result: Slowly, at rates of a few inches to a few feet a year, a hillock with an icy core grows from the former lake.

But now, though he can't argue with the result, a Japanese researcher disputes the process. According to Hisashi Ozawa of the Nagaoka Institute of Snow and Ice Studies in Japan, it's not the ice. It's the water.

Ozawa has found that the soil beneath arctic pingos contains supercooled water--liquid water colder than 32 degrees F. or 0 degrees C. It can't freeze there, he says, because the soil is made up of grains so tiny that there isn't room between them for ice crystals to form.

Now, Ozawa wouldn't run into many arguments about water in and under pingos. In fact, they've been seen to erupt water (and ice chunks), which isn't surprising if the water is under pressure as theory demands. But he would run into controversy with the rest of his views. What makes a pingo grow, he says, has nothing to do with traditional views of frost heaving. It's not from the expansion of water as it solidifies. Instead--and he states that his laboratory experiments prove it--Ozawa says pingos grow because they tap the jittery energy of the supercooled water molecules. No mechanical work is involved whatsoever.

I hope Ozawa comes to Alaska some day and gives a talk about his ideas, if only so I can hear the debate about what makes pingos grow. But however they do it, it's one of the wonders of the north. Elsewhere, they only grow molehills into mountains; here, we're willing to start with lakes.