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:

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 wedges melt.

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

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 down-wind shore.

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 collapses further.

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

If conditions are right, the cycle will begin again.

Return to top
 

Animation of the cycle:

The full cycle in action: