Pingo growth

Why does flat Arctic ground heave up a hill of solid ice with no mountain-building anywhere near it?

On the flattest imaginable ground — a delta, a plain of thaw lakes, nothing you could call relief for a hundred kilometres — there are hills. Conical, steep-sided, sometimes tens of metres high, often with a crater at the summit. Cut one open and it is ice, a single massive body of it, with a thin skin of frozen soil stretched over the top.

Nothing is pushing here. There is no fault, no fold, no magma, and the climate is uniform for hundreds of kilometres around while the hills are not. So a hill has been built by something available on flat cold ground, acting at a particular spot and not its neighbours. What is that something, and what picks the spot?

Start with the only force nearby that could plausibly lift anything: freezing. Water expands when it becomes ice, which is unusual and is the whole engine here. The size of the effect is worth deriving — ice is about 917 kilograms per cubic metre against water's thousand, and a thousand divided by 917 is a little over 1.09, so a given mass of water occupies about nine per cent more volume once frozen.

Nine per cent is not much. Freeze a metre of wet soil and you lift the surface by centimetres. So a first attempt fails: uniform freezing cannot build a hill of metres, and certainly cannot build one here and not five hundred metres away.

Which tells us what to look for. The expansion must be concentrated — collected from a large volume of ground and delivered to one small place. How? Ask what freezing does to water it cannot displace. If a front advances into saturated sediment with somewhere for the pore water to drain, nothing dramatic occurs. If the water has nowhere to go, the expansion is accommodated as pressure, and pressure drives the remaining liquid toward whatever is weakest.

Now we need a way to seal the pocket, and the Arctic supplies one. Permafrost — ground that has stayed below zero for at least two consecutive years — is essentially impermeable. Under a lake deep enough not to freeze to its bed, the water insulates the ground below, so a thawed zone persists right through the permafrost: a talik, a column of unfrozen saturated sediment inside a frozen medium.

And then the contingent event that picks the spot. The lake drains — a river cuts its bank, or the shore breaches into a lower basin. The insulating water is gone, the floor is exposed to the Arctic winter, and permafrost aggrades downward from the new surface and inward from the sides. The talik is closed in from every direction at once.

Follow the water. It cannot escape sideways or downward, because it is surrounded by frozen ground; and as the front advances the volume available to it shrinks while the freezing expansion adds to the squeeze. So the pore water is expelled toward the centre and upward, to wherever the new cap is thinnest, and there it is injected and freezes. That growing body of ice lifts the frozen ground above it, and goes on lifting as long as the trapped talik has liquid water left to give.

The ending is implied by the geometry. A dome cannot grow without stretching its cap, and frozen ground in tension eventually cracks. Cracks open along the summit, exposing the ice core to summer air, and the hill destroys itself — leaving a pond ringed by a raised rampart, as diagnostic as the intact hills.

Two clarifications, and one is a distinction that is easy to blur.

The first: this is not the frost heave that sorts stones into circles and polygons on Arctic ground. That works by cryosuction, drawing water along a thermal gradient through unfrozen films toward a front, building many thin segregation lenses through a soil column and lifting the surface by small amounts each year in both directions. A pingo is the opposite in its plumbing — pressure expelling water rather than suction drawing it, one massive body of injection ice rather than a stack of lenses, and a one-way growth that does not reverse each summer. Same climate, same phase change, different mechanism.

The second: the account above is the closed-system or hydrostatic kind, the type of flat delta country. There is a second family. On slopes in discontinuous permafrost, groundwater from higher ground arrives under artesian head through unfrozen paths and freezes near the cold surface, building the same landform from an open supply. That matters, because "there is a pingo here" does not by itself tell you which mechanism ran.

Which is what makes the claim testable. The closed-system model says the water was already there, as pore water in a former lake basin, and that growth is self-limiting because the talik eventually freezes solid. So it predicts these pingos in drained lake basins on flat continuous permafrost, with cores of massive injection ice whose chemistry matches shallow basin pore water. What would refute it: coring such pingos and finding stacked segregation lenses with soil partings instead of a single injected body, or finding the ice chemistry matches deep regional groundwater from outside the basin — either of which would mean the closed system was never closed.

One number I will not offer is a count of how many exist, since published totals depend on what a survey counts as a pingo and how it treats collapsed ones. The figure often quoted for the tallest in the Mackenzie Delta is around fifty metres, which I am recalling rather than deriving.

• Why ice-wedge polygons, built by thermal contraction rather than pressure, produce a network instead of a mound.

Every term the collection defines is gathered in the glossary.