Arctic National Wildlife Refuge
Alaska Region   

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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:

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:

active layer over frozen ground - USFWSA cut-away view of the tundra in summer. The active layer is thawed.

cracks form in soil - USFWSWinter 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.

ice freezes in cracks - USFWSDuring 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.

active layer thaws but ice remains in frozen ground - USFWSIn summer, the active layer and the tops of the ice wedges melt.

cracks expand - USFWSEach winter, cracks form again in the same places...

cracks freeze creating larger ice wedges - USFWSand each spring, additional water enters and enlarges the ice wedges as the freezing water expands.

cracks freeze creating larger ice wedges - USFWSThis cycle of crack, melt, and freeze continues to enlarge the wedges year by year...

soil is pushed up on each side of cracks - USFWSuntil 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.

crack widens at soil surface to expose ice wedge below - USFWSIf the ice is exposed, a wedge may begin to melt.

crack widens more and ice wedge melts deeper - USFWSAs more ice is exposed, the ice wedge and the active layer melt lower...

soil and water collect in opening - USFWSuntil a pond begins to form.

developing pond expands - USFWSThe pond water holds heat from the summer sun, so the active layer melts deeper beneath the water.

developing pond expands - USFWSSeen from above, these lakes (called thermokarst lakes) can become longer in one direction when prevailing winds blow waves against the down-wind shore.

pond begins to drain when soil edge deteriorates - USFWSThe lake side may break down, causing the lake to drain.

developing pond expands - USFWSWithout its insulating cover of water, the active layer begins to refreeze.

leaving deep area of thawed, moist soil - USFWSIn winter, the surface freezes over a thawed remnant of the active layer.

active layer freezes in winter over warmer, moist soil below - USFWSThe very wet soil continues to freeze within the permafrost layer, even in summer.

unfrozen water - USFWSAs the unfrozen area continues to contract, the unfrozen water is squeezed under great pressure.

water pushed upward under pressure - USFWSEventually, the water is under such pressure that it pushes upward (the direction of least resistance)...

unfrozen water collects under root mat of active layer and freezes - USFWSuntil the unfrozen water collects under the root mat, and freezes, creating a pingo.

soil cracks open to expose ice - USFWSIf the root mat cracks open enough to expose the ice, the pingo top begins to melt.

ice core melts - USFWSAs the ice core continues to melt, the pingo collapses further.

pingo disappears from tundra - USFWSContinued melting over many years removes most traces of the pingo.

cycle begins again with cracks in tundra - USFWSIf conditions are right, the cycle will begin again.

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Animation of the cycle:

animation of ice wedge and pingo creation - USFWSThe full cycle in action:

September 12, 2008