Ice wedge

Ice wedges in Sprengisandur, Iceland

A melting pingo with surrounding ice wedge polygons near Tuktoyaktuk, Canada

An ice wedge is a crack in the ground formed by a narrow or thin piece of ice that measures up to 3–4 meters in length at ground level and extends downwards into the ground up to long meters meters. During the winter months, the water in the ground freezes and expands. Once temperatures reach −17 degrees Celsius or lower, the ice that has already formed acts like a solid and expands to form cracks in the surface known as ice wedges. As this process continues over many years, ice wedges can grow up to the size of a swimming pool. The ice wedge usually appears in a polygonal pattern known as ice wedge polygons. The cracks can also be filled with materials other than ice, especially sand, then it's called sand wedge.

Formation

The origin of ice wedges has many theories but only one has consistently been backed by most prominent scientists: the thermal contraction theory.

Thermal contraction theory

The Thermal Contraction Theory states that during the winter months, thermal contraction cracks form only a few cm wide and a couple of metres deep because of the extreme cold weather. Over the next few months, the snow melts and the remaining water fills the cracks and the permafrost below the surface freezes it. These tiny cracks turn into permafrost. Once the summer months arrive, the permafrost expands; the fact of horizontal compression produces upturning of the frozen sediment by plastic deformation. The next winter the cold refreezes and cracks the already forming ice wedge and opens a way for the eventual melting snow to fill the empty crack. The mean annual air temperature thought needed to form ice wedges is −6° to −8°C or colder.

Forms

There are three different forms of ice wedges: active, inactive and casts. All three forms are prevalent today and can be found in different parts of the world.

Active

Active ice wedges are those that are still evolving and growing. During each year, a layer of ice will be added if cracking occurs, but cracking need not occur every year to be considered active. The zone in which most ice wedges remain active is along the permafrost zone. The amount of active ice wedges that is cracking yearly is consistently declining and becoming inactive.

Inactive

Inactive ice wedges are wedges that are no longer cracking and growing. Throughout the winter months, the wedge does not split and therefore in the summer no new water is added.

Casts

In areas of past permafrost, ice wedges have melted and are no longer filled with ice. The wedge, which is now empty, is filled with sediment and dirt from the surrounding walls. These are called ice wedge casts and are used to calculate the climate of hundreds of thousands of years ago.