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:
A cut-away view of the tundra in summer. The active layer is thawed.
Winter 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.
During 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.
In summer, the active layer and the tops of the ice
Each winter, cracks form again in the same
and each spring, additional water enters and
enlarges the ice wedges as the freezing water expands.
This cycle of crack, melt, and freeze continues to
enlarge the wedges year by year...
until 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.
If the ice is exposed, a wedge may begin to melt.
As more ice is exposed, the ice wedge and the active layer melt lower...
until a pond begins to form.
The pond water holds heat from the summer sun, so the active layer melts deeper beneath the water.
Seen from above, these lakes (called thermokarst lakes) can become longer in one direction when prevailing winds blow waves against the
The lake side may break down, causing the lake to drain.
Without its insulating cover of water, the active layer begins to refreeze.
In winter, the surface freezes over a thawed
remnant of the active layer.
The very wet soil continues to freeze within the permafrost layer, even in summer.
As the unfrozen area continues to contract, the unfrozen water is squeezed under great pressure.
Eventually, the water is under such pressure that it pushes upward (the direction of least resistance)...
until the unfrozen water collects under the root mat, and freezes, creating a pingo.
If the root mat cracks open enough to expose the ice, the pingo top begins to melt.
As the ice core continues to melt, the pingo
Continued melting over many years removes
most traces of the pingo.
If conditions are right, the cycle will begin again.
Animation of the cycle:
The full cycle in action:
September 12, 2008