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Ice Wedges, Polygons, and Pingos

As the arctic soil freezes and thaws over many hundred years, it is cracked and buckled to create ice wedges, polygons, thermocarst lakes, and pingos. 

This web page includes:
Steps of the cycle
Animation of the cycle 


Steps of the cycle:

There are a number of ways that polygons, arctic lakes, and pingos form. Here's one way this cycle works in the northern part of the Refuge:

permcy 1A cut-away view of the tundra in summer. The active layer is thawed.

permcy 2Winter cold causes the soil to shrink, and cracks to form. The active layer is frozen, so it acts just like the permafrost soils beneath it.

permcy 3During warm spring days, water seeps into the cracks. It freezes and expands when it is chilled by the still-frozen soil. The frozen water forms wedges of ice in the soil.

permcy 4In summer, the active layer and the tops of the ice wedges melt.

permcy 5Each winter, cracks form again in the same places...

permcy 6and each spring, additional water enters and enlarges the ice wedges as the freezing water expands.

permcy 7This cycle of crack, melt, and freeze continues to enlarge the wedges year by year...

permcy 8until the soil above the wedges is pushed up, forming ridges. If you look down from above, these ridges create a blocky pattern on the ground, called polygons.

permcy 9If the ice is exposed, a wedge may begin to melt.

permcy 10As more ice is exposed, the ice wedge and the active layer melt lower...

permcy 11until a pond begins to form.

permcy 12The pond water holds heat from the summer sun, so the active layer melts deeper beneath the water.

permcy 13Seen from above, these lakes (called thermokarst lakes) can become longer in one direction when prevailing winds blow waves against the down-wind shore.

permcy 14The lake side may break down, causing the lake to drain.

permcy 15Without its insulating cover of water, the active layer begins to refreeze.

permcy 16In winter, the surface freezes over a thawed remnant of the active layer.

permcy 17The very wet soil continues to freeze within the permafrost layer, even in summer.

permcy 18As the unfrozen area continues to contract, the unfrozen water is squeezed under great pressure.

permcy 19Eventually, the water is under such pressure that it pushes upward (the direction of least resistance)...

permcy 20until the unfrozen water collects under the root mat, and freezes, creating a pingo.

permcy 21If the root mat cracks open enough to expose the ice, the pingo top begins to melt.

permcy 22As the ice core continues to melt, the pingo collapses further.

permcy 23Continued melting over many years removes most traces of the pingo.

permcy 24If conditions are right, the cycle will begin again.

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Animation of the cycle:

permafrost ground formations animationThe full cycle in action:
 

Page Photo Credits — All photos courtesy of USFWS unless otherwise noted.
Last Updated: Jan 08, 2014
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