Why Is There a Hill in the Middle of This Flat Tundra? A Lens of Ice Pushed It Up
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A conical mound rising abruptly from flat permafrost ground is built by a growing core of pure ice beneath it, and when the ice melts the hill collapses into a ringed pond.
What the feature is
The form is a rounded or conical hill, from a few metres to seventy metres high and up to several hundred metres across, standing on otherwise flat ground in permafrost regions. Beneath the soil and vegetation of the surface lies a core of almost pure ice, which is what holds the hill up. The summit is frequently cracked open where the surface has been stretched by growth, and those cracks expose the ice and begin the collapse. The name is from Inuvialuktun and means a small hill.
The two ways they form
Both routes trap water where it must freeze in place:
- •A drained shallow lake leaves saturated ground that begins to freeze
- •Freezing from the sides and top traps water in a shrinking pocket
- •That water freezes, expands and lifts the surface above it
- •Or groundwater under pressure flows towards a low point
- •Meets permafrost, freezes there and accumulates as a growing lens
- •The first kind is closed and local, the second fed from elsewhere
How they die
The collapse is as characteristic as the growth and leaves a recognisable scar. As the mound rises, the covering of soil is stretched thin and cracks open at the summit, exposing the ice core to summer air and to running water. The ice melts from the top down, the summit falls in, and the hill becomes a crater. Eventually the whole core is gone and what remains is a shallow pond surrounded by a raised rim of the collapsed covering material. Those rimmed ponds are common across arctic lowlands and mark hills that no longer exist.
The neighbouring ice landforms
Frozen ground builds a family of shapes and placing this one among them helps in reading an arctic landscape. Repeated freezing and thawing sorts stones into rings, stripes and polygons on flat ground, producing patterns that look artificial and are not. Wedges of ice grow downwards along contraction cracks and outline enormous polygons across whole plains. Small hummocks a metre or so high cover large areas where the surface layer has been churned. Lobes of saturated soil creep downslope over the frozen layer beneath. The feature here is simply the largest single structure the same processes produce.
Why they are studied now
The features have become indicators of a changing climate and a practical hazard at the same time. Collapsed remains found far south of the present permafrost limit, including in East Anglia and in Wales, mark ground that was frozen during the last glaciation and allow the former extent of permafrost to be mapped. Current collapse rates in the Arctic are monitored as a measure of warming ground. And a collapsing mound releases methane that was trapped beneath it, which is one of several feedbacks that make thawing permafrost a concern beyond the region itself.
The takeaway
A hill in flat permafrost ground stands on a core of nearly pure ice, formed either where a drained lake bed froze inwards and trapped water, or where pressurised groundwater froze on reaching frozen ground. Growth stretches and cracks the summit, which lets the core melt, leaving a pond with a raised rim. Such rimmed ponds map where permafrost used to reach.